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.
Will be waiting for the update. 🙂
Thanks Sean, that explains a lot 😉
I’ll try to be online when the event starts (at 7 p.m. here) and listen to you guys.
Thanks for the heads up. I will try to tune in to this announcement. I am continually more and more glad I am going into cosmology as a grad student next year. The good reports just keep coming. 🙂
Aaaah, this is just a public relations stunt by NASA and by Sean.
even back when the universe was matter-dominated?
Public relations stunts in science are important… it’s what keeps funding agenscies paying attention, it makes funding agencies happy to do it, and it attracts the attention of administrators far more than actual scientific papers. All of which is important.
Anyway, it’s possible there’s some cool new result here 🙂
I, alas, will be driving my wife to an appointment during the pres srelease, but I’m fully capable of reading about it later (and reading any astro-ph papers that are out on it — are any yet?) Will the audio stream be archived, or will it be one of those annoying things that you can “only” get live?
-Rob
I am looking forward to watching the press conference, today
The astro-ph paper can’t appear until after the NASA announcement, but as far as I know it’s otherwise ready to go. And it seems NASA does generally archive and make available old audio streams.
So having effectively ruled out the results of the Friedman equations(?), we are back with Einstein’s cosmological constant, and “anti- gravity?”
Maybe you can explain what “anti-gravity” means as well in our understanding of the universe?
I admit, it’s just a PR stunt by NASA and, more importantly, by me. They didn’t want to make a big deal out of it — they were all “If ordinary people on the street know what we’re doing, that could ruin everything!” But, sensing a valuable opportunity to get my face on the radio, I insisted that they stage a press carnival, or I would release those grainy films I took of the fake Moon landing.
Question:
Is Dark Matter in places where ordinary particles are not? And how could you know that? And say you don’t know the answer, then how can you estimate the amount of DM present in the universe?
Some notation: DM(dark matter), DE(dark energy),RS(regular stuff).
If DM and RS happen to be seen together, then they know of each others presence. So there is some degree of freedom, which allows of some level of interaction between these two forms of matter. Is that true?
Then if DM and RS tend not to do anything spectacular like expanding or lowering their temps due to expansion, etc. then we can believe that they dont even know about the expansion. Which means that they do not communicate to any level with DE. But if DE is a fundamental property of spacetime, at the singularity of the big bang, DE and DM and RS(or their then forms) must have been aware of each others existence since more or less its the same. So what sense did DM and RS lose? What symmetry broke irreversibly?
Is it a spacetime function or a tensor component that is monotonously decreasing or increasing? Is DE locally unchanging at all, and thus unaware of the expansion it is causing from out point of observation? Are the DM and RS clusters just small and insignificant perturbations in the local DE universe? Or maybe, are these insignificant perturbations, what drives the DE changing?
But if these perturbations indeed drive a change in DE then it is just another form of interaction between it and the DM and RS. So…these then are coupled…and you expect to have been driving each others dynamics…and if that should be the case, then DE has been changing and you would expect to be able to detect that…
So whats the catch here? DE was positive, 0, negative?
Joseph, where are you starting school and where are you coming from?
…or I would release those grainy films I took of the fake Moon landing.
With your cellphone, right?
-Rob
If science were just for scientists, it would be a rather selfish game.
Huzzah to Sean for his publicity stunts and blogging!
I’m not sure how interesting these lastest observations of supernovae are. As far as I can tell noone would whole heartedly believe in dark energy from supernovae observations alone. There are loads of major issues in determining the expansion rate of the universe (therefore seeing the dark energy influence) from supernovae. First (and most importantly) it is assumed that they all have the same intrinsic brightness, which in fact they don’t, but you can perform a fudge called the stretch factor to make this so. However we don’t understand fully why their intrinsic brightnesses should be the same so these observations are based on empiral relations. This means we have no idea whether these relations evolve with redshift and given this data is from supernovae at very high redshift can we trust it?
Another interesting issue which becomes important when determining with precision the equation of state w from high redshift supernovae is the need to correct the redshift of the supernovae for its peculiar velocity. The galaxy which the supernova is in will also be moving due to gravitational effects from other galaxies in addition to the expansion of the universe, the redshift measured is a sum of these two things. This effect is really only important for low redshift supernovae who’s expansion velocities are smaller. However the high redshift supernovae rely on accurate calibration from the ones at low redshift. One could imagine that a bunch of low redshift supernovae over a large area of sky could be moving in the same direction towards a large more distant cluster in a ‘bulk flow’. This would create a correlated error in the supernovae observations which could bias w either too high or too low and you could imagine also it has an effect on measuring w(z). As far as I am aware this effect is not accounted for.
