Blogs / Cosmic Variance

Greetings from Oxford, a charming little town across the Atlantic with its very own university. It’s in the United Kingdom, a small island nation recognized for its steak and kidney pie and other contributions to world cuisine. What you may not know is that the UK has also produced quite a few influential philosophers and cosmologists, making it an ideal venue for a small conference that aims to bring these two groups together.

The proximate reason for this particular conference is George Ellis’s 70th birthday party. Ellis is of course a well-known general relativist, cosmologist, and author. Although the idea of a birthday conference for respected scientists is quite an established one, Ellis had the idea of a focused and interdisciplinary meeting that might actually be useful, rather than just bringing together all of his friends and collaborators for a big party. It’s to his credit that they invited as many multiverse-boosters as multiverse-skeptics. (I would go for the party, myself.)

George is currently very interested and concerned by the popularity of the multiverse idea in modern cosmology. He’s worried, as many others are (not me, especially), that the idea of a multiverse is intrinsically untestable, and represents a break with the standard idea of what constitutes “science.” So he and the organizing committee have asked a collection of scientists and philosophers with very different perspectives on the idea to come together and hash things out.

It appears as if there is working wireless here in the conference room, so I’ll make some attempt to blog very briefly about what the different speakers are saying. If all goes well, I’ll be updating this post over the next three days. I won’t always agree with everyone, of course, but I’ll try to fairly represent what they are saying.

Saturday night:

Like any good British undertaking, we begin in the pub. I introduce some of the philosophers to Andrei Linde, who entertains us by giving an argument for solipsism based on the Wheeler-deWitt equation. The man can command a room, that’s all I’m saying.

(If you must know the argument: the ordinary Schrodinger equation tells us that the rate of change of the wave function is given by the energy. But for a closed universe in general relativity, the energy is exactly zero — so there is no time evolution, nothing happens. But you can divide the universe into “you” and “the rest.” Your own energy is not zero, so the energy of the rest of the universe is not zero, and therefore it obeys the standard Schrodinger equation with ordinary time evolution. So the only way to make the universe real is to consider yourself separate from it.)

Sunday morning: Cosmology

9:00: Ellis gives the opening remarks. Cosmology is in a fantastic data-rich era, but it is also coming up against the limits of measurement. In the quest for ever deeper explanation, increasingly speculative proposals are being made, which are sometimes untestable even in principle. The multiverse is the most obvious example.

Question: are these proposals science? Or do they attempt to change the definition of what “science” is? Does the search for explanatory power trump testability?

The questions aren’t only relevant to the multiverse. We need to understand the dividing line between science and non-science to properly classify standard cosmology, inflation, natural selection, Intelligent Design, astrology, parapsychology. Which are science?

9:30: Joe Silk gives an introduction to the state of cosmology today. Just to remind us of where we really are, he concentrates on the data-driven parts of the field: dark matter, primordial nucleosynthesis, background radiation, large-scale structure, dark energy, etc.

Silk’s expertise is in galaxy formation, so he naturally spends a good amount of time on that. Theory and numerical simulations are gradually making progress on this tough problem. One outstanding puzzle: why are spiral galaxies so thin? Probably improved simulations will crack this before too long.

10:30: Andrei Linde talks about inflation and the multiverse. The story is laden with irony: inflation was invented to help explain why the universe looks uniform, but taking it seriously leads you to eternal inflation, in which space on extremely large (unobservable) scales is highly non-uniform — the multiverse. The mechanism underlying eternal inflation is just the same quantum fluctuations that give rise to the density fluctuations observed in large-scale structure and the microwave background. The fluctuations we see are small, but at earlier times (and therefore on larger scales) they could easily have been very large — large enough to give rise to different “pocket universes” with different local laws of physics.

Linde represents the strong pro-multiverse view: “An enormously large number of possible types of compactification which exist e.g. in the theory of superstrings should be considered a virtue.” He said that in 1986, and continues to believe it. String theorists were only forced to take all these compactifications seriously by the intervention of a surprising experimental result: the acceleration of the universe, which implied that there was no magic formula that set the vacuum energy exactly to zero. Combining the string theory landscape with eternal inflation gives life to the multiverse, which among other things offers an anthropic solution to the cosmological constant problem.

