$H_0$ Tension on the Light of Supermassive Black Hole Shadows Data

TL;DR

This paper explores how supermassive black hole shadows can serve as a new, independent probe to measure the Hubble constant, potentially reducing systematic uncertainties and improving precision in cosmological parameter estimation.

Contribution

It proposes a method to use black hole shadow observations to constrain $H_0$, breaking parameter degeneracies and achieving higher precision than current techniques.

Findings

Black hole shadows can be characterized by angular size and mass.

Forecasting at high redshifts can reduce measurement uncertainty to about 4%.

Method can improve $H_0$ measurement precision in future experiments.

Abstract

Cosmological tensions in current times have opened a wide door to study new probes to constrain cosmological parameters, specifically, to determine the value of the Hubble constant $H_0$ through independent techniques. The two standard methods to measure/infer $H_0$ rely on: (i) anchored observables for the distance ladder, and (ii) establishing the relationship of the $H_0$ to the angular size of the sound horizon in the recombination era assuming a standard Cosmological Constant Cold Dark Matter ($\Lambda$CDM) cosmology. However, the former requires a calibration with observables at nearby distances, while the latter is not a direct measurement and is model-dependent. The physics behind these aspects restrains our possibilities in selecting a calibration method that can help minimise the systematic effects or in considering a fixed cosmological model background. Anticipating the possibility of deeply exploring the physics of new nearby observables such as the recently detected black hole shadows, in this paper we propose standard rules to extend the studies related to these observables. Supermassive black hole shadows can be characterised by two parameters: the angular size of the shadow and the black hole mass. We found that it is possible to break the degeneracy between these parameters by forecasting and fixing certain conditions at high(er) redshifts, i.e., instead of considering the $\approx$10\% precision from the EHT array, our results reach a $\approx 4\%$, a precision that could be achievable in experiments in the near future.