Huge Cold Spot in Early Universe Not so Cold After All?

September 23, 2009

Since researchers found a billion-light-year-wide cold spot in the universe in 2004, theories have abounded: It's a cosmic defect, a mysterious supervoid, or even an "imprint" of a parallel universe.

But according to a new analysis, the spot is merely a statistical artifact, and one that's really nothing special.

"I think [our findings] bring into question all the hype about the cold spot," said study co-author Dragan Huterer of the University of Michigan.

Just a Byproduct

Huterer and colleague Ray Zhang took a new look at a well-known temperature map of the cosmic microwave background, or CMB, the ancient and diffuse afterglow of the big bang.

(Related: "Proof of Big Bang Seen by Space Probe, Scientists Say.")

The map revealed the CMB has an average temperature of about 2.7 Kelvin (-455 degrees Fahrenheit, or -270 degrees Celsius). A few small patches are slightly warmer or cooler than average, but they vary by less than a microkelvin.

But initial studies of the CMB map revealed one region nestled in the constellation Eridanus that appears to be unusually cold—about 70 microkelvins cooler than average. The spot is the one place in the universe where the temperature deviates drastically.

This was a startling finding, since accepted models of the early universe say that the big bang created an initially uniform cosmic landscape, when viewed on large scales.

Possible explanations have so far included a huge cloud that's soaking up CMB radiation, a gaping hole devoid of matter, or even the footprint of another universe that was once entangled with our own.

Keep It Simple?

Now Huterer and Zhang are ready to throw cold water on these theories. They argue that the spot is no more than a byproduct of the statistical tool commonly used to examine CMB data, called—yes, really—the Spherical Mexican Hat Wavelets method.

In their study, posted to the preprint arXiv Web site, Huterer and Zhang analyzed the cold spot using two simpler statistical tools.

Their new findings show that the cold spot's temperature deviation from the CMB average is, after all, no more or less than anywhere else in the universe. (See some of the pictures used to study the origins of the universe.)

But not all statistical tools are created equal, argues Patricio Vielva, an astrophysicist at the University of Cantabria in Spain. Vielva still thinks the cold spot is unusual.

The alternative tools used in the study are simpler, Vielva said. But unlike the Spherical Mexican Hat Wavelets method, the tools are too simple to adequately probe fluctuations in CMB temperature.

For instance, Vielva said, the Mexican Hat method is better suited for finding the roundish shapes that constitute hot and cool patches in the CMB radiation, and it can better scan the CMB at different scales.

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