Early Universe Physics Insensitive and Uncalibrated Cosmic Standards: Constraints on Ω<sub>m</sub> and Implications for the Hubble Tension
Weikang Lin, Xingang Chen, Katherine J. Mack
Abstract
Abstract To further gain insight into whether pre-recombination models can resolve the Hubble tension, we explore constraints on the evolution of the cosmic background that are insensitive to early universe physics. The analysis of the 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, 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 Ω m = 0.302 ± 0.008 (68% C.L.), assuming a flat Λ cold dark matter universe after recombination. When we combine these with other nonlocal 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 nonlocal, observations, and the local measurements that cannot be resolved by modifying pre-recombination early universe physics.