The universe is thought to have consistently become cooler over time. But about 1.5 billion years ago, the cosmos overheated in a massive temperature spike, caused by runaway black holes that pumped tons of ultraviolet radiation into the universe.

The universe's initial temperature was north of 10^28 Kelvin, and even a billion years later it was still about 8,000 degrees Celsius. Considering the universe's average temperature now hovers slightly above absolute zero, that's incredibly hot. But physicists always assumed that the universe's temperature change was a steady downward slope, and at any given moment the universe's temperature would never again be hotter.

Cambridge researcher George Becker and his team have discovered it's not nearly as simple as that. They studied the temperatures of ancient gas clouds in the furthest reaches of the observable universe. While the clouds from a billion years after the Big Bang were indeed 8,000 degrees Celsius, those from three and a half billion years after the Big Bang were considerably hotter, about 12,000 degrees. What could cause this weird spike?


As with most strange things in the universe, the answer is black holes. Early black holes at the center of the universe's first galaxies swallowed up tons of surrounding material, releasing vast swaths of ultraviolet radiation as a byproduct. These black holes are known as quasars, and they're some of the brightest light sources in the entire universe.

This period in the universe's history was the first in which quasars became common, but why was this the only period in which they were able to raise the universe's temperature so dramatically? The researchers suspect it's because the intergalactic gas clouds common at the time were stuffed with helium atoms. When the ultraviolet light hit the helium, it would have sent electrons flying.


This would have created a massive game of cosmic pinball, in which particles continually collided with each other, which warmed up the gas. Once the helium supplies were exhausted, the universe could begin to cool down again.