The universe is getting hotter, study

The temperature has increased about 10-fold over the last 10 billion years.


The cosmic thermal history is quantified by the evolution of the mean thermal energy density in the universe. It is driven by the growth of structures as baryons get shocked heated in collapsing dark matter halos.

While probing the thermal history of the universe over the last 10 billion years, scientists found that the mean temperature of gas across the universe has increased more than ten times over that time period and reached about 2 million degrees Kelvin today, approximately 4 million degrees Fahrenheit.

In 2019, Jim Peebles, Nobel Laureate in Physics, theorized how large-scale structures form in the universe. Here, the universe’s large-scale structure means the global patterns of galaxies and galaxy clusters on scales beyond individual galaxies. It is formed by the gravitational collapse of dark matter and gas. This new study directly confirms the seminal work by Jim Peebles.

Yi-Kuan Chiang, lead author of the study, said, “As the universe evolves, gravity pulls dark matter and gas in space together into galaxies and clusters of galaxies. The drag is violent — so violent that more and more gas is shocked and heated up.”

“The findings showed scientists how to clock the progress of cosmic structure formation by “checking the temperature” of the universe.”

Using a newly developed method, scientists could estimate the temperature of gas farther away from Earth. They then compared them to gases closer to Earth and near the present time. The results showed that the universe is getting hotter over time due to the gravitational collapse of cosmic structure, and the heating will likely continue.

The data on light, obtained through two missions, Planck and the Sloan Digital Sky Survey, aided in understanding the evolution of the universe’s temperature. Scientists combined the data from both missions and evaluated the distances of the hot gases near and far via measuring redshift.

Along with a method to estimate temperature from light, scientists could measure the mean temperature of gases in the early universe. They then compared that means with the mean temperature of gases closer to Earth — gases today.

Scientists found that those gases reach temperatures of about 2 million degrees Kelvin — approximately 4 million degrees Fahrenheit, around objects closer to Earth. That is about ten times the temperature of the gases around things farther away and further back in time.

Chiang said“The universe is warming because of the natural process of galaxy and structure formation. It is unrelated to the warming of Earth. These phenomena are happening on very different scales. They are not at all connected.”

Journal Reference:

  1. Chiang, Yi-Kuan et al. The Cosmic Thermal History Probed by Sunyaev-Zeldovich Effect Tomography. DOI: 10.3847/1538-4357/abb403
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