Distant foreground and the Hubble constant tension

TL;DR

This paper explores how a distant foreground, like cold grey dust, could contaminate the CMB and explain the discrepancy between local and Planck-derived measurements of the Hubble constant, by increasing CMB fluctuation dispersion.

Contribution

It introduces the idea that distant foreground contamination, specifically cold grey dust, can alter CMB measurements and potentially resolve the Hubble constant tension.

Findings

Increased CMB dispersion reduces the estimated Hubble constant.

Distant grey dust could be an undetectable contaminant affecting cosmological parameters.

This mechanism offers a possible explanation for the Hubble tension.

Abstract

It is possible to explain the discrepancy (tension) between the local measurement of the cosmological parameter $H_0$ (the Hubble constant) and its value derived from the Planck-mission measurements of the Cosmic Microwave Background (CMB) by considering contamination of the CMB by emission from some medium surrounding distant extragalactic sources (a distant foreground), such as extremely cold coarse-grain (grey) dust. As any other foreground, it would alter the CMB power spectrum and contribute to the dispersion of CMB temperature fluctuations. By generating random samples of CMB with different dispersions, we have checked that the increased dispersion leads to a smaller estimated value of $H_0$, the rest of the cosmological model parameters remaining fixed. This might explain the reduced value of the {\it Planck}-derived parameter $H_0$ with respect to the local measurements. The cold grey dust for some time has been suspected to populate intergalactic space and it is known to be almost undetectable, except for the effect of dimming remote extragalactic sources.