Constraining Modified Gravity Using Galaxy Cluster Dynamics

Abstract

Various sources of evidence, from galaxy rotation curves to the formation of cosmic structure, suggest the necessity of additional, unobserved, mass in the universe. Generally, these phenomena are attributed to "dark matter;" however, another potential approach is the modification of the theory of gravity. Modified Newtonian Dynamics (MOND) is the most popular of these and has successfully matched observations on galaxy scales and made several successful, a priori unexpected, predictions therein; however, the paradigm fails to match observations on larger scales, specifically in groups and clusters of galaxies. To rectify these tensions, various branches of MOND theory extensions have emerged; however, many of these introduce mathematical abstractions or additional free parameters which make these extensions difficult to refute or confirm. In this work, two such branches - EMOND and MOND plus Dark Matter (MOND+DM) - are challenged using observational and theoretical constraints which emerge from galaxy clusters. We apply the theoretical framework of these extensions to predict observable deviations from the LCDM paradigm, which could serve as observational tests of the MOND extensions in question. CHANDRA observations of a collection of galaxy clusters are used to constrain the asymptotic behavior of the cluster temperature profiles in each MOND branch. The assumption of hydrostatic equilibrium implies very tight constraints on the MOND phenomenology and introduces some non-trivial points of failure in both branches. We demonstrate that the MOND+DM paradigm may constrained by the imposition of hydrostatic equilibrium in galaxy clusters and several results are derived to that effect. We show that these constraints could be tested with upcoming X-ray missions, including XRISM. Furthermore, we illustrate significant inconsistencies in the EMOND paradigm which suggest that extension to be non-viable in clusters of galaxies. Modified gravity theories in the MOND research program continue to produce intriguing results on galaxy scales; however, classical theories of MOND gravity continue to fail on the cluster scale. In galaxy clusters, this work has demonstrated that MOND extensions make refutable predictions which should be detectable with current or upcoming observational technologies, signifying that each of these theories could be tested by observing missions. While the failure of these extensions doesn't necessarily indicate that MOND is fundamentally flawed, showing that these extensions fail to produce viable results clearly places the burden of progression on MOND theorists to establish MOND paradigms in which such phenomenology is no longer predicted.

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Recommended citation: Diggins, Eliza C. (2024). “Constraining Modified Gravity Using Galaxy Cluster Dynamics.” Retrieved from eliza-diggins.github.io.