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I am a graduate student studying astronomy at the University of Washington.

The Astronomist.
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April 12, 2012
09:49 PM
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Disassociate Galaxy Clusters

by The Astronomist in The Astronomist.

A dissociative galaxy cluster is a cluster of galaxies that just can't keep it together any longer. This may sound like an unnecessary anthropomorphication of galaxies, but it is actually a description of galaxy clusters which have collided and experienced stratification of their constituent parts. In the standard and successful model of cosmology the largest scale structures in the universe, like super clusters of thousands of galaxies, form via the merger of filamentary structures composed of smaller clusters of galaxies. Gravity keeps pulling clusters together along highways of galaxy clusters. Occasionally it is expected and observed that galaxy clusters meet each other head on in cosmic train wrecks moving at thousands of kilometers per second. These traumatic merging events scar the galaxy clusters for life. Their post traumatic stress afflictions include hot shocked X-ray gas and galaxies displaced from their gas halos. Lets consider the three main constituents of a galaxy cluster: stars, gas, and dark matter.Clusters are made of aggregates of hundreds or thousands of galaxies and each galaxy is made of hundreds of billions of stars. The stars of the galaxy cluster are conspicuous in that they shine and are observable in pictures, but they account for only about 5% or less of the cluster's mass. The luminous stars of galaxies don't interact much during a collision with another cluster of galaxies and so they act like people in two crowds which are moving in opposite directions. Stars are part of the cosmic ghost train.The gas in galaxy clusters accounts for about 10% of the regular (or baryonic) mass in clusters. Gas does interact during a collision. The gas clouds in colliding galaxy clusters slams together like two waves of water meeting and stalls out, but not without undergoing a process known as shock heating first which raises the gas temperature to millions of degrees.Gas is part of the cosmic train wreck.The dark matter in galaxy clusters is the most dominant part of the cluster by mass making up about 90% the mass. Dark matter does not interact much. The dark matter halos travel right through each other like ghosts when two clusters collide. However, it is possible that the dark matter does interact slightly and dissociative collisions are a powerful tool in constraining this dark matter interaction. The dark matter halos of the colliding clusters should sail right past each other like two ghost trains, but if the trains slow down even in the slightest it may indicate something strange.These so called dissociation mergers are difficult to observe and analyze. They require telescopes in space, follow up observations on the ground, observations in multiple wavelength regimes, and algorithms to predict the distribution of dark matter. So far there are six such dissociation mergers systems detected. You would think it would be obvious to spot some of the most massive structures in the universe smashing into each other, but spotting galaxy clusters is actually very difficult because of their great distance. Perhaps in an optical survey, like that in the image below taken by the Hubble Space telescope, over densities of galaxies are detected. In practice many times it is easier to first identify galaxy clusters through their gas content because the gas content is more massive than the stellar component. Many new clusters are identified by observing the cluster gas's effect in the microwave regime or in the X-ray regime. In the image below taken by the the NASA Chandra X-ray observatory the hot intracluster gas is seen in pink. This image corresponds to exactly the same field of view on the sky as the optical image above.It may dawn on you that by the very definition of dark matter there is no telescope which can observe it directly. The only in way in which dark matter interacts strongly is through gravity and thus that is how astronomers look for it. Through theoretical predictions and confirmed observations we know that gravity bends light and thus massive galaxy clusters will bend the light of even more distant galaxies. Thus through weak gravitational lensing the dark matter betrays its presence. A careful statistical analysis of galaxy shapes in the optical image above reveals that the galaxies which are confirmed not to be in the foreground cluster are slightly distorted in shape via the gravitational force of the dark matter which is in the foreground. A reconstruction of the total mass in the clusters is shown in the image below where the parts of the cluster which have the most mass are shown in blue. This image corresponds to exactly the same field of view on the sky as optical and X-ray images above.Finally, a superposition of all the data allows us to glimpse at what a crisis this merging cluster is in. Note that the optical image remains in its original color, the gas is in pink, and the mass is in blue. The image below is known as the Musket Ball Cluster. The actual collision of galaxies occurred about 700 million years ago. We can rewind the collisions in our heads and envision that blue/optical cluster on the right of the image was once on the left and so the blue/optical cluster on the left of the image was once on the right; the clusters collided head on and the gas stopped dead at the center, but the galaxies and dark matter hardly stopped. There are several other images below of other dissociative cluster mergers with the same color scheme. Notice the different morphologies and distributions of mass, stars, and gas. The collisions are not always so straight forward.Musket Ball Cluster. X-ray: NASA/CXC/UCDavis/W.Dawson et al; Optical: NASA/STScI/UCDavis/W.Dawson et al.Train Wreck Cluster. X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al.Bullet Cluster. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.The awesome thing about these cosmic mergers is how they can constrain the dark matter self-interaction cross-section. That is, exactly who much does dark matter interact with itself? The interpretation of these collisions is not always simple such as in the Train Wreck Cluster (seen above) where there seems to be an extra dark matter core not associated with any bright galaxy at the center of the image, but nonetheless these mergers can be thought of as astrophysical laboratories of dark matter. It would be ver... Read more »

Dawson, W., Wittman, D., Jee, M., Gee, P., Hughes, J., Tyson, J., Schmidt, S., Thorman, P., Bradač, M., Miyazaki, S.... (2012) DISCOVERY OF A DISSOCIATIVE GALAXY CLUSTER MERGER WITH LARGE PHYSICAL SEPARATION. The Astrophysical Journal, 747(2). DOI: 10.1088/2041-8205/747/2/L42

Jee, M., Mahdavi, A., Hoekstra, H., Babul, A., Dalcanton, J., Carroll, P., & Capak, P. (2012) A STUDY OF THE DARK CORE IN A520 WITH THE : THE MYSTERY DEEPENS . The Astrophysical Journal, 747(2), 96. DOI: 10.1088/0004-637X/747/2/96

Markevitch, M., Gonzalez, A., Clowe, D., Vikhlinin, A., Forman, W., Jones, C., Murray, S., & Tucker, W. (2004) Direct Constraints on the Dark Matter Self‐Interaction Cross Section from the Merging Galaxy Cluster 1E 0657−56. The Astrophysical Journal, 606(2), 819-824. DOI: 10.1086/383178

December 12, 2011
03:10 PM
1,356 views

The First Quantum Computer

by The Astronomist in The Astronomist.

In a nondescript office park outside Vancouver with views of snow capped mountains in the distance is a mirrored business park where very special work is being done. The company is D-Wave, the quantum computing company. D-Wave's mission is to build a computer which will solve humanity's grandest challenges.D-Wave aims to develop the first quantum computer in the world, perhaps they already have. The advent of quantum computers would be a sea change in the world that would allow for breaking of cryptography, better artificial intelligence, and exponential increases in computing speed for certain applications. The idea for quantum computers has been bubbling since Richard Feynman first proposed that the best way to simulate quantum phenomena would be with quantum systems themselves, but it has been exceedingly difficult to engineer a computer than can manipulate the possibilities of quantum information processing. Hardly a decade ago D-Wave began with a misstep which is the origin of their name. D-Wave got its name from their first idea which would have used yttrium barium copper oxide (YBCO) which is a charcoal looking material with a superconducting temperature above that of the boiling point of liquid nitrogen. This means that YBCO is the standard science lab demonstration of superconducting magnetic levitation. Ultimately the crystalline structure of YBCO was found to be an imperfect material, but the cloverleaf d-wave atomic orbital that lends YBCO its superconducting properties stuck as D-Wave's name. The vision of D-Wave did not change, but their approach did. They realized they would have to engineer and build the majority of the technology necessary to create a quantum computer themselves. They even built built their own superconducting electronics foundry to perform the electron beam lithography and metallic thin film evaporation processes necessary to create the qubit microchips at the heart of their machine.I recently got to visit D-Wave, the factory of quantum dreams, for myself. The business park that D-Wave is in is so nondescript that we drove right by it at first. I was expecting lasers and other blinking lights, but instead our University of Washington rented van pulled into the wrong parking lot which we narrowly reversed out of. In the van were several other quantum aficionados, students, and professors, mostly from computer science who were curious at what a quantum computer actually looks like. I am going to cut the suspense and tell you now that a quantum computer looks like a really big black refrigerator or maybe a small room. The chip at the heart of the room is cooled to a few milikelvin, colder than interstellar space, and that is where superconducting circuits count electric quantum sheep. The tour began with us milling around a conference room and our guide, a young scientist and engineer, was holding in his hand a wafer which held hundreds of quantum processors. I took a picture and after I left that conference room they did not let me take any more pictures.Entering the laboratory it suddenly dawned on me that this wasn't just a place for quantum dreams it was real and observable. The entire notion of a quantum computer was more tangible. A quantum computer is a machine which uses quantum properties like entanglement to perform computations on data.The biggest similarity between a quantum computer and a regular computer is that they both perform algorithms to manipulate data. The data, or bits, of a quantum computer are known as qubits. A qubit is not limited to the values of 0 or 1 as in a classical computer but can be in a superposition of these states simultaneously. Sometimes a quantum computer doesn't even give you the same answer to the exact same question. Weird. The best way to conceive of a quantum computing may be to imagine a computation where each possible output of the problem has either positive or negative probability amplitudes (a strange quantum idea there) and when the amplitudes for wrong answers cancel to zero and right answers are reinforced.The power of quantum computers is nicely understood within the theoretical framework of computational complexity theory. Say for example that I give you the number 4.60941636 × 1018 and ask for the prime factors of this number. Now if someone were to give you the prime factors you could verify them as correct very quickly, but what if I asked you to generate the prime factors for me (I dare you. I have the answer. I challenge you). The quintessential problem here is the P versus NP question which asks whether if a problem can be verified quickly can it also be solved quickly. Quickly is defined as polynomial time meaning that the algorithm scales as the number of some inputs to some power. Computational complexity theory basically attempts to categorize different kinds of problems depending on how fast a solution can be found as the size of the problem grows. A P class problem is one in which the solution can be found within polynomial time. A NP class problem is one in which the solution can be verified in polynomial time. So if I ask you for the prime factors of my number above that is an NP problem because given the numbers you could verify the answer quickly, but it would be very difficult to calculate the numbers just given the number. It is an open question, but it appears likely that all P problems are a subset of NP. This means that problems verifiable in polynomial time are not necessarily solved in polynomial time. The issue is that for some very interesting problems in the real world we could verify the answer if we stumbled upon it, but we won't even be able stumble upon the answer in a time shorter than the age of the universe with current computers and algorithms. What we know we know and what we think we know is a sea of confusion, but the popular opinion and where people would take their wagers is that P is not equal to NP.Suddenly, with mystique and spooky actions at a distance, quantum computing comes swooping in and claims to be able to solve some NP problems and all P problems very quickly. A general quantum computer would belong to the complexity class of BQP. There is a grand question at hand, is BQP in NP? (More generally, is BQP contained anywhere in the polynomial hierarchy? The polynomial hierarchy is a complexity class which generalizes P and NP problems to a particular kind of perfect abstract computer with the ability to solve decision problems in a single step. See this paper here on BQP and the Polynomial Hierarchy by Scott Anaronson who is a outspoken critic of D-Wave) At this time we cannot even claim to have evidence that BQP is not part of NP, but most scientists close to the problem think that BQP is not a subset of NP. Quantum computing researchers are trying to get better evidence that quantum computers cannot solve NP-complete problems in polynomial time (if NP was a subset of BQP then the polynomial hierarchy collapses). A reasonable wager I would take is that P is a (proper) subset of BQP and BQP is itself is a (proper) subset of NP. This claim has not been rigorously proved but it is suspected to be true and further there are some NP problems which it has been shown to be true for such as prime factorization and some combinatoric problems.There might be an elephant in the room here. The D-Wave architecture is almost certainly attacking a NP complete problem and reasonable logic says that quantum computers will solve P problems and some NP problems, but not NP complete problems (this is also not proven, but suspected). An NP complete problem is a problem in which the time it takes to compute the answer may reach into millions or billions of years even for moderately large versions of the problem. Thus we don't know if this particular quantum computer D-Wave has built even allows us to do anything efficiently we couldn't already do on a classical computer efficiently; it doesn't seem to be a BQP class computer thus it cannot for example solve prime factorization cryptography problems. So, yes it is a quantum machine, but we don't have any evidence it is an interesting machine. At the same time we don't have any evidence it is an uninteresting machine. It is not general purpose enough to be clear it a a big deal, nor is it so trivial it is totally uninteresting.The D-Wave lab was bigger than I expected and it was at once more cluttered and more precise than I thought it would be. It turns out the entire process of quantum computing follows this trend. There are a lot of factors they contend with and on the tour I saw people dead focused with their eyes on a microscope executing precise wiring, coders working in pairs, theoreticians gesturing at a chaotic white board, and even automated processes being carried on by computers with appropriately looking futuristic displays. The engineering problems D-Wave faces include circuit design, fabrication, cryogenics, magnetic shielding and so on. There is too much to discuss here so I will focus on what I think are scientifically the two most interesting parts of the D-Wave quantum computer which are the qubit physics and the quantum algorithm which they implement; in fact these two par... Read more »

Harris, R., Johansson, J., Berkley, A., Johnson, M., Lanting, T., Han, S., Bunyk, P., Ladizinsky, E., Oh, T., Perminov, I.... (2010) Experimental demonstration of a robust and scalable flux qubit. Physical Review B, 81(13). DOI: 10.1103/PhysRevB.81.134510

Harris, R., Johnson, M., Han, S., Berkley, A., Johansson, J., Bunyk, P., Ladizinsky, E., Govorkov, S., Thom, M., Uchaikin, S.... (2008) Probing Noise in Flux Qubits via Macroscopic Resonant Tunneling. Physical Review Letters, 101(11). DOI: 10.1103/PhysRevLett.101.117003

July 27, 2011
04:26 AM
736 views

A Cubic Millimeter of Your Brain

by The Astronomist in The Astronomist.

