Novel method puts a number on universe's matter and energy

This mysterious dark matter continues to puzzle scientists, who hypothesize it comprises yet-to-be-discovered subatomic particles.

The science of counting galaxies

The study, explained in a press release, breaks new ground by leveraging a well-established technique involving galaxy clusters to pin down the universe's matter density. "The total number of these clusters—called 'cluster abundance'—is a sensitive indicator of the amount of matter in the universe," says co-author Gillian Wilson, Professor of Physics at UC Merced.

Anatoly Klypin from the University of Virginia also notes a challenge in this area. He points out that accurately determining the mass of a galaxy cluster is difficult, particularly because the majority of the matter is dark and, therefore, not detectable by telescopes.

Mass-richness relation (MRR)

To bypass this challenge, the research team turned to an ingenious solution: counting the number of galaxies in each cluster as a proxy for its mass. Known as mass-richness relation (MRR), this method allowed them to extrapolate the total mass of the galaxy clusters under observation. Data for this innovative technique came from the Sloan Digital Sky Survey.

Tomoaki Ishiyama, a key contributor from Chiba University, stated that by employing MRR, they could juxtapose observational findings with numerical models. The results produced an optimal alignment, confirming that the universe is made up of 31% matter. These findings align with observations of the cosmic microwave background (CMB) made by the Planck satellite.

What sets this study apart is its pioneering use of spectroscopy—a technique that separates radiation into different bands or colors—to precisely measure the distance to each cluster and identify the galaxies genuinely part of it. This eliminates background or foreground noise, enabling a higher level of accuracy than ever before.

Shaping the future of cosmology

The team's success marks a paradigm shift, proving that the MRR technique is a formidable ally in our quest to understand the universe. It promises to enhance our knowledge further when applied to upcoming deep-field imaging and spectroscopic galaxy surveys, such as those conducted with the Subaru Telescope, Dark Energy Survey, Dark Energy Spectroscopic Instrument, Euclid Telescope, eROSITA Telescope, and the much-anticipated James Webb Space Telescope.

Study abstract:

The cluster mass–richness relation (MRR) is an observationally efficient and potentially powerful cosmological tool for constraining the matter density Ωm and the amplitude of fluctuations σ8 using the cluster abundance technique. We derive the MRR relation using GalWCat19, a publicly available galaxy cluster catalog we created from the Sloan Digital Sky Survey-DR13 spectroscopic data set. In the MRR, cluster mass scales with richness as \mathrm{log}{M}_{200}=\alpha +\beta \mathrm{log}{N}_{200}. We find that the MRR we derive is consistent with both the IllustrisTNG and mini-Uchuu cosmological numerical simulations, with a slope of β ≈ 1. We use the MRR we derived to estimate cluster masses from the GalWCat19 catalog, which we then use to set constraints on Ωm and σ8. Utilizing the all-member MRR, we obtain constraints of Ωm = {0.31}_{-0.03}^{+0.04} and σ8 = {0.82}_{-0.04}^{+0.05}, and utilizing the red member MRR only, we obtain Ωm = {0.31}_{-0.03}^{+0.04} and σ8 = {0.81}_{-0.04}^{+0.05}. Our constraints on Ωm and σ8 are consistent and very competitive with the Planck 2018 results.