Scientists at the University of California at Riverside have precisely measured the total amount of matter making up the cosmos, concluding that dark energy, the mysterious force speeding up the expansion of the universe, accounts for 69 percent of the total mass-energy budget with normal and dark matter contributing the remaining 31.5 (+/- 1.3) percent.
Some 80 percent of the latter is made up of dark matter, the nature of which is unknown, with the remaining 20 percent made up of the particles, atoms and molecules familiar to human beings as gas, dust, planets, stars and galaxies.
“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 Mohamed Abdullah, a graduate student in the UCR Department of Physics and Astronomy and lead author of a paper in the Astrophysical Journal.
“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.”
To come with an accurate measure of the mass in the universe, the researchers developed a tool known at GalWeight to measure the mass of a galaxy cluster using the orbits of its member galaxies. They then compiled a catalogue of galaxy clusters based on the Sloan Digital Sky Survey and compared the number of clusters with the results of numerical simulations.
“We have succeeded in making one of the most precise measurements ever made using the galaxy cluster technique,” said co-author Gillian Wilson, a professor of physics and astronomy at UCR. “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.”
Combining their results with those obtained by other researchers using different techniques, the UCR team was able to determine what they conclude is the best combined value of 31.5 percent.