The Size of the Proton Measured with Lasers

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. The new research relies on the fact that the energy levels allowed within an atom depend upon the quantum mechanical interaction of the proton and the electron (or in the case of this recent experiment the exotic muon particle). Each atom has its own energy levels and corresponding spectral lines like a fingerprint. Understanding the spectra produced by atoms was historically very important, to stress this Hansch called the simple hydrogen spectrum the 'Rosseta stone of atoms.' Tiny discrepancies in the expected spectra of the atom in experiment compared to theory have led to major advances in fundamental knowledge. The breakthrough that allowed for exploration of these discrepancies in the behavior of atoms occurred exactly 50 years ago with the development of the laser.

The Hansch lecture on the heartbeat of light is available to watch on the Lindau conference website here. It is at least worth watching what he explains at minute 18 on the nonlinear self organization of light pulses in pulsed lasers in analogy to mechanical pendulums. He shows a video of ten mechanical pendulums in a row with staggered frequencies ranging from 30 to 39 cycles per minute. Each pendulum corresponds to one of the frequencies present in a laser cavity and at the first moment all the lasers are in phase such that constructive interference occurs corresponding to a laser pulse or a large transfer of energy. Quickly the pendulums get out of phase and although they look chaotic there are smaller emerging and disappearing patterns. If you wait long enough the pendulums briefly line up in phase again and this is when the laser when emit the next big flash of light. This demonstration is lovely because it underlies all of physics, if you are a physicist you probably can immediately visualize what I am describing, if you are not a physicist you may have to see the video to visualize what I am talking about, but everyone will appreciate the beauty of the simple demonstration which was effective enough to illicit a round of applause from the audience.

Historically precision measurement of the hydrogen energy levels were difficult because Doppler broadening is large for the particularly light weight hydrogen atom. Hansch explained that with lasers you can pick out hydrogen that is standing still or at most moving sideways using saturation spectroscopy. The development of lasers allowed physicists for the first time to see single fine structure components in atoms particularity the Lamb shift discerning the 2S state where the electron comes close to the proton and the 2P state where it stays away. The saturation spectroscopy technique allows the Lamb splitting of energy levels to be seen plainly. The Lamb shift depends on the size of the proton, but to probe the proton size more finely tricks are needed.

The notion of size for a particle like the proton that resides in the realm quantum mechanics is tricky to define, but there are two classic ways of measuring its radius: scattering of electrons from a hydrogen atom or by looking at the exact energy levels of a hydrogen atom. The size of the proton has been based mainly on the precision spectroscopy of atomic hydrogen and calculations from bound-state quantum electrodynamics. It is known that a hydrogenic atom with a smaller Bohr radius would enhance the effects related to the finite size of the proton, that is to say a proton interacting with a bound oppositely charged massive particle would demonstrate effects in Lamb shift due to a contribution from the proton's size. A collaborative team of scientists lead by Randalf Pohl have spectroscopically measured the Lamb shift of muonic hydrogen and found the charge radius of the proton is 4% smaller than the previously accepted value.

Muonic hydrogen is like regular hydrogen but the electron has been swapped for a muon. The experiment called for muonic hydrogen in which a muon travels around the proton with a radius 200 times smaller than that of hydrogen constructed from and electron. A muon is an elementary particle similar to the electron, is has the the same negative charge and spin, but it is about 200 times more massive than an electron. The muon 'orbits' so close to the proton in fact that it actually spends some portion of its orbit within the radius of the proton. Muons decay quickly and creating muonic hydrogen is a task that could only be undertaken at the Paul Scherrer institute in Switzerland which is the sole location in the world where a muon beam of sufficient intensity could be generated.

The Lamb shift is the result of angular momentum conservation within the atom. The 2S state of hydrogen has zero angular momentum and the 2P state has an angular momentum of one (don't ask about units). As mentioned earlier the result is that in the 2S state the electron comes close to the proton and in the 2P state it stays away.

The experiment worked as follows. The researchers created muonic hydrogen at the Paul Scherrer Institute with equipment constructed especially for the experiment (it took ten years to build). Once the muonic hydrogen is created the researchers shine in a tunable infrared laser with a frequency corresponding to the splitting between the 2S and 2P states. The laser will excite some of the muons from the 2S into the 2P state (when the muonic hydrogen is created a tiny fraction of of it is naturally produced in the 2S state), but the muons will quickly decay to the ground state emitting a powerful x-ray in the process. The researchers measured the amount of x-rays emitted at each specific frequency they had their infrared laser tuned to and the exact frequency which generated the most x-ray flux is the Lamb shift measurement they made.

