Geometric Constraints on the Pre-Recombination Expansion History from the Hubble Tension

Abstract

I perform a model-independent reconstruction of the background pre-recombination expansion history of the Universe. I find that purely early-time resolutions to the Hubble tension, satisfying the geometric CMB constraints, exist at the background level. This class of solutions requires a smooth transition around matter-radiation equality, characterized by a $\simeq 15\%$ expansion rate enhancement prior to recombination. This result serves as a blueprint for future model-building approaches, providing a background stress-test for Hubble tension proposals.

Comments: 5 pages, 1 figure (+ supplementary material)