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How hot is the Universe today? How hot was it before? A new study by an international team of researchers, including members of the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), suggests that the average gas temperature in large structures in the Universe increased about 3 times over last 8 billion years, to reach about two million Kelvin today.
The large-scale structure of the Universe refers to the global model of how galaxies and clusters of galaxies are distributed in space. This cosmic web was formed from tiny irregularities in the distribution of matter in the early Universe, which were amplified by the gravitational pull. “As the Universe evolves, gravity pulls dark matter and gas into space together into galaxies and galaxy clusters,” said Yi-Kuan Chiang, lead author of the study and researcher at Ohio State. University Center for Cosmology and AstroParticle Physics. “The resistance is violent, so violent that more and more gasoline is shocked and heated.”
This heated gas can then be used to measure the average temperature of the Universe over cosmic time. Specifically, the researchers used the so-called “Sunyaev-Zeldovich” effect, named after Rashid Sunyaev, director emeritus at the Max Planck Institute of Astrophysics, and Soviet-era physicist Yakov Zeldovich, who first predicted this phenomenon. in theory. This effect occurs when low-energy photons of the cosmic microwave background radiation are scattered by hot electrons in the large-scale structure of the Universe. Scattering transfers energy from electrons to photons, making hot electron gas visible. The intensity of the Sunyaev-Zeldovich effect is proportional to the thermal pressure of the gas, which in turn is proportional to the temperature of the electrons.
While this measurement is simple in principle, collecting the necessary data was an important undertaking. The study, which was published in Astrophysical Journal, was developed in collaboration with researchers from Kavli IPMU, Ohio State University, Johns Hopkins University and the Max Planck Institute for Astrophysics.
The researchers used data collected from two observatories, the Planck satellite and the Sloan Digital Sky Survey (SDSS). Planck is the European Space Agency mission that measured the cosmic microwave background radiation. SDSS collected detailed images and light spectra of galaxies. By combining the two datasets, the scientists were able to measure the amount of thermal pressure around the positions of galaxies and galaxy clusters.
“It took astronomers more than 15 years to collect the necessary data using one telescope on the ground and one in space,” said Brice Ménard, who led the analysis with Chiang. Ménard, who has been a visiting researcher at Kavli’s IPMU since 2011, added: “From an analytical point of view, our team spent four years developing the algorithms needed to extract the signal from this data.”
Furthermore, the interpretation of the data required a physical model, provided by Ryu Makiya, a researcher at Kavli’s IPMU. “By combining the latest data with a cutting-edge theoretical model, we were able to reveal how the temperature of the Universe evolved and how it was linked to the formation of the large-scale structure of the Universe,” said Makiya. “The next goal is to understand the details of the physics of thermal and non-thermal phenomena.”
Chiang, of Ohio Stated University, added, “Our new measurement provides direct confirmation of the fundamental work of Jim Peebles – the 2019 Nobel Prize in Physics – who presented the theory of the emergence of a large-scale structure of Universe. “
The study determined that about eight billion years ago (with a redshift of z = 1), the average temperature of electrons was around 700,000 Kelvin, which now rises to around two million Kelvin. Furthermore, scientists have determined that its evolution is almost entirely driven by the growth of structures, as gas is heated by shock in the collapse of large-scale structures.
In 2000, Eiichiro Komatsu, Principal Investigator at IPMU Kavli and Director in the Department of Physical Cosmology at the Max Planck Institute for Astrophysics, was also involved in an earlier effort to calculate how the temperature of the Universe has evolved. “For 20 years, we have been studying how to measure it using the Sunyaev-Zeldovich effect,” he recalled. “Now we have finally measured the temperature of the Universe, not only thanks to the remarkable advances in observational data, but also thanks to the efforts of brilliant young scientists such as Yi-Kuan Chiang and Ryu Makiya. This is very satisfying, ”Komatsu added.
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