The new measurement is discrepant with previous results, but the team has done such a careful job of measurement, the first results indicating a discrepancy were discovered six years ago but the results were held, that theoreticians are questioning the accuracy of fundamental constants like the Rydberg constant and basic theories of quantum electrodynamics. There is some more discussion and interpretation of the experiment over at Uncertain Principles on how the proton is even smaller than we thought. I think that it is a very cool discovery. Hansch commented on discovery during his lecture with an anecdote about Arthuer Schawlow (who received the 1981 Nobel for his work with laser spectroscopy). Schawlow would ask students in the hallways at Stanford, 'What have you discovered?' and Hansch says the message to students is, 'I am not here to learn something old, I am here to discover something new.'


References:

Pohl R, Antognini A, Nez F, Amaro FD, Biraben F, Cardoso JM, Covita DS, Dax A, Dhawan S, Fernandes LM, Giesen A, Graf T, Hänsch TW, Indelicato P, Julien L, Kao CY, Knowles P, Le Bigot EO, Liu YW, Lopes JA, Ludhova L, Monteiro CM, Mulhauser F, Nebel T, Rabinowitz P, Dos Santos JM, Schaller LA, Schuhmann K, Schwob C, Taqqu D, Veloso JF, & Kottmann F (2010). The size of the proton. Nature, 466 (7303), 213-6 PMID: 20613837

Microwave Sky Seen by Planck

The first image of the microwave sky was released today by the Planck collaboration. The image is the result of a year of observations from the Planck satellite. How far we have come since the first image of the cosmic microwave background by COBE! The most prominent aspect of the image is the bright band across sky caused by diffuse gas and dust emission from our own Milky Way. Also visible are local clouds of gas, nearby galaxies such as Andromeda, and more distant galaxies which host supermassive black holes in their center. The more subtle variations which will be visible when the foregrounds are removed are tiny temperature fluctuations which carry information about the cosmic microwave background and primordial density fluctuations seeded by the Big Bang. However, scientists are waiting to dive into detailed analysis of the multi-frequency data ranging from 30 GHz to 857 GHz until all of the foregrounds and telescope systematics can be understood. Ultimately the Planck data will give us the most precise constraints humans have ever had on the parameters of our cosmos.

Planck is a major step forward in cosmic microwave background (CMB) observations because it measures polarization of microwave photons. The polarization of photons may carry information about the universe from inflation or when the CMB was generated 400,000 years after the Big Bang. Generally when an electromagnetic wave or photon is incident upon a free electron the scattered photon is polarized perpendicularly to the incident direction. Different regions of the CMB may have a net linear polarization generated when radiation from perpendicular directions in the sky has different intensities. Different directions in the sky have different intensities dependent upon perturbations; there are three kinds of perturbations 1) scalar perturbations due to density fluctuations, 2) vector perturbations due to vorticity (like cosmic strings or defects, although these are not likely to be detected), 3) and tensor perturbations due to gravity waves. The Planck mission will be the first CMB space satellite to measure the as of yet unseen gravity wave or "B-mode" poarlization which will reveal the physics of primordial gravity waves when the Universe was in existence for just 10^-36 seconds.

The History of the Universe

LINDAU, Germany — John Mather is humble when describing his measurements of the cosmic microwave background radiation despite the fact that Steven Hawking described this measurement as possibly the most important discovery humans have ever made. The cosmic microwave background radiation is the remnant glow of the Big Bang; it is the primary evidence.  Mather is careful to place his work in context next to the original work of Penzias and Wilson who made the first measurement of the cosmic microwave background radiation.

