Blogs / Cosmic Variance

The prizes were awarded this evening at Harvard’s annual IgNobel Prize ceremony. This year’s theme was “Risk”. Among the winners are:

Veterinary Medicine: Catherine Bertenshaw and Peter Rowlinson, Newcastle, for showing that cows who have names give more milk than nameless cows. (Not sure what that has to do with risk…)

Peace: Stefan Bolliger (sp?), et al., Univ. of Bern, Switzerland, for determining whether it is better to be smashed over the head with a full or empty bottle of beer. (The empty ones work better!)

Public Health: Ilena Badnar of the University of Chicago for inventing a brassiere that can be converted to a pair of gas masks. Paul Krugman with a pink bra cup on his face…woah.

Biology: Fumiyake Yamaguchi et al. for demonstrsating that the feces of giant pandas can be used to reduce kitchen waste by 90%.

It’s great that Little Miss Sweetie Poo keeps the speeches short by running up and yelling “please stop! I’m bored!”

Hopefully the whole list and the video will be up on their web site soon!

Back for the third and final day of the Philosophy and Cosmology conference in honor of George Ellis’s birthday. I’ll have great memories of my time in Oxford, almost all of which was spent inside this lecture hall. See previous reports of Day One, Day Two.

It’s become clear along the way that I am not as accurate when I’m trying to represent philosophers as opposed to physicists; the vocabularies and concerns are just slightly different and less familiar to me. So take things with an appropriate grain of salt.

Tuesday morning: The Case for Multiverses

9:00: Bernard Carr, one of the original champions of the anthropic principle, has been instructed to talk on “How we know multiverses exist.” Not necessarily the title he would have chosen. Of course we don’t observe a multiverse directly; but we might observe it indirectly, or infer it theoretically. We should be careful to define “multiverse,” not to mention “exist.”

There certainly has been a change, even just since 2001, in the attitude of the community toward the multiverse. Quotes Frank Wilczek, who tells a parable about how multiverse advocates have gone from voices in the wilderness to prophets. That doesn’t mean the idea is right, of course.

Carr is less interested in insisting that the multiverse does exist, and more interested in defending the proposition that it might exist, and that taking it seriously is perfectly respectable science. Remember history: August Comte in 1859 scoffed at the idea we would ever know what stars were made of. Observational breakthroughs can be hard to predict. Rutherford: “Don’t let me hear anyone use the word `Universe’ in my department!” Cosmology wasn’t respectable. For what it’s worth, the idea that what we currently see is the whole universe has repeatedly been wrong.

So how do we know a multiverse exists? Maybe we could hop in a wormhole or something, but let’s not be so optimistic. There are reasons to think that multiverses exist: for example, if we find ourselves near some anthropic cutoff for certain parameters. More interesting, there could be semi-direct observational evidence — bubble collisions, or perhaps giant voids. Discovering extra dimensions would be good evidence for the theories on which the multiverse is often based.

The only direct observations that currently exists that might bear directly on multiverses is the prediction of giant voids and dark flows by Laura Mersini-Houghton and collaborators.

Carr believes that the indirect evidence from finely-tuned coupling constants is actually stronger. Existence of planets requires a very specific relationship between strength of gravity and electromagnetism, which happens to exist in the real world. There is a similar gravity/weak tuning needed to make supernovae and heavy elements. Admittedly, many physicists dislike the multiverse and find it just as unpalatable as God. But ultimately, multiverse ideas will become normal science by linking up with observations; we just don’t know how long it will take.

9:45: George Ellis follows Carr’s talk with what we’ve been waiting for a while — a strong skeptical take on the multiverse idea.

There are lots of types of multiverses: many-worlds, separated by space or time, or completely disjoint. Anthropic arguments are what make the idea go. The project is to make the apparently improbable become probable.

The very nature of the scientific enterprise is at stake: multiverse proponents are proposing that we weaken the idea of scientific proof. Science is about two things: testability and explanatory power. Is it worth giving up the former to achieve the latter?

The abstract notion of a multiverse doesn’t get you anything; you need a specific model, with a distribution of probabilities. (Does Harry Potter exist somewhere in your multiverse?) But if there is some process that generates universes, how do you test that process? Domains beyond our particle horizon are unobservable. How far should we expect to be able to extrapolate? Into a region which, in principle, we will never be able to observe.

