A team led by scientists at the University of California, Riverside (UCR), USA, has precisely measured the total amount of matter in the Universe.
As published in the Astrophysical Journal, the team determined 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: “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 metre.
“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 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.
Present-day galaxy clusters have formed from matter that has collapsed over billions of years, because of this, the number of clusters observed at the present time is very sensitive to cosmological conditions and the total amount of matter.
To overcome this, the UCR-led team of astronomers first developed ‘GalWeight’, a cosmological tool to measure 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 create ‘GalWCat19’, a publicly available catalogue of galaxy clusters. Finally, they compared the number of clusters in their new catalogue with simulations to determine the total amount of matter in the Universe.
By combining their measurement with those from the other teams, the UCR-led team was 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.