The cosmic microwave background (CMB) was first measured by Penzias and Wilson in 1965, but it was predicted decades earlier independently by several astrophysicists. You can read about the journey Penzias and Wilson took to making their discovery in this previous post on pigeon waste, cosmic melodies and noise in scientific communication. Cosmologists were not content with the first tenuous measurement of the 2.7 kelvin background, but they would have to wait until the Cosmic Microwave Background Explorer (COBE) was launched in 1989 to measure the CMB to one part in 100,000 or 30 millionths of a degree difference in temperature. In 2006 John Mather and George Smoot received the Nobel Prize in physics for their discovery of the blackbody form and anisotropy of the CMB. Mather and Smoot's precision measurements indicated that the Big Bang produced radiation that was perfectly consistent with the theoretical predictions for a blackbody and that the anisotropy, or spatial variations, of the relic radiation were extremely miniscule. The observations fit the theory so well that when plotting the data the error bars must be enlarged to make them visible.

Mather told the attendees the entire history of the Universe during a morning lecture. First, there was the Big Bang. Then a brief period of stupendous growth occurred known as inflation. The early Universe was extremely hot and contained simple particles of matter as well as antimatter; the matter and antimatter annihilated upon contact until only one part per billion of of the early universe was antimatter (this is good for us, because we are made of normal matter, but a mystery to cosmologists). Within the first 3 minutes the formation of Helium nuclei had occurred. The Universe remained in a dense fog of mostly free protons, electrons, and  Helium nuclei until about 400000 years after the Big Bang. At this point the Universe had cooled enough that electrons could become captured by the free protons and Helium nuclei to form neutral atoms. The photons which up until this point had been scattering off of the free particles suddenly found that they could effectively travel the entire distance of the universe before having another scattering. These photons cooled as they traversed the expanding Universe until they encountered the detectors on COBE.

If Mather is the stoic scientist, then Smoot is the adventuring explorer. In a break away afternoon session Smoot had the opportunity to tell young scientists a few more details about the CMB and the ramifications. The minor variations in the CMB are quantum fluctuations that were super sized during the period of inflation. Smoot says that our own galaxy was a quantum fluctuation at one time. Through analysis of the CMB with the technique of spherical harmonics Smoot is keen to to stress that the early Universe is extremely linear and that deviations from the known amount of dark matter, dark energy, or age of the universe creatures significant inconsistencies with the data and the theory. 

Every galaxy we observe today is related to the small perturbations present in the early Universe. The cold spots in the CMB are slightly denser than the surrounding areas and so as the universe evolved gravity's long range attractive forces meant that over densities were inherently unstable. The over densities grew larger and larger until galaxies, clusters, and super clusters formed. Today, astronomers are measuring the result of this growth of structure through galaxy surveys such as as the Sloan Digital Sky Survey. The observed distribution of galaxies is perfectly consistent with the theories.

Telling the entire history of the Universe must be a humbling job. Astronomers, Mather says, actually have a simple job of describing the universe, galaxies, stars, and places where life may form. Astronomers don't actually have to say how life formed, but there are researchers and Nobel Laureates here at Lindau who are trying to answer that exact question. Mather finished his talk with more questions than answers. How did we get here? Are we alone? What happens next?

The Nobel Laureate Meetings in Lindau

I have been on blog sabbatical for some time, but I am going to be back in full force in about two weeks blogging from a lake in southern Germany. I have been invited to Lindau Germany to cover the 60th Meeting of Nobel Laureates in about two weeks. You can follow what I have to say along with several other bloggers on the Lindau Blog and you can also submit an original and stimulating question you would like to ask a Nobel Laureate.

One of the goals of the Lindau conference is to facilitate dialogue between Nobel laureates and young scientists. The conference will consist of lectures by the Nobel laureates followed by panel discussions between the young scientists and laureates. There will be an exchange of ideas on basic research as well as applications in fields such as medicine, physics, chemistry and economics. It is great to see this atmosphere of exchange between the various generations and the encouragement to young scientists for the future.  Personally, I am very excited to hear the talks and have the opportunity to ask Nobel laureates questions. I could use some recommendations on good questions to ask a Nobel laureate. Some questions I have thought of

• What was an assumption you or your field had that has turned out to be completely wrong?
• As a young researcher, how can we deal with the reality of daily failures and obstacles?
• What kind of fundamental research could lead to unexpected applications in daily life?

So, what would you ask?

Busy

I am overwhelmingly busy right now. I wont have any time for The Astronomist until the middle of June or so because apparently taking painful qualifying exams builds character. In the meantime here are some diversions...

