Galactic-center S-Stars as a prospective test of the Einstein Equivalence Principle

TL;DR

This paper proposes using spectroscopic and astrometric observations of S-stars near pericenter in the Galactic center to test the Einstein Equivalence Principle with existing instruments, leveraging relativistic effects at high speeds.

Contribution

It introduces a novel observational method to test the Einstein Equivalence Principle using S-stars' relativistic orbits around the Galactic center.

Findings

Spectroscopic monitoring can detect relativistic effects in S-star spectra.

Existing instruments are sufficient for performing this test.

The method provides a new way to verify fundamental physics in strong gravity regimes.

Abstract

The S-Stars in the Galactic-center region are found to be on near-perfect Keplerian orbits around presumably a supermassive black hole, with periods of 15-50 yr. Since these stars reach a few percent of light speed at pericenter, various relativistic effects are expected, and have been discussed in the literature. We argue that an elegant test of the Einstein equivalence principle should be possible with existing instruments, through spectroscopic monitoring of an S-star concentrated during the months around pericenter, supplemented with an already-adequate astrometric determination of the inclination. In essence, the spectrum of an S-star can be considered a heterogeneous ensemble of clocks in a freely-falling frame, which near pericenter is moving at relativistic speeds.