Still, there are issues, especially the measure problem: how do you compare different quantities when they’re all infinitely big? (E.g. number of different kinds of observers in the multiverse.) Linde doesn’t think any of the currently proposed measures are completely satisfactory, including the ones he’s invented. A big problem with Boltzmann brains.

Another problem is what we mean by “us,” when we’re trying to predict “what observers like us are likely to see.” Are we talking about carbon-based life, or information-processing computers? Help, philosophers!

Linde thinks that the multiverse shows tendencies, although not cut-or-dried predictions. It prefers a cosmological constant to quintessence, and increases the probability that axions rather than WIMPs are the dark matter. Findings to the contrary would be blows to the multiverse idea. Most strongly, without extreme fine-tuning, the multiverse would not be able to simultaneously explain large tensor modes in the CMB and low-energy supersymmetry.

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Last week saw the first public release of data from the refurbished Hubble Space Telescope, and its new imaging camera (Wide Field Camera 3, or “WFC3″).

Over the past week, four papers have shown up on astro-ph using new WFC3 data of the Hubble Ultra Deep Field (see the prescient comment by Brian Mingus in the original blog post):

Bouwens et al
Oesch et al
Bunker et al
McLure et al

All of these papers are based upon data released on September 9th, from a large “Treasury” program to extend the wavelength coverage of the Hubble Deep Field. The first two papers were produced by the team that actually proposed the observations, and the second two were from groups that were sitting around eagerly waiting for the first group’s data to be publicly released¹.

All of the papers deal with the statistics and properties of extremely high redshift (i.e. distant and young) galaxies. The dominant technique for finding high redshift galaxies has been looking for “drop out” galaxies. These are galaxies that have essentially zero flux in blue filters, due to absorption from intergalactic gaseous Hydrogen, and significant flux in all redder filters (at wavelengths that are largely unaffected by the same gas). Higher redshift galaxies “drop out” of progressively redder filters, because the rest frame (un-redshifted) wavelength at which the gas absorbs the galaxy’s light appears redshifted to longer and longer (redder) wavelengths for highly redshifted galaxies. This technique was pioneered by Guhathakurta, Tyson, & Majewski in 1990, and put on the map as a technique for galaxy selection by Chuck Steidel throughout the 90’s.

These early works looked for “U-band dropouts”, which turn out to be star forming galaxies at a redshift of 3 (about 2.5 billion years after the big bang). Subsequently, people had the bright idea to just keep pushing the drop out technique to redder wavelengths, to look for ever more distant galaxies. However, this comes at a cost, since more distant galaxies tend to be much fainter, so you need to work harder to have statistically significant detections in the red filters, and strong contraints on the absence of detections in bluer filters. The new WFC3 data pushes this technique out of the optical and into the infrared, selecting galaxies at redshifts from 6 to 9, when the universe was 0.5-1 billion years old. (Note: in the picture above, each successive column shows an image taken at redder and redder wavelengths. The likely distant galaxies are those that show up in the rightmost three columns but none of the leftmost columns)

The new papers all find that at these early times, the star formation rate of the universe is on the rise. This isn’t too surprising, given that you’re getting so close to the beginning of the universe — early on, structure hasn’t really had much time to form, so naturally you should find that fewer galaxies have yet had time to go about their business.

All in all, these are nice results doing just what the new data was designed to do.

¹ In Jackson Hole, I once saw a bald eagle sitting high in a tree above a stream. There was an osprey sitting lower down the same tree. The river guide said the eagle waits for the osprey to catch a fish, and then just steals the fish from the osprey.

² After posting this, I found a nice write-up by Ron Cowan here, as well as discussion of the result from the always lovely Peter Coles here. There’s also a cute discussion of the stress involved in such publications over at andxyl’s place.

