The Sciences

Everything I know about the universe I did not learn from newspaper headlines

Cosmic VarianceBy Sean CarrollMar 27, 2006 4:47 PM


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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:

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.

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