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

TL;DR

This paper reconstructs the early Universe's expansion history to identify geometric constraints on resolving the Hubble tension, revealing that a smooth, 15% expansion rate increase before recombination can address the discrepancy.

Contribution

It provides a model-independent analysis showing early-time solutions with a smooth transition can resolve the Hubble tension within geometric constraints.

Findings

Purely early-time solutions exist at the background level.

A 15% expansion rate enhancement before recombination is required.

The results serve as a blueprint for future Hubble tension models.

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.