The Hubble Tension Is Hard To Resolve

The simplest solutions to the disparities in measurements of the Hubble constant at early and late times in the history of the universe probably won't work.

The Hubble tension has now grown to a level of significance which can no longer be ignored and calls for a solution which, despite a huge number of attempts, has so far eluded us. Significant efforts in the literature have focused on early-time modifications of ΛCDM, introducing new physics operating prior to recombination and reducing the sound horizon. 

In this opinion paper I argue that early-time new physics alone will always fall short of fully solving the Hubble tension. I base my arguments on seven independent hints, related to 1) the ages of the oldest astrophysical objects, 2) considerations on the sound horizon-Hubble constant degeneracy directions in cosmological data, 3) the important role of cosmic chronometers, 4) a number of ``descending trends'' observed in a wide variety of low-redshift datasets, 5) the early integrated Sachs-Wolfe effect as an early-time consistency test of ΛCDM, 6) early-Universe physics insensitive and uncalibrated cosmic standard constraints on the matter density, and finally 7) equality wavenumber-based constraints on the Hubble constant from galaxy power spectrum measurements. 

I argue that a promising way forward should ultimately involve a combination of early- and late-time (but non-local -- in a cosmological sense, i.e. at high redshift) new physics, as well as local (i.e. at z∼0) new physics, and I conclude by providing reflections with regards to potentially interesting models which may also help with the S8 tension.

Sunny Vagnozzi, "Seven hints that early-time new physics alone is not sufficient to solve the Hubble tension" arXiv:2308.16628 (August 31, 2023) (accepted for publication in Universe).