A primary goal in cosmology is to accurately measure the total amount of matter in the universe, a daunting exercise for even the most mathematician. A team led by scientists from the University of California at Riverside did just that.
Reporting in the Astrophysical JournalThe team found that matter makes up 31% of the total amount of matter and energy in the universe, with the remainder made up of dark energy.
“To put this amount of matter into context: If all the matter in the universe were evenly distributed over space, this would correspond to an average mass density that corresponds to only about six hydrogen atoms per cubic meter,” said first author Mohamed Abdullah. a PhD student in the UCR Physics and Astronomy Department. “However, since we know that 80% of matter is actually dark matter, most of that matter is actually not made up of hydrogen atoms, but of a type of matter that cosmologists don’t yet understand.”
Abdullah explained that a proven technique for determining the total amount of matter in the universe is to compare the observed number and mass of galaxy clusters per unit volume with predictions from numerical simulations. Since today’s galaxy clusters have formed from matter that collapsed under its own gravity over billions of years, the number of clusters currently observed is very sensitive to cosmological conditions and especially to the total amount of matter.
“A higher percentage of matter would result in more clusters,” said Abdullah. “The ‘Goldilocks’ challenge for our team was to measure the number of clusters and then determine which answer was ‘just right’. However, measuring the mass of a galaxy cluster accurately is difficult because most of the Matter is dark and we cannot see it with telescopes. “
To overcome this difficulty, the team of astronomers led by the UCR first developed “GalWeight”, a cosmological tool for measuring the mass of a galaxy cluster based on the orbits of its member galaxies. Researchers then used their tool on observations from the Sloan Digital Sky Survey (SDSS) to create “GalWCat19”, a publicly available catalog of galaxy clusters. Finally, they compared the number of clusters in their new catalog with simulations to determine the total amount of matter in the universe.
“We managed to take one of the most accurate measurements ever made with the galaxy cluster technique,” said co-author Gillian Wilson, professor of physics and astronomy at UCR, in whose laboratory Abdullah works. “Furthermore, this is the first use of the galaxy orbit technique to achieve a value consistent with that obtained by teams using non-cluster techniques such as background cosmic microwave anisotropies, acoustic baryons, Type Ia or supernovae Gravitational lenses used. “
“A big advantage of using our GalWeight galaxy orbit technique was that our team was able to determine a mass for each cluster individually rather than relying on more indirect statistical methods,” said third co-author Anatoly Klypin, Expert in numerical simulations and cosmology.
By combining their measurements with those of the other teams using different techniques, the UCR-led team found a best combined value and concluded that matter was 31.5 ± 1.3% of the total amount of matter and energy in the Universe.
The research paper is entitled “Cosmological Limitations of Ωm and σ8 from Cluster Frequencies Using the Optical-Spectroscopic SDSS Catalog GalWCat19”.
The universe is the same everywhere – even more than cosmologists have predicted
Mohamed H. Abdullah et al., Cosmological restrictions for Ω m and σ 8 from cluster frequencies using the optical-spectroscopic SDSS catalog GalWCat19, The astrophysical journal (2020). DOI: 10.3847 / 1538-4357 / aba619
Provided by the University of California – Riverside
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