Shortly after the Big Bang, in the Planck era, the universe was about 10³² Kelvin hot. Afterwards it expanded rapidly and cooled down as the energy spread over an ever larger space. By and large, this process should continue as long as the universe expands – an end to the expansion is not yet in sight, on the contrary.

But there is a process that counteracts this cooling – at least temporarily. It is easy to understand. When 10,000 people go home from a football stadium, the mood cools down initially as the density of people decreases. But when a few fans get together to celebrate, things get hotter again. The formation of structures is also responsible for the warming of the cosmos. When gas masses clump together with dark matter halos to form galaxies, their particles heat up.

Astronomers proved this now also in a new study, which was published in the Astrophysical journal. It examines the thermal history of the universe in the last 10 billion years and found out that the average temperature of the gas in the entire universe rose in this period by more than ten times – to today about 2 million Kelvin (4 million degrees Fahrenheit).

“Our new measurement is a direct confirmation of the groundbreaking work of Jim Peebles – winner of the 2019 Nobel Prize for Physics – who has laid out the theory of the origin of large-scale structure in the universe,” says Yi-Kuan Chiang, lead author of the study. The large-scale structure of the Universe refers to the global patterns of galaxies and clusters of galaxies on scales beyond individual galaxies. It results from the gravitational collapse of dark matter and gas.

“As the universe evolves, gravity pulls dark matter and gas in space together to form galaxies and galaxy clusters,” Chiang says. “The resistance is fierce – so fierce that more and more gas is shocked and heated up.” The results, Chiang says, showed scientists how to determine the progress of cosmic structure formation by measuring the temperature of the universe.

To do this, the researchers used a new method that allowed them to estimate the temperature of gases that are further away from the Earth – that is, farther back in time – and compare them with gases that are closer to the Earth and closer to the present time. To understand how the temperature of the universe has changed over time, the researchers used data from two missions, Planck and the Sloan Digital Sky Survey. They combined the data and evaluated the distances of the hot gases near and far by measuring redshift.

This warming is not related to global warming on Earth. “These phenomena occur on very different scales,” Chiang says. “They are not connected at all.”

The study was conducted in collaboration with researchers from the Kavli Institute for Physics and Mathematics of the Universe, Johns Hopkins University and the Max Planck Institute for Astrophysics.