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.

Among my recent peregrinations was a jaunt up to Santa Barbara, where I gave two talks in a row (although in different buildings, and to somewhat different audiences). Both were about attempts to weasel out of the need for dark stuff in the universe by trying to modify gravity.

The first talk, a high-energy theory seminar, was on trying to do away with dark energy by modifying gravity. I used an antiquated technology called “overhead transparencies” to give the talk itself, so there is no electronic record. If I get a chance sometime soon, I’ll post a summary of the different models I talked about.

The subsequent talk was over at the Kavli Institute for Theoretical Physics. There was a program on gravitational lensing going on, and they had asked Jim Hartle to give an overview of attempts to replace dark matter with modified gravity. Jim decided that he would be happier if I gave the talk, so it was all arranged to happen on a day I’d be visiting SB anyway. (Don’t feel bad for me; it was fun to give the talks, and they took me to a nice dinner afterwards.) I’m not really an expert on theories of gravity that do away with dark matter, but I’ve dabbled here and there, so I was able to put together a respectable colloquium-level talk.

And here it is. You can see the slides from the talk, as well as hear what I’m saying. I started somewhat lethargically, as it’s hard to switch gears quickly from one talk to another, but we built up some momentum by the end. I started quite broadly with the idea of different “gravitational degrees of freedom,” and worked my up to Bekenstein’s TeVeS model (a relativistic version of Milgrom’s MOND), explaining the empirical difficulties with clusters of galaxies, the cosmic microwave background, and most recently the Bullet Cluster. We can’t say that the idea is ruled out, but the evidence that dark matter of some sort exists is overwhelming, which removes much of the motivation for modifying gravity.

The KITP is firmly in the vanguard of putting talks online, both audio/video and reproductions of the slides. By now they have quite the extensive collection of past talks, from technical seminars to informal discussions to public lectures. Some recent categories of interest:

• String Phenomenology Program
• Applications of Gravitational Lensing Program
• Public Lectures
• Blackboard Lunches (informal introductions at a slightly more accessible level)
• Journalist in Residence (including a couple of discussions on “The String Wars,” led by George Johnson)

On Friday I’ll be at Villanova, my alma mater, giving a general talk to undergraduates on what science is all about. I’m not sure if it will be recorded, but if the yet-to-be-written slides turn out okay, I’ll put them online.

Scott Aaronson has thown down a gauntlet by claiming that theoretical computer science, “by any objective standard, has contributed at least as much over the last 30 years as (say) particle physics or cosmology to humankind’s basic picture of the universe.” Obviously the truth-value of such a statement will depend on what counts as our “basic picture of the universe,” but Scott was good enough to provide an explanation of the most important things that TCS has taught us, which is quite fascinating. (More here.) Apparently, if super-intelligent aliens landed and were able to pack boxes in our car trunks very efficiently, they could also prove the Riemann hypothesis. Although the car-packing might be more useful.

There are important issues of empiricism vs. idealism here. The kinds of questions addressed by “theoretical computer science” are in fact logical questions, addressable on the basis of pure mathematics. They are true of any conceivable world, not just the actual world in which we happen to live. What physics teaches us about, on the other hand, are empirical features of the contingent world in which we find ourselves — features that didn’t have to be true a priori. Spacetime didn’t have to be curved, after all; for that matter, the Earth didn’t have to go around the Sun (to the extent that it does). Those are just things that appear to be true of our universe, at least locally.

But let’s grant the hypothesis that our “picture of the universe” consists both of logical truths and empirical ones. Can we defend the honor of particle physics and cosmology here? What have we really contributed over the last 30 years to our basic picture of the universe? It’s not fair to include great insights that are part of some specific theory, but not yet established as true things about reality — so I wouldn’t include, for example, anomalies canceling in string theory, or the Strominger-Vafa explanation for microstates in black holes, or inflationary cosmology. And I wouldn’t include experimental findings that are important but not quite foundation-shaking — so neutrino masses don’t qualify.

With these very tough standards, I think there are two achievements that I would put up against anything in terms of contributions to our basic picture of the universe:

1. An inventory of what the universe is made of. That’s pretty important, no? In units of energy density, it’s about 5% ordinary matter, 25% dark matter, 70% dark energy. We didn’t know that 30 years ago, and now we do. We can’t claim to fully understand it, but the evidence in favor of the basic picture is extremely strong. I’m including within this item things like “it’s been 14 billion years since the Big Bang,” which is pretty important in its own right. I thought of a separate item referring to the need for primordial scale-free perturbations and the growth of structure via gravitational instability — I think that one is arguably at the proper level of importance, but it’s a close call.
2. The holographic principle. I’m using this as a catch-all for a number of insights, some of which are in the context of string theory, but they are robust enough to be pretty much guaranteed to be part of the final picture whether it involves string theory or not. The germ of the holographic principle is the idea that the number of degrees of freedom inside some region is not proportional to the volume of the region, but rather to the area of its boundary — an insight originally suggested by the behavior of Hawking radiation from black holes. But it goes way beyond that; for example, there can be dualities that establish the equivalence of two different theories defined in different numbers of dimensions (ala AdS/CFT). This establishes once and for all that spacetime is emergent — the underlying notion of a spacetime manifold is not a fundamental feature of reality, but just a good approximation in a certain part of parameter space. People have speculated about this for years, but now it’s actually been established in certain well-defined circumstances.

A short list, but we have every reason to be proud of it. These are insights, I would wager, that will still be part of our basic picture of reality two hundred years from now. Any other suggestions?

Confession time: I didn’t successfully figure out what these two crescents were, until I read the explanation at Astronomy Picture of the Day. Can you?

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