How much matter is there in the universe? An international team of researchers has succeeded in measuring this crucial quantity and thereby validating an earlier measurement. Their study shows that matter, including dark matter, accounts for 31% of all matter and energy in the universe. The rest is dark energy.
Dr. Mohamed Abdullah, a researcher at the National Research Institute of Astronomy and Geophysics in Egypt and Chiba University in Japan, says that only 20% of this matter is called “baryonic.” This includes things like stars, galaxies and atoms. The remaining 80% consists of dark matter, a still mysterious substance.
To reach these conclusions, the team used a proven methodology. They compared the number and mass of observed galaxy clusters with predictions from numerical simulations. Gillian Wilson, Abdullah’s doctoral advisor and professor of physics at UC Merced, explains that the number of these clusters depends very much on the total amount of matter in the universe.
Anatoly Klypin of the University of Virginia adds that it is difficult to accurately measure the mass of a galaxy cluster. This is primarily because most of the matter in these clusters is dark matter, which is invisible to telescopes. The team therefore had to use an indirect indicator: the number of galaxies in each cluster. The more massive a cluster is, the more galaxies it contains.
The researchers therefore estimated the total mass of each star cluster in their sample from the Sloan Digital Sky Survey. The data was consistent with a universe made up of 31% matter. Tomoaki Ishiyama of the University of Chiba says these results agree perfectly with previous observations from the Planck satellite.
The study also demonstrates the power of spectroscopy to determine the distance of each cluster and identify the galaxies that are gravitationally bound to it. This level of precision was not achievable with the more rudimentary imaging techniques used in previous studies.