Are there more connections in a cubic millimeter of your brain than there are stars in the Milky Way? We are going to answer that question in a moment, but first take a look at this image of hippocampal neurons in a mouse's brain. It is an actual color image from a transgenic mouse in which fluorescent protein variations are expressed quasi-randomly in different neurons. This kind of image is known as a brainbow and is aesthetically awesome further it may we one way to empirically examine a volume of the brain (tomography) to answer this question.In reality mapping even an entire cubic millimeter of the brain is extremely daunting task, but we can still answer this question. First, I know that there are different kinds of neurons that vary in size and that some neurons can have a soma (the big part that has the nucleus from which the dendrites extend) spanning a millimeter in size. Thus if you picked a random cubic millimeter of brain you could run right into the heart of a neuron and you would find very few connections. Given this fact, we can very easily answer this question with a resounding no, however, this seems like an unsatisfactory trite approach. So I looked up some numbers on how many neurons are in the brain, how many connections are in the brain, and how many stars are in the Milky Way. Lets answer the question using the 'average' number of connections per cubic millimeter.How many neurons and connections there are in the brain? This is kind of a tricky question and I am not a nuerobiologist so I have gone to several resources for the answer. Professor of Computational Neuroscience at MIT Sebastung Seung says in a TED talkyour brain contains 100 billion neurons and 10,000 times as many connectionsProfessor of Molecular Cellular Physiology at Stanford Stephen Smith says in a press release on brain imaging thatIn a human, there are more than 125 trillion synapses just in the cerebral cortex aloneRené Marois from the Center for Integrative and Cognitive Neurosciences at Vanderbilt Vision Research Center states in a recent paper [1]The human brain is heralded for its staggering complexity and processing capacity: its hundred billion neurons and several hundred trillion synaptic connections can process and exchange prodigious amounts of information over a distributed neural network in the matter of milliseconds.I have enough expert sources now to confidently say these experiments agree that the human brain has some 100 billion neurons (1011). The number of connections seems less precise, but it is at least several 100 trillion connections (1014) as judged by Marios and Smith and as much as 1015 as judged by Seung.The number of connections in the brain is tricky to define. We may define a synaptic connection as each place the neuron touches another neuron and a synapse is present. It doesn't seem to make sense to simply count incidental contact. Further, there is the question of whether we should count redundant contacts between neurons. We can obtain an upper bound on the number of connections in the brain by considering the case in which every neuron is connected to every other neuron. Coincidentally the operation of connecting every node in a network with every other node is a process I am familiar with from cross correlating radio signals. Anyways, the equation we are looking for is N(N+1)/2 where N is the number of nodes in the network. Thus, for our N=1011 neurons the maximum number of non-redundant connections is about 1022. This maximum bound is huge! But how huge is it really? Hilariously, while searching for an answer to my original question I found a message board pondering the grand statementThere are more connections in the brain than atoms in the Universe.A really clever person pointed out thatTheoretically, if we took all the atoms in the universe; wouldn't that include the atoms within the brain?People have this feeling that the number of connections between items can be much larger than the number of actual items in the collection and while this intuition is true the idea that there are more connections in the brain than there are atoms in the universe is absurd. Lets put it in perspective that a few grams of any substance, like water, is measured units of moles. A mole is standard unit of measurement corresponding to the absolute 6.02 x 1023. Thus even a drop of water contains more atoms than there are connections in the brain.Now we need to know how many neurons and connections are in an average cubic millimeter of the brain. How big is the brain? John S. Allen of the Department of Neurology at University of Iowa stated in a recent paper that[2] The mean total brain volumes found here (1,273.6 cc for men, and 1,131.1 cc for women) are very comparable to the results from other high-resolution MRI-volumetric studies.We can take the volume of the brain as 1000cc as a low estimate (which will only over estimate the density of connections).The final thing we need to know to answer the question at hand is the number of stars in the Milky Way. Like every other number we have been working with it is rather uncertain. Even if we define a star as only those spheres of gas which are large enough to fuse hydrogen at some point in their lifetime we don't know the answer because we can't see the multitudes of dim stars. There are probably at least 500 billion star like objects in the Milky Way. Lets take 100 billion as the number to be conservative. Finally, lets bring all the numbers together. One cubic millimeter is 1/1000 of a cubic centimeter and 1/1000000 (10-6) of the entire volume of the brain. We can scale the total number of connections in the brain (using the high estimate of 1015 connections in the brain) then we find that there are 109 connections in a cubic millimeter of the brain. The 109 connections in a cubic millimeter of the brain is two orders of magnitude smaller than a low estimate of the number of stars in the Milky Way. No, on average there are not more connections in a cubic millimeter of your brain than there are stars in the Milky Way. My first response to this question was bullshit! This question (or rather statement) is made by David Eagleman here at a TEDx talk and here on the Colbert Report. Colbert also called out Eagleman when he dropped this factoid, but it didn't stop the interview. I have also contacted some actual neuroscientists to see what they thought of this statement and they agree with me that it is not true. Maybe there is special part of the brain particularly more dense in connections than the brain on average, but that would be misleading like saying the density of the Milky Way is that of water because, you know, certain parts of the Milky Way are water. The better statement would be to say that there are are more connections in the brain than there are stars in the Milky Way. As Colbert would say, I am putting you on notice Eagleman.While we are on the subject I want to mention my favorite talk about the brain which mixes just the right amount of wonder and fact. It is the TED talk I mentioned earlier by Sebastian Seung on what he calls the connectome - the serious of connections in your brain between neurons which physically dictates how you think. In the video he discusses another volume tomography technique in the brain using a cube of mouse brain tissue just 6 microns on a side. It is another great visualization for what is actually in a cubic millimeter of your brain. ... Read more »

Marois, R., & Ivanoff, J. (2005) Capacity limits of information processing in the brain. Trends in Cognitive Sciences, 9(6), 296-305. DOI: 10.1016/j.tics.2005.04.010

Allen, J., Damasio, H., & Grabowski, T. (2002) Normal neuroanatomical variation in the human brain: An MRI-volumetric study. American Journal of Physical Anthropology, 118(4), 341-358. DOI: 10.1002/ajpa.10092

March 25, 2011
11:01 PM
1,081 views

Naming the Unknown #1

by The Astronomist in The Astronomist.

Naming the Unknown is a new series where I highlight interesting papers in astrophysics. Research papers which I find compelling or of general interest will be spotlighted. The title 'Naming the Unknown' comes from accusation that cosmologists have simply begun to come up with names for those things which are not understood; yet, I do not think that anyone would claim that science is at times anything other than coming up with names for the unknown. Scientists define the unknown in terms of the other unknowns and as time passes the first unknown has a context, but ultimately all we have a is a self referential group of symbols that isn't necessarily any more logically sound than where we begun. The relation of fundamental particles (μ+ compared to a π+, obvious relation no?), the definition words in a dictionary, and all information suffers from the flaw of self referential formalisms. I digress. On to the first paper.* * * Fermi Bubbles: Giant, Multibillion-Year-Old Reservoirs of Galactic Center Cosmic Rays. Roland M. Crocker , and Felix Aharonian. DOI: 10.1103/PhysRevLett.106.101102The Fermi telescope recently discovered evidence for giant gamma ray lobes associated with our Milky Way (you should see my first posted about the Fermi Bubbles here if you are not familiar with this remarkable discovery). The original paper on the Fermi bubbles was an observation description of the evidence for the bubbles; it was found that there are lobes extending above and below the plane of the Milky Way symmetrically with an extent of ~10 kpc and a unique energy spectrum. Several possible formation scenarios for bubbles were put forward, but no single theory was advanced a definitive. In a paper published this month in Physical Review Letters Roland Crocker and Felix Aharonian conclude that the bubbles are naturally explained due to a population of relic cosmic ray protons and heavy ions injected into the bubbles by high density star formation in the galactic center. There are two general lines of thought as to the source of the bubbles. In one scenario the black hole at the center of our milky way is somehow responsible for the gamma ray lobes. The black hole paradigm can further be broken down into two sub-categories: tidal disruption of a star and active galactic nuclei.The tidal disruption of a star occurs when the super massive black hole at the center of our Milky Way, Sagatarius A*, disrupts, or basically eats a wandering star. The tidal disruption of stars by black holes is viable and certainly does occur (see work by Guillochon et al. 2009) with the release of energy, hot plasma, wind, and shocks which could heat up the halo and produces thermal x-rays (see ongoing work by Cheng et al. 2011, unpublished). However, this explanation for the Fermi bubbles is slightly ad hoc and it would have to occur on a periodic basis to account for the bubbles.Saggatarius A* is dormant now, but if a star cluster or gas cloud fell/accreted into it in the past it may have undergone an active galactic nuclei like phase which could emit sufficient radiation and cosmic rays to explain the bubbles. This active galactic nuclei scenario would also have to occur periodically (10 million years or so) to explain the presence of the bubble.Lets forget super massive black holes and look at a simple alternative. Crocker and Aharonian invoke ongoing star formation in the galactic center to explain the hard-spectrum, uniform intensity, vast extension, and the energetics seen in the bubbles. Extremely long time scale star formation (on the order a billion years) will have with it an associated cosmic ray population which will be injected into the bubbles by a wind. Cosmic rays are hadrons (mainly protons and some heaver ions) accelerated by non thermal process such as supernovae shocks which move at extremely high velocities and thus carry lots of energy. The cosmic rays will lose energy primarily though collisions with other protons (pp collisions) in the low density plasma of the bubbles and subsequently produce gamma rays electrons, positrons, and neutrinos (intermediate meson particles are also created). This would explain the gamma ray emission at >100 Mev seen form the galactic plane. Under many conditions this kind of gamma ray emission would be expected to trace the underlying ambient density of matter with which the cosmic rays are colliding with, however, in the case of the Fermi bubbles the time scale for proton collisions and time scale for the particles to escape from the system are comparable. The bubbles would be a saturated system wherein the gamma ray luminosity is only proportional to the power injected independent of the gas density; this is a vital point in explaining the morphology of the lobes: they have a hard spectrum out to their edge and then end abruptly.Based on IRAS satellite data the galactic center star formation rate is ~0.08 solar masses per year in turn implying a rate of ~0.04 supernovae per century. These supernovae inject power at a rate of 1039 ergs per second into cosmic rays. These cosmic rays are removed the from the immediate vicinity of the galactic center and transported into the bubble regions by a super wind generated by the ongoing star formation and supernovae themselves. A wind such as this is observed commonly in many other star forming galaxies such as NGC 3079. The wind speed is ~1200 kilometers per second and has sufficient velocity to escape locally, but it has been shown that the wind should stall at a height less than ~15 kiloparsecs above the plane and this would explain the exact height of the bubbles.That continuous star formation (and subsequent supernovae) could be responsible for the Fermi bubbles is an Occam's razor kind of solution. It reproduces a number of observations seen in the bubbles and predicts some further properties. For example the electrons and positrons which are created along with the gamma ray emission will synchrotron radiate because of ambient magnetic fields with a luminosity of ~1026 ergs per second which is exactly what is seen in the 20-60 Ghz band by the WMAP satellite (the so called WMAP haze). This kind of after the fact observation is not so impressive, but the authors make various predictions which will be testable in future observations.***A large aside on the mess of publishing, press releases, and open access. When NASA made the press release on the Fermi bubbles and I first blogged about them I stated that I didn't have anything to go on besides the press release because no paper was available. How wrong was I! The first paper was published in The Astrophyical Journal, titled Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?, in November of 2010 (at the same time as the press release), but it was posted on the arxiv on the 29th of may 2010. This paper I mentioned here today was published the 16th of August 2010 on the arxiv and then published just a few days ago in March in physical review letters. It is astounding how long the peer review process took for each of these papers, but it is deplorable that NASA doesn't make readily available links to the actual paper. I could have told you about the Fermi bubbles and given a natural explanation for them about a year ago if I had been on top of this.... Read more »

Crocker, R., & Aharonian, F. (2011) Fermi Bubbles: Giant, Multibillion-Year-Old Reservoirs of Galactic Center Cosmic Rays. Physical Review Letters, 106(10). DOI: 10.1103/PhysRevLett.106.101102

February 25, 2011
06:27 PM
924 views

Fusion for the Future: NIF

by The Astronomist in The Astronomist.