In the good old days we accepted the Cosmological Principle, and assumed things continued uniformly forever beyond our observable horizon. Completely untestable, of course. If all the steps in the extrapolation are perfectly tenable, extrapolations are fine — but that’s not the case here. In particular, the physics of eternal inflation (gravity plus quantum field theory, Coleman-de Luccia tunneling) has never been tested. It’s unknown physics used to infer an unobservable realm. Inflation itself is not yet a well-defined theory, and not all versions of inflation are eternal. We haven’t even found a scalar field!

There is a claim that a multiverse is implied by the fine-tuning of the universe to allow life. At best a weak consistency test. Can never actually do statistical tests on the purported ensemble. Another claim is that the local universe, if it’s inside a bubble, should have a slight negative curvature — but that’s easily avoided by super-Hubble perturbations, so it’s not a strong prediction. We could, however, falsify eternal inflation by observing that we live in a “small” (topologically compact) universe. But if we don’t, it certainly doesn’t prove that eternal inflation is right. Finally, it’s true that we might someday see signatures of bubble collisions in the microwave background. But if we don’t, then what? Again, not a firm prediction.

Ultimately: explanation and testability are both important, but one shouldn’t overwhelm the other. “The multiverse theory can’t make any prediction because it can explain anything at all.” Beware! If we redefine science to accommodate the multiverse, all sorts of pseudo-science might sneak inside the tent.

There are also political/sociological issues. Orthodoxy is based on the beliefs held by elites. Consider the story of Peter Coles, who tried to claim back in the 1990’s that the matter density was only 30% of the critical density. He was threatened by a cosmological bigwig, who told him he’d be regarded as a crank if he kept it up. On a related note, we have to admit that even scientists base beliefs on philosophical agendas and rationalize after the fact. That’s often what’s going on when scientists invoke “beauty” as a criterion.

Multiverse theories invoke “a profligate excess of existential multiplicity” in order to explain a small number of features of the universe we actually see. It’s a possible explanation of fine tuning, but is not uniquely defined, is not scientifically testable, and in the end “simply postpones the ultimate metaphysical question.” Nevertheless — if we accumulated enough consistency tests, he’d be happy to eventually become convinced.

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With little fanfare, last week the Tevatron at Fermilab, and the two experiments CDF and D0, emerged from an 11-week shutdown for what will likely be the final run of the collider, which is over 20 years old. In the past year, the machine has regularly set new records for luminosity (essentially the number of collisions per second) and delivered over 2 fb^-1 (inverse femtobarns) of proton-antiproton collisions at a center of mass energy of 1.96 TeV to the experiments, still the highest in the world. The startup has gone very smoothly, and the Tevatron delivered a solid load of data to the experiments last week.

This funny unit, inverse femtobarns, allows us to calculate how many collision events of a certain type to expect. Take top quark pair production, for example. For protons colliding with antiprotons at Tevatron energies, we can calculate (or measure) what we call the production cross section. This cross-section is in fact expressed as an area, a very small area, since protons and antiprotons are so small. One “barn” is 10^-28 m², and the cross section for top pair production is about 7 x 10^-12 barns, or 7 pb. By multiplying the cross-section times the integrated luminosity we can get the number of top quark pairs events produced. To get the number we actually observe in the detector, we need to take into account the efficiency for reconstructing them.

The plot here shows the history of what we call Run 2 at the Tevatron. After a very slow start in 2001, following a five-year shutdown to upgrade the whole complex, the collider set new luminosity records year after year, and has nearly delivered 7 fb^-1. This final run is expected to last two years, until the end of 2011, by which point we hope to have recorded another 5 fb^-1, nearly doubling the present sample.

This past year had been expected to be the year of the LHC at CERN. But the magnet quench incident of one year ago caused a delay of over a year in repairs and retrofits. It is still expected that the LHC will return to commissioning in November of this year, possibly colliding protons on protons before the end of the calendar year, albeit at low energies. When it does come online at higher energies, probably early next year, it is expected that the LHC will deliver no more than about 0.2 fb^-1 at a collision energy of 10 TeV, five times that of the Tevatron. Even with such a small sample, there could be striking discoveries at the LHC which are out of reach for the Tevatron, simply because the LHC energy is so much larger.

