Astrophysicists Measure Total Amount of Matter in Universe | Astronomy

Using data on a sample of 756 galaxy clusters identified in the Sloan Digital Sky Survey (SDSS), a team of astrophysicists from the United States and Egypt has determined that matter makes up 31% of the total amount of matter and energy in the Universe, with the remainder consisting of dark energy.

Abdullah et al. determined that matter makes up about 31% of the total amount of matter and energy in the Universe. Image credit: Mohamed Abdullah, University of California Riverside.

“To put that amount of matter in context, if all the matter in the Universe were spread out evenly across space, it would correspond to an average mass density equal to only about six hydrogen atoms per cubic meter,” said study first author Mohamed Abdullah, a graduate student in the Department of Physics and Astronomy at the University of California Riverside and the Department of Astronomy at the Egypt’s National Research Institute of Astronomy and Geophysics.

“However, since we know 80% of matter is actually dark matter, in reality, most of this matter consists not of hydrogen atoms but rather of a type of matter which cosmologists don’t yet understand.”

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 own gravity, the number of clusters observed at the present time is very sensitive to cosmological conditions and, in particular, the total amount of matter.

“A higher percentage of matter would result in more clusters,” Abdullah said.

“The ‘Goldilocks’ challenge for our team was to measure the number of clusters and then determine which answer was ‘just right’.”

“But it is difficult to measure the mass of any galaxy cluster accurately because most of the matter is dark so we can’t see it with telescopes.”

To overcome this difficulty, Abdullah and colleagues developed a cosmological tool, dubbed GalWeight, to measure the mass of a galaxy cluster using the orbits of its member galaxies.

The researchers applied their tool to SDSS observations to create a catalog of 1,800 galaxy clusters.

They then compared the data on 756 clusters from their catalog with simulations.

By combining their measurements with those from the other teams that used different techniques, they were able to determine a best combined value, concluding that matter makes up 31.5±1.3% of the total amount of matter and energy in the Universe.

“We have succeeded in making one of the most precise measurements ever made using the galaxy cluster technique,” said study co-author Professor Gillian Wilson, a researcher in the Department of Physics and Astronomy at the University of California Riverside.

“Moreover, this is the first use of the galaxy orbit technique which has obtained a 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.”

“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,” said co-author Dr. Anatoly Klypin, a researcher in the Astronomy Department at New Mexico State University and the Department of Astronomy at the University of Virginia.

The team’s results appear in the Astrophysical Journal.


Mohamed H. Abdullah et al. 2020. Cosmological Constraints on Ω m and σ 8 from Cluster Abundances Using the GalWCat19 Optical-spectroscopic SDSS Catalog. ApJ 901, 90; doi: 10.3847/1538-4357/aba619

This article is based on a press-release provided by the University of California Riverside.

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