A flexible parameterization to test early physics solutions to the Hubble tension with future CMB data

TL;DR

This paper proposes a flexible, model-agnostic framework to detect early-universe physics modifications that could resolve the Hubble tension, using future CMB data with high precision.

Contribution

It introduces a four-parameter fluid model capturing deviations from $ ext{Lambda}$CDM relevant before recombination, enabling robust testing of new physics scenarios.

Findings

Forecasts constraints with Simons Observatory data.

Demonstrates potential to detect a range of models.

Shows consistency with Planck data but allows for high Hubble values.

Abstract

One approach to reconciling local measurements of a high expansion rate with observations of acoustic oscillations in the CMB and galaxy clustering (the "Hubble tension") is to introduce additional contributions to the $\Lambda$CDM model that are relevant before recombination. While numerous possibilities exist, none are currently well-motivated or preferred by data. However, future CMB experiments, which will measure acoustic peaks to much smaller scales and resolve polarization signals with higher signal-to-noise over large sky areas, should detect almost any such modification at high significance. We propose a model-agnostic method to capture most relevant possible deviations from $\Lambda$CDM due to additional non-interacting components, while remaining sufficiently constraining to enable detection across various scenarios. The phenomenological model uses a fluid model with four parameters governing additional density contributions that peak at different redshifts, and two sound speed parameters. We forecast possible constraints with Simons Observatory, explore parameter degeneracies that arise in $\Lambda$CDM, and demonstrate that this method could detect a range of specific models. Which of the new parameters gets excited can indicate the nature of any new physics, while the generality of the model allows for testing with future data in a way that should not be plagued by a posteriori choices or publication bias. When testing our model with Planck data, we find good consistency with the $\Lambda$CDM model, but the data also allows for large Hubble parameter, especially if the sound speed of an additional component is not too different to that of radiation.