Fusion is only 50 years away and it will solve all of the worlds energy problems. That is the good news. The bad news is that it has been 50 years away for the last 50 years. If that situation is maddening to you then you are not alone. Leonardo Mascheroni, a retired Los Alamos National Laboratory physicist, wanted funding to build a colossal laser for producing energy from fusion and was willing to trade the United States' nuclear weapons secrets to realize his dream. Mascheroni was recently indicted on charges of treason concerning selling nuclear arms secrets and is awaiting trial sometime this year. In the meantime the United States is pressing forward with a completely separate laser fusion project called the National Ignition Facility or the NIF which uses 192 lasers fired in unison to recreate the energy source of the stars harnessed on Earth.In this post I am going to talk about the basics of fusion and the NIF. I also have questions and answers with a physicist on the project, Siegfried Glenzer, at the end of the post. I asked him some hard questions not just about the science, but also about the politics going on around the project. Physicists would like their experiments and budgets to work in a vacuum, but alas they never are. I deeply thank doctor Glenzer for answering my questions. What is fusion?Fusion is the joining of two or more separate atomic nuclei into a larger nuclei. Fusion can create energy because the mass of the input and output nuclei are not necessarily equal in mass. Specifically, if the mass of the output nuclei is less than the total mass of the input nuclei then the mass difference is made up by the production of energy as Einstein taught us E=mc2 (conversely if the output nuclei are more in total mass than the input nuclei then the reaction would consume energy). In particular, stars like our Sun fuse lighter elements into heavier elements up until the point the star is attempting fusion of iron which does not produce energy because iron has the largest binding energy per nucleon. Actually fusion processes in stars normally involve several intermediate nuclei or elements. The most important process for our Sun is the proton-proton chain which fuses four hydrogen nuclei, 11H, to form a single helium nuclei 42He with a mass difference of ΔM. Einstein's mass energy relation shows us how much energy this process releases.4 ⋅ 11H -42He = ΔMΔM c2 ≈ 27 MeVThe key to joining two nuclei together is overcoming the repulsive electric Coulomb force between nuclei. The positive charge on nuclei repel each other until the two nuclei actually meet and then the attractive short range strong nuclear force takes over to bind the two nuclei into new larger nuclei. The fewer the number of protons in the nuclei the easier it is to fuse. The repulsive force between nuclei may be overcome in several ways. Inside stars heat and pressure, which comes from the stars gravitational contraction, occasionally forces two nuclei close enough together for them to fuse and all together the star burns consistently for a very long time. The more massive the star the hotter and denser it is at the center so larger nuclei can be fused. The production of heavier elements by stars fusing hydrogen is essentially the origin of all elements heavier than lithium; massive stars occasionally explode, and thus we are all made of stardust. The input elements for the first fusion reactors will be the hydrogen isotopes of deuterium (H with a neutron) and tritium (H with two neutrons) because this reaction has the highest nuclear cross section and high energy yield.Why is fusion important?Fusion is very important; this is the kind of physics that future presidents should understand. In this post I am focusing on the basics of fusion and the prospects for the National Ignition Facility and a in a future post I will talk about another project known as ITER. I should clarify that there are effectively many different kinds of fusion machines and an important distinction is net energy positive and net energy negative machines. The ratio of fusion power to input power (often denoted Q in the field) must be positive to have a viable energy solution. There exist at this moment very many fusion machines which take more input power than they make in output power (they have a fractional Q value). Some of the current machines seem fantastic like 'table top' pyroelectric fusion devices, but the reality is that they take energy to run and have no foreseeable future in the energy game. These devices play a role as portable neutron generators in labs for various research purposes or in security as nuclear material detectors. Net energy positive machines have not yet been invented. The NIF will not produce energy, but will be a testbed for fusion technologies. The fusion technology goal is the sustainable production of energy from abundant raw elements such as hydrogen, helium, or related isotopes (Helium 3, deuterium, tritium). Fusion using these light elements is cheap, safe, and green. Fusion is cheap (however the technology development is expensive!) because the raw elements like hydrogen are abundant, further as a consequence of this virtually infinite supply (one in every 6,500 atoms on Earth is a deuterium atom) it can be considered a renewable energy. Fusion is safe because when a fusion nuclear reactor malfunctions unlike a fission nuclear reactor the reaction will snuff itself out rather than proceed uncontrollably to the point of a thermonuclear explosion. Finally, fusion is green or environmentally friendly because it produces no climate altering products.There are so many reasons fusion is important. Fusion is the future. It is the next step in humanity's technological evolution. This video from the BBC Horizons series with physicist Brian Cox gives a cursory look at the NIF, and puts the entire endeavor into perspective (and to boot in finishes with The Kinks This Time Tomorrow which has the most appropriate lyrics ever).How do we use fusion to make energy?Under the correct conditions of incredibly high density, pressure, and temperature a self sustaining fusion process can occur. These conditions are of course exactly what you find at the center of a star, but on Earth these conditions are engineered via the use of confinement and heating mechanisms. The NIF will use a symphony of lasers to simultaneously heat and compress a pellet of deuterium and tritium to simulate the conditions inside of a star. A deuterium and tritium target has been chosen for this first experiment because the fusion cross section between deuterons and tritons is three orders of magnitude larger than for any other atoms. Other fusion projects like ITER will use a toroidal (or doughnut shaped) chamber known as a tokamak to confine a deuterium and tritium plasma which is then heated through magnetic field confinement or radio frequency heating kind of like a big nuclear microwave. Once the fusion process is begun radiation and fast neutrons will be emitted which will be absorbed by the walls of the machine in order to gather heat to drive a steam-turbine generator to produce electricity pretty much just like every power plant.How does NIF work?It all starts with a single primordial laser source with very low power which is slightly preamplified and split into 48 parts. These pulses are then amplified by a factor of 10 billion in another set of preamplifiers then they are split into 192 parts and sent to the main amplifier. Then electrical energy stored in capacitors is dumped into 7680 xenon flashtubes which operate pretty much like the flash on your camera, except they are over 6 feet tall and take 30 kilojoules of input power each. The bright incoherent full spectrum light from the flashtubes passes through Neodymium doped glass and in a stupendously inefficient process amplifies the laser beams. The lasers bounce back and fourth a few times and finally go through the amplifier and the main optics system again before heading to the target chamber. At this point the primordial laser has been amplified by a factor of 1015 (in the video below he says quadrillion which apparently doesn't even have an agreed upon meaning, I think 1015 is right). The beams trave... Read more »

Glenzer, S., MacGowan, B., Michel, P., Meezan, N., Suter, L., Dixit, S., Kline, J., Kyrala, G., Bradley, D., Callahan, D.... (2010) Symmetric Inertial Confinement Fusion Implosions at Ultra-High Laser Energies. Science, 327(5970), 1228-1231. DOI: 10.1126/science.1185634

January 21, 2011
08:41 PM
1,283 views

The Universe and Life is asymmetric: Chirality

by The Astronomist in The Astronomist.

The shadow of symmetry haunts physics. Symmetry is invoked to understand nature concisely, but broken symmetry is invoked to understand nature completely. Physics is filled with examples of shattered symmetries: there is more matter than antimatter, neutrinos only come in the left handed spin flavor, and quantum processes break symmetries constantly, but nature also violates symmetry in chemistry and biology in a very clever manner. Chemistry and biology are subjects I do no normally touch upon, but I am intrigued by the curious circumstance of life on Earth: many molecules are not superimposable upon their mirror images, a property called chirality, and life on Earth has a preference for these chiral mirror configurations. Physics and life is inherently asymmetric.That something is not identical to its mirror image is a property known as chirality. Hands (etymologically the word chirality is derived from the Greek word for hand), spiral galaxies, and the DNA helix are all examples of chiral objects. In particle physics chirality is actually an abstract notion defined by transformations of the particle with respect to their right of left handed representation in the Poincaré group. In chemistry chirality is well described by analogy to your hands wherein left and right hands cannot be superposed on each other even though the fingers are the same and match up. This article is an exploration of chirality in biochemistry. I want to ask what makes life chiral, why is life chiral, and how did life become chiral. In order to supplement my limited knowledge of the subject I interviewed a world expert and author of over twenty papers on the subject, Robert Compton, who I must give a deep thanks to for being willing to answer my silly questions.It is important to accept that the concept of symmetry is tinted by the human notion of harmonious or aesthetically pleasing forms, but the strict mathematical interpretation of symmetry relies upon metrics of geometry. To this end many seemingly symmetric forms in the living natural world are actually examples of broken symmetries: spiral tree trunks, the human form, and sea shells (which generally only coil in one particular direction according to species). The remarkable thing is that this macro asymmetry can be traced back to a micro asymmetry in the chemistry of life. The arrangement of atoms in a molecule defines the function of that molecule, but even molecules with the same chemical configuration can behave differently as in the case of chiral molecules which are like mirror images of the same molecule that come in 'left' and 'right' handed forms. The great asymmetry of life is that all living organisms on Earth almost exclusively utilize the left handed (or levorotatory) configuration for amino acids and the right handed (or dextrorotatory) configuration for sugars belonging to DNA or RNA.Perhaps it is trivial or obvious that life is chiral when looking at the nautilus, but this obvious chirality is a macroscopic feature which belies the fine arrangement of atoms which defines the chirality of biomolecules.Different structural forms of compounds with the same molecular formula are known as isomers to chemists. A stereoisomer is an isomeric molecule which has the identical constitution and sequence of bonded atoms, but has a different three-dimensional geometry in space. An enantiomer is one specific steroisomer of the two possible mirror images that are non-superposable. The dominance of the left handed chiral enantiomer in biology is a massive blow to the idea that nature is perfectly symmetric and is an unsolved mystery as to why nature is this way.Many molecules are chiral, however because molecules are constantly vibrating the instantaneous structure of a molecule may lack the exact structure or symmetry seen in an ideal model. Regardless, enantiomers have identical chemical properties except when they react with other molecules which are also enantiomeric in which case chiral forces yield a difference in behavior. Further, and perhaps more important for biology, particularly astrobiology, is that enantiomers have identical physical properties except with respect to the way they interact with plane-polarized or circularly polarized light or other chiral compounds. A pure enantiomer compound will rotate the plane of a monochromatic plane-polarized light by a certain angle in one direction, say clockwise, while the other enantiomer form of the compound will rotate the light by an equal amount in the opposite direction. Things that rotate light are said to be optically active. Measurements with a polarimeter allow chemists to determine if a compound is chiral or not. Polarization of light by organic compounds was discovered in 1815 by the French physicist and chemist Jean-Baptiste Biot. He found that organically produced chemical solutions consistently rotated plane polarized light, but laboratory synthesized chemicals did not reproduce the rotation. Beyond conjectures he had no explanation for the phenomena.Years later Louis Pasteur preformed a similar experiment with tartaric acid produced from grapes and tartaric acid synthesized in the lab. Pasteur went further and somehow used tweezers and a microscope (I do not conceive to understand how) to separate the tartaric acid crystals which he produced in the laboratory into piles of left and right handed molecules. He found that polarized light was rotated by the left handed molecules that he had selected in the same way the polarized light was rotated by the organic tartaric acid. He concluded that chiral molecules are responsible for the rotation of polarized light.So chiral molecules rotate light, but actually so does an individual achiral molecule! In an ensemble of achiral molecules each individual molecule may rotate the plane of the polarization, but the net rotation averaged over the ensemble will result in zero rotation. A mixture of two enantiomers in a 1:1 ratio (which is what you get when you create chemicals in the lab) is optically inactive because the rotation results in a zero net polarization rotation. When a reagent or catalyst is optically active the chiral product will also be optically active, or in the presence of chiral forces such as circularly polarized light this may also induce optical activity via enantimoeric excess in the products as well. Generally you can get optically active compounds in two ways 1) The reagent in already optically active. 2) The reaction of achiral but optically inactive precursors in a chiral optically active environment occurs. It takes an optically active molecule or chiral force to produce a product that is optically active.Most chemical reactions are not enantiomerically selective so that the initial reason for a completely homochiral biology on Earth remains a mystery just as when Biot and Pasteur discovered chirality through optical activity. Of course chirality is simply geometric in nature and thus this geometric asymmetry is what makes life chiral. Any molecule that contains a tetrahedral carbon or other central atom bonded to four different atoms or constituents will exist in enantiomeric forms; given that all biological molecules are at least this complicated, then (almost?) all biological molecules exist in enantiomeric forms. It may be that the chirality of biomolcules is simply a consequence of the emergent complexity of basic physics. The conditions necessary for a solution initially containing near equal number of chiral forms to evolve towards pure chirality has been explored (see Frederick Frank 1953) and is plausible. A tiny initial imbalance has spiraled out of control and now each successive generation of biomolecules on Earth is produced by the previous generation of chiral reagents, thus this is the why life is chiral. None of this explains how life is chiral, but a common answer is that because life can be chiral it is chiral.From this persepctive this topic is not so interesting, honestly. I have come to the conclusion that chriality is as it must be given that each generation of life is spawned from the previous generation under conditions which do have enantiomeric selection forces present. The question is why was left handed chirality chosen for life on our Earth?Now actually, the chirality of biomolecules is not just ... Read more »

