How much “stuff” is there in the universe? You’d think that would be easy to figure out. But it’s not. Astronomers add up what they can detect and still find that there is more to the universe than meets the eye. So, what is “out there” and how do they explain it all?
According to astronomer Mohamed Abdullah (National Research Institute of Astronomy and Geophysics in Egypt and Chiba University (Japan)), the universe has dark and visible components. Matter makes up only 31 percent of the known universe. The rest is dark energy, which remains largely unknown. “Cosmologists believe that only about 20% of this total mass is made up of ordinary or ‘baryonic’ matter, which includes stars, galaxies, atoms and life,” he said. “About 80% (of all matter) is made up of dark matter, the mysterious nature of which is not yet known, but may consist of some as yet undiscovered subatomic particle.”
Determining the composition of the universe using galaxy clusters
The best measurements of the “stuff of the cosmos” come from the Planck satellite, which mapped the universe. He studied the cosmic microwave background, a remnant of radiation left over from the Big Bang, about 13.8 billion years ago. Planck’s measurements allowed astronomers to come up with a “gold standard” for measuring the total mass in the universe. However, it is always good to check against Planck using other methods.
Abdullah and a team of scientists did just that. They used a different method called the Cluster Mass-Richness Relation. Basically, it measures the number of galaxy members in a cluster to determine the mass of the cluster. According to astronomer and team member Gillian Wilson, it offers a way to measure cosmic matter. “Because current galaxy clusters formed from matter that collapsed under its own gravity over billions of years, the number of currently observed clusters, the so-called ‘cluster abundance’, is very sensitive to cosmological conditions and, in particular, the total amount of matter,” she said, noting that the method compares the observed number and mass of galaxies per unit volume with predictions from numerical simulations.
This is not an easy method because it is difficult to accurately measure the mass of any galaxy cluster. Much of the mass of the cluster is dark matter. In other words, what you see in the cluster isn’t necessarily all you get. So the team had to be smart. They took advantage of the fact that more massive clusters contain more galaxies than less massive ones. Since all galaxies have bright stars in them, the number of galaxies contained in each cluster is used to estimate the total mass. Essentially, the team measured the number of galaxies in each cluster in their sample and then used that information to estimate the total mass of each cluster.
The corresponding Planck
The result of all the measurements and simulations corresponded almost exactly to the Planck numbers for mass in the universe. They came up with a universe that is 31% matter and 69% dark energy. It also seems to agree with other work the team has done measuring galaxy masses. To get their results, Mohamed’s team was able to use spectroscopic studies of the clusters to determine their distances. The observations also allowed them to determine which galaxies were members of specific clusters.
Simulations were also essential to this work. Observations from the Sloan Digital Sky Survey allowed the team to compile a catalog of galaxy clusters called “GalWeight”. They then compared the clusters in the catalog with their simulations. The result was a calculation of the total mass in the universe based on the mass abundance relation.
The technique is robust enough to be used as new astronomical data comes in from different instruments. According to Wilson, the team’s work shows that the MRR technique goes beyond their work. “The MRR technique can be applied to new data sets that are available from large wide-field and deep imaging and spectroscopic galaxy surveys such as the Dark Energy Survey, the Dark Energy Spectroscopic Instrument, the Euclid Telescope, the eROSITA Telescope and the James Webb Space Telescope,” she said. he said.
The results also show that cluster abundance is a competitive technique for constraining cosmological parameters. It also complements techniques that are not focused on clusters. These include CMB anisotropy, baryon acoustic oscillations, type Ia supernovae or gravitational lensing. Each is also a useful tool in measuring various characteristics of the universe.