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Theory Group
Theory Group Seminar , 24 January 2023

Michael Toomey

Addressing the Hubble and S8 Tensions with a Kinetically Mixed Dark Sector


We are now in the era of precision cosmology and have reached the point where there is no longer cosmological concordance in the ΛCDM model between data sets spanning cosmic history. Nowhere is this best exemplified than in the discrepancy between the expansion rate today, the so-called “Hubble constant” H0, as directly measured by late-Universe probes and that inferred from the early-Universe using the standard model. With recent measurements putting the tension in excess of 5σ and countless models put forth at restoring concordance, there is a strong impetus to narrow and improve on the most promising candidates. Arguably, the most successful model to-date is Early Dark Energy (EDE) which posits a new very light scalar field that becomes dynamical around recombination. Unfortunately, while this model is successful at reducing the tension in the expansion rate, it suffers from two major shortcomings. First, if the potential of the scalar field is to have the correct late-Universe dynamics it must be fine-tuned and secondly, the model exacerbates the S8 (large-scale structure) tension. In this work we will present a new kinetically mixed dark sector model (KMIX) which, inspired by the most general string theory axions, is theoretically well motivated and requires no fine tuning. The axion in our theory, which behaves similarly to EDE around recombination, also plays a role in the late-time Universe by modifying the growth of structure. Thus, our model naturally has the ingredients necessary for alleviating the S8 and H0 tensions simultaneously.

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