Mysterious cold streak puzzles astronomers in cosmic background
A peculiar cold region in the cosmic microwave background, known as the Cold Spot, continues to baffle scientists despite years of study. Discovered over a decade ago, this anomaly appears cooler than its surroundings and may hint at large-scale structures in the universe. Researchers remain uncertain about its cause, with theories ranging from statistical flukes to massive voids.
The cosmic microwave background (CMB) is the faint glow left over from the Big Bang, providing a snapshot of the early universe. Within this uniform radiation, astronomers identified an odd feature in 2007: the Cold Spot, a circular patch about 5 degrees across in the constellation Eridanus. This region is approximately 70 microkelvins cooler than the average CMB temperature of 2.725 kelvins, a deviation that stands out against the expected uniformity.
Initial observations came from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) satellite, which mapped the CMB in detail starting in 2001. The anomaly was confirmed and refined by the European Space Agency's Planck satellite, launched in 2009, whose higher-resolution data showed the Cold Spot's sharp edges and internal structure. As one researcher noted in a New Scientist analysis, 'There is an odd streak in the universe and we still don't know why,' highlighting the ongoing mystery.
Several explanations have been proposed. One leading theory suggests the Cold Spot results from the integrated Sachs-Wolfe effect, where photons from the CMB pass through a vast supervoid—a region with fewer galaxies and less matter—causing them to lose energy and appear cooler. Estimates place this potential Eridanus supervoid at about 1.8 billion light-years in diameter, making it one of the largest known structures. However, confirming such a void is challenging, as it lies billions of light-years away and requires deep surveys of distant galaxies.
Alternative ideas include a cosmic texture or bubble collision from the early universe, though these remain speculative. Statistical analyses suggest the Cold Spot's rarity: in a standard cosmological model, such a feature should occur only once in 50 random skies. Recent studies, including those using Planck's full dataset released in 2018, have not resolved the debate, with some papers arguing it could be a rare but natural fluctuation.
The implications are significant for cosmology. If tied to a supervoid, the Cold Spot challenges models of cosmic structure formation, potentially indicating clumpier matter distribution than predicted. Conversely, if a fluke, it underscores the limits of current observations. Ongoing telescope efforts, like those from the Atacama Cosmology Telescope, aim to probe deeper, but for now, the streak endures as an enigmatic mark on the universe's ancient light.