A Quantitative Test of the No-Hair Theorem with Sgr A* using stars, pulsars, and the Event Horizon Telescope

TL;DR

This paper discusses a comprehensive approach to testing the no-hair theorem of black holes by combining stellar, pulsar, and horizon-scale observations of Sgr A* to improve measurement accuracy.

Contribution

It proposes a multi-faceted observational strategy combining different data types to rigorously test the black hole no-hair theorem.

Findings

Combined observations reduce systematic biases.

Different measurement methods have nearly orthogonal uncertainties.

Approach enhances the precision of black hole property measurements.

Abstract

The black hole in the center of the Milky Way, Sgr A*, has the largest mass-to-distance ratio among all known black holes in the Universe. This property makes Sgr A* the optimal target for testing the gravitational no-hair theorem. In the near future, major developments in instrumentation will provide the tools for high-precision studies of its spacetime via observations of relativistic effects in stellar orbits, in the timing of pulsars, and in horizon-scale images of its accretion flow. We explore here the prospect of measuring the properties of the black-hole spacetime using all these three types of observations. We show that the correlated uncertainties in the measurements of the black-hole spin and quadrupole moment using the orbits of stars and pulsars are nearly orthogonal to those obtained from measuring the shape and size of the shadow the black hole casts on the surrounding emission. Combining these three types of observations will, therefore, allow us to assess and quantify systematic biases and uncertainties in each measurement and lead to a highly accurate, quantitative test of the gravitational no-hair theorem.