Scientists might have discovered the source of the mysterious Cold Spot in the Cosmic Microwave Background: an enormous supervoid.
The Cold Spot is an intriguing oddity in the Cosmic Microwave Background (CMB) radiation leftover from the Big Bang. Cosmologists have proposed a number of explanations, running the gamut from entirely new physics to parallel universes. Now a new study announces the detection of a supervoid, a gigantic hole in the universe’s web of galaxies, which might explain the anomaly.
During its nine-year stint mapping the tiny temperature fluctuations in the CMB, the Wilkinson Microwave Anisotropy Probe (WMAP) revealed several interesting anomalies. The most significant of these is the Cold Spot, a large region spanning 10 degrees in Eridanus that is about four times cooler than the average temperature fluctuation in the CMB. It’s unlikely, though not impossible, that the Cold Spot could have originated from the primordial density fluctuations that created the rest of the CMB’s spotted pattern.
If density fluctuations are to blame, then the Cold Spot's very existence could pose a challenge to the current model of inflation. So cosmologists have sought alternative explanations. Now István Szapudi (University of Hawaii) and colleagues have analyzed galaxies in the WISE-2MASS catalog to discover a vast supervoid that could be responsible for the Cold Spot.
The large-scale structure of the universe resembles a web, where comparatively empty regions called voids separate clusters of galaxies. As light climbs up and down these gravitational hills and valleys, the expansion of space causes photons to gain and lose energy in an unequal way called the Integrated Sachs-Wolfe Effect. So an extremely large void could leave a cool fingerprint on the CMB.
Cosmologists had suggested that a supervoid spanning hundreds of millions of light-years could cause the Cold Spot, but previous searches had come up empty — in part because they had searched the distant, early universe.
Szapudi’s team combined data from the WISE-2MASS catalogue and Pan-STARRS1, a robotic telescope that images the full sky once per week, to survey closer galaxies instead. They mapped relatively nearby galaxies lying within the Cold Spot's boundaries and found the density decreased near the center of the Cold Spot for galaxies existing 11.1 billion years after the Big Bang.
The supervoid appears to be roughly spherical, though its internal structure may be more complex, containing smaller voids and filaments. Szapudi’s team estimates that the supervoid, possibly the largest yet discovered, spans about 900 million light-years. According to Mark Neyrinck (Johns Hopkins University), this is “the clearest evidence yet that there is a supervoid that substantially affects the Cosmic Microwave Background.”
But there are still a few questions that remain unanswered. One is the strength of a supervoid’s impact on the CMB: the initial calculation for the supervoid Szapudi’s team spotted does not fully account for the CMB temperature drop in the Cold Spot.
Another pressing question is supervoids’ prevalence. If a larger survey of the sky revealed that they were fairly common, then a single supervoid would be a less likely explanation for the anomalous Cold Spot. “With a larger sample, it would be clearer that this supervoid is the only thing on the sky capable of making a big imprint like the Cold Spot,” explains Neyrinck. “This would make the issue basically indisputable.”
Szapudi et al. "The Cold Spot in the Cosmic Microwave Background: the Shadow of a Supervoid," to appear in the proceedings of the Moriond Cosmology Conference 2014.
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