Robert N. Compton, Richard M. Pagni, & Volume 48, 2002, Pages 219-261. (2002) The chirality of biomolecules. Advances In Atomic, Molecular, and Optical Physics, 219-261. info:/

December 16, 2010
08:56 PM
949 views

Supermassive Black Holes

by The Astronomist in The Astronomist.

A black hole is a massive object with such powerful gravity that not even light may escape from it. Black holes only have three unique properties which are mass, charge, and spin. At one time black holes were a speculative phenomena, but astronomers now understand that black holes are a relatively common and important occurrence in our Universe, unfortunately the public and science fiction still seems to be in dark. There are a lot of misconceptions about black holes:If the Sun was replaced by an equal mass black hole then the Earth would fall into it. In reality we would continue to orbit about the center of mass of the new black hole object and about eight minutes later after the Sun was removed it would be dark on Earth (that is how long it takes light from the sun to get to us). Supermassive black holes distort the space time around them in a very extreme way. Actually, the larger the black hole the smaller the distortion and tidal forces around it such that a very massive black hole hardly distorts the space time outside its event horizon, of course, every black holes does very strange things with space time inside the event horizon.Tiny black holes created by particle accelerators like the LHC may destroy Earth. The truth is that these tiny black holes would evaporate extremely rapidly via Hawking Radiation and they have such a small gravitational interaction they would not pull in any other matter.Black holes lead to extra dimensions. Why don't you jump into one and tell astrophysicists what you find?Black holes are strange objects. Massive black holes shape the evolution of galaxies, charged black holes are thought not to exist, and spinning black holes pull space itself around at close to the speed of light. According to Einstein mass tells space time how to curve and space time tells mass how to curve. So all mass distorts space in a small manner, but black holes create what are known as singularities where the mathematics which describes the curvature of space time breaks down.You can solve the equations of general relativity and see how black holes arise, but they also arise in the context of standard Newtonian physics. Consider a rocket launching from earth into space. In order for the rocket to free itself from the gravitational pull of Earth it must be moving at the escape velocity (the actual situation is much more complicated than this, but suffice to say it's... complicated). We can find the escape velocity for any object of mass, m, away from a more massive object of mass M by setting the object's kinetic energy, 1/2 m v 2, equal to the potential energy of the object in the massive object's gravitational field, GMm/r, where G is Newton's gravitational constant. A terrible thing happens when you consider what happens if the object had a velocity equivalent to the speed of light, c. Suddenly you find there in a mass and radius combination which creates and object so dense from which not even light can escape. The radius to which an object must be compressed to form a black hole is known as the Shwarzchild radius or the event horizon.Karl Schwarzchild was a German mathematician who was working as an artillery lieutenant during World War I when he was the first person to solve the Einstein field equations. He found a solution corresponding to a physical object with strange properties: apparently matter and energy could enter, but not exit ( again it is actually more complicated as Hawking has taught us, but I digress ). His solution known as the Schwarzschild metric is a rather succinct description of the spacetime around a non-spinning black hole:The details of this equation aren't important here. I have shown it for two reasons. First, it really is a thing of beauty that some of you will hopefully appreciate. Second, is the realization that as the radius of the object r approaches the Schwarzchild radius rs the denominator becomes zero and dividing by zero is next to impossible. This is the singularity. The singularity is unavoidable as no mathematical trick or coordinate transformation can rid the Schwarzchild metric of all singularities.The astronomy of supermassive black holesObservations of other galaxies, particularly nearby galaxies, reveal compelling evidence that nearly all galaxies have a super massive black hole at their center. The Milky Way has a supermassive black hole at its center known as Sgr A*. How these black holes form is not well understood, but it must have been long ago when the universe was very young such that the black holes had time to grow. They may form from the collapse of some of the first stars created in the Universe known as Pop III stars or they may form from the monolithic collapse of a cold could of gas in the early Universe. Many galaxies emit very high energy radiation from their centers (ultraviolet, X-ray or gamma ray) that is easily explained by a super massive black hole at their center. These galaxies are said to have Active Galactic Nuclei (AGN), but our Milky Way does not emit such high energy radiation. This an interesting and lucky fact that astronomers have explained by noting that if gas is actively falling onto a black hole it forms and accretion disk which heats up to very high temperatures. Thus matter falling into a black hole emits massive amounts of powerful radiation just before it falls into the black hole. Active galactic nuclei are especially correlated with galaxies which have just undergone mergers with other galaxies which are gas rich; the theory, which is backed up by simulations and observations, says that when gas rich galaxies merge gas funnels into the central super massive black holes and induces a period of high luminosity. The black hole at the center of our Milky Way also undergoes periods of gas accretion or luminosity spikes in the X-ray to radio bands. One such event in May of 2003 was a powerful flare seen located just a few milli-arcseconds from the position of Sgr A* which corresponds to less than ten Schwarzschild radii from the black hole position.Observing supermassive black holesHow do we know that we have really observed black holes not something else? This is a hard question and astronomers may often talk about compact massive objects instead of black holes if they are not certain about a particular object in the sky. In the case of the supermassive black hole at the center or our Milky Way more direct observations are possible. Astronomers have been watching Sgr A* for decades, but only with modern instruments has powerful evidence come to light. Observations of flare events close to the Schwarzschild radius of a suspected black hole like the event described above are one way. Also watching the gravitational influence of the dark object is another:The movie above was made with high resolution near-infrared observations over many years (in the bottom left corner is the year of the observations) on the 10 meter Keck telescope in Mauna Kea by a group at UCLA led by Andrea Gehez. These stars are right at the center of the Milky Way in the immediate vicinity of Sgr A* and they are moving in a Keplerian orbit around a central mass which is unseen in optical light. The stars are moving at speeds up to 1400 km/s (or 3 million mph) and so using Kepler's laws of motion the central object is estimated to be four million solar masses. The object is also very minute. Given this great mass and small size astrophysicists only ... Read more »

Gebhardt, K., Bender, R., Bower, G., Dressler, A., Faber, S., Filippenko, A., Green, R., Grillmair, C., Ho, L., Kormendy, J.... (2000) A Relationship between Nuclear Black Hole Mass and Galaxy Velocity Dispersion. The Astrophysical Journal, 539(1). DOI: 10.1086/312840

Genzel, R., Schödel, R., Ott, T., Eckart, A., Alexander, T., Lacombe, F., Rouan, D., & Aschenbach, B. (2003) Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre. Nature, 425(6961), 934-937. DOI: 10.1038/nature02065

Ghez, A., Salim, S., Weinberg, N., Lu, J., Do, T., Dunn, J., Matthews, K., Morris, M., Yelda, S., Becklin, E.... (2008) Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits. The Astrophysical Journal, 689(2), 1044-1062. DOI: 10.1086/592738

October 30, 2010
10:05 PM
1,256 views

Very precise pulsar measurements

by The Astronomist in The Astronomist.

A neutron star is made of neutrons, right? Astrophysicists ponder this question and forge theory after theory, but the only thing they conclude with certainty is that a neutron star by any other name would still be made of the densest form of matter know to exist in our Universe. Under certain conditions a star which has exhausted all of its fuel and is sufficiently massive will not be able to support its own weight with pressure support (as in a regular star) or with electron degeneracy support (as in a white dwarf) such that electrons and protons merge to form neutrons because it is a more energetically favorable arrangement of the matter. A neutron star is a sort of massive atomic nucleus, but without charge. The actual composition and detailed properties of neutron star are still theoretically uncertain.New measurements of the pulsating neutron star and helium-oxygen-carbon white dwarf binary system J1614-2230 reported in a Nature letter are the highest precision determinations of a neutron star's mass to date. The data comes from the massive Green Bank Telescope using the new Green Bank Ultimate Pulsar Processing instrument which accurately records the time of arrival of each radio pulse sent out by the rapidly rotating neutron star (which is a pulsar). The quality of this instrument, having over a tera op of computing power, and the size of the telescope, 100 meters, made this measurement possible. For a quick rundown of this result you can watch these quick movies on the scientific implications and the technology behind the discovery which were created by the NRAO.The analysis uses a general relativistic effect involving the time delay of light known as the Shapiro delay effect. When a light ray passes a massive object it follows a curved path. General relativity says that curvature of light rays can only take place when the velocity of the propagation of the light rays also varies with position. The Shapiro delay increases the light travel time through the curved space-time near a massive body. The equation to determine the time delay effect is delightfully simple.The delay depends on the mass, M, of the time delaying body between the source and the observer, the gravitational constant G, the speed of light, c, and the geometry of the system. The geometry is that light has to be passing near the gravitating body before it gets to the observer for the effect to occur at all so the vector that points from the observer to the source, R, and the vector that points from the observer to the gravitating mass are vital. Pulsar J1614-2230 is a nearly edge on, 89 degrees, system meaning that when the white dwarf passes in front of the pulsar during the binary orbit the Shapiro effect will be very strong. I ran a quick calculation of the time delay and found it to be exactly on the order of a few microseconds. The first figure in the paper shows the geometry and the measured effect.With this data in hand the standard Keplerian orbital parameters are calculated for this clean binary system and the masses of the objects are calculated. The mass of the neutron star was found to be 1.97 +/- 0.04 solar masses which is the most precise measurement of neutron star mass to date. Unfortunately this measurement technique does not provide any information about the radius of the neutron star, but because the mass was so high it already set a limit on the equation of state of the neutron star matter. This means that we can begin to answer what a neutron star is really made of. Different kinds of matter have a different behavior as you add more mass to them which is intuitive if you thought how discrepant with respect to size a planet made out of cotton candy versus rock would be. This result indicates that exotic models of hadronic matter including hyperons, kaon condensates are ruled out. Condensed quark matter is not ruled out, but highly constrained with this data. This is a big deal for particle physicists because this kind of system is an experiment that could never be carried out in a lab, but is necessary to probe fundamental physics.This cool result on neutron stars glosses over another application of precise pulsar measurements that the authors of this Nature paper regarded as noise. The plot above is very neat and clean, but before the data looks like that a timing analysis must take into account the time delays associated with many more mundane effects. Effects that change the time of arrival of the pulsar include the variations in the Euclidean distance between the Earth and the pulsar resulting from Earth’s orbital motion, the proper motion of the pulsar, and its binary motion, dispersive delays in the interstellar medium, and time dilation of clocks in the observatory and pulsar frames and along the propagation path. The Earth's orbital motion about the solar system barycenter (known as the Roemer delay) is up to 500 seconds and so must be removed from the data. The powerful thing is that the Earth's orbital motion tells us about the mass and orbits of all the bodies in our solar system. A paper published in the Astrophysical Journal states that with ten years of careful observation of 20 pulsars the masses and orbits of solar system bodies could be determined better than with any other method and even undiscovered trans-Neptunian objects could be found.Precise pulsar measurements are powerful. The first extrasolar planet ever discovered was actually made with pulsar measurements. Pulsars can tell us about the nature of neutron stars, the properties of own solar system, oh and even gravitational waves. If only astronomers had the money to build a pulsar timing array... ReferencesDemorest PB, Pennucci T, Ransom SM, Roberts MS, & Hessels JW (2010). A two-solar-mass neutron star measured using Shapiro delay. Nature, 467 (7319), 1081-3 PMID: 20981094D. J. Champion, G. B. Hobbs, R. N. Manchester, R. T. Edwards, D. C. Backer, M. Bailes, N. D. R. Bhat, S. Burke-Spolaor, W. Coles, P. B. Demorest, R. D. Ferdman, W. M. Folkner, A. W. Hotan, M. Kramer, A. N. Lommen, D. J. Nice, M. B. Purver, J. M. Sarkissian, I. H. Stairs, W. van Straten, J. P. W. Verbiest, & D. R. B. Yardley (2010). Measuring the mass of solar system planets using pulsar timing ApJ arXiv: 1008.3607v1... Read more »