I’m also worried about the fact that only 13 of the 21 have been spectroscopically confirmed as Type Ia (the ones to use). Also does this mean they don’t have spectroscopic redshifts (ie accurate) for these either?
Overall I think we have to wait for results from other probes such as weak lensing or the baryon acoustic oscillations before we really have an idea whether or not dark energy is evolving with time. As for the nature of dark energy itself I hope it’s modified gravity, not just for the sake of Sean’s research, but I think it’s psychologically nicer than weird fields and vacuum energy.
Long post, probably made no sense, nevermind.
Alex,
I agree with you that SNe alone don’t tell the whole story. But two major points of the research are:
1. Riess now has spectra of several high redshift SNe and has shown that they look the same as low redshift SNe; in other words there’s no obvious sign of evolution.
2. With the high redshift SNe you can attempt to divide w(z) into “bins” and get constraints in each bin; with this new data you find that w 1 to pretty good confidence.
Peculiar velocities are included in the error budget for all SNe, but you’re right that bulk motions could mess up the calibration. Bulk motions would show up as an anisotropy in inferred distance-redshift relation for those SNe, however, and to my knowledge this hasn’t been seen at any significant level.
I don’t know about the spectroscopic confirmation stuff off the top of my head. I don’t think unconfirmed Ia were included in the “Gold” sample used for the cosmological analysis.
Oops, in point two above it’s supposed to be “w < 0 for z > 1 to pretty good confidence”. Forgot about HTML not liking < and >.
Alex,
Evidence of cosmic acceleration doesn’t kick in until z>0.1, so a bias in the low redshift SNe used for calibration would have little effect on measuring w(z). Such a bias could affect the measurement of the expansion rate H_0, and people do think about this.
I’m not an expert on type Ia SNe, but from attending various colloquia, my understanding is that there is no evidence for any major evolution. If there is some slight evolution going on, the next generation of supernova surveys WILL have to worry about it. The error bars Sean mentions are relatively huge, so there’s no need to worry right now.
I disagree with what I just said about low z SNe – they could indeed bias w(z).
Aaaaah….it’s all hopeless. What is “energy” if not simply a bookkeeping device we all invented? Some quantity associated with time translation invariance which is conserved.
Ooooooohhh…we’ll never find the answer. It’s all hopeless.
:*****(
I knew it was a PR stunt. NASA’s just in it for the money and the fame and to meet girls (and boys too).
Alex — rather than proprer motion, gravitational lensing is more likely to be a systematic to worry about for really-high-z supernovae.
For proper motion to be a significnat systematic at those sorts of redshifts, the galaxies would have to be moving relativistically, or at the very least implausibly fast.
There *are* some ideas as to why the intrinsic brightness should always be the same. The core reason is that the Chandrasaekar mass is the same for all supernovae, but there are a lot of supernova theorists who’ve gone farther with that.
We do have reasons to believe that the stretch/magnitude relationship is the same at high-z and low-z. Last time I worked on this in detail (the Knop 2003 paper), things at z~0.7 and z~0 looked very consistent. It’s more plausible that the demographics of stretches will change than that the stretch/magnitude relationship would chang.e
I have to report the emergence of a certain temptation to send in annonymous hostile comments, just to enjoy Sean’s response. The phrase “valuable opportunity to get my face on the radio” is pretty memorable…
(No worries, I’ll resist the temptation)
A Three Ring Circus
Anti-gravity…hmmmm…..speeding up? Why?
Then the universe is “fluctuating or oscillating,” between the “curvature parameters?”
Speculating about what Cosmologists are doing and thought, hey, from the layman, might as well throw the above in as to what one might think the universe is doing from this analysis?
I didn’t hear any drum roll or, “ta da,” before the top hat came off. 🙂
It was nice to read some updates on dark energy and what the cosmologists are finding out about it. Thank you Sean. So is the idea that DE is just the cosmological constant now stronger ? Also I was wondering what happened to the old idea of Bruno Zumino , that tried to explain a vanishing/almost vanishing cosmological constant from SUSY ? (Since the ground state must be 0 when SUSY is a strict symmetry at some scale.)
Chaz,
I am coming from BYU. I don’t know where I am going yet. I apply in a month. I am just crossing my fingers I get in somewhere with a good thesis advisor. My time spent at Los Alamos taught me there is nothing more important than a good advisor and research group. I am applying to schools with strong theoretical Cosmology programs.