If you haven’t heard that Planck has seen first light, you haven’t been reading the right cosmology blogs: see Andrew Jaffe, Peter Coles, and Planck’s own Twitter feed. Planck is of course the European Space Agency’s microwave background satellite experiment, which was launched back in May. Since then it’s been tumbling in space about once every minute, doing a leisurely scan of the sky. The survey is not nearly completed, but all systems seem to be running smoothly.

Here’s the region it’s looked at so far, superimposed over a visual-light map of the Milky Way:

And here’s a zoom in on one region, as seen in two different wavelengths:

So far the scientists are playing with the data to learn about the instrument, not so much about the microwave background. Andrew predicts a big splash of papers from Planck in August 2012. We’ll be looking for a bunch of things: Are the overall features of the CMB consistent with predictions from inflation? Are there “non-Gaussian” features indicating extra power in some regions? Is the strength of the perturbations equal on all scales, or does it gradually diminish at smaller distances? Did we learn anything surprising from the polarization, such as tensor modes that could come from inflation or an overall rotation that could come from quintessence? Does the universe have a preferred direction?

I’m sure it will be front-page news, whatever that news turns out to be. Stay tuned.

Almost a year ago we talked about dark photons — the idea that there was a new force, almost exactly like ordinary electromagnetism, except that it coupled only to dark matter and not to ordinary matter. It turns out to be surprisingly hard to rule such a proposal out on the basis of known astrophysical data, although I suspect that it could be tightly constrained if people did high-precision simulations of the evolution of structure in such a model.

In fact our original idea wasn’t merely the idea of dark photons, it was dark atoms — having dark matter bear a close family resemblance to ordinary matter, all the way to having most of its mass be in the form of composite objects consisting of one positively-charged dark particle (a “dark proton”) and one negatively-charged dark particle (a “dark electron”). We thought about it a very tiny bit, but didn’t pursue the idea and only mentioned it in passing at the very end of our paper. There is an informal rule in theoretical physics that you should only invoke the tooth fairy (propose an extremely speculative idea or hope for some possible but unprovable result) once per paper, so we stuck with only a single kind of charged dark particle.

But once someone invokes the tooth fairy in their paper, anyone who writes another paper gets to invoke the tooth fairy for themselves. (That’s just how the rule works.) And the good news is that it’s now been done:

Atomic Dark Matter
Authors: David E. Kaplan, Gordan Z. Krnjaic, Keith R. Rehermann, Christopher M. Wells

Abstract: We propose that dark matter is dominantly comprised of atomic bound states. We build a simple model and map the parameter space that results in the early universe formation of hydrogen-like dark atoms. We find that atomic dark matter has interesting implications for cosmology as well as direct detection: Protohalo formation can be suppressed below $M_{proto} \sim 10^3 – 10^6 M_{\odot}$ for weak scale dark matter due to Ion-Radiation interactions in the dark sector. Moreover, weak-scale dark atoms can accommodate hyperfine splittings of order $100 \kev$, consistent with the inelastic dark matter interpretation of the DAMA data while naturally evading direct detection bounds.

(Note that one of the authors has been a guest-blogger here at CV.) It looks like a great paper, and they seem to have done a careful job at chasing down some of the interesting implications of dark atoms. In fact the idea might be more robust than that of the one in our paper; the fact that dark atoms are neutral lets you slip loose of some of the more inconvenient observational bounds. And the last sentence of the abstract points to an intriguing consequence: by giving the dark matter particles some structure, you might be able to explain the intriguing DAMA results while remaining consistent with other (thus far negative) direct searches for dark matter. Stay tuned; that dark sector may turn out to be a pretty exciting place after all.

Remember a few months back, when we were all excited about the Space Shuttle taking a crew of astronauts to fix and upgrade the Hubble Space Telescope (HST)? At the time, it looked like the repairs worked well (as in, nothing obvious went wrong, electronics woke up and said “hi”, etc). However, until the instruments actually take data, one never knows.

Well, now we know.

The new and refurbished instruments officially kick astronomical butt.

