A top goal in cosmology is to exactly measure the entire amount of matter in universe, a difficult exercise for even the foremost mathematically proficient. A team led by scientists at University of California, Riverside, has now done just that.
“To put that quantity of matter in context, if all the matter in universe were spread-out evenly across space, it might correspond to a mean mass density adequate to only about 6 hydrogen atoms per cubic meter,” said first author Mohamed Abdullah, a graduate student in UCR Department of Physics and Astronomy. “However, since we all know 80% of matter is dark matter , actually , most of this matter consists not of hydrogen atoms but rather of a kind of matter which cosmologists don’t yet understand.”
Abdullah defined that one well-proven technique for determining the entire amount of matter in universe is to compare the observed number & 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 amount of clusters observed at this time is extremely sensitive to cosmological conditions &, especially , the entire amount of matter.
“A higher percentage of matter would result in much more clusters,” Abdullah said. “The ‘Goldilocks‘ challenge for our team was to calculate the amount of clusters then determine which answer was ‘just right.’ But it’s difficult to calculate the mass of any galaxy cluster accurately because most of the matter is dark so we can not see it with telescopes.”
To overcome this difficulty, the UCR-led team of astronomers first developed “GalWeight“, a cosmological tool to calculate the mass of a galaxy cluster using the orbits of its member galaxies. The researchers then applied their tool to observations from the Sloan Digital Sky Survey (SDSS) to make “GalWCat19,” a publicly available catalog of galaxy clusters. Finally, they compared the amount of clusters in there new catalog with simulations to find out the entire amount of matter in the universe.
“We have succeeded in making one among the foremost precise measurements ever made using the galaxy cluster technique,” said coauthor Gillian Wilson, a professor of physics & astronomy at UCR in whose lab Abdullah works. “Moreover, this is often the 1st use of the galaxy orbit technique which has obtained a value in agreement with those obtained by teams who used noncluster techniques like 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 ready to determine a mass for every cluster individually instead of believe more indirect, statistical methods,” said the 3rd coauthor Anatoly Klypin, an expert in numerical simulations & cosmology.
By combining their measurement with those from another teams that used different techniques, the UCR-led team was ready to determine a best combined value, concluding that matter makes up 31.5±1.3% of the entire amount of matter & energy in universe.
The research paper is titled “Cosmological Constraints on Ωm & σ8 from Cluster Abundances using the GalWCat19 Optical-spectroscopic SDSS Catalog.”