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2Physics Quote:
"Stars with a mass of more than about 8 times the solar mass usually end in a supernova explosion. Before and during this explosion new elements, stable and radioactive, are formed by nuclear reactions and a large fraction of their mass is ejected with high velocities into the surrounding space. Most of the new elements are in the mass range until Fe, because there the nuclear binding energies are the largest. If such an explosion happens close to the sun it can be expected that part of the debris might enter the solar system and therefore should leave a signature on the planets and their moons." -- Thomas Faestermann, Gunther Korschinek (Read Full Article: "Recent Supernova Debris on the Moon" )

Wednesday, July 25, 2007

"Changing Constants, Dark Energy and the Absorption of 21 cm Radiation" -- By Ben Wandelt

Ben Wandelt [Photo credit: Department of Physics, University of Illinois/Thompson-McClellan Photography]

Rishi Khatri and Ben Wandelt have recently proposed a new technique for testing the constancy of the fine structure constant across cosmic time scales using what may prove to be the ultimate astronomical resource for fundamental physics. In this invited article, Ben Wandelt explains the motivation for this work and the physical origin of this treasure trove of information.

Author: Ben Wandelt
Affiliation: Center for Theoretical Astrophysics, University of Illinois at Urbana-Champaign

What makes Constants of Nature so special? From a theorist's perspective constants are necessary evils that ought to be overcome. The Standard Model has 19 “fundamental constants,” and that is ignoring the non-zero neutrino masses which bring the total count to a whopping 25. That's 25 numbers that need to be measured as input for the theory. A major motivating factor in the search of a fundamental theory beyond the Standard Model is to explain the values of these constants in terms of some underlying but as yet unseen structure.

What's more, not even the constancy of Constants of Nature (CoNs) is guaranteed (Uzan 2003). Maybe the quantities we think of as constants are actually dynamic but vary slowly enough that we haven't noticed. Or even if constant in time, these numbers may be varying across cosmic distances.

Quite contrary to taking the constancy of the CoNs for granted, one can argue that it is actually surprising. String theorists tell us these constants are related to the properties of the space spanned by the additional, small dimensions beyond our observed four (3 space + 1 time). These properties could well be dynamical (after all, we have known since Hubble that the 3 large dimensions are growing) so why aren't the 'constants' changing? This perspective places the onus on us to justify why the small sizes are at least approximately constant. So the modern, 21st century viewpoint is that it would be much more natural if the CoNs were not constant but varying—either spatially or with time.

By way of example consider the cosmic density of dark energy. The 20th century view was in terms of “vacuum energy,” a property of empty space that is predicted by the Standard Model of particle physics. This is qualitatively compelling, but quantitatively catastrophically wrong. More recently three main categories of attempts emerged to explain that particular constant (and ignore the vacuum energy problem). The first category explains dark energy as some new and exotic form of matter. The second category of explanations sees the acceleration of the Universe as evidence that our understanding of Gravity is incomplete.

The third argues that the dark energy density is just another CoN, the “cosmological constant,” which appears as an additive term in Einstein's equations of general relativity and therefore increases the rate of the expansion of the Universe. This possibility was originally suggested by Einstein himself. While this is quite an economical way of modeling all currently observed effects of the universal acceleration it is also hugely unsatisfactory as an actual explanation—somewhat analogous to a boss explaining the size of your salary as “Because that's the number and that's it.” The attempt to turn this into an actual explanation through the pseudo-anthropic reasoning associated with string-theoretic landscape arguments corresponds to your boss adding “Because if you wanted to earn any more than that you wouldn't be here to ask me this question.”

If we consider the cosmic density of dark energy as another CoN that appears in Einstein's equation, it should also somehow arise from the underlying fundamental theory, like the other constants. By the identical argument we went through before we should in fact be surprised by its constancy. Hence most of the theoretical activity takes place within categories one and two, endowing this supposedly constant CoN with dynamical properties that can in principle be tested by observation.