Even better, I’ve been hearing rumors of killer numbers, at least for the imaging cameras — throughputs that are 15% better than measured from the ground, electronic read noises that are lower than before the instruments broke a few years back. I have no numbers on the spectrographs, but the press release photos show some nice looking spectra, which is a zillion percent improvement on the pre-repair state of the telescope, for which the only spectroscopic capabilities were grisms (which only the most studly of spectroscopists dare to use). Phil will probably have more, given his history with one of the refurbished spectrographs.

(For the near-infrared channel of the new imager WFC3, I can personally verify awesomeness. We got some imaging during the last month, and had to sign non-disclosure agreements that we would keep our mouths shut, which was nearly impossible because the data quality was insane.)

Anyways, everyone involved in making this happen should be very proud!

The arrow of time wasn’t the only big science problem garnering media attention last week: there was also a claim that dark energy doesn’t exist. See Space.com (really just a press release), USA Today, and a bizarre op-ed in the Telegraph saying that maybe this means global warming isn’t real either, so there.

The reports are referring to a paper by mathematicians Blake Temple and Joel Smoller, which is behind a paywall at PNAS but publicly available on the arxiv. (And folks wonder why journals are dying.) Now, some of my best friends are mathematicians, and in this paper they do the kind of thing that mathematicians are trained to do: they solve some equations. In particular, they solve Einstein’s equation of general relativity, for the particular case of a giant spherical “wave” in the universe. So instead of a universe that looks basically the same (on large scales) throughout space, they consider a universe with a special point, so that the density changes as you move away from that point.

Then — here’s the important part — they put the Earth right at that point, or close enough. And then they say, “Hey! In a universe like that, if we look at how fast distant galaxies and supernovae are receding from us, we can fit the data without any dark energy!” That is, they can cook up a result for distance vs. redshift in this model that looks like it would in a smooth model with dark energy, even though there’s nothing but ordinary (and dark) matter in their cosmology.

There are three things to note about this result. First, it’s already known; see e.g. Kolb, Marra, and Matarrese, or Clifton, Ferreira, and Land. In fact, I would argue that it’s kind of obvious. When we observe distant galaxies, we don’t see the full three dimensions of space at every moment in time; we can only look back along our own light cone. If the universe isn’t homogeneous, but is only spherically symmetric around our location, I can arrange the velocities of galaxies along that past light cone to do whatever I want. We could have them spell out “Cosmic Variance” in Morse code if we so desired. So it’s not very surprising we could reconstruct the observed distance vs. redshift curve of an accelerating universe; you don’t have to solve Einstein’s equation to do that.

Second, do you really want to put us right at the center of the universe? That’s hard to rule out on the basis of data — although people are working on it. So it’s definitely a possibility to keep in mind. But it seems a bit of a backwards step from Copernicus and all that. Most of us would like to save this as a move of last resort, at least while there are alternatives available.

Third, there are perfectly decent alternatives available! Namely, dark energy, and in particular the cosmological constant. This idea not only fits the data from supernovae concerning the distance vs. redshift relation, but a bunch of other data as well (cosmic microwave background, cluster abundances, baryon acoustic oscillations, etc.), which this new paper doesn’t bother with. People should not be afraid of dark energy. Remember that the problem with the cosmological constant isn’t that it’s mysterious and ill-motivated — it’s that it’s too small! The naive theoretical prediction is larger than what’s required by observation by a factor of 10¹²⁰. That’s a puzzle, no doubt, but setting it equal to zero doesn’t make the puzzle go away — then it’s smaller than the theoretical prediction by a factor of infinity.

The cosmological constant should exist, and it fits the data. It might not be the right answer, and we should certainly keep looking for alternatives. But my money is on Λ.

Over the last couple of months I’ve been working on a new project with an undergraduate student and a postdoc. I’m not really ready to talk about the details yet, but one thing that became clear during our work was that we would understand the results of our numerical solutions much better if we had a movie of them. This is pretty standard these days and (particularly when you have a smart, motivated and fast student) one can obtain quite sophisticated animated solutions that allow one to develop a feeling for rather unintuitive results.

