The new WMAP results have told us a lot about the universe. The basic findings are:
- The LambdaCDM model — a universe comprised of about 4% ordinary matter, 22% dark matter, and 74% dark energy — passes yet another test. The data fit quite well, and we have some new constraints on the cosmological parameters.
- There is some evidence that primordial perturbations, the small ripples in density that later grew into stars and galaxies, did not have precisely the same amplitude on all scales. More quantitatively, the scalar spectral index n was measured to be 0.951 +0.015/-0.019 (updated — see comment below), whereas purely scale-free behavior would be n=1. It’s not as statistically significant as we would like, but it’s something.
- Reionization, the process in which electrons were ripped from ambient hydrogen atoms when the first stars turned on, happened a little bit later than the first-year WMAP data seemed to indicate. This is an important input to our understanding of the “dark ages” between the early universe and the bright galaxies we see today.
All of this is very exciting to professional cosmologists. But consider the perspective of a newspaper that wants to convey that excitement to a popular audience. The data on LambdaCDM are important, but verifying that a known model is still consistent might not seem like earth-shattering news. The information about reionization is new, but early stars don’t quite have the origin-of-the-universe kind of implications that really seem exciting to the reader on the street. But, intriguingly, the slight scale dependence of the density perturbations fits very well with the predictions of the inflationary universe scenario. In this story, the tiny ripples in the primordial universe have their origin in quantum-mechanical fluctuations during the period when the universe is “inflating” (expanding quasi-exponentially at ultra-high energies). Since the expansion rate during inflation does gradually change with time, the amout of such fluctuations gradually evolves from scale to scale. Inflation traces back to the very earliest times about which we can sensibly speak (and long before we have any reliable data), so that is definitely something that could get the juices flowing.
So a lot of stories focused on the support for inflation as the centerpiece of the WMAP narrative. Which is fine, as far as it goes, but needs to be treated with some caveats. First, of course, even in the most generous reading, the purported detection of scale dependence was only at a level of about 3.3 standard deviations, which is not a reliable discovery by most standards in physics. (In particle-physics lingo, it’s “evidence for,” not “discovery of,” which would require 5 standard deviations.) More importantly, even if there had been incontrovertible evidence for scale dependence, that would by no means prove that inflation was right beyond reasonable doubt; it fits well into the inflation story, but certainly doesn’t preclude the possibility of other stories. And finally, it should go without saying that the evidence being discussed is somewhat indirect; it’s not like we’re looking directly at what the universe was doing 10-30 seconds after the Big Bang. (The cosmic microwave background is a snapshot of the universe about 380,000 years after the Big Bang, quite a while later.)
But those subtleties are hard to get across in a few words, and the resulting stories in the press showed evidence of the struggle between conveying the (undeniable) excitement and getting the story precisely correct. Indeed, the tension was evident right in the press release from Goddard Space Flight Center. There’s principal investigator Chuck Bennett, choosing his words with care:
WMAP polarization data allow scientists to discriminate between competing models of inflation for the first time. This is a milestone in cosmology. “We can now distinguish between different versions of what happened within the first trillionth of a second of the universe,” said WMAP Principal Investigator Charles Bennett of the Johns Hopkins University in Baltimore. “The longer WMAP observes, the more it reveals about how our universe grew from microscopic quantum fluctuations to the vast expanses of stars and galaxies we see today.”
Actually, it’s not the first data that allow us to discriminate between different models, although it is some of the most precise data to date. But the idea of “distinguishing between different versions of what happened” is a very good one, and a nice way to tell the story. Sadly, in the next sentence the possibility that inflation is not right seems to have been abandoned, as he speaks with apparent confidence about the origin of galaxies in quantum fluctuations.
This urge to overstate the case is evident elsewhere, as well. In the New York Times we read:
The reason, Dr. Spergel explained, is that the force driving inflation is falling as it proceeds. The smaller bumps would be produced later and so a little less forcefully than the bigger ones.
That, in fact, is exactly what the Wilkinson probe has measured. Dr. Spergel said, “It’s very consistent with simplest inflation models, just what inflation models say we should see.”
Michael Turner, a cosmologist at the University of Chicago, called the results, “the first smoking gun evidence for inflation.”
Here, David Spergel is being very careful to stress that the data are consistent with simple models, which is quite different from saying that it verifies those models are correct. Michael Turner is much less cautious, as “smoking gun evidence” would lead you to believe that the case was closed, which it definitely is not. It’s just very difficult to simultaneously be a cautious scientist and convey an accurate sense of the very real excitement that cosmologists have when examining these data.
If the quotes are ambiguous, the headlines are worse. Let’s face it, “Satellite Gathers Useful Data” wouldn’t sell a lot of newspapers. So many places went for the idea that we had actually observed the extremely early universe, rather than made some observations that constrained theories of the extremely early universe. So we get:
- Astronomers glimpse newborn universe
- NASA probe captures ‘cosmic growth spurt’
- Astronomers Detect First Split-Second of the Universe
- NASA probe peers back to an instant after Big Bang
- University physicists see origins of cosmos
Really, WMAP did not see the origin of the cosmos, any more than seeing an infant is the same as seeing someone being born. But it’s not hard to figure out where they got the idea — the NASA press release is titled “NASA Satellite Glimpses Universe’s First Trillionth of a Second.”