The media love this sort of race, and have portrayed it as a race to discover the particle the media has heard the most about – the Higgs boson. With 12 fb^-1, even combining all the search modes and channels, and combining the data from both experiments, a standard model Higgs boson might be seen at the three standard deviation level, but almost certainly not the five standard deviation level, which is the gold standard in the field. The LHC won’t be able to see a standard model Higgs boson with the initial sample either. It will take a year or two at higher luminosity, probably starting in 2012, to get there.

To my mind, if there is a race, it is a race for the unknown. What I worry about, what I literally lie awake thinking about, is whether we are looking at the Tevatron data exactly the right way. People have searched for many different new physics signals at the Tevatron, but there has been no unambiguous observation of anything beyond the standard model. To get a five sigma discovery with the remainder of the Tevatron data, it would have to be the case that there is already about a three sigma excess in the data we have. But have we looked at everything?

Nevertheless, a hard-core of dedicated, talented, and very new-physics-hungry physicists will continue to operate the detectors and analyze the data to come, myself among them. Like many, I am playing both sides: when LHC data come we’ll analyze that, too.

The previous post on the Philosophy and Cosmology conference in Oxford was growing to unseemly length, so I’ll give each of the three days its separate post.

Monday morning: The Case for Multiverses

9:00: We start today as we ended yesterday: with a talk by Martin Rees, who has done quite a bit to popularize the idea of a multiverse. He wants to argue that thinking about the multiverse doesn’t represent any sort of departure from the usual way we do science.

The Big Bang model, from 1 second to today, is as uncontroversial as anything a geologist does. Easily falsifiable, but it passes all tests. How far does the domain of physical cosmology extend? We only see the universe out to the microwave background, but nothing happens out there — it seems pretty uniform, suggesting that conditions inside extend pretty far outside. Could be very far, but hard to say for sure.

Some people want to talk only about the observable universe. Those folks need aversion therapy. After all, whether a particular distant galaxy eventually becomes observable depends on details of cosmic history. There’s no sharp epistemological distinction between the observable and unobservable parts of the universe. We need to ask whether quantities characterizing our observable part of the universe are truly universal, or merely local.

So: what values of these parameters are consistent with some kind of complexity? (No need to explicitly invoke the “A-word.”) Need gravity, and the weaker the better. Need at least one very large number; in our universe it’s the ratio of gravity to electromagnetic forces between elementary particles. Also need departure from thermodynamic equilibrium. Also: matter/antimatter symmetry, and some kind of non-trivial chemistry. (Tuning between electromagnetic and nuclear forces?) At least one star, arguably a second-generation star so that we have heavy elements. We also need a tuned cosmic expansion rate, to let the universe last long enough without being completely emptied out, and some non-zero fluctuations in density from place to place.

If the amplitude of density perturbations were much smaller, the universe would be anemic: you would have fewer first-generation stars, and perhaps no second-generation stars. If the amplitude were much larger, we would form huge black holes very early, and again we might not get stars. But ten times the observed amplitude would actually be kind of interesting. Given an amplitude of density perturbations, there’s an upper limit on the cosmological constant, so that structure can form. Again, larger perturbations would allow for a significantly larger cosmological constant — why don’t we live in such a universe? Similar arguments can be made about the ratio of dark matter to ordinary matter.

Having said all that, we need a fundamental theory to get anywhere. It should either determine all constants of nature uniquely, in which case anthropic reasoning has no role, or it allows ranges of parameters within the physical universe, in which case anthropics are unavoidable.

10:00: Next up, Philip Candelas to talk about probabilities in the landscape. The title he actually puts on the screen is: “Calabi-Yau Manifolds with Small Hodge Numbers, or A Des Res in the Landscape.”

A Calabi-Yau is the kind of manifold you need in string theory to compactly ten dimensions down to four, picked out among all possible manifolds by the requirement that we preserve supersymmetry. There are many examples, and you can characterize them by topological invariants as well as by continuous parameters. But there is a special corner in the space of Calabi-Yau’s where certain topological invariants (Hodge numbers) are relatively small; these seem like promising places to think about phenomenology — e.g. there are three generations of elementary particles.

Different embeddings lead to different gauge groups in four dimensions: E₆, SO(10), or SU(5). Various models with three generations can be found. Putting flux on the Calabi-Yau can break the gauge group down to the Standard Model, sometimes with additional U(1)’s.

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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.