Dark Sector Cosmic Fluid Interactions as a Solution to the Observed Disagreement in Measurements of the Hubble Expansion Rate

The standard cosmological model, known as the ΛCDM model, is the simplest model that accurately describes a variety of aspects of the Universe, including the cosmic microwave background (CMB), large scale structure, and accelerated expansion. Independent observational data, including data from the CMB, baryon acoustic oscillations (BAO), and supernovae type Ia (SNe Ia) provide significant statistical support for the ΛCDM model. Despite this support the Hubble expansion rate determined from these observations is inconsistent with direct measurements, presenting a tension of over 4σ. In this examination we attempt to alleviate this tension by looking at an important assumption of the ΛCDM model, the assumption that the energy densities of the different cosmic fluid components evolve independently. To test this we consider pairwise interactions between dark sector cosmic fluid components by introducing terms which allow for energy exchange between components to the right hand side of the Friedmann fluid equations. Making use of Markov Chain Monte Carlo methods we find that energy exchange between cold dark matter (CDM) and dark energy can correct for the discrepancy between CMB measurements of the Hubble expansion rate and direct measurements, but that adding the BAO measurements to the analysis prevents this tension from being fully alleviated. Our findings suggest dark sector cosmic fluid interactions are a strong candidate for physics beyond ΛCDM and warrant additional research.