Of course none of these aesthetic or theoretical arguments for what constitutes a satisfying explanation holds any water if it cannot be tested. And in fact, there are two sorts of tests: laboratory tests and astronomical observations. For definiteness, let's focus the discussion on a particular CoN, the most accurately measured CoN, the fine structure constant α. This number tells us the strength of the force that will act on an electric charge when it is placed in an electromagnetic field. If you have heard about the charge of the electron you have already encountered this constant in a slightly different form. Since charge has units (Coulomb), one could always redefine the units to change the value. So the relevant number is a dimensionless combination of the charge of the electron with other CoNs. This gives α ≈ 1/137.

Over the years, the value of α has been measured in laboratory experiments to about 10 digits of accuracy. Using the extreme precision of atomic fountains, the value of α was measured over 5 years and found to have changed by less than 1 part in 1015 per year [Marion et al. 2003].

Laboratory experiments do have their distinct advantages: the setup is under complete control and repeatable. However, they suffer from the very short lever arm of human time scales. Astronomical observations provide a much longer lever arm in time. The best current observations use quasar absorption lines and limit the variation to a similar accuracy when put in terms of yearly variation—but these measurements constrain variation over the last 12 billion years, the time it took the Universe to expand by a factor of 2.

In fact, using such quasar data, one group has claimed a detection of a change in α of 0.001% over the last 12 billion years [Webb et al. 2001]—though this claim is certainly controversial [Chand et al. 2006], but things may become interesting at that level.

My graduate student Rishi Khatri and I have discovered a new astronomical probe of the fine structure constant that is likely the ultimate astronomical resource of information for probing its time variation. Compared to the quasar data our technique probes α at an even earlier epoch, only a few million years after the Big Bang, when the Universe went from 200 times smaller to 30 times smaller than it is today. And in principle, if some technological hurdles can be overcome, there is enough information to measure α to nine digits of accuracy 13.7 billion years in the past! This would be 10,000 more sensitive than the best laboratory measurements.

What is this treasure trove of information? It arrives at the Earth in the form of long wavelength radio waves between 6 meters and 42 meters long. Theses radio waves started out their lives between 0.5 cm and 3 cm long, as part of the cosmic microwave background that was emitted when the hot plasma of the early Universe transformed into neutral hydrogen gas. As the Universe expands, these waves stretch proportionally. After about 7 million years, the ones with the longest initial wavelength first stretch to a magic wavelength: 21 cm. At this wavelength these waves resonate with hydrogen atoms: they have just the right energy to be absorbed by its electron. Waves that are absorbed are removed from the cosmic microwave background and can be seen as an absorption line (similar to the well-known Fraunhofer lines in the solar spectrum). As the Universe expands during the next 120 million years, waves that were initially shorter stretch to 21 cm and are similarly absorbed by hydrogen. After this time, light from the first stars heat the hydrogen to the point that it can no longer absorb these waves [Loeb and Zaldarriaga 2004]

It turns out that the amount of absorption is extremely sensitive to the value of α. Therefore, the spectrum of absorption lines we expect to see in the radio waves is an accurate record of the value of α during this epoch. We could even look for variations of α within this epoch, and check for spatial variations of α and other 'constants.' I argued above that these variations are expected on general grounds, but they are also predicted by specific string-theory inspired models for dark energy such as the chameleon model.

The tests we propose are uniquely promising to constrain fundamental physics models with astronomical observations. Important technological hurdles have to be overcome to realize measurements of the radio wave spectrum at the required level of accuracy. Still, the next time you see snow on your analog TV you might consider that some of what you see is due to long wavelength radio waves have reached you from the early Universe, having traveled to you across the gulf of cosmic time and carry in them the signature that may reveal the fundamental theory of Nature.

Chand H. et al. 2006, Astron. Astrophys. 451, 45.
Khatri, R. and Wandelt, B. D. 2007, Physical Review Letters 98, 111301. Abstract
Loeb, A. and Zaldarriaga, M. 2004, Physical Review Letters 92, 211301. Abstract
Marion, H. et al. 2003, Phys.Rev.Lett. 90, 150801. Abstract
Uzan, J.-P. 2003, Reviews of Modern Physics, vol. 75, 403. Abstract
Webb,J. K. et al. 2001, Physical Review Letters 87, 091301. Abstract

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At 11:51 AM, Anonymous Anonymous said...

This is certainly a clever idea but finally it would depend on how carefully one can take into account all astrophysical factors. The work that comes to my mind is of Joe Taylor's (1993 Nobel) painstakingly-done analysis of Hulse-Taylor pulsar over ten years to finally conclude the evidence of gravitational waves. Somehow I guess this work would also demand a rogorous analysis.

