admin 
One of the most interesting and important questions in cosmology is: “How much matter is there in the universe?” An international team, including scientists from Chiba University, succeeded in measuring the total amount of matter for the second time. In a report published in the Astrophysical Journal, the team determined that matter accounts for 31% of the total amount of matter and energy in the universe, with the remainder being dark energy.
“Cosmologists believe that only about 20% of all matter is ordinary or ‘baryonic’ matter, which includes stars, galaxies, atoms and life,” first author first, Dr. Mohamed Abdullah, a researcher at the National Research Institute, explains. Institute of Astronomy and Geophysics-Egypt, Chiba University, Japan.“About 80% is made up of dark matter, the mysterious nature of which is unknown but may include unknown subatomic particles.” Co-author Gillian Wilson explains:
“The team used a proven technique to determine the total amount of matter in the universe, which involves comparing the observed quantity and mass of astronomical clusters. galaxies per unit volume with predictions from numerical simulations”.Abdullah’s former graduate advisor, professor of physics and vice chancellor for research, innovation and economic development at UC Merced.
“The number of clusters observed today, called ‘cluster abundance,’ is very sensitive to cosmic conditions and especially to the total amount of matter.”
“A higher percentage of total matter in the universe would cause more clusters to form,” says Anatoly Klypin of the University of Virginia. “But it is difficult to accurately measure the mass of a galaxy cluster because most of the matter is dark and we cannot see it directly with telescopes.”
To overcome this difficulty, the team was forced to use an indirect tracer of cluster mass. They relied upon the fact that more massive clusters contain more galaxies than less massive clusters (mass richness relation: MRR). Because galaxies consist of luminous stars, the number of galaxies in each cluster can be utilized as a way of indirectly determining its total mass.
By measuring the number of galaxies in each cluster in their sample from the Sloan Digital Sky Survey, the team was able to estimate the total mass of each of the clusters. They were then able to compare the observed number and mass of galaxy clusters per unit volume against predictions from numerical simulations.
The best-fit match between observations and simulations was with a universe consisting of 31% of the total matter, a value that was in excellent agreement with that obtained using cosmic microwave background (CMB) observations from the Planck satellite. Notably, CMB is a completely independent technique.
“We have succeeded in making the first measurement of matter density using the MRR, which is in excellent agreement with that obtained by the Planck team using the CMB method,” says Tomoaki Ishiyama from Chiba University. “This work further demonstrates that cluster abundance is a competitive technique for constraining cosmological parameters and complementary to non-cluster techniques such as CMB anisotropies, baryon acoustic oscillations, Type Ia supernovae, or gravitational lensing.”
The team credits their achievement as being the first to successfully utilize spectroscopy, the technique that separates radiation into a spectrum of individual bands or colors, to precisely determine the distance to each cluster and the true member galaxies that are gravitationally bound to the cluster rather than background or foreground interlopers along the line of sight.
Previous studies that attempted to use the MRR technique relied on much cruder and less accurate imaging techniques, such as using pictures of the sky taken at some wavelengths, to determine the distance to each cluster and the nearby galaxies that were true members.
The paper, published in The Astrophysical Journal, not only demonstrates that the MRR technique is a powerful tool for determining cosmological parameters but also explains how it can be applied to new datasets that are available from large, wide, and deep-field imaging, and spectroscopic galaxy surveys such as those performed with Subaru Telescope, Dark Energy Survey, Dark Energy Spectroscopic Instrument, Euclid Telescope, eROSITA Telescope, and the James Webb Space Telescope.