• The Myth of Gravity
Erik Verlinde considers gravity as information. His theory implies that gravity is nothing more than the result of a system maximizing its entropy. See the full paper here.
• The Holographic Universe
Craig Hogan’s suggests nature deals with quantum mechanics and spacetime through the holographic universe. See a paper that discusses holographic noise in gravity wave interferometers here.
• The World Cup
Something completely different, but also completely awesome, and the physics of soccer is quite interesting too. Check out the video below that just begins to capture a fraction of the spirit of the World Cup held in South Africa this summer.

Dark Matter Confronts Observations

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 before

The 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 matter

Dark 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 WIMPs

Recent 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 do not yet know where the positron excess originates. The interpretation of the photon background can be similarly difficult. It is well known that the cosmic x-ray background is the cumulative emission from active galactic nuclei (it can be modeled with a single power-law), but on top of this background local astrophysical phenomena and or dark matter annihilation may contribute to the signal. Astrophysicists are curious to know if dark matter annihilation is necessary or do cosmic rays account for all the gamma rays in diffuse galactic radiation? The most promising and ongoing source of data to constrain WIMPs comes from the Fermi collaboration. Despite initial excitement in the astrophysics community and the blogosphere about the possible origins of the anomalies (if you don't read the physics blogosphere see here or here) the Fermi collaboration now states their results on the measured cosmic-ray spectra are consistent with a standard model of diffuse galactic gamma-ray emission,

The new result, based on the initial ten months of science observations, significantly improves and extends our knowledge of the isotropic diffuse emission at the high-energy end. The observed emission is softer and lower in intensity than measurements with EGRET had indicated previously. Fermi’s observed spectrum is consistent with a single power-law description over nearly three orders of magnitude in energy

In conclusion the prospect for direct astrophysical observations of dark matter is overwhelmed by uncertainty in the backgrounds and only more careful observations will improve this situation. I should reiterate that the uncertainty is in the detection of dark matter signature, and not the presence of gravitating dark matter.

Making dark matter consistent with observations

Despite the uncertainty in observations we may still draw conclusions and place bounds on possible dark matter candidates. The observed relic density implies that the dark matter annihilation cross section times the relative velocity at freeze-out is very small (see equation above). Unfortunately, given this predicted annihilation cross section the positron signal would be 2 to 3 orders of magnitude smaller than the observed cosmic ray excesses! In order for the WIMP model to fit the various observations a cross section much larger than that predicted by the relic abundance is necessary. Various unknown forces (the dark sector) could increase the annihilation rate. Some researchers have suggested the neutralino WIMP with Sommerfeld enhanced annihilations may explain the observed cosmic ray excesses. Sommerfeld enhancement increases the cross section of particles at low velocities similarly to classical gravitational enchantment. To wrap your head around this imagine dark matter particles as strangers quickly passing in the street; dark matter particles and strangers don't bother to hang out with each other at this speed, but if they stop at a coffee shop then the possibility that strangers would have some kind of interaction is greatly enhanced. If dark matter particles are jetting around too fast they wont have any interactions, however, if the particles slow down a significant interaction may occur between them. The classical gravitational enhancement analogy can quantify Sommerfeld enhancement a little more. Consider a particle encountering a star of radius R and mass M so the cross section should just be πR² in the case that the particle directly hits the star, but as the particle approaches the star at a velocity ν it is trivial to show using conservation of energy and angular momentum that the cross section is increased to

where ν_esc²=2GM/R. The Sommerfeld enhancement is the quantum extension of this classical phenomena. If you work through the entire derivation the result is that the enhacement leads to a cross section that scales at low energies as σν ∝ 1/ν. It turns out that the velocity of dark matter particles is a factor of ~10^{- 3} less now compared to what it was at freeze out. It is a miracle of dark matter that Sommerfeld enhancement provides an elegant mechanism for boosting annihilations now to exactly the amount needed.

