Hubble tension

• ArXiv:2406.15548

Given the abundance of precision cosmological data now available, modifications to the standard ΛCDM that do not conflict with observations are surprisingly difficult to realize. The “least unlikely” type of modification that is allowed (Knox & Millea 2020) increases the expansion rate of the Universe before recombination. This modification decreases the physical sound horizon measured by the CMB, requiring an attendant decrease in the distance to the last scattering that is obtained through an increase in 𝐻0. One possible mechanism for achieving this increased expansion at early times is “Early Dark Energy” (EDE), where a new cosmological energy source with an equation of state similar to dark energy contributes ∼ 10% of the critical density at its time of maximal contribution (𝑧 ∼ 3000; see Poulin et al. 2023 for a recent review).