Climbing out of the shadows: building the distance ladder with black hole images

TL;DR

This paper proposes using supermassive black hole shadows as a new method to measure cosmological distances, potentially improving the precision of the Hubble constant estimation without relying on traditional distance calibrators.

Contribution

It introduces a novel approach of using SMBH shadows as distance indicators, enabling independent and potentially more accurate measurements of the Hubble constant.

Findings

Hubble constant can be constrained with ~10% precision using current ground-based interferometers.

Future interferometers could achieve ~4% precision in Hubble constant measurements.

SMBH shadow observations offer a promising alternative to traditional distance ladder methods.

Abstract

In the era of precision cosmology it has became crucial to find new and competitive probes to estimate cosmological parameters, in an effort of finding answers to the current cosmological tensions/discrepancies. In this work, we show the possibility of using observations of Super Massive Black Hole (SMBH) shadows as an anchor for the distance ladder, substituting the sources usually exploited for such purpose, such as Cepheid variable stars. Compared to the standard approaches, the use of SMBH has the advantage of not needing to be anchored with distance calibrators outside the Hubble flow since the shadows physical size can be estimated knowing the mass of the SMBH. Furthermore, SMBH are supposed to inhabit the center of all galaxies which, in principle, means that we can measure the size of the shadows in any Supernova type Ia host galaxy. Under the assumption that the mass of the SMBH can be accurately and reliably estimated, we find that the Hubble constant can be constrained with a $\approx10\%$ precision even considering current experimental design of ground-based interferometers. By constructing a SMBH catalogue based on a specific choice of the SMBH Mass Function (BHMF), we forecast the constraints on the Hubble constant, finding that a precision of $\approx4\%$ may be within reach of future interferometers.