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The Sciences

Dark Energy: Still a Puzzle

Cosmic VarianceBy Sean CarrollAugust 28, 2009 5:34 PM


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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 (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 10^120. 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.

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