Demorest PB, Pennucci T, Ransom SM, Roberts MS, & Hessels JW. (2010) A two-solar-mass neutron star measured using Shapiro delay. Nature, 467(7319), 1081-3. PMID: 20981094

D. J. Champion, G. B. Hobbs, R. N. Manchester, R. T. Edwards, D. C. Backer, M. Bailes, N. D. R. Bhat, S. Burke-Spolaor, W. Coles, P. B. Demorest.... (2010) Measuring the mass of solar system planets using pulsar timing. ApJ. arXiv: 1008.3607v1

October 16, 2010
09:37 PM
1,246 views

The Goldilocks Planet

by The Astronomist in The Astronomist.

Once upon a time there was a planet named Earth. It orbited exactly one astronomical unity away from a G2V type star. Billions of years went by and Earth found that it lived right in the habitable zone where liquid water was maintained on it surface and life spontaneously arose. Pretty soon life on Earth became restless, questioned its own existence, and looked for life on Gliese 581. Earthlings found many planets and exclaimed, 'Gliese 581 b is too hot, Gilese 581 c is slightly too hot, Gliese 581 d is slightly too cold, Gliese 581 e is way too hot, Gliese 581 f is too cold, but Gliese 581 g is just right!' so the story goes.Gliese 581 is an unassuming star: it is relativity close at 20 light years away (the 87th closest cataloged star to earth), it is only a third the mass of the sun, and it is relativity quiet in terms of stellar activity (which is beneficial for life because flares scorch planets). It is the sixth planet from Gliese 581 denoted merely as g that harbors so much potential. It is not to hot, not too cold, it is just right. It is the Goldilocks planet. Vogt et al. 2010 recently reported on the discovery of this planet which is a 3.1 Earth mass (or larger) planet orbiting in the habitable zone of the M3V type star Gliese 581. The problem is that this planet may not exist.The MediaI did not immediately discuss Gliese 581 here at The Astronomist because I wanted to read the paper before weighing in. However the authors were compelled to issue a press release about their findings before making their peer reviewed paper available. After I finally looked at the paper I was somewhat disappointed. The whole thing was a science journalism media circus. A selection of some of my favorite excerpts:“Found: An Earth like Planet, at Last” Time magazine“The chances of life on this planet are 100 percent,” Steven Vogt“Could contain more gold than we could ever imagine” PR Fire"Are the Gliesans going to Hell?" Huffington Post"An Alderaan Moment: Earth-Like planet disappears" Death+TaxesMedia coverage is a double edged sword for science. Scientists strive to come up with compelling results that often fall flat when presented to the media or public, but other stories make waves disproportionately large relative to their scientific impact. Arguably the discovery of an earth like planet should have been accompanied by a much larger amount of media attention. The misinformation spread about Gliese 581 g is a symptom of the real problem which is the science. This is not a confirmed planet detection, there is no evidence this planet has a hospitable atmosphere anything like Earth, and there is certainly no evidence of life. This wont stop the media from speculating or spreading dangerous misinformation (like the idea we could travel there if we trash Earth). The quote from Steven Vogt is an example of poor journalism where he prefaced the statement by saying, 'my own personal feeling is that the chances of life on this planet are 100 percent,' but the crucial context was thrown away for headlines.The ScienceAll the planets around Gliese 581 were discovered using the radial velocity technique. In any gravitationally bound system the bodies orbit their common center of mass. It is a subtle effect in a star-planet system where the central star dominates the mass. The central star will move at a characteristic speed depending on the orbits of the planets around it. The movement of the star is measured through the Doppler shift of the light emitted by the star. Modern instruments are super sensitive to even the smallest movements of stars down to as little as 1 m/s. Observations of the radial velocity of the star over a period of time (usually several years) is analyzed using Fourier analysis. The Fourier analysis identifies periodic signals in the data corresponding to the orbital period of the planet or planets.The researchers used two data sets spanning almost two decades. Most of the data came from the researcher's own instrument HIRES, and additional data came from a Swiss group with the HARPS instrument. The HIRES data spans a larger time range, but the HARPS data is more precise. This combined data set is how the researchers identified two new planets f and g.The ProblemsA little after this new Goldilocks planet was announced the Swiss group announced that they could find no evidence of Gliese 581 g in their data. Does this mean it doesn't exist? Well this is tricky. A planetary researcher in my department, Rory Barnes, spoke to the New York times before the Swiss group had spoke up and said that the planet looked like the 'real deal'. After the announcement was made I spoke to Barnes again and he said that he would have to hold off further judgment until more information was available. The onus of proof in science is upon those who make extraordinary claims. Vogt et al. were only able to find this planet by combing the available data sets; they actually state in their paper that they did not detect the planet in either of the data sets independently, only in combination. The damning part of the Swiss groups statement is that they say they have much more data available at this point that Vogt et al. did no have access to during their analysis. When the Swiss team forces planet g to fit their complete data they actually get a negative fit indicating that planet g really isn't there. The thing about this paper that I am least happy with is the quoted false alarm probability. The false alarm probability appears to be 1% based on the figures in the paper (see figure 3 specifically), but in the text it is quoted as ~10-5. I don't know what is going on.Then there is their error analysis (warning this is about to get technical feel free to skip this paragraph). Vogt et al. used the peaks in the power spectrum to identify the planets in the system then subtracted off the highest power modes corresponding to the planets they had found. The power spectrum for each planet carried with it a false alarm probability, but once the planet had been subtracted out of the power spectrum its false alarm probability was washed away (you can see this happening in figure 3). They compound their errors after the 1st, 2nd, 3rd, 4th, and 5th planets which have varying false alarm rates. The proper way to do this is a joint fit model to all planets in the system using Bayesian analysis.The strangest thing about all this is that when this paper was first submitted to The Astrophysical Journal the Swiss group was reviewing the paper and it was rejected. This Vogt paper meta chronicles its own history and discusses why it was retracted previously over concern of systematics. Unfortunatly the quality of the paper may not have improved. The Swiss group has actually leveled one specific concern, Vogt used perfectly circular orbits to find planet g, but the evidence shows the orbits are probably slightly elliptical. In fact in 2009 Vogt used elliptical orbits, but in this new paper circular orbits have been adopted. The image above illustrates this and makes a pictorial argument as to how circular vs elliptical orbits could introduce errors.The discovery of an Earth-like planet seems imminent. I do not know if this is it. I will hold off further judgment until more information was available.References:... Read more »

Steven S. Vogt, R. Paul Butler, Eugenio J. Rivera, Nader Haghighipour, Gregory W. Henry, . (2010) The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581. ApJ accepted. info:/arXiv: 1009.5733v1

September 13, 2010
06:38 PM
1,258 views

Magnetic Fields in Cosmology

by The Astronomist in The Astronomist.

The existence of magnetic fields on cosmologically large scales is an unsolved problem in astrophysics. Theory favors a universe that did not begin with any magnetic fields present and classical magnetohydrodynamics restricts the spontaneous emergence of a magnetic state under the influence of ideal forces. In a paper entitled Twisting Space-Time: Relativistic Origin of Seed Magnetic Field and Vorticity appearing Physical Review letters Swadesh Mahajan and Zensho Yoshida propose a universal magnetic field generating effect using ideal special relativistic fluid dynamics. Mahajan and Yoshida's insight was that in describing magnetic fields, which are mathematically equivalent to a vorticity, a careful application of ideal dynamics in the framework of distortions caused by special relativity may result in the spontaneous emergence of a magnetic state in contrast to the previous theoretical result.Magnetic fields are found to be important in every scale hierarchy of the universe. Most notably detailed images of galaxies paradoxically display regions of chaotic turbulence and beautiful grand coherent designs at once. Thus it is clear that turbulent motion on scales below hundreds of parsecs does not necessarily destroy coherent optical or magnetic features over scales of kiloparsecs. Indeed, magnetic fields are indirectly observed at optical and radio wavelengths by detecting the polarization of the electromagnetic field through the Faraday effect and also by the Zeeman splitting effect. The Faraday effect is the rotation of the linear polarization vector of light which occurs when polarized radiation passes through a magnetized and ionized medium. Radio observations are the most powerful technique and by measuring both the dispersion and polarization rotation the mean of the magnetic field along the line of sight can be measured. Such observations indicate a wide range of magnetic field are present in astrophysics. The image at right below shows the magnetic fields present in M51 which are likely similar in structure and strength to that of the Milky Way.The total radio continuum emission from the "whirlpool" galaxy M51 (distance estimates range between 13 and 30 million light years) is strongest at the inner edges of the optical spiral arms, probably due to the compression of magnetic fields by density waves. The vectors give the orientations of the regular magnetic fields as derived from the polarized emission. The field lines follow nicely the optical spiral arms. Unexpectedly, strong polarized emission is observed also between the optical arms which indicates the action of a dynamo. This image was observed with the VLA in its most compact configuration at 6cm radio wavelength (broadband continuum). As the VLA cannot detect the diffuse, large-scale radio emission, data from the Effelsberg 100-m telescope in Germany at the same wavelength was added. Investigator(s): Rainer Beck (MPIfR Bonn, Germany), Cathy Horellou (Onsala Space Observatory). Image courtesy of NRAO/AUIMicroguass fields are present in galaxies at scales of a few kiloparsecs and on the much larger scales of megaparsecs ordered fields of perhaps a few orders of magnitude less are present in galaxy clusters. Magnetic fields in astronomy are controlled by induction of partially ionized gas. A common model for creating these magnetic fields is the dynamo effect wherein an electrically conductive fluid accelerated by some kinetic force generates convective motions in the fluid; it is plausible that a turbulent hydromagnetic dynamo of some kind coupled to an inverse cascade of magnetic energy wold give rise to regular galactic magnetic fields. Following the basic dynamo theory magnetic field lines can be simulated for galaxies which are consistent with observations. The dynamo theory is actually a mechanism for maintaining or growing fields rather than creating them, but it is expected that minuscule primordial magnetic field seeds in the early universe of cosmological origin drive the magnetic fields observed today.The magnetic dynamo and the primordial magnetic seed theories are both unsatisfactory. The model wherein the the large scale magnetic field in galaxies is the result of the twisting of a cosmological magnetic fields by galactic differential rotation is not satisfactory because a primordial field wound up by differential rotation ultimately decays in an effect known as flux expulsion. The primordial seed theory must explain the presence of large magnetic fields in higher redshift objects when the universe was much younger when the fields should not have had sufficient time to grow. Researchers disagree over what initial primordial field strength is necessary to create the magnetic fields seen today; estimates vary from as large as 10-9 gauss [1] to 10-30 gauss [2], but either way an alternative model would be welcome.Mahajan and Yoshida's work was motivated by the search for a universal mechanism for magnetic field generation. They key to creating a magnetic field is the vorticity of an ionized material which is analyzed in this paper with topological constraints. In mathematical terms fundamental cosmology requires a topological constraint on the vorticity of the universe (consider that you wouldn't expect the universe to have a preferred rotation), however this constraint can be broken by the application of special relativity. The problem of magnetic fields lies in the fact that vorticity must vanish for every ideal force such as the entropy conserving thermodynamic forces (this can be proven though the governing Hamiltonian dynamics of an ideal fluid where ultimately Kelvin's circulation theorem shows that if the initial state has no circulation the later sate will also be vorticity-free). Introduction of the Lorentz factor γ=(1-(v/c)2)-1/2 from special relativity destroys the exactness of the ideal thermodynamic force and allows spontaneous vorticity. The authors find a new term that provides a magnetic field growing mechanism as long as the kinetic energy is inhomogeneous. The authors mechanism can provide a finite seed for even mildly relativistic flows. They provide an example for very standard parameters (electron density n=1010 cm3, temperature T= 20 eV and velocity, v, compared to c of v/c=10-2) and find their relativistic drive mechanism remains dominate over other effects until magnetic fields of 1 gauss or so which is much larger than most magnetic fields ever observed, thus the relativistic drive is the only dominant effect. The relativistic drive mechanism will likely help us understand, among other things, the origin of magnetic fields in astrophysical and cosmic settings.References:[1] Beck, R., Brandenburg, A., Moss, D., Shukurov, A., & Sokoloff, D. (1996). GALACTIC MAGNETISM: Recent Developments and Perspectives Annual Review of Astronomy and Astrophysics, 34 (1), 155-206 DOI: 10.1146/annurev.astro.34.1.155[2] ... Read more »