Having said this, I must confess that my own research isn’t one of the areas of science that typically lends itself to spectacular and artistic visualization. And I’m always therefore a little jealous of those people whose results allow dramatic representations.

A number of these are being featured in Wired’s Best Science Visualization Videos of 2009. These are drawn from across all of science and what they all have in common, aside from their usefulness in their respective fields, is their great beauty.

The closest one of these to my own area is the quite stunning movie of the simulation of a type Ia supernova explosion, credited to Brad Gallagher, George Jordan, Dean Townsley, Robert Fisher, Nathan Hearn, Jim Truan and Don Lamb.

A good theoretical description of these objects is certainly fascinating astrophysics in its own right. However, as we’ve discussed many times on this blog, it is also an important step in understanding how, and the extent to which, type Ia supernovae can serve as standardizable candles, with which we may track the expansion history of the universe. The current understanding of this has been enough to discover the fact that the universe is accelerating, but our future plans are to exploit it further, to help provide insight into the origin of cosmic acceleration. A detailed understanding of how supernova explosions occur would be a valuable contribution to this quest.

And they’re just lovely to watch.

We’re all waiting for the LHC to restart. Current plans call for collisions later this year, but at lower energies than originally hoped.

Why is it so hard to say for sure? Here’s a nice article in the CERN Bulletin that lays out some of the difficulties.

Due to the huge amount of inter-dependency between different areas of work in the LHC, even a small change can necessitate a complete overhaul of the schedule. For example, something as simple as cleaning a water cooling tower – required regularly by Swiss law to prevent Legionella – has a huge impact on the planning: “When you clean the water tanks it means we don’t have water-cooling for the compressors, that means we can’t run the cryogenics, so the temperature starts to go up,” explains Myers. “If a sector gets above 100 K, then the expansion effects of heating can cause problems, and we could have to replace parts.”

That may be cold comfort (get it? cold comfort!), but it’s the real world. I have no strong opinions about the job CERN is doing, except to recognize that this is the most complicated machine ever built, so patience is probably called for. The particles and interactions are going to be the same next year as they were last year. (Or if they’re not, that would be even more interesting.)

A paper just appeared in Physical Review Letters with a provocative title: “A Quantum Solution to the Arrow-of-Time Dilemma,” by Lorenzo Maccone. Actually just “Quantum…”, not “A Quantum…”, because among the various idiosyncrasies of PRL is that paper titles do not begin with articles. Don’t ask me why.

But a solution to the arrow-of-time dilemma would certainly be nice, quantum or otherwise, so the paper has received a bit of attention (Focus, Ars Technica). Unfortunately, I don’t think this paper qualifies.

The arrow-of-time dilemma, you will recall, arises from the tension between the apparent reversibility of the fundamental laws of physics (putting aside collapse of the wave function for the moment) and the obvious irreversibility of the macroscopic world. The latter is manifested by the growth of entropy with time, as codified in the Second Law of Thermodynamics. So a solution to this dilemma would be an explanation of how reversible laws on small scales can give rise to irreversible behavior on large scales.

The answer isn’t actually that mysterious, it’s just unsatisfying. Namely, the early universe was in a state of extremely low entropy. If you accept that, everything else follows from the nineteenth-century work of Boltzmann and others. The problem then is, why should the universe be like that? Why should the state of the universe be so different at one end of time than at the other? Why isn’t the universe just in a high-entropy state almost all the time, as we would expect if its state were chosen randomly? Some of us have ideas, but the problem is certainly unsolved.

So you might like to do better, and that’s what Maccone tries to do in this paper. He forgets about cosmology, and tries to explain the arrow of time using nothing more than ordinary quantum mechanics, plus some ideas from information theory.

