Early-Universe-Physics Insensitive and Uncalibrated Cosmic Standards: Constraints on $\Omega_{\rm{m}}$ and Implications for the Hubble Tension

TL;DR

This paper introduces a method to constrain the cosmic background evolution using early-universe-insensitive standards, providing robust measurements of matter density and implications for the Hubble tension.

Contribution

It demonstrates that certain cosmic measurements are insensitive to early-universe physics, enabling more robust constraints on cosmological parameters like m and H0.

Findings

Constraints on m consistent with Planck 2018

Reduced sensitivity of H0 to early-universe physics

Tension persists between post-recombination and local H0 measurements

Abstract

To further shed light on whether pre-recombination models can resolve the Hubble tension, we explore constraints on the cosmic background evolution that are insensitive to early-universe physics. The analysis of the cosmic microwave background (CMB) anisotropy has been thought to highly rely on early-universe physics. However, we show that the fact that the sound horizon at recombination being close to that at the end of the drag epoch is insensitive to early-universe physics. This allows us to link the absolute sizes of the two horizons and treat them as free parameters. Jointly, the CMB peak angular size, Baryon Acoustic Oscillations (BAO), and Type Ia supernovae can be used as "early-universe-physics insensitive and uncalibrated cosmic standards", which measure the cosmic history from recombination to today. They can set strong and robust constraints on the post-recombination cosmic background, especially the matter density parameter with $\Omega_{\rm{m}}=0.302\pm0.008$ ($68\%$ C.L.) assuming a flat $\Lambda$CDM after recombination. When we combine these with other non-local observations, we obtain several constraints on $H_0$ with significantly reduced sensitivity to early-universe physics. These are all more consistent with the Planck 2018 result than the local measurement results such as those based on Cepheids. This suggests a tension between the post-recombination, but non-local, observations and the local measurements which cannot be resolved by modifying pre-recombination early universe physics.