The Most Precise Measurement Of How Much Matter There Is In The Universe

This photo shows the galaxy cluster Cl 0024+17 (ZwCl 0024+1652) as seen by Hubble. NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)


A classic question in astronomy is how much stuff is in the Universe? Researchers have now put a new limit on the answer.

Astronomers measure this as a fraction of the critical density of the universe, a useful parameter that is used as a benchmark to describe the geometry of the cosmos. Observations over time have shown that the total matter-energy density of the universe is very, very close to this critical density. Now a new study published in The Astrophysical Journal suggests that about 31.5 percent of that value is due to matter.

“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,” first author Mohamed Abdullah, a graduate researcher in the UC Riverside, said in a statement. “However, since we know 80 percent 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.”

Fractions of the matter-energy density of the Universe. UCR/Mohamed Abdullah

Estimating how much matter is in the Universe is not an easy task, so the team had to get creative. The number of galaxy clusters in the local universe depends on the total amount of matter. If there were more matter, there would be more clusters. The challenge then is to measure how massive those clusters are and find a total.

“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," said Abdullah.

The team developed a cosmological tool called GalWeight, which uses the motion of the galaxies in clusters to find a total mass. Those values were then inputted into a cosmological simulation from which the team was able to extract a final estimate.

“We have succeeded in making one of the most precise measurements ever made using the galaxy cluster technique,” explained co-author Gillian Wilson, a professor of physics and astronomy at UCR in whose lab Abdullah works. “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 noncluster techniques such as cosmic microwave background anisotropies, baryon acoustic oscillations, Type Ia supernovae, or gravitational lensing.”



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