I don’t think that there’s anything wrong with the actual technical results in the paper — at a cursory glance, it looks fine to me. What I don’t agree with is the claim that it explains the arrow of time. Let’s just quote the abstract in full:

The arrow of time dilemma: the laws of physics are invariant for time inversion, whereas the familiar phenomena we see everyday are not (i.e. entropy increases). I show that, within a quantum mechanical framework, all phenomena which leave a trail of information behind (and hence can be studied by physics) are those where entropy necessarily increases or remains constant. All phenomena where the entropy decreases must not leave any information of their having happened. This situation is completely indistinguishable from their not having happened at all. In the light of this observation, the second law of thermodynamics is reduced to a mere tautology: physics cannot study those processes where entropy has decreased, even if they were commonplace.

So the claim is that entropy necessarily increases in “all phenomena which leave a trail of information behind” — i.e., any time something happens for which we can possibly have a memory of it happening. So if entropy decreases, we can have no recollection that it happened; therefore we always find that entropy seems to be increasing. Q.E.D.

But that doesn’t really address the problem. The fact that we “remember” the direction of time in which entropy is lower, if any such direction exists, is pretty well-established among people who think about these things, going all the way back to Boltzmann. (Chapter Nine.) But in the real world, we don’t simply see entropy increasing; we see it increase by a lot. The early universe has an entropy of 10⁸⁸ or less; the current universe has an entropy of 10¹⁰¹ or more, for an increase of more than a factor of 10¹³ — a giant number. And it increases in a consistent way throughout our observable universe. It’s not just that we have an arrow of time — it’s that we have an arrow of time that stretches coherently over an enormous region of space and time.

This paper has nothing to say about that. If you don’t have some explanation for why the early universe had a low entropy, you would expect it to have a high entropy. Then you would expect to see small fluctuations around that high-entropy state. And, indeed, if any complex observers were to arise in the course of one of those fluctuations, they would “remember” the direction of time with lower entropy. The problem is that small fluctuations are much more likely than large ones, so you predict with overwhelming confidence that those observers should find themselves in the smallest fluctuations possible, freak observers surrounded by an otherwise high-entropy state. They would be, to coin a pithy phrase, Boltzmann brains. Back to square one.

Again, everything about Maccone’s paper seems right to me, except for the grand claims about the arrow of time. It looks like a perfectly reasonable and interesting result in quantum information theory. But if you assume a low-entropy initial condition for the universe, you don’t really need any such fancy results — everything follows the path set out by Boltzmann years ago. And if you don’t assume that, you don’t really explain our universe. So the dilemma lives on.

I’ve posted before about Galaxy Zoo, and see also Phil Plait’s posts introducting the zoo, oncounterintuitive results and what do to if you find something weird.

They had a pretty cool collective result recently, discovering tiny galaxies that are rapidly forming stars, that they call “green pea galaxies” (which is apparently a different sort of beast from the non-galaxy I called a green crayon in my initial post) — see here for more or here for the paper itself.

Anyways, Galaxy Zoo has just announced a new project, the supernova zoo.

This time, not only are we classifying galaxies, but we’re hunting supernovae : exploding stars. Images of likely supernova candidates captured by a telescope in California are being fed to our website at http://supernovae.galaxyzoo.org”. Astronomers are standing by in the Canary Islands to follow up on the most exciting possibilities, but
first we need your help to decide where to point the telescope. Please take the time to go to the site, read the tutorial – and then start hunting.

I have to say I love the image of “astronomers standing by”. I tried it (classifying supernovae, not standing by) — it’s pretty cool, and actually a bit challenging. What you actually do is answer a series of questions about a supernova candidate, to determine whether it’s a good candidate or just a messed up image.

Here’s an obvious good one:

And one where the candidate (right image) is distorted so it doesn’t look like a star.

To my mind it’s not nearly as interesting as the galaxy zoo, because images of galaxies are just way cooler than low-resolution images of supernovae (supernovae remants are another story of course). But it’s a great use of human eyes, and a pretty good way to waste a few lazy August afternoons. It will definitely be interesting to see if the data is useful enough to help the supernovae followup substantially.

Have at it! Maybe they’ll name the next supernovae “SN2009cosmicvariancereaders”.