Dark Energy Has Long Been Dark-Energy-Like

By Sean Carroll | November 16, 2006 2:20 am

Thursday (“today,” for most of you) at 1:00 p.m. Eastern, there will be a NASA Media Teleconference to discuss some new observations relevant to the behavior of dark energy at high redshifts (z > 1). Participants will be actual astronomers Adam Riess and Lou Strolger, as well as theorist poseurs Mario Livio and myself. If the press release is to be believed, the whole thing will be available in live audio stream, and some pictures and descriptions will be made public once the telecon starts.

I’m not supposed to give away what’s going on, and might not have a chance to do an immediate post, but at some point I’ll update this post to explain it. If you read the press release, it says the point is “to announce the discovery that dark energy has been an ever-present constituent of space for most of the universe’s history.” Which means that the dark energy was acting dark-energy-like (a negative equation of state, or very slow evolution of the energy density) even back when the universe was matter-dominated.

Update: The short version is that Adam Riess and collaborators have used Hubble Space Telescope observations to discover 21 new supernovae, 13 of which are spectroscopically confirmed as Type Ia (the standardizable-candle kind) with redshifts z > 1. Using these, they place new constraints on the evolution of the dark energy density, in particular on the behavior of dark energy during the epoch when the universe was matter-dominated. The result is that the dark energy component seems to have been negative-pressure even back then; more specifically, w(z > 1) = -0.8+0.6-1.0, and w(z > 1) < 0 at 98% confidence.


Longer version: Dark energy, which is apparently about 70% of the energy of the universe (with about 25% dark matter and 5% ordinary matter), is characterized by two features — it’s distributed smoothly throughout space, and maintains nearly-constant density as the universe expands. This latter quality, persistence of the energy density, is sometimes translated as “negative pressure,” since the law of energy conservation relates the rate of change of the energy density to (ρ + p), where ρ is the energy density and p is the pressure. Thus, if p = -ρ, the density is strictly constant; that’s vacuum energy, or the cosmological constant. But it could evolve just a little bit, and we wouldn’t have noticed yet. So we invent an “equation-of-state parameter” w = p/ρ. Then w = -1 implies that the dark energy density is constant; w > -1 implies that the density is decreasing, while w < -1 means that it’s increasing.

In the recent universe, supernova observations convince us that w = -1+0.1-0.1; so the density is close to constant. But there are puzzles in the dark-energy game; why is the vacuum energy so small, and why are the densities of matter and dark energy comparable, even though matter evolves noticeably while dark energy is close to constant? So it’s certainly conceivable that the behavior of the dark energy was different in the past — in particular, that the density of what we now know as dark energy used to behave similarly to that of matter, fading away as the universe expanded, and only recently switched over to an appreciably negative value of w.

These new observations speak against that possibility. They include measurements of supernovae at high redshifts, back when the density of matter was higher than that of dark energy. They then constrain the value of w as it was back then, at redshifts greater than one (when the universe was less than half its current size). And the answer is … the dark energy was still dark-energy-like! That is, it had a negative pressure, and its energy density wasn’t evolving very much. It was in the process of catching up to the matter density, not “tracking” it in some sneaky way.

Of course, to get such a result requires some assumptions. Riess et al. consider three different “priors” — assumed behaviors for the dark energy. The “weak” prior makes no assumptions at all about what the dark energy was doing at redshifts greater than 1.8, and draws correspondingly weak conclusions. The “strong” prior uses data from the microwave background, along with the assumption (which is really not that strong) that the dark energy wasn’t actually dominating at those very high redshifts. That’s the prior under which the above results were obtained. The “strongest” prior imagines that we can extrapolate the behavior of the equation-of-state parameter linearly back in time — that’s a very strong prior indeed, and probably not realistic.

So everything is consistent with a perfectly constant vacuum energy. No big surprise, right? But everything about dark energy is a surprise, and we need to constantly be questioning all of our assumptions. The coincidence scandal is a real puzzle, and the idea that dark energy used to behave differently and has changed its nature recently is a perfectly reasonable one. We don’t yet know what the dark energy is or why it has the density it does, but every new piece of information nudges us a bit further down the road to really understanding it.

Update: The Riess et al. paper is now available as astro-ph/0611572. The link to the data is broken, but I think it means to go here.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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