... But any bright idea finally finds a way to prove its worth. Good luck to Prof. Wandelt!

At 10:34 PM, Anonymous Matt Richter said...

Thanks to Prof. Wandelt for this nice account of the work. I read the PRL paper earlier and did not understand lot of things but I knew that the idea was simple and so must be great.

Now, I can use this article to give a journal club talk on the PRL paper.

At 8:32 PM, Anonymous Igor Yakushin said...

Interesting article. Somehow missed it in PRL.

Good ideas for a Laboratory experiment that can shed light on any aspect of cosmology is so hard to come by that we must give a serious look at this.

Would like to read updates on further work.

At 10:01 PM, Anonymous Anonymous said...

It would have been great if Prof. Wandelt had elaborated on the 'technological hurdles' that he foresees.

I could not fully agree with his view on the advantages of Laboratory experiments. It's not safe to say that 'the setup is under complete control and repeatable'. The recent failure of PVLAS experiment to reproduce the result that had confirmed earlier the existence of Axions somehow comes to my mind. As far as cosmology is concerned, personally I trust COBE-like experiments and observations from space telescopes more than those performed in earth-based Laboratory.

For any such sensitive experiment, my opinion is: Do the experiment in space (... and get rid of those unwarranted and unpredictable disturbances) and analyze the data in your office.

'Nature' of 80s-90s is full of false alarms of the existence of extra-solar system planets -- all false alarms coming from other interferences from earth-based laboratories. Look at LIGO -- it seems unless LISA satellites fly in sky, there's no hope of detecting gravitational waves.

At 4:04 PM, Blogger Ben Wandelt said...

First of all, thank you for your very encouraging comments so far! I would like to reply to a few of the questions and points made here.

"Anonymous" asked about the technological hurdles I mentioned. There are several: First, because these waves have very long wavelengths (several meters), one needs very widely spaced antennas. Then, the antennas need to cover enough area to receive enough signal. The measurement accuracy is limited by the thermal noise due to long wavelength radio emission from electrons accelerated in the Galactic magnetic field. So to start getting to this information would require building a next generation radio array beyond SKA, the Square Kilometer Array.

A measurement at the ultimate level of sensitivity would be very ambitious. We would need radio antennas separated by hundreds of thousands of kilometers. This would therefore have to be done in space, but is certainly not out of the question.

Even in the 1970s NASA engineers were imagining 100km large radio telescopes in space, made out of extremely fine wire mesh. Now, 30 years on, several governments are looking for scientific reasons to go to the Moon. A large radio array on the moon, paired with large area antennas in orbit around Earth would have the required resolution.

Such a radio installation would of course have many other hugely interesting applications - for example it would find every single radio galaxy in the observable Universe.

The other point made by "Anonymous" was about space observations versus ground based experiments. As you can tell from the preceding discussion, and from my involvement in Planck, the next generation CMB satellite, I agree. Space is a much more benign environment for sensitive astronomical observations but it has to be said that (on top of being a lot cheaper) ground observations are generally very important as pathfinders and to demonstrate new detector technologies that will then fly on the next space mission.

The distinction I was trying to make in the article was between laboratory experiments and astronomical observations. An experimenter has the setup of the experiment under complete control and can repeat the measurement at will. In astronomical observations, powerful as they are, we only have one sky and have to work with what we find.

Keep the comments/questions coming - that's the beauty of an online publication!

All the best,


At 8:38 PM, Anonymous Santanu Dey said...

I'm a graduate student from TIFR, India.. just starting... Not yet decided on a problem.

This article was very motivating for a rookie like me. It would be great if scientists like you write more of such articles.

At 10:25 PM, Anonymous Francois Bondu said...

Prof. Wandelt, if you do not wish to keep it a secret until your next publication, may we know what the current activities of yours and your team are to follow up this idea?

I visited your homepage but couldn't guess it.

At 10:00 PM, Blogger Ben Wandelt said...

Hi Francois -

We submitted a paper proposing a test of string theory based on 21cm radiation... Watch my home page (http://cosmos.astro.uiuc.edu) for an update.



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