Interestingly, some of the headlines were misleading in the opposite sense, by being less exciting than the truth:
We already have plenty of evidence for the Big Bang! Some more of that would be anticlimactic indeed. And, needless to say, the fact that the universe is expanding is not exactly hot news. I know what they’re all trying to say, but can’t but feeling that if people had a better general idea about what we already know about cosmology, they wouldn’t be tempted to write headlines like this.
I have great sympathy for everyone involved in the process of bringing a story like this to the public — from the scientists working on the project, to the outside scientists who help interpret the results for reporters, to the journalists themselves, to the headline-writers with the unenviable task of squeezing some subtle thoughts into just a few words. But the readers need to take some of these overly enthusiastic declarations with a grain of salt. If you want the real scoop, you have to go beyond the newspaper headlines. For example, by reading blogs.
The significance of detection of scale dependence from WMAP data alone is not 2.5 standard deviations. It is 3.3 sd (ns=0.951 +0.015/-0.019). When WMAP 3-year data is combined with 2dFGRS, SDSS, ACBAR, Boomerang, CBI, VSA and supernova data, the significance goes up to 4.8 sd (ns=0.938 +0.013/-0.018). This assumes the standard LCDM model. See the parameter table on the LAMBDA archive:
http://lambda.gsfc.nasa.gov/product/map/current/params/lcdm_all.cfm
At least there wasn’t a headline reading, “New data from expensive satellite tells us nothing we didn’t already know; cosmology one big waste of time.” See, it could have been worse…
Seriously, this is excellent analysis of the reality of getting science coverage in the press. The competition for column inches is immense, especially with the pending nuptials of celestial bodies like “Brangelina” and Jen and Vince vying for space. And it’s true that most people aren’t that clear on the various different “models” of the early universe. I don’t write often about pure cosmology, but when I do, it always takes me several hours of research just to get all the conflicting threads straightened out. If I struggle with it, the average layperson will struggle even more… if they bother to try and figure it out at all.
So yeah — better awareness in the public about these things is needed. But that’s not going to be found in daily newspapers or online coverage. Absolutely, people should be reading blogs like this one… looking up resources on the Web (and there are many good ones out there)… that sort of thing. Newspapers just report the basic facts; they’re just the initial front lines in keeping the public informed.
Posts like this one can help a lot. And if someone can point me to an online FAQ that neatly lays out all the competing models, theories, etc. of current cosmological theory, yay!
Rob Knop wrote:
Sean, does the WMAP data, or any other obsevation, measure a decrease of energy density 1/size^3, or is it just assumed?
JT– You’re right, thanks. I had mixed up the “+” error bar and the “-” error bar, which are of course not symmetric. I will change the post to reflect this. Adding in the other sources of data is ultimately what you want to do, but I wouldn’t want to emphasize that at this point, since it’s tricky to be sure that you have the relative normalizations straight.
Doug– It is assumed (since that’s what GR predicts), and then you see if you can fit the data. If the mass density evolved very differently from 1/a^3, the data would be wildly off; for example, the distance to the surface of last scattering would be very different.
Jennifer, we are huge fans of Brad and Angelina here at CV. The temptation to do a Brangelina photo of the day is hard to resist.
I don’t know of any handy comparison chart for competing cosmological models. Different people have different notions of what constitutes a “cosmological model” — for some it’s the relative abundance of different consitutents (matter, radiation, vacuum), for others it’s more about structure formation (cold dark matter vs. hot, inflation vs. topological defects), for yet others it’s more about the very early times (inflation vs. the cyclic universe or something else). And the data and theories keep shifting around a bit, so it’s hard to keep a helpful table up to data. We’ve pretty much focused in on a universe that is spatially flat, dominated by dark energy and cold dark matter, with truly primordial perturbations (i.e. not continuously generated by cosmic strings or suchlike) that have approximately equal amplitudes on all scales, with the properties that the perturbations are “adiabatic” (correlated between radiation and matter) and “Gaussian” (oriented randomly). That’s what’s implied by “Standard LambdaCDM model.” The origin of the perturbations, and the relative abundances of the different consituents, are still mysterious.
NL & Rob & Sean (posts 17, 18, and 20) –
A helpful way to “integrate by eye” is simply to plot energy per logarithmic frequency (or wavelength) interval, such as in figure 1 of http://arxiv.org/abs/astro-ph/0105539 . This makes it clear that the CMB wins the energy-density-in-photons contest by at least an order of magnitude.
I’m glad to see “Mr and Mrs Smith” finally got the recognition it deserved here at Cosmic Variance. “Who’s your daddy now?” indeed…. 🙂
Sean,
Thanks for that answer. Of course, it assumes that there is a surface of last scattering playing, but I understand. A long time ago, though, when you and Adam Riess were Ira Flato’s guests, he was asking for ideas about dark energy that could be tested, and I suggested the change in energy density over time would be a good one, but I never heard back from him.
