Since the day the first Wilkinson Microwave Anisotropy Probe (WMAP) data were released, in 2003, all manner of cosmic microwave background (CMB) anomalies have been reported; there’s been the cold spot that might be a window into a parallel universe, the “Axis of Evil”, pawprints of local interstellar neutral hydrogen, and much, much more.
But do the WMAP data really, truly, absolutely contain evidence of anomalies, things that just do not fit within the six-parameters-and-a-model the WMAP team recently reported?
In a word, no.
Every second year since 2003 the WMAP science team has published a set of papers on their analyses of the cumulative data, and their findings (with the mission due to end later this year, their next set will, sadly, be their last). With time and experience – not to mention inputs from the thousands of other researchers who have picked over the data – the team has not only amassed a lot more data, but has also come to understand how WMAP operates far better. As a consequence, not only are the published results – such as limits on the nature of dark energy, and the number of different kinds of neutrinos – more stringent and robust, but the team has also become very au fait with the various anomalies reported.
For the first time, the team has examined these anomalies, in detail, and has concluded that the answer to the question, in their words, “are there potential deviations from ΛCDM within the context of the allowed parameter ranges of the existing WMAP observations?” is “no”.
The reported anomalies the team examined are many – two prominent cold spots, strength of the quadrupole, lack of large angular scale CMB power, alignment of the quadrupole and octupole components, hemispherical or dipole power asymmetry, to name but a handful – but the reasons for the apparent anomalies are few.
“Human eyes and brains are excellent at detecting visual patterns, but poor at assessing probabilities. Features seen in the WMAP maps, such as the large Cold Spot I near the Galactic center region, can stand out as unusual. However, the likelihood of such features can not be discerned by visual inspection of our particular realization of the universe,” they write, and “Monte Carlo simulations are an invaluable way to determine the expected deviations within the ΛCDM model. Claims of anomalies without Monte Carlo simulations are necessarily weak claims”.
An amusing example: Stephen Hawking’s initials (“SH”) can be clearly seen in the WMAP sky map. “The “S” and “H” are in roughly the same font size and style, and both letters are aligned neatly along a line of fixed Galactic latitude,” the team says; “A calculation would show that the probability of this particular occurrence is vanishingly small. Yet, there is no case to made for a non-standard cosmology despite this extraordinarily low probability event,” they dryly note.
Many of the reports of WMAP CMB anomalies would likely make for good teaching material, as they illustrate well the many traps that you can so easily fall into when doing after-the-fact (a posteriori) statistical analyses. Or, as the team puts it in regard to the Stephen Hawking initials: “It is clear that the combined selection of looking for initials, these particular initials, and their alignment and location are all a posteriori choices. For a rich data set, as is the case with WMAP, there are a lot of data and a lot of ways of analyzing the data.”
And what happens when you have a lot of data? Low probability events are guaranteed to occur! “For example, it is not unexpected to find a 2σ feature when analyzing a rich data set in a number of different ways. However, to assess whether a particular 2σ feature is interesting, one is often tempted to narrow in on it to isolate its behavior. That process involves a posteriori choices that amplify the apparent significance of the feature.”
So, does the team conclude that all this anomaly hunting is a waste of effort? Absolutely not! I’ll quote from the team’s own conclusion: “The search for oddities in the data is essential for testing the model. The success of the model makes these searches even more important. A detection of any highly significant a posteriori feature could become a serious challenge for the model. The less significant features discussed in this paper provided the motivation for considering alternative models and developing new analysis of WMAP (and soon Planck) data. The oddities have triggered proposed new observations that can further test the models. It is often difficult to assess the statistical claims. It may well be that an oddity could be found that motivates a new theory, which then could be tested as a hypothesis against ΛCDM. The data support these comparisons. Of course, other cosmological measurements must also play a role in testing new hypotheses. No CMB anomaly reported to date has caused the scientific community to adopt a new standard model of cosmology, but claimed anomalies have been used to provoke thought and to search for improved theories.”
Primary source: Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies? (arXiv:1001.4758). The five other Seven-Year WMAP papers are: Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation (arXiv:1001.4538), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Planets and Celestial Calibration Sources (arXiv:1001.4731), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results (arXiv:1001.4744), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Power Spectra and WMAP-Derived Parameters (arXiv:1001.4635), and Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission (arXiv:1001.4555). Also check out the official WMAP website.