Searching for motivation: new physics and the Hubble tension

In recent years, discrepancies between CMB-inferred and local measurements of certain cosmological parameters have begun to appear. These tensions may ultimately be due to some additional ingredient beyond the Lambda-CDM model.

In this talk, we focus on the tension on the value of the Hubble expansion parameter between early and late time observations. 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.