Universe is Getting Hotter, Says New Study
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The mean temperature of gas across the Universe has increased more than 10 times over the last 10 billion years and reached about 2 million Kelvin today, according to new research published in the Astrophysical Journal.

As the Universe evolves, matter concentrations are surrounded by gas halos getting hotter and bigger. Image credit: D. Nelson / Illustris Collaboration.

“Our new measurement provides a direct confirmation of the seminal work by Jim Peebles, who laid out the theory of how the large-scale structure forms in the Universe,” said lead author Dr. Yi-Kuan Chiang, a researcher in the Center for Cosmology and AstroParticle Physics at the Ohio State University.

The large-scale structure of the Universe refers to 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.

“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,” Dr. Chiang said.

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

Dr. Chiang and colleagues used a new method that allowed them to estimate the temperature of gas farther away from Earth — which means further back in time — and compare them to gases closer to Earth and near the present time.

Using data collected by ESA’s Planck satellite and the Sloan Digital Sky Survey, they estimated the redshift of gas concentrations seen in images of microwave light going back in time all the way to 10 billion years ago.

They found that the gases in the present-day Universe reach temperatures of about 2 million Kelvin around objects closer to Earth. That is about 10 times the temperature of the gases around objects farther away and further back in time.

This trend is also predicted by numerical simulations showing how dark matter and the atoms present in the gas evolve with time.

“The Universe is warming because of the natural process of galaxy and structure formation,” Dr. Chiang said.

“It is unrelated to the warming on Earth. These phenomena are happening on very different scales. They are not at all connected.”

“We have measured temperatures throughout the history of the Universe,” said co-author Professor Brice Ménard, a researcher in the Kavli Institute for the Physics and Mathematics of the Universe and the Department of Physics & Astronomy at Johns Hopkins University.

“As time has gone on, all those clusters of galaxies are getting hotter and hotter because their gravity pulls more and more gas toward them.”

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Yi-Kuan Chiang et al. 2020. The Cosmic Thermal History Probed by Sunyaev-Zeldovich Effect Tomography. ApJ 902, 56; doi: 10.3847/1538-4357/abb403

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