Beck, R., Brandenburg, A., Moss, D., Shukurov, A., & Sokoloff, D. (1996) GALACTIC MAGNETISM: Recent Developments and Perspectives. Annual Review of Astronomy and Astrophysics, 34(1), 155-206. DOI: 10.1146/annurev.astro.34.1.155

Davis, A., Lilley, M., & Törnkvist, O. (1999) Relaxing the bounds on primordial magnetic seed fields. Physical Review D, 60(2). DOI: 10.1103/PhysRevD.60.021301

Mahajan, S., & Yoshida, Z. (2010) Twisting Space-Time: Relativistic Origin of Seed Magnetic Field and Vorticity. Physical Review Letters, 105(9). DOI: 10.1103/PhysRevLett.105.095005

August 20, 2010
01:40 PM
1,419 views

Cylons and Smelloscopes: False Positives and False Negatives in the Search for Extraterrestrial Life

by The Astronomist in The Astronomist.

Are there planets outside of our solar system? Is there life on other planets? Is life on other planets like life on Earth? These are questions that astronomers, astrobiologists, chemists, and geologists are trying to answer with current experiments. In order to answer these questions we must observe distant planets and we must determine what life on those planets may be like. Detecting extrasolar planets is tricky enough, but imaging what alien life is like may well be stranger than science fiction. Yesterday evening I attended a lecture sponsored by the Seattle Astronomical Society given by Shawn Domagal-Goldman titled Cylons and Smelloscopes: False Positives and False Negatives in the Search for Extraterrestrial Life. It was an excellent lecture and filled with interesting topics. Shawn touched on the philosophical problem of defining life in the broadest of senses (is Number Six alive?) and he pointed out that the verification of life on distant planets faces technical challenges and basic scientific limitations (a smelloscope sure would help!).Dimitar Sasselov set off minor shock waves of gossip and rumors in the media and astronomy communities when claimed that the NASA Kepler mission had found 140 Earth-like planets a few weeks ago during a talk he gave at the TED Global 2010 meeting in Oxford. The media thought we had found earth's twin, but astronomers knew that Sasselov had exaggerated the situation. Sasselov had to post a redaction of sorts on the Kepler blog in order to clarify what he said. What he should have said is that the Kepler mission will find and verify the presence of potentially habitable planets and that Kepler currently had 140 candidate extrasolar planets. The candidates are not confirmed and so a pessimistic outcome could be that half of the candidates will be false. The difficulty in finding extrasolar planets or life is fraught with false positive and false negatives. A false positive is a detection that seems like exactly what you were looking for, and maybe it is, but the detection was either bad data or you were looking for the wrong thing. A false negative is a detection which you conclude is not what you were looking for, but either your data was fouled or your detection threshold was too constrictive.How do we find planets outside of our solar system? There are at least five methods to find planets: Doppler shift, astrometric measurement, transit method, gravitational microlensing, and direct detection. Shawn discussed in depth the Kepler mission that is currently monitoring more than 150,000 stars in the direction of the Cygnus constellation for any signs of extrasolar planets that may be orbiting those stars. So, what method does Kepler use to find planets? It watches for eclipses! When a planet orbiting a distant star crosses in front of the star some of the light from the host star is blocked. The planet will transit (astronomers often use the world transit not eclipse for exoplanets) in front of the the star once an orbit and thus the period of orbit can be determined. A secondary eclipse also occurs when the day side of the planet is blocked by the star. The video below illustrates the whole process.Yes, there are planets outside of our solar system. The current exoplanet detection count is 473 and counting; you can watch that count go up over at Planet Quest. Kepler may double that number, but more importantly it has the ability to find earth size planets. Most of the planets found to date have been large, hot, and inhospitable to most kinds of life anyone can fathom. How do we detect signs of life on other planets? Astronomers look for bio-markers in the planet's atmosphere. Bio-markers are molecular signatures of certain compounds that could not be produced by non-biological process; bio-markers indicate that dynamic non-equilibrium chemistry is present on the surface of that planet. Astronomers can measure the light emitted as a function of wavelength, the spectra, that a planet emits to determine the molecular species present in the atmosphere. For example the Earth's atmosphere has the spectral signature of water which means it has conditions in which life as we know it can thrive. If we found an earth size planet that had water in its atmosphere which wasn't too hot we would say we had found a habitable planet. If we found oxygen or ozone (03) in an atmosphere it would almost certainly mean life was present on the planet because 03 is quickly removed from atmospheres through standard geological processes such as oxidation of iron, but it may remain present in an atmosphere if it is continually replenished by the photosynthesis mechanism of algae and plants. One of the topics Shawn talked about in his talk and a focus of his research was the problem of being certain that non-biological processes are not creating the oxygen rich atmospheres. The runaway greenhouse effect combined with the photo-disassociation of carbon dioxide can produce oxygen in a similar way to biological life. This is where the smelloscope would be useful: ozone along with other non-equilibrium species such as nitrous oxide and methane in specific ratios would be the scent we are looking for. Bio-signatures were not present on the early Earth. In fact the Earth probably looked a lot more like Venus. The diagram above shows that Venus, Earth, and Mars all have distinct spectral features that tell us about their atmospheres. The hardest part of looking for bio-signatures is that we do not have a telescope that is sensitive enough. Trying to take the spectra of a planet orbiting a bright star is like trying to tell the color of the wings on a gnat hovering around a spotlight on the moon. Like a baseball player holding up one hand to block the sun from his eyes as he focuses on the ball an occulter or star shade working with an existing telescope in space would do the trick. The current funding situation in astronomy is dire, but there is hope that a mission called New Worlds will one day work with the James Webb Space Telescope to allow us to take a closer look at planets which Kepler is finding.Is there life on other planets? We don't know and it may be a more complicated question than is suspected. There is a bias towards looking for life that is similar to what life on Earth is like. There is a bias towards looking for life that alters its host planet's atmosphere significantly enough to detect it with telescopes on earth. There is a bias towards looking for life that is alive as we define it. These biases may lead to false negatives in the search for life, but as Shawn pointed out the possibilities for life to exist are much grander than our imaginations so we do the best we can. Also, despite the difficulties for finding life on other planets and the gulf between the public's perception of aliens and reality scientists are taking this as a serious venture. Scientists from diverse fields are coming together to forge a path forward. One such project is the Virtual Planet Laboratory which employs scientists in fields such as geology, chemistry, biology, and astronomy. The Virtual Planet Laboratory is a team of scientists who are building computer simulated planets to discover the likely range of planetary environments for planets around other stars so we can better look for habitable planets and distinguish between planets with and without life. However, we can't even discern with certainty the presence of life on Mars or Europa at this point, what hope do we have for finding life on distant planets?I think there is a lot of hope and I am not alone in that sentiment. I don't search for planets or life in my research, but I think that the search for life, pa... Read more »

Beichman, C. A., Woolf, N. J., & Lindensmith, C. A. (1999) The Terrestrial Planet Finder (TPF) : a NASA Origins Program to search for habitable planets. JPL publication. info:/

July 29, 2010
03:51 PM
1,198 views

Hubble Bubble

by The Astronomist in The Astronomist.

The Copernican principle holds that humans are not privileged observers of the Universe. Copernicus stated that the Earth is not at the center of the solar system or at any particularly special position in the heavens. Modern cosmology has extended this idea to reason that the earth does not occupy any unique position in the Universe. Modern philosophy of science pushes the principle even further to conclude that every observer (even if they be they little green men) should reason as if they were the most standard observer. However, despite all these humble and rational thoughts it is still tempting to explain certain aspects of modern cosmology that seem finely tuned as consequences of observer selection effects. Namely I am speaking of dark energy or the accelerated cosmological expansion which supposedly could be explained if we occupy a privileged position near the center of a large, nonlinear, and nearly spherical void in mass density. The idea that the region of the cosmos around us could be a void is colloquially known in astronomy as the Hubble bubble. Technically a Hubble bubble is defined as a region of space wherein there is an observed departure of the local value of the Hubble constant from its cosmologically averaged value.Lets speculate a little further on what it would be like to live in a Hubble bubble. In the standard cosmological model of the Universe the structures we see today like galaxies and clusters of galaxies (and similarly the structures we don't see like the massive dark matter halos the visible matter is embedded in) formed from tiny primordial quantum fluctuations in the early universe. The fluctuations were random variations in density such that locations which were over-dense formed galaxies and those which were under-dense formed voids. It is possible, in fact statistically quite acceptable that there are voids of various sizes in the Universe. These voids would become increasingly under-dense as the Universe evolved and equivalently over-dense regions of the Universe became increasingly over-dense. Inside the void matter would expand outward due to the gravitational pull of matter in surrounding dense regions and thus an observer at the center of the void would see an accelerated expansion of matter outward. Now it is also possible that our entire observable Universe is a Hubble bubble, but that really flies in the face in all of cosmology. It is unfounded, absurd, and really the whole idea of a Hubble bubble may explain dark energy, but is hardly a very good explanation. The Hubble Bubble is wildly speculative and precision cosmology has almost completely defeated it as a credible explanation. First, as the framework of cosmology has been successful resting on the Copernican principle it seems odd to throw it out now. It is odd and largely misguided. First, the probability of producing a void of necessary magnitude; to mimic aspects of dark energy is extremely small in the standard structure formation models. Second, the probability of an observer being at the center (the only location where the expansion effect would be noticed) is extremely low. Finally, the void would need to be close to spherical to match the observed spatial smoothness (or isotropy) of the universe. These qualitative arguments and many more quantitative arguments from precision cosmology data are laid forth in a recent paper by A. Moss, J. Zibin, and D. Scoot titled Precision Cosmology Defeats Void Models for Acceleration. The abstract follows:The suggestion that we occupy a privileged position near the center of a large, nonlinear, and nearly spherical void has recently attracted much attention as an alternative to dark energy. Putting aside the philosophical problems with this scenario, we perform the most complete and up-to-date comparison with cosmological data. We use supernovae and the full cosmic microwave background spectrum as the basis of our analysis. We also include constraints from radial baryonic acoustic oscillations, the local Hubble rate, age, big bang nucleosynthesis, the Compton y-distortion, and for the first time include the local amplitude of matter fluctuations, σ8. These all paint a consistent picture in which voids are in severe tension with the data. In particular, void models predict a very low local Hubble rate, suffer from an "old age problem", and predict much less local structure than is observed. The paper makes several quantitative arguments against the plausibility any kind of void model for cosmic acceleration by drawing together an impressive amount of cosmological data and technical expertise, however, they don't ever mention the term Hubble Bubble. A 2007 paper by Conley et al. takes the Hubble Bubble paradigm head on: Is There Evidence for a Hubble Bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies. In Conley et al. they explore how dust effects the colors of type Ia supernovae because they reason if the dust can be modeled as a purely local Milky Way effect then the supernovae data would actually favor the Hubble Bubble. Of course, despite difficulties the analysis, they find that in their parametrization there is evidence for more than the simply effect of local Milky Way dust implying doom for the Hubble Bubble. So the Hubble Bubble has been burst.References:Adam Moss, James P. Zibin, & Douglas Scott (2010). Precision Cosmology Defeats Void Models for Acceleration arXiv preprint arXiv: 1007.3725v1Conley, A., Carlberg, R., Guy, J., Howell, D., Jha, S., Riess, A., & Sullivan, M. (2007). Is There Evidence for a Hubble Bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies The Astrophysical Journal, 664 (1) DOI: 10.1086/520625... Read more »

Adam Moss, James P. Zibin, & Douglas Scott. (2010) Precision Cosmology Defeats Void Models for Acceleration. arXiv preprint. arXiv: 1007.3725v1

Conley, A., Carlberg, R., Guy, J., Howell, D., Jha, S., Riess, A., & Sullivan, M. (2007) Is There Evidence for a Hubble Bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies. The Astrophysical Journal, 664(1). DOI: 10.1086/520625

July 11, 2010
07:00 PM
1,331 views

The Size of the Proton Measured with Lasers

by Alexander in The Astronomist.

