One of the most pressing questions in cosmology is: "How much matter is there in the universe?" An international team of scientists has now succeeded in measuring the total amount of matter for the second time. The team reports in The Astrophysical Journal that they determined that matter makes up 31 percent of the total matter and energy in the universe, with dark energy making up the remainder.
First author Dr. Mohammed Abdullah, a researcher at the Egyptian National Institute of Astronomy and Geophysics at Chiba University in Japan, explained: "Cosmologists believe that only about 20% of total matter is made of conventional matter or 'baryon' matter, which includes stars, galaxies, atoms and life." About 80% is made of dark matter, whose mysterious properties are not yet understood, but may be composed of some as-yet-undiscovered subatomic particles. (see picture). "
"The team used a well-established technique to determine the total amount of matter in the universe by comparing the observed number and mass of galaxy clusters per unit volume with predictions from numerical simulations," said co-author Gillian Wilson, Abdullah's former graduate advisor, professor of physics and vice chancellor for research, innovation and economic development at UC Merced. "The number of star clusters currently observed, the so-called 'cluster abundance', is very sensitive to cosmological conditions, especially the amount of matter."
Anatoly Klypin of the University of Virginia said: "The higher the proportion of total matter in the universe, the more star clusters will be formed. But it is difficult to accurately measure the mass of any galaxy cluster because most of the matter is dark matter, which we cannot see directly with telescopes."
To overcome this difficulty, the team had to use an indirect tracker of galaxy cluster masses. They rely on the fact that more massive star clusters contain more galaxies than less massive star clusters (mass richness relationship: MRR). Since galaxies are made up of luminous stars, the number of galaxies in each cluster can be used to indirectly determine its total mass. By measuring the number of galaxies in each cluster in the Sloan Digital Sky Survey sample, the team was able to estimate the total mass of each cluster. They then compared the observed number and mass of galaxy clusters per unit volume with those predicted by numerical simulations.
The best fit between the observational results and the simulation results is that the universe is composed of 31% of the total matter. This value is in good agreement with the Cosmic Microwave Background (CMB) observations of the Planck satellite. It is worth noting that CMB is a completely independent technology.
Validation and Technology
Tomoaki Ishiyama of Chiba University said: "We successfully measured the density of matter using MRR for the first time, which is in good agreement with the results obtained by the Planck team using the CMB method. This work further demonstrates that cluster abundance is a competitive technique for constraining cosmological parameters and is complementary to non-cluster techniques such as CMB anisotropy, baryon acoustic oscillations, type Ia supernovae or gravitational lensing."
The team believes their results are the first to successfully use spectroscopy - a technique that separates radiation into individual bands, or colors of the spectrum - to accurately determine the distance to each cluster and the true member galaxies that are gravitationally bound to the cluster, rather than background or foreground distractors along the line of sight. Previous studies that have attempted to use MRR techniques have relied on much cruder and less precise imaging techniques, such as using photos of the sky taken at certain wavelengths, to determine the distance of each cluster to its actual member galaxies.
Conclusions and future applications
The paper, published in the Astrophysical Journal on September 13, not only demonstrates that MRR technology is a powerful tool for determining cosmological parameters, but also explains how it can be applied to new data sets obtained from large, wide-field and deep-field imaging and spectroscopic galaxy surveys such as those conducted by the Subaru Telescope, the Dark Energy Survey, the Dark Energy Spectrograph, the Euclid Telescope, the eROSITA Telescope, and the James Webb Space Telescope.