However, a satisfactory solution to the dark matter problem must not only have dark matter annihilating at the correct rate, but it must also produce the right density and structure on all cosmological scales which is consistent with observations. In a paper appearing in Physical Review Letters, Jonathan L. Feng, Manoj Kaplinghat, and Hai-Bo Yu of the University of California, Irvine, point out some problems with these explanations, in thier own words from the abstract:

Dark matter with Sommerfeld enhanced annihilation has been proposed to explain observed cosmic ray positron excesses in the 10 GeV to TeV energy range. We show that the required enhancement implies thermal relic densities that are too small to be all of dark matter. We also show that the dark matter is sufficiently self-interacting that observations of elliptical galactic dark matter halos exclude large Sommerfeld enhancement for light force carriers. Resonant Sommerfeld enhancement does not modify these conclusions, and the astrophysical boosts required to resolve these discrepancies are disfavored, especially when significant self-interactions suppress halo substructure.

The authors find that the relic density cannot be achieved in any Sommerfeld enhanced interaction scenario even with additional resonant boosting terms. What is more, they find that the Sommerfeld enhancement would create significant changes in dark matter halos because the additional interactions would isotropize the velocity dispersion and create spherical halos that contradict observations of NGC 720. The effects of dark matter self interactions can be seen in almost any x-ray image of a galaxy or cluster galaxies. For example, the image above of the galaxy cluster CL0025+1654 shows the dark matter (shown in blue) has an approximately thermal distribution and an elliptical shape. Subhalos with high density are also predicted to be present (in the image above the subhalos are not resolved, but many N-body simulations have predicted them to be there) with lower velocity dispersions than the thermal bulk of the halo, such that the Sommerfeld enhanced cross sections make these components especially important. Further observations of halos and subhalos will place even more constraints on dark matter self interactions.

I must comment on the authors findings on spherical halos. I would agree that enhanced interactions can create significant changes in the shapes of dark matter halos, but I caution that analysis of a single galaxy is not sufficient to constrain dark matter. Observations of our own Milky Way indicate that our dark matter halo probably looks like a beach ball flattened perpendicular to the visible plane of the galaxy. This would corroborate the authors findings, however, I have had discussions with colleagues on dark matter forming spherical halos. Finally, it should be obvious that any dark matter model that calls for heavily interacting dark matter would produce wildly inconsistent results. One theory that is been proposed is that there is a hidden world of mirror-matter that acts just like regular matter, but it doesn't interact with regular matter through any force but gravity. Mirror galaxies, mirror stars, and mirror planets are completely inconsistent with observations! The experiments discussed above placed constraints on the type and mass of dark matter particles that would rule out such mirror-matter. I did not include or discuss the DAMA experiment's observations earlier because they had not been reproduced independently by any other group until very recently. The CoGeNT experiment now has a confirmation of the DAMA signal and this may change things by allowing the considered masses of dark matter candidates to be reduced considerably (this still doesn't allow for mirror-matter although some are implying it does). I refuse to equivocate my opinion that there is no hidden sector which is an exact copy of the standard model.

A survey of the lessons and knowns about dark matter indicates that we need more data. Dark matter models motivated from cosmic positron observations require an enhanced cross section to reproduce the data, but this enhanced cross section is inconsistent with the relic dark matter density and observations of dark mater halos. We need observers to determine if the cosmic positron excess is caused by a local astrophysical source such as a pulsar. We need theorists to develop a simple model for dark matter that is motivated from fundamental particle physics that make testable predictions for future observations.


References:

Abdo, A., et al. (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

Visions of space flight

We won't be getting out of the hood anytime soon.

It's the planetary neighborhood I am talking about here. The stars may beckon but it's an interplanetary, rather than interstellar culture that we will likely inhabit for hundreds if not thousands of years in the future. Baring the miracle of a "warp drive," the stars are simply too far away in space and time (via the theory of relativity) for a true interstellar culture to develop. The solar system with its 8 planets, 166 moons and countless asteroids and comets is likely to be our home -- our only home -- for a long, long time.

We should consider the implications of these limitations on coherent human cultures in space because today the president unveils his new plans for NASA.

The Obama administration made headlines recently when it reversed direction on NASA's Bush-era push to return to the Moon. The new plan turns to hungry young private space ventures to give us access to Near Earth Orbit. Stepping back on any present space mission the plan calls for development of the next generation of space technologies for the next generation of space exploration. But critics fault the Obama plan for its lack of any clear goals for these new technologies. Without a bold choice of destination -- Mars is the obvious choice) -- critics say the human space program will simply drift.

more by Adam Frank here. Also President Obama will announce plans for the future of Nasa at the Kennedy Space Center in Florida today.

Reaching the stars is easy