Why a giant ‘cold spot’ in the cosmic microwave background has long confounded astronomers

Leftover light from the young cosmos has a major flaw, and we simply don’t know how to fix it. It’s the cold spot. It’s just way too vast and way too cold. Astronomers are unsure what it is, but they mostly are in agreement that it’s worth examining.

The cosmic microwave background (CMB) was spawned when our cosmos was only 380,000 years old. At the time, our universe was roughly a million times smaller than it is today and had a temperature of over 10,000 kelvins (9,700 degrees Celsius, or 17,500 degrees Fahrenheit), meaning all of the gas was plasma. As the cosmos expanded, it cooled, and the plasma became neutral. In the course, it released a flood of white-hot light. Over the billions of years since, that light has cooled down and stretched to a temperature of about 3 kelvins (minus 454 F, or minus 270 C), putting that radiation in the microwave band of the electromagnetic spectrum.

The CMB is nearly perfectly uniform, but there are minute differences in temperature to about 1 part per million, and those imperfections, which seem like splotches of various shapes and sizes, are the spiciest part about it. We can’t guess exactly what the fluctuations will be, which precise spots will be cold and which spots will be hot. That’s because the light we’re seeing is coming from a part of the cosmos that is now pulled away from observable view. This means we have to depend on statistics to understand the CMB. We can’t say what splotches will appear where; we can only employ physics to understand the average size of splotches and how cold or hot they might be, on average.

The cold spot

Just about everything with the CMB is hunky dory. We understand quiet well where the splotches come from, and over the decades, we’ve fabricated increasingly refined telescopes and satellites for getting a better look. Actually, the detection and measurement of the CMB is one of the major success stories in science.And then there’s the cold spot.Now there are loads of cold spots in the CMB. But there’s one that stands out. It stands out visuallyeven. If you take a look at a map of the CMB — where the entire sphere of the sky is compressed into a strange, vaguely oval shape — it’s down and a little to the right. In the sky, it’s in the direction of the constellation Eridanus. This cold spot is oddly cold, about 70 microkelvins colder than average. Average run-of-the-mill cold spot are only 18 microkelvins colder than average. Its 5 degrees across while average cold spot is less than 1 degree.