Gravitational Theories near the Galactic Center

Abstract

Abstract Upcoming Extremely Large Telescopes (ELTs) are promising probes of gravity in or near the galactic center (GC). Effects of alternative theories of gravity, namely the Brans–Dicke theory (BDT) and f(R) gravity, are studied near the GC black hole by calculating departure from general relativity (GR) in periastron advance of the S stars and light deflection. For these estimations, black hole spin and quadrupole moments are taken in the ranges χ = 0.1–2.0 and , respectively. Periastron advance ( ) has been calculated for hypothetical S stars with orbital period one-fifth of S0-2 and eccentricity e = 0.8. The difference between BDT and GR ( ) lies in the range 10−3–2.3 μas yr−1, even for a large departure from GR. The difference between quadrupoles and J 2 = 2.0 lies in the range . These ranges are not only outside the astrometric capability of the ELTs, but are also contaminated by stellar perturbations. Parameter degeneracy among χ, J 2, and is discussed. For black hole–S-star distances, D LS = 100 and 50 au, the difference in light deflection between BDT and GR lies in the range , making it difficult to distinguish them. From the relation between scalaron mass, in f(R) gravity, and calculated , it is found that can form a stable “dark cloud” near the black hole. Scalarons with are found to bring close to the astrometric range of the ELTs. Prospects for these scalarons in the tests of gravity are discussed.