Measuring the Cosmic Balance: Astrophysicists Quantify Matter, Dark Matter, and Dark Energy in the Universe

A research team’s approach to gauging the mass of galaxy clusters hinges on counting their constituent galaxies. Within the realm of cosmology, a critical inquiry centers on quantifying the universe’s total matter content. A multinational consortium of scientists has now achieved this feat for the second time. As outlined in their report in The Astrophysical Journal, the team determined that matter constitutes 31% of the combined matter and energy in the universe, with dark energy constituting the remainder.

Dr. Mohamed Abdullah, the lead author and a researcher at the National Research Institute of Astronomy and Geophysics-Egypt, Chiba University, Japan, elucidates, “Cosmologists posit that merely about 20% of this total matter consists of regular or ‘baryonic’ matter, encompassing stars, galaxies, atoms, and life. The remaining 80% is dark matter, an enigmatic substance whose true nature remains undiscovered, possibly comprising as-yet-undetected subatomic particles” (refer to Figure 1).

The team employed a well-established technique to ascertain the universe’s overall matter content, which involves comparing the observed quantity and mass of galaxy clusters per unit volume with predictions derived from numerical simulations, as explained by co-author Gillian Wilson, former graduate advisor to Abdullah and Professor of Physics and Vice Chancellor for research, innovation, and economic development at UC Merced. Wilson adds, “The present-day number of observed clusters, referred to as ‘cluster abundance,’ is profoundly sensitive to cosmological conditions, particularly the total quantity of matter.”

Anatoly Klypin, from the University of Virginia, further elaborates, “A higher proportion of the universe’s total matter would result in the formation of more clusters. Nevertheless, accurately measuring the mass of any galaxy cluster is challenging, given that most of the matter is dark and remains invisible to telescopic observations.”

To surmount this challenge, the team relied on an indirect tracer of cluster mass, leveraging the fact that more massive clusters harbor a greater number of galaxies than less massive ones, a relationship known as the mass richness relation (MRR). Since galaxies are composed of luminous stars, the number of galaxies in each cluster serves as an indirect means of estimating its total mass. By quantifying the number of galaxies in each cluster within their sample from the Sloan Digital Sky Survey, the team estimated the total mass of each cluster and subsequently compared it to numerical simulation predictions.

The most congruent match between observations and simulations was achieved when the universe was deemed to consist of 31% matter, a figure harmonizing remarkably well with measurements from the Planck satellite’s cosmic microwave background (CMB) observations. Notably, the CMB approach is entirely independent.

Tomoaki Ishiyama of Chiba University emphasizes, “Our accomplishment marks the first measurement of matter density using the MRR, aligning closely with the findings of the Planck team utilizing the CMB technique. This work underscores that cluster abundance presents a robust method for constraining cosmological parameters, complementing other non-cluster techniques such as CMB anisotropies, baryon acoustic oscillations, Type Ia supernovae, and gravitational lensing.”

The team attributes their success to the innovative use of spectroscopy, a technique that disperses radiation into its constituent spectral bands or colors, for precise distance determination to each cluster and identification of the true member galaxies gravitationally associated with the cluster. This contrasts with earlier studies, which employed coarser and less precise imaging techniques, such as multi-wavelength sky pictures, to ascertain cluster distances and identify neighboring galaxies as true members.

In conclusion, the paper, published on September 13 in The Astrophysical Journal, not only highlights the potency of the MRR technique in constraining cosmological parameters but also outlines its potential application to newly available datasets from expansive, wide, and deep-field imaging and spectroscopic galaxy surveys, including those conducted by instruments such as the Subaru Telescope, Dark Energy Survey, Dark Energy Spectroscopic Instrument, Euclid Telescope, eROSITA Telescope, and the James Webb Space Telescope.

This research has received support from the IAAR Research Support Program at Chiba University, Japan, MEXT/JSPS KAKENHI (Grant Numbers JP19KK0344, JP21H01122, and JP21F51024), MEXT under the “Program for Promoting Research on the Supercomputer Fugaku” (JPMXP1020200109), JICFuS, the National Science Foundation, and NASA.

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