Measuring the quantity – or rather the total proportion – of matter within the Universe represents to this day one of the main challenges of cosmology. Achieving this goal is far from simple and poses many challenges for the best astrophysicists, physicists and mathematicians active in the field. Recently, a team led by researchers from the University of California at Riverside, achieved this goal. The researchers determined that matter accounts for 31.5% of the total amount of matter and energy in the Universe, with the remainder being dark energy.
« To put this amount of matter into context, if all the matter in the universe were evenly distributed in space, it would correspond to an average mass density of only about six hydrogen atoms per cubic meter. Said lead author of the study, Mohamed Abdullah, a graduate student in the Department of Physics and Astronomy at UCR.
« However, since we know that 80% of matter is actually dark matter, in reality most of this matter is not made up of hydrogen atoms but rather of a type of matter that cosmologists do not understand yet “. The study was published on September 25 in the journal The Astrophysical Journal.
How do astronomers calculate the total amount of matter?
A well-proven technique for determining the total amount of matter in the universe is to compare the number and mass of observed galaxy clusters per unit volume with predictions from numerical simulations.
Because today’s galaxy clusters have formed from matter that collapsed over billions of years under its own gravity, the number of clusters seen today is very sensitive to cosmological conditions and, in in particular, to the total amount of material. ” A higher percentage of material would result in more clusters “Said Abdullah. ” The challenge for our team was to measure the number of clusters and determine which was the correct answer. ».
But it’s difficult to accurately measure the mass of a cluster of galaxies, because the majority of matter is dark, so we can’t see it with our telescopes. To overcome this difficulty, the team of astronomers led by the UCR first developed “GalWeight”, a cosmological tool for measuring the mass of a cluster of galaxies using the orbits of its member galaxies.
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A comparison with simulations
The researchers then applied their tool to observations from the Sloan Digital Sky Survey (SDSS) to create “GalWCat19”, a catalog of galaxy clusters accessible to the public. Finally, they compared the number of clusters in their new catalog with simulations to determine the total amount of matter in the Universe.
« A huge benefit of using our GalWeight galaxy orbit technique is that our team was able to determine a mass for each cluster individually rather than relying on more indirect statistical methods. Said co-author Anatoly Klypin, an expert in numerical simulations and cosmology.
By combining their measurements with those of other teams that used different techniques, the team led by the RCU was able to determine the best combined value, concluding that material represents 31.5 ± 1.3% of the total amount of material. and energy in the Universe.
Source : The Astrophysical Journal
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