A little over a week ago in Lindau, Germany Theordor Hanch hinted at new measurements of the size of the proton which may impact the fundamental theory of quantum electrodynamics. Hansch's lecture was an overview of the history of lasers progressing from our realization of the wave/particle duality nature of light to new research published in Nature on the size of the proton.... Read more »

Pohl R, Antognini A, Nez F, Amaro FD, Biraben F, Cardoso JM, Covita DS, Dax A, Dhawan S, Fernandes LM.... (2010) The size of the proton. Nature, 466(7303), 213-6. PMID: 20613837

May 5, 2010
12:51 AM
1,403 views

Dark Matter Confronts Observations

by The Astronomist in The Astronomist.

Dark matter is like the Rome of astronomy, all observations lead to dark matter. The problem is that physicists and astronomers, don't know what it actually is. The observations which support dark matter come from many different independent observations, so it is not just some observational error. The observations which corroborate the dark matter paradigm make for a fantastic discussion, but for right now I would like to focus on explanations for what dark matter may be. Specifically, what kind of particles are dark matter?Dark Matter is not like any particle beforeThe most attractive candidate theory for dark matter would be simple, it would be motivated from fundamental particle physics, and it would make testable predictions for future observations. Whatever dark matter particles are we know that dark matter does not interact with photons, is electrically neutral, is highly non-relativistic, and it is dissipationless; so basically that means dark matter is boring. To drive at what dark matter really is I would first ask, can even begin to understand the observations without tying ourselves to any specific models for dark matter's nature? I do not know. I am merely going to outline the fundamentals of a plausible dark matter model and address how it confronts observations. There are dark matter particles theories invoked from supersymmtery, universal extra dimensions, and branes; the most common dark matter candidate which I will focus on here is the so called weakly interacting massive particle (WIMP). If dark matter is a WIMP, then we still have only narrowed down what kind of particle it is to a zoo of particles. If we want a pure motivation for an unknown dark matter particle we could go back to the early 1930's when Enrico Fermi developed a theory of beta decay that involved the neutrino and necessitated a new mass scale in nature; this mass scale introduces new fundamental particles that would interact weakly with regular matter and presto, WIMPs. It turns out that WIMPs, particularly supersymmetric neutralinos, are a compelling fit given the current data, but as more precises observations accumulate the theory faces scrutiny. There was a plague of rumors of an affirmative dark matter detection a year ago which were of course unfounded (it took much restraint not to throw in my lot with the speculations). Today dark matter looms as a unsolved problem because its existence is not in question, only its origin.A plausible theory for dark matterDark matter particles zoom right through matter without interacting, similarly to neutrinos, but unlike neutrinos dark matter particles are very massive. The gravitational potential of the largest structures in the universe, like galaxies and galaxy clusters, is dominated by dark matter. Dark matter is everywhere (yes, even here on Earth dark matter is present and it does have some gravitational effect, but it is infinitely more feeble compared to the Earth or the Sun), but dark matter doesn't clump, it doesn't form dark molecules, and it doesn't form dark galaxies. Regular matter that you or I are made of (known as baryonic matter) is gravitationally attracted to dark matter (non-baryonic matter), but dark matter dominates because there is so much of it. It is hard to notice dark matter because it only interacts with regular matter through gravity for the most part and gravity is the weakest of forces. Dark matter remains like giant clouds in which a galaxy or a cluster of galaxies resides within. Dark matter is different from regular matter because regular matter can be detected through electromagnetic waves which dark matter shouldn't produce, unless dark matter self annihilates.If dark matter is thermal weakly interacting massive particles (WIMPs) then it may produce observable signals when it self annihilates. A popular model for the WIMP which would self annihilate is the neutralino (see figure at right). Self annihilation is exactly what it sounds like, like two identical but opposite forces meeting, the result is an explosion of energy and particles (the interaction conserves energy, momentum, and other quantum numbers). WIMP self annihilations into positrons and electrons could be detected by cosmic ray detectors. These self annihilations and the observable signal would be rare though. If they were common we would see a lot more signal. In the early universe these annihilations would have been much more common such that if there was a primordial abundance of dark matter it would have self annihilated to be consistent with the much lower density of dark matter we observe today (to wrap your head around this take my contrived analogy of a king who has many identical twin sons all born at once, in time they might murder each other until there was enough land for the remaining sons to all have just enough). Mathematically this is stated that the density of dark matter today, Ω, is proportional to the inverse of the particle cross section times relative velocity at freeze-out, σν, (think of this as probability the particles would interact). Today dark matter is said to have frozen out at its current density because on average a dark matter particle will travel the entire distance across the universe (this distance changes also invoking the Hubble scaling h) before interacting with another dark matter particle:This is known as the WIMP miracle. Particle physics independently predicts a particle with the right density to be the dark matter that astronomers observe.Evidence for WIMPsRecent observations from experiments and collaborations including ATIC, Fermi/GLAST, PAMELA, and others have observed an excess over the expected background of cosmic ray positrons (I am lumping these experiments together, but they actually detect subtly different kinds of particles and found different kinds of anomalies; for example PAMELA detected an upturn in the positron fraction e+/(e++e-) from 10-100 Gev while ATIC detected and excess in the total e++e- count at energies of 300-800 Gev). The reality is that anomalies are not unexpected given the uncertainty in astrophysical foregrounds which vary with energy.Cosmic rays are charged particles (like positrons, e+,and electrons, e-) fired like bullets moving close to the speed of light at random throughout the universe and may be created by any of your favorite high energy astrophysical sources like magnetars, super massive black holes, supernovae, and so forth. Cosmic ray particles (any very fast moving charged particles) can interact with the galactic magnetic field, the interstellar medium, or the interstellar radiation field. These interactions generally cause the particles to lose energy and that energy is emitted in the form of observable photons. Thus dark matter will leave an astronomical signature not only in the form of cosmic rays from direct annihilation into positrons or electrons as discussed above, but also in the form of scattering of photons. The reason for all this talk about cosmic rays is that now we can see that the dark matter models explaining the PAMELA, ATIC, and Fermi results would also produce other observables, for example, an excess of gamma ray photons from the galactic center (they would be focused from the galactic center because that is where the dark matter would be densest and annihilating most rapidly) at energies 100 GeV and more. Another signal that may come from dark matter was seen in observations made by the WMAP satellite which detected residual microwave emission from the galactic center known as the 'WMAP haze'.You can't just ask a particle where it came from. The electrons and positrons of WIMP annihilation are cosmic rays with an exotic origin, but because there may be an undetected 'local' pulsar producing the cosmic ray anomalies being detected we... Read more »

Abdo, A., Ackermann, M., Ajello, M., Atwood, W., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Baughman, B., Bechtol, K.... (2010) Spectrum of the Isotropic Diffuse Gamma-Ray Emission Derived from First-Year Fermi Large Area Telescope Data. Physical Review Letters, 104(10). DOI: 10.1103/PhysRevLett.104.101101

Nima Arkani-Hamed, Douglas P. Finkbeiner, Tracy R. Slatyer, & Neal Weiner. (2008) A Theory of Dark Matter. Phys.Rev.D79:015014,2009. arXiv: 0810.0713v3

Feng, J., Kaplinghat, M., & Yu, H. (2010) Halo-Shape and Relic-Density Exclusions of Sommerfeld-Enhanced Dark Matter Explanations of Cosmic Ray Excesses. Physical Review Letters, 104(15). DOI: 10.1103/PhysRevLett.104.151301

March 26, 2010
02:50 PM
1,598 views

On the Phenomena of Lightning

by Alexander in The Astronomist.

Thunderstorms are epic demonstrations of nature that can be quite fascinating when they aren't terrifying. The study of thunderstorms, in particular lightning, is of obvious practical interest, but also there is also a purely aesthetic and amusing aspect to them.... Read more »

Siingh, D., Singh, A., Patel, R., Singh, R., Singh, R., Veenadhari, B., & Mukherjee, M. (2009) Thunderstorms, Lightning, Sprites and Magnetospheric Whistler-Mode Radio Waves. Surveys in Geophysics, 29(6), 499-551. DOI: 10.1007/s10712-008-9053-z

February 19, 2010
03:47 AM
555 views

An Upper Limit On Not Knowing What the F*** They're Doing

by The Astronomist in The Astronomist.

First, I should say that the Supernova Cosmology Group and others using Type Ia supernova as standard candles are very precise in their work and I don't seriously doubt their results as they have been very consistent with other observations. There is though the one dark shadow looming over all their results and that is systematic error. Cosmologists use Type Ia supernova as a lighthouse in the dark because we can assume that all lighthouses have the same intrinsic luminosity and therefore any difference in observed luminosity is due solely to the distance from us. Thus by observing distant supernovae and recording their various properties such as luminosity and recession velocity from us we can plot their velocity versus distance and we can learn about the expansion of our universe and the cosmological constant. However, we assumed that we knew their intrinsic luminosity, but of course there are always unknown unknowns:As we know, There are known knowns. There are things we know we know. We also know There are known unknowns. That is to say We know there are some things We do not know. But there are also unknown unknowns, The ones we don’t know We don’t know.—Donald Rumsfeld, Feb. 12, 2002, Department of Defense news briefingToday I read two things online that I really enjoyed and I realized that they are actually very connected. On The Blog of Steve Shwartz I read that No One Knows What the F*** They're Doing (or "The 3 Types of Knowledge") and couldn't agree more (for example, I certainly don't know what I am doing). And in Nature I read about An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate (and the arXiv preprint here) which raised questions about possible systematics in the use of supernovae in cosmology. The abstract from the nature article:There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 107 years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of ~30–50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.So, what the researchers found using Chandra data is observational evidence that type Ia supernovae are not simply explosions of Chandrasekhar mass white dwarfs, which would have been the simple case. The 'classic' picture is that when the amount of material accreted onto a white dwarf exceeds the Chandrasekhar mass the dwarf explodes:The new Chandra results indicate that some Type Ia supernovae probably originate from the collision of white dwarf binaries. The collision occurs because the stars radiate away gravitational waves and move inevitably closer. The result is an explosion of two stars that are near the Chandrasekhar mass so the observed luminosity may not be so standard:There is at least one caveat to the results and the explanation given above. The Chandra observations were focused on elliptical galaxies and on the the center of one spiral galaxy because these areas had minimal amounts of gas and dust which block X-rays from reaching detectors. To summarize the results, the dominant mechanism for Type Ia supernovae in the elliptical early type galaxies Chandra observed is white dwarf mergers and not mass accretion. The take away point is that cosmologists need to take into account the galaxy type when using supernovae as standard candles because elliptical and spiral galaxies have different supernova progenitors; the supernova cosmology surveys have only used a small fraction of supernova from elliptical galaxies though, so it wont really change current results! So all that worry to discover nothing so troubling, but perhaps we gain assurance that soon even more distant standard candles can be trusted (like the GRB as a standard candle) despite that we can never really place anything more than an upper limit on unknown unknowns.References:Marat Gilfanov, & Akos Bogdan (2010). An upper limit on the contribution of accreting white dwarfs to the typeIa supernova rate Nature, 18 February 2010, Vol.463, p.924 arXiv: 1002.3359v1... Read more »

Marat Gilfanov, & Akos Bogdan. (2010) An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate. Nature, 18 February 2010, Vol.463, p.924. arXiv: 1002.3359v1

February 19, 2010
12:05 AM
579 views

An Upper Limit On Not Knowing What the F*** They're Doing

by The Astronomist in The Astronomist.

First, I should say that the Supernova Cosmology Group and others using Type Ia supernova as standard candles are very precise in their work and I don't seriously doubt their results as they have been very consistent with other observations. There is though the one dark shadow looming over all their results and that is systematic error.... Read more »

Marat Gilfanov, & Akos Bogdan. (2010) An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate. Nature, 18 February 2010, Vol.463, p.924. arXiv: 1002.3359v1

February 10, 2010
11:22 PM
1,549 views

The Cosmos isn't strange, people are strange

by The Astronomist in The Astronomist.

