Searching for motivation: new physics and the Hubble tension

In recent years, discrepancies between CMB-inferred and local measurements of certain cosmological parameters have appeared. These tensions may ultimately be due to some additional physics beyond what is described by the Lambda-CDM (Lambda-Cold Dark Matter) model. In this talk, we focus on the tension between early and late time measurements of the Hubble expansion parameter. After reviewing the basics and most popular solutions, we introduce two well-motivated scenarios which rely on light scalar fields and address some shortcomings of early dark energy constructions.

The first scenario features a scalar field non-minimally coupled to gravity, which induces a change in the value of the Newton constant from the early epochs to today. Crucially, such a field naturally undergoes a dynamical transition around the epoch of matter-radiation equality, in contrast to other scenarios which suffer from a coincidence problem. Secondly, we discuss an ultra-light axion model, with resonant decay to dark gauge fields. This scenario enjoys the axionic shift symmetry without requiring non-standard axion potentials. We present fits to cosmological data for both models and discuss their limitations.

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