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Cosmic Cold Spot Just a Trick of the Light

The famous cold spot in the cosmic microwave background is an artifact of the statistical methods used to find it, according to a new analysis.

The echo of the Big Bang bathes our universe in a steady afterglow of microwaves that extends evenly in all directions. Various space telescopes have measured the distribution of the so-called cosmic microwave background with fantastic accuracy, the most recent being the WMAP spacecraft.

We’ve long known that the temperature of these microwaves is 2.7 K but WMAP established that this varies by no more than about 10^-5 across the entire sky. That fits exactly with our vision of the Big Bang as an entirely homogeneous event–there ought to be no “preferred” directions in spacetime

But in 2004, astronomers found a region of the cosmic microwave background in the southern hemisphere in the direction of the constellation of Eridanus that is significantly colder than the rest by about 70 microkelvin. Such a cold spot should not exist if the Big Bang really were a well behaved, even event.

Since then astronomers, have puzzled long and hard over the origin of this cold spot, suggesting variously that it is caused by a supervoid, that it is a remnant of an early phase transition in the universe and, most controversial of all, that it is a window into a parallel universe.

Now Ray Zhang and Dragan Huterer at the University of Michigan in Ann Arbor say that the cold spot is simply an artifact of the statistical method–called Spherical Mexican Hat Wavelets–used to analyse the WMAP data. Use a different method of analysis and the cold spot disappears (or at least is no colder than expected). They say:

“We trace this apparent discrepancy to the fact that WMAP cold spot’s temperature profile just happens to favor the particular profile given by the wavelet.”

And they conclude:

“We find no compelling evidence for the anomalously cold spot in WMAP at scales between 2 and 8 degrees.”

That may resolve the mystery but it makes a damp squib out of what once seemed one of the most exciting discoveries in astrophysics. Shame!

Ref: arxiv.org/abs/0908.3988: Disks in the Sky: A Reassessment of the WMAP “Cold Spot”

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