The cosmos isn't strange, people are strange. The universe on the largest of scales is actually simple compared to the complexities of the human mind or even the weather. In a statistical sense all current observations indicate that universe is homogeneous and isotropic everywhere. The best evidence for this statement is the cosmic microwave background (CMB) radiation which is light from the big bang that has traveled unimpeded through the universe since recombination. A simple and consistent model for the universe is that just after the big bang an inflationary field with quantum fluctuations rapidly expanded. These fluctuations seeded the CMB with a Gaussian random field of temperature perturbations. The seventh year Wilkinson Microwave Anisotropy Probe (WMAP) data is consistent with an inflationary ΛCDM model that specifies just six parameters (see Larson et al. 2010): the baryon density Ωb, the cold dark matter density Ωc, a cosmological constant ΩΛ, a spectral index of scalar fluctuations ns, the optical depth to reionization τ , and the scalar fluctuation amplitude Δ2R. These results are not new. They are further refinements on previous WMAP data which have all been consistent with the ΛCDM model showing that the universe is flat, with a nearly (but not exactly) scale invariant fluctuation spectrum seeded by quantum flucuations during inflation, with Gaussian random phases, and with statistical isotropy over the entire sky. When WMAP data are combined with additional cosmological data, the ΛCDM model remains robust, and stronger constraints are placed on allowed parameters.However, if you keep looking closer you can find surprises in the data. The human mind is a readily adept tool at recognizing patterns so a visual inspection of the WMAP image is always a good idea. You can find many statistically unlikely events in the WMAP sky map. Exactly what is hidden in the cosmic microwave background is like asking what you see in the clouds, but some claim to see the secret masters of the universe at work. In fact you can see Steven Hawking's initials the WMAP image!This seems like an outrageous claim so the first thing I did when I heard this was to look at my desktop, which of course is an image of the WMAP sky, and indeed this is no ruse. All I have done in the above image is to outline what was already present; to corroborate this I encourage you to observe the original images of the microwave sky from the WMAP collaboration.There are many other strange occurrences in the WMAP sky which are not so easily observed by the casual observer or with the human eye. However when you go bowling with the CMB even statistics can lead you astray. You have to ask yourself is physics cognitively biased? Some striking visual anomalies that cosmologists have pointed out include the extremely large cold spot at the center, the four blue ridges in the lower hemisphere, the 'SH' initials, etc. Despite these observations the standard cosmological interpretation (see Komatsu et al. 2010) is, not surprisingly, standard. There are no anomalies to account for, but many researchers are searching for them. The situation is similar to particle physics where if the LHC finds the Higg's boson at the expected energy range then the standard model is validated, but if something unexpected is found new physics, answers to open questions, or a new direction for discovery other than the standard model may be opened (indeed, some find the prospect of merely finding the Higg's boson a disappointment). For example if there is statistically significant support for a hemispherical or dipole power asymmetry across the sky this could point to evidence for a unique inflaton field and, out on a limb here, evidence for physics beyond our universe. The real question is whether or not there are anomolies in the data which are significant. In Bennet et al 2010 the prospect for CMB anomalies is seriously addressed. An excerpt from the abstractIn this paper we examine potential anomalies and present analyses and assessments of their significance. In most cases we find that claimed anomalies depend on posterior selection of some aspect or subset of the data. Compared with sky simulations based on the best fit model, one can select for low probability features of the WMAP data. Low probability features are expected, but it is not usually straightforward to determine whether any particular low probability feature is the result of the a posteriori selection or non-standard cosmology. Hypothesis testing could, of course, always reveal an alternate model that is statistically favored, but there is currently no model that is more compelling. We find that two cold spots on the map are normal CMB fluctuations. We also find that that the amplitude of the quadrupole is well within the expected 95% confidence range and therefore is not anomalously low. We find no significant anomaly with a lack of large angular scale CMB power for the best-fit CDM model. We examine in detail the properties of the power spectrum data with respect to the CDM model and find no significant anomalies. The quadrupole and octupole components of the CMB sky are remarkably aligned, but we find that this is not due to any single map feature; it results from the statistical combination of the full sky anisotropy pattern. It may be due, in part, to chance alignments between the primary and secondary anisotropy, but this only shifts the coincidence from within the last scattering surface to between it and the local matter density distribution. This alignment has been known for years and yet no theory has replaced CDM as more compelling. We examine claims of a hemispherical or dipole power asymmetry across the sky and find that the evidence for these claim is not statistically significant. We confirm the claim of a strong quadrupolar power asymmetry effect, but there is considerable evidence that the effect is not cosmological. The likely explanation is an insufficient handling of beam asymmetries. We conclude that there is no compelling evidence for deviations from the CDM model, which is generally an acceptable statistical fit to WMAP and other cosmological data.In case no one ever told you, the answer to any question asked in a paper's title is no. So if you make an arbitrary decision on how to run statistical analysis on your data (like something as a trivial as a tuned bin size in a histogram) this imposes a posterior selection on the data which will likely effect your conclusion. In order to draw conclusions about such a complicated observation (I began by saying this was all simple and I maintain that, but the instrument and detectors taking the observations are not simple) you must run Monte Carlo simulations to determine expected deviations from your model. So in conclusion cosmic variance limited data will necessarily show probabilistically unlikely events, and the cosmos isn't strange people are strange. Though this certainly isn't the end of probing the CMB with the PLANCK mission currently flying and CMB polarization just being explored. We still want to know how closely natures matches our theory, exactly or not exactly? And I will give you a hint here, you can always measure again so if you have to ask...References:C. L. Bennett, R. S. Hill, G. Hinshaw, D. Larson, K. M. Smith, J. Dunkley, B. Gold, M. Halpern, N. Jarosik, A. Kogut, E. Komatsu, M. Limon, S. S. Meyer, M. R. Nolta, N. Odegard, L. Page, D. N. Spergel, G. S. Tucker, J. L. Weiland, E. Wollack, & E. L. Wright (2010). Se... Read more »

C. L. Bennett, R. S. Hill, G. Hinshaw, D. Larson, K. M. Smith, J. Dunkley, B. Gold, M. Halpern, N. Jarosik, A. Kogut.... (2010) Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies?. ApJ. arXiv: 1001.4758v1

February 8, 2010
02:42 AM
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The Cosmological Constant and the Dark Sector

by The Astronomist in The Astronomist.

What is the phenomenology of the dark sector? That is my question. The dark sector refers to dark energy and dark matter, which are two distinct phenomena which seem to have no direct connection other than in name. In this post I am going to talk about the cosmological constant, dark energy, and look at some landmark literature on the subject. I am going to show the origin of the 10120 order of magnitude error that results from the quantum field theory prediction and cosmological observation. I am not going to shy away from the equations and science so while you may need some advanced knowledge of math or physics to grasp everything I hope that anyone could at least see the light at the end of this dark tunnel. I am going to outline the physicists theoretical case and the astronomers observational case, and we will see how deceiving the cosmos can be.Dark energy is a form of energy attributed to the nature of empty space which increases the rate of expansion of the universe; that is if you observe a distant galaxy not only is it moving away from you in time, but the rate at which it recedes from you is accelerating. In the last 30 years or so a wide range of observations have corroborated a model of the universe wherein a majority of energy is attributed to the dark sector. The current consensus is that there is a dark energy component of our universe that represents 2/3 of the entire energy content of the universe that explains the observed cosmic acceleration. This dark energy can lead to other strange phenomena such as repulsive gravity and ultimately a universe that tears itself apart.The composition of the cosmos.The classic and simplest explanation for dark energy is the cosmological constant. The cosmological constant was originally introduced by Einstein as a term in his gravitational field equations in order to allow a steady state non-empty universe solution to his equations. The cosmological constant introduces a non-zero vacuum energy into the universe. This vacuum energy acts as a negative pressure (conversely a negative vacuum energy would result in a positive pressure) and this vacuum energy is known as dark energy. The idea of a vacuum containing energy is very much expected by physicists, but the observed value of the vacuum energy is what is surprising as we will see. The cosmological constant represents the particularly simple case of constant vaccum energy and is represented by the Greek character lambda (Λ). A seminal paper (also see Weinberg 1989 or for more recent general reviews see Carroll 2000, Frieman et al. 2008, and Peebels & Ratra 2002) on the topic was published in 1992 by Carrol, Press & Turner. The abstractThe cosmological constant problem is examined in the context of both astronomy and physics. Effects of a nonzero cosmological constant are discussed with reference to expansion dynamics, the age of the universe, distance measures, comoving density of objects, growth of linear perturbations, and gravitational lens probabilities. The observational status of the cosmological constant is reviewed, with attention given to the existence of high-redshift objects, age derivation from globular clusters and cosmic nuclear data, dynamical tests of ΩΛ, quasar absorption line statistics, gravitational lensing, and astrophysics of distant objects. Finally, possible solutions to the physicist's cosmological constant problem are examined.Roughly following Carrol et al. (1992) I will explore further the origin of the cosmological consant and the question of why the observed vaccum energy is so small in compairon to the scales of predicted by fundamental physics. We start with the Friedman equation derived from Einstein's field equations. It relates the Hubble parameter, H, to the scale factor, a, and other basic quantities.Where the dot denotes a time derivative, G is the gravitational constant, ρM is the cosmological density of matter, and k is the curvature parameter which can take on values of -1,0, and +1 corresponding to a negative, flat, and hyperbolic universe geometries respectively. The Friedman equation can be viewed in terms of the contribnutions from matter (ρM), curvature (k), and vacuum energy (Λ). It is customary to parametrize these quantities in terms of their fractional value at the current epoch, that is today. We denote the current values with a subscript 0. For example the current value of Hubbles Constant is H0~70 km/s/Mpc.So in total then we have simplified the problem to the statement that ΩM+Ωk+ΩΛ=1 for consistency with the Friedman equation. The astronomers cosmological constant problem is whether a nonzero ΩΛ is required to achieve consistency.The physicists cosmological constant problem begins with the statement that there are virtual vacuum states present in a vacuum due to the Heisenberg uncertainty principle. For example consider a relativistic field as the collection of harmonic oscillators of all possible frequencies, ω. The vaccum has a zero-point energy E0 (for a scalar field φ of spinless bosons with mass m) which is the sum of contributions over all possible modes of the field, i.e. over all wave vectors k.We preform the sum by considering the system of in a box of side length L and letting L tend to ∞. An appropriate periodic boundary condition implies λj=L/nj for some integer nj with a wave vector kj=2π/λj. In the range ( kj, kj + dkj) there are dkj Lj/(2 π) discrete values of kj such that the sum becomes the integral:The energy density of the vaccum is simply this ground state energy divided by the volume, L3. In order to properly obtain an answer we must use ωk2=k2+m2/h2 and most importantly we impose a cutoff at a maximum wave vector kmax»m/h (note that I must use h where I mean hbar here). The result is thenThe vaccum energy density can be shown to apporach infinity as kmax (the physicist will recall ultraviolet catastrophe) appraoches infinity therefore it is expected that there is some cutoff value near the Planck energy. The logical choice is to choose kmax=E/h. The resultant prediction for this vaccum zero-point energy density is that ρvac ≈ 1074 GeV 4h-3 ≈ 1092 g/cm3, however observational cosmology has constrainted ρobs≈ 10-47 GeV 4h-3 ≈ 10-29 g/cm3. The difference between the predicted and observed value is 120 orders of magnitude. This discrepancy is devastatingly imcomprensible large and can only be described as an EPIC FAIL. However, it may be misleading to chracterize the discrepancy this way since energy density can be expressed as a mass scale to the fourth power. Writing ρλ= Mvac4 we find the difference is only 1030. The theoretical predictions from quantum field theory have been sound in predicting vacuum effects such as the Casimir force so ... Read more »

Carroll, Sean M., Press, William H., & Turner, Edwin L. (1992) The cosmological constant. ARA, 499-542. info:/

Will J. Percival, Beth A. Reid, Daniel J. Eisenstein, Neta A. Bahcall, Tamas Budavari, Joshua A. Frieman, Masataka Fukugita, James E. Gunn, Zeljko Ivezic, Gillian R. Knapp.... (2009) Baryon Acoustic Oscillations in the Sloan Digital Sky Survey Data Release 7 Galaxy Sample. MNRAS. arXiv: 0907.1660v3

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