Combining pre- and post-recombination new physics to address cosmological tensions: case study with varying electron mass and sign-switching cosmological constant

TL;DR

This study investigates whether combining pre- and post-recombination new physics, specifically varying electron mass and a sign-switching cosmological constant, can resolve the Hubble tension, ultimately finding it unsuccessful but providing valuable insights.

Contribution

The paper presents a concrete case study of combining two different new physics models to address the Hubble tension and analyzes why this approach fails, offering general lessons for future model building.

Findings

No successful solution to the Hubble tension was found with the combined models.

The matter density parameter $oldsymbol{ extOmega_m}$ critically influences the models' effects.

Lessons on parameter space and the importance of $oldsymbol{ extOmega_m}$ in tension resolution were derived.

Abstract

It has recently been argued that the Hubble tension may call for a combination of both pre- and post-recombination new physics. Motivated by these considerations, we provide one of the first concrete case studies aimed at constructing such a viable combination. We consider models that have individually worked best on either end of recombination so far: a spatially uniform time-varying electron mass leading to earlier recombination (also adding non-zero spatial curvature), and a sign-switching cosmological constant inducing an AdS-to-dS transition within the $\Lambda_{\rm s}$CDM model. When confronted against Cosmic Microwave Background (CMB), Baryon Acoustic Oscillations, and Type Ia Supernovae data, we show that no combination of these ingredients can successfully solve the Hubble tension. We find that the matter density parameter $\Omega_m$ plays a critical role, driving important physical scales in opposite directions: the AdS-to-dS transition requires a larger $\Omega_m$ to maintain the CMB acoustic scale fixed, whereas the varying electron mass requires a smaller $\Omega_m$ to maintain the redshift of matter-radiation equality fixed. Despite the overall failure, we use our results to draw general model-building lessons, highlighting the importance of assessing tension-solving directions in the parameter space of new physics parameters and how these correlate with shifts in other standard parameters, while underscoring the crucial role of $\Omega_m$ in this sense.