Determining the Hubble Constant without the Sound Horizon Scale: Measurements from CMB Lensing

TL;DR

This paper explores how CMB lensing measurements can constrain the Hubble constant independently of the sound horizon scale, offering a potential way to address existing tensions in cosmological measurements.

Contribution

It introduces a method to determine H0 from CMB lensing data without relying on the sound horizon scale, providing an alternative approach to resolve measurement tensions.

Findings

Current CMB lensing data yields H0 = 73.5 ± 5.3 km/s/Mpc.

Future CMB surveys may improve constraints but face limitations.

Galaxy power spectrum methods could enhance future measurements.

Abstract

Measurements of the Hubble constant, $H_0$, from the cosmic distance ladder are currently in tension with the value inferred from Planck observations of the CMB and other high redshift datasets if a flat $\Lambda$CDM cosmological model is assumed. One of the few promising theoretical resolutions of this tension is to invoke new physics that changes the sound horizon scale in the early universe; this can bring CMB and BAO constraints on $H_0$ into better agreement with local measurements. In this paper, we discuss how a measurement of the Hubble constant can be made from the CMB without using information from the sound horizon scale, $r_s$. In particular, we show how measurements of the CMB lensing power spectrum can be used to place interesting constraints on $H_0$ when combined with measurements of either supernovae or galaxy weak lensing, which constrain the matter density parameter. The constraints arise from the sensitivity of the CMB lensing power spectrum to the horizon scale at matter-radiation equality (in projection); this scale could have a different dependence on new physics than the sound horizon. From an analysis of current CMB lensing data from Planck and Pantheon supernovae with conservative external priors, we derive an $r_s$-independent constraint of $H_0 = 73.5\pm 5.3$ km/s/Mpc. Forecasts for future CMB surveys indicate that improving constraints beyond an error of $\sigma(H_0) = 3$ km/s/Mpc will be difficult with CMB lensing, although applying similar methods to the galaxy power spectrum may allow for further improvements.