A new WMAP data and paper release is always a little Christmas for cosmologists. Among the latest set of papers based on seven years of operation, released last month, one concerning whether there are any Cosmic Microwave Background Anomalies suggests that the WMAP team has an affinity for bowling. Why? Because they set 'em up... and then the knock 'em down.
A bit of background: The WMAP dataset contains the finest map we yet have of oldest primordial light in existence -- the first photons released after the Big Bang that were able to make it all the way to us, living over 13 billion years later. As a result, to say that people have pored over these data with tremendous care and effort is to understate the case.
The simplest models for the early Universe in the dominant inflationary paradigm predict that the fluctuations in the temperature of the light from the CMB will be well described by a Gaussian random field. Thus, one of the most intriguing things to look for in the data are deviations from pure Gaussianity, which would signal that something extra and interesting was going on in the early Universe. This can be exciting, but comes with a critical caveat: statistics can be an unfaithful friend to the eager data-miner. This is because a purely random field on which 100 tests are done will, on average, produce at least one result that is only 1% probable in the underlying model. Hence, any anomaly that is found in the data must be provably much rarer than any random fluctuation if one is to believe that it is truly a deviation from the theory.
Given human nature and the hopes of glory that rest in each of our hearts, perhaps what happened was inevitable. As time passed, various experts on the WMAP data began to uncover strange anomalies in those data that appeared, upon investigation, to be just the sort of rarer-than-random things that would be the mark of a real discovery: Right away, people noticed that there was an huge cold spot smack dab in the middle of the map. Then the map seemed not to have enough structure on the largest scales (in technical terms, it had anomalously low power in its quadrupole, and seems to lack sufficient correlation on wide angular scales) Then the quadrupole was found to align with the octupole, defining a special direction in space that was cleverly named by one of its earliest investigators as the "axis of evil." And then two halves of the sky were found to have starkly differing levels of small-scale temperature correlation, a so-called power asymmetry.
It's critical to note that these anomaly-discovering experts never included members of the actual WMAP team. The team was itself admirably impartial throughout the hoopla that attended each discovery, presumably well aware that approval from the team would be tantamount to a final verdict on the discovery and that dismissal without careful analysis would be irresponsible. Nonetheless, they have clearly been keeping close watch on these developments. This new paper is their long-gestated answer to all these claims.
In short, their result is negative: they don't believe the anomalies are real. They reach this conclusion through a careful study of each one separately. First, they give a concise review of each anomaly and the opposing arguments regarding its significance. This gives the paper an almost Thomistic style; by the middle, I was half expecting a sed contra to pop up. Then, they improve and deepen the original analysis, taking into account other developments in the literature. In each case but one, they conclude that the anomaly is less statistically significant than it at first seemed, moving each out of the "rarer-than-random" column and onto the "almost certainly just a fluke" side of the ledger.
I'm sure you're curious now to know what the last anomaly standing is. It's a bit of a curious and technical one: a quadrupolar "effect" in the two-point power spectrum. That is, the way the temperatures of the map are correlated depends a bit on where they are located on the sky, in a pattern that has four distinct quadrants. However, before you start writing your paper on the asymmetric Universe, it turns out that even this Official WMAP Approved anomaly probably isn't exciting evidence for new early Universe physics. The critical evidence disfavoring an early Universe origin is that the effect isn't equally present in different frequency bands, which true CMB effects must be; and the preferred direction that the effect aligns is suspiciously similar to the ecliptic pole -- the direction in the sky picked out by our (and the WMAP satellite's) orbit around the sun.
All of which is very reasonable and proper, and a bit disappointing. I, like anyone, am always eager for the Universe to throw us a little loop, to find a bit of an Easter egg left from the Universe's earliest days. But as the WMAP team points out in their conclusion, we humans are simply hardwired to discern patterns, so much so that we do it too well -- finding patterns where none really are. Their example, a fun tidbit with which I'll close, is pointing out the appearance of the initials "S. H." in the map, which is certainly (?) not evidence for new physics, unless Stephen Hawking has something very surprising up his sleeve.