Scientists precisely measure the total amount of matter in the universe

UC Riverside-led team’s technique relied on determining the mass of galaxy clusters.


80% of matter is dark matter, in reality, most of this matter consists not of hydrogen atoms but instead of a type of matter which cosmologists don’t yet understand.

Measuring the total amount of matter in the universe is a topmost goal in cosmology.

In a new study, a team led by scientists at the University of California, Riverside, has measured the universe’s total amount of matter. Scientists reported that matter makes up 31% of the total amount of matter and energy in the universe, with the remainder consisting of dark energy.

First author Mohamed Abdullah, a graduate student in the UCR Department of Physics and Astronomy, said, “one well-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. Because present-day galaxy clusters have formed from matter that has collapsed over billions of years under its gravity, the number of clusters observed at present is susceptible to cosmological conditions and, in particular, the total amount of matter.”

“A higher percentage of matter would result in more clusters. The ‘Goldilocks’ challenge for our team was to measure the number of clusters and determine which answer was ‘just right.’ But it is difficult to accurately measure the mass of any galaxy cluster because most of the matter is dark, so we can’t see it with telescopes.”

Scientists overcame this difficulty by developing a cosmological tool to measure the mass of a galaxy cluster using its member galaxies’ orbits.

Scientists then applied their tool to 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 universe’s total amount of matter.

Like Goldilocks, the team compared the number of galaxy clusters they measured with predictions from numerical simulations to determine which answer was “just right.” (UCR/Mohamed Abdullah)

Coauthor Gillian Wilson, a physics and astronomy professor at UCR in whose lab Abdullah works, said, “We have succeeded in making one of the most precise measurements ever made using the galaxy cluster technique. Moreover, this is the first use of the galaxy orbit technique, which has obtained value in agreement with those obtained by teams who used non-cluster techniques such as cosmic microwave background anisotropies, baryon acoustic oscillations, Type Ia supernovae, or gravitational lensing.”

The third coauthor Anatoly Klypin said“A huge advantage of using our GalWeight galaxy orbit technique was that our team was able to determine a mass for each cluster individually rather than rely on more indirect, statistical methods.”

Scientists later combined their measurements with those from the other teams. They determined that matter makes up 31.5±1.3% of the universe’s total amount of matter and energy. 

Journal Reference:
  1. Mohamed H. Abdullah et al. Cosmological Constraints on Ω m and σ eight from Cluster Abundances Using the GalWCat19 Optical-spectroscopic SDSS Catalog. DOI: 10.3847/1538-4357/aba619


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