Assuming there was a way to measure it, what effect would the finding that the energy density of the universe was remaining constant over time have on the standard cosmology?
Doug, any model in which the energy density of matter evolved in some non-standard way would presumably have effects on all sorts of things — the Hubble diagram (luminosity distance vs. redshift), number counts of clusters and galaxies, evolution of the power spectrum, baryon oscillations, the CMB power spectrum, perhaps even nucleosynthesis. LambdaCDM passes all of those tests, so any alternative would have to do comparably well to be taken seriously. Something as dramatically non-standard as a constant matter density is ruled out a million ways from Sunday.
Sean,
Thanks.
Question: if the CMB was an absolute perfect black body, would not the universe have forever remained in a permanent Dark Age, in effect, succumbed to an eternal heat death devoid of any repolarization/reionization signature? If the answer to this question is roughly in the affirmative, can one safely argue that the primary reason the universe travels an evolutionary course be fundamentally linked to the imperfections(as opposed to the perfections) of the CMB? Furthermore, if the answer to this question is somewhat yes, would the “smoking gun” of the universe’s evolutionary pathway be embedded in the “imperfect parameters” involving the inhomogeneities/anisotrpies of the CMB- in contrast to the “perfect parameters” involving the homogeneities/isotropies of the CMB? Therefore, assuming this “broad-in-scope” line of reasoning is vaguely valid, then in order to uncover the origins of the universe, one must uncover the origins of the imperfections ( the inhomogeneities/anisotropies) of the CMB minus the random noise of the perfections (the homogeneities/isotropies) of the CMB. PLease respond gently, I am an obvious layperson.
Cynthia, yes, it is deviations from perfect smoothness of the primordial plasma that grow into galaxies and clusters today. (Not really “deviations from a blackbody” — the CMB is very close to a blackbody at every point, although the temperature fluctuates from place to place.) If it weren’t for primordial perturbations, structures would have developed *much* more slowly, although there would always be some perturbations, if only because of thermal/quantum fluctuations. However, since there is a cosmolgical constant that can suppress structure growth at late times, we can certainly imagine that if the perturbations were substantially smaller, essentially no structures would ever have formed.
Although most of the newspaper headlines regarding the recent WMAP results arguably exaggerated the importance of the new results, I think there is another school of thought regarding recent advances in cosmology which is equally if not more frustrating to witness. This is the “cosmology will never be a science” or “anti-Big Bang” school of thought. These people criticize the fact that many cosmologists are calling the standard 6-parameter cosmological model simple. They reason that a model with 6-parameters is complicated and unaesthetic. They say that cosmology will never be a science but then, in a contradictory manner, offer scientific evidence for their own cosmology theories.
I certainly think peer review and skepticism of new results are crucial aspects of the scientific process; however, I don’t know why some people seem so unwilling to accept the notion that we may actually be getting somewhere in terms of understanding the sweep of cosmic evolution. I don’t think cosmology is solved, but I would like to believe (and I think the currrent evidence is bearing this out) that we are finally at the point where we can claim to have a good understanding of cosmic evolution even if we don’t know all of the details. Just because something is big doesn’t necessarily mean that it is harder to understand. Unlike archeologists, cosmologists and astronomers can directly observe past objects of study.
Sean, thanks for your generosity in clearing-up my faulty reasoning regarding the blackbody of the CMB. Phrased succinctly, the perturbations of the primordial plasma – not the deviations from the blackbody – are correlated with the inhomogeneities/anisotropies of the CMB. Let me share the convoluted pathway of my faulty reasoning: when Kenneth Ford in his popular text “The Quantum World” characterized blackholes as the most perfect blackbodies in Nature, my faulty reasoning linked the “more perfect” blackbody of a blackhole to the “less perfect” blackbody of the CMB. I further faultered by linking the “less perfect” blackbody of the CMB to the inhomogeneities/anisotropies of the CMB. A superb example of bad science on my part: perhaps the Bush administration would be eager to appoint me as a new addition to their illustrious science team?
I’m even more of a layperson than the “obvious layperson” who commented above, but I got a lot out of Sean’s post, and I thank him for it. Having worked in journalism for a while, as one who was presumed to have more exertise in a certain area (the arts) than my average reader, I’m aware of the difficulties in explaining complex matters in a short space, and, however he came by it, I think Sean is too.
That reminds me of something else. Elsewhere in the blogosphere I’ve been reading lately about a controversy in the New York theater world; compared to some of that, the discussions I see here, when I have time to drop in, are models of civility.
If anyone has a peripheral interest in the CMB, one should view Hiranya Peiris’s (u of Chicago) excellent talk at the Space Telescope Institute (spring 2006 series). She presented the CMB with the appropriate digestible mix of scale and depth. K C Cole (comment #21) implied that Michael Turner ( u of Chicago) mis-stepped by over-referencing “smoking guns” as “inflationary features” embedded in the WMAP data. However, Hiranya Peivis – as a fellow constituent at the U of Chicago – makes up nicely for any assumed pitfalls in Michael Turner’s media relations.
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