The common temperature of the universe was getting hotter

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For almost a century, astronomers have understood that the universe is in a state of expansion. Since the 1990s, they have understood that the rate of expansion has been accelerating for four billion years. As this progresses and the galaxy clusters and filaments of the universe move further apart, scientists theorize that the mean temperature of the universe will gradually decrease.

However, according to new research from the Center for Cosmology and Astroparticle Physics (CCAPP) at Ohio State University, the universe does appear to be actually getting hotter over time. After studying the thermal history of the universe over the past 10 billion years, the team concluded that the mean temperature of cosmic gas has increased more than ten times, and is now around 2.2 million K (~ 2.2 ° C ; 4 million ° F).

The study describing its results, "The Cosmic Thermal History Examined by Sunyaev-Zeldovich Effect Tomography," recently appeared in the Astrophysical Journal. The study was led by Yi-Kuan Chiang, a research fellow at CCAP, and included members of the Kavli Institute for Physics and Mathematics of the Universe (Kavli IPMU), Johns Hopkins University, and the Max Planck Institute for Astrophysics.

An artist concept of the Planck spacecraft. Credits: ESA / NASA / JPL-Caltech

For their study, the team examined thermal data on the large-scale structure (LSS) of the universe. This refers to patterns of galaxies and matter on the largest cosmic scale that is the result of the gravitational collapse of dark matter and gas. As Dr. Chiang in an Ohio State press release stated:

“Our new measurement provides direct confirmation of the groundbreaking work of Jim Peebles – the 2019 Nobel Prize winner in physics – who set out the theory of how the large-scale structure in the universe is formed. As the universe evolves, gravity pulls dark matter and gas in space together into galaxies and galaxy clusters. The resistance is violent – so fierce that more and more gas is shocked and heated. "

To measure thermal changes over the past 10 billion years, Chiang and his colleagues combined data from ESA's Planck Infrared Astronomical Satellite and the Sloan Digital Sky Survey (SDSS). While Planck was the first European mission to measure the cosmic microwave background (CMB) temperature, SDSS is a massive multispectral survey that has produced the most detailed 3D maps of the universe.

From these data sets, the team correlated eight of Planck's sky intensity maps with two million spectroscopic redshift references from the SDSS. The team combined redshift measurements (which are routinely used to determine how quickly objects are moving away from us) and temperature estimates based on light, and compared the temperature of more distant gas clouds (farther back in time) to those closer to Earth lie.

All-sky data obtained from ESA's Planck mission showing the different wavelengths. Photo credit: ESA

From this, the research team was able to confirm that the mean temperature of gases in the early universe (approx. 4 billion after the Big Bang) was lower than it is today. This is apparently due to the gravitational collapse of the cosmic structure over time, a trend that will continue and intensify as the expansion of the universe accelerates further.

As Chiang summarized, the universe warms up due to the natural process of galaxy and structure formation and has nothing to do with temperature changes here on Earth:

“As the universe evolves, gravity pulls dark matter and gas in space together into galaxies and galaxy clusters. The resistance is violent – so fierce that more and more gas is shocked and heated. These phenomena occur on very different scales. They are not connected at all. "

In the past, many astronomers have argued that the cosmos would continue to cool as it expanded, which would inevitably lead to "big chill" (or "big freeze"). In contrast, Chiang and his co-workers showed that scientists can follow the evolution of cosmic structure formation by "checking the temperature" of the universe.

A section of the 3D map created by BOSS. The rectangle on the far left shows a section of 1000 square meters in the sky with almost 120,000 galaxies or about 10% of the total measurement. Photo credit: Jeremy Tinker / SDSS-III

These results could also have implications for theories that accept “cosmic cooling” as a foregone conclusion. On the one hand, it has been suggested that a possible solution to the Fermi Paradox is for extraterrestrial intelligences (ETIs) to be inactive, waiting for the universe to improve (the aesthetic hypothesis).

The argument is based in part on the thermodynamics of computation (the Landauer principle) and says that advanced species with a cooling universe can get far more out of their megastructures. If the cosmos gets hotter over time, does this mean that life becomes less likely over time due to the increased cosmic rays?

Assuming that there is no mechanism to maintain a particular thermal equilibrium, would this mean that the universe does not end in a “big chill” but rather in a “big blaze”? Robert Frost famously wrote: "Some say the world will end in fire, others say in ice." Only time will tell which of these prove to be correct and what impact this could have on life in the future …

Further reading: Ohio State News, The Astrophysical Journal

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