Hubble Tension Persists as Coma Cluster Highlights Distance Discrepancies
One of the key questions in modern cosmology is determining the precise value of the Hubble parameter (H₀), which describes the linear relationship between an object's recession velocity and its distance. In principle, one could measure various objects in the sky and perform a linear regression to find its value. Indeed, such measurements identify a H₀ ≈ 73–74 km/s/Mpc. However, more recent observations by the Planck satellite, which mapped the cosmic microwave background (CMB), predicts a lower value of H₀ ≈ 67.4 ± 0.5 km/s/Mpc when evolved to the present using our most reliable cosmological model, known as ΛCDM. This discrepancy has come to be known as the Hubble tension.
This discrepancy persists, as new research led by Daniel Scolnic provides an even higher value for the Hubble parameter. The study leverages recently released data collected by the Dark Energy Spectroscopic Instrument (DESI), which gathered the spectra of galaxies located within the Coma cluster. This data set also included distance estimates based on the Fundamental Plane (FP), which empirically relates the relative velocity of galaxies within a cluster to their surface brightness and angular radius. This relationship can be used to derive intrinsic properties of a galaxy, allowing the team to determine rough distance measures to each target, as well as an uncallibrated estimate for the Hubble parameter.
Scolnic et al. expanded on these findings by calibrating the DESI data with Type Ia supernovae to obtain a reliable distance to the Coma cluster. The team used a historic dataset of 13 high-quality supernovae, selected from a potential sample of 32 that had occurred in the cluster over the past few decades. These supernovae were used to determine a mean intrinsic brightness, which, when compared to their observed brightness, enabled a precise determination of the cluster's distance. The results were further refined by incorporating the Hubble Space Telescope distance ladder for callibration. Their work yields a distance to the Coma cluster of 98.5 ± 2.2 Mpc, consistent with previous measurements but with significantly higher precision. This in turn corresponds to a Hubble parameter of 76.5 ± 2.2 km s−1 Mpc−1 based on the DESI FP relation.
This value reinforces that the discrepancy between local measurements and cosmological predictions for the Hubble constant is a broader issue, not confined to any single method. As more data from upcoming surveys, such as those from JWST and continued spectroscopic follow-ups of supernovae in clusters like Coma, become available, the uncertainty in these distance measurements should decrease, potentially shedding new light on the nature of the Hubble tension.
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Journal Source: D. Scolnic et al, The Hubble Tension in Our Own Backyard: DESI and the Nearness of the Coma Cluster, The Astrophysical Journal Letters, Vol. 979, No. 1, (2025), DOI:https://doi.org/10.3847/2041-8213/ada0bd
Cover Image Credit: Marilyn Chung/Lawrence Berkeley National Laboratory