THE GALACTIC CENTER: A TESTING GROUND FOR DARK MATTER AND GRAVITY THEORIES

Abstract:

The discovery of stars orbiting very close to the supermassive black hole at the Galactic Center of the Milky Way, Sagittarius A*, opened a new way to test its gravitational field and constrain the environment around it. Moreover, data collected on stars can also be combined with the near horizon images taken by the Event Horizon Telescope, producing the perfect laboratory to test the theory of gravity and constrain dark matter models.

The latter represents one of the major challenges in physics, since there are several experimental observations proving its existence in the Universe, but none of the proposed models can be seen as conclusive. In this doctoral thesis we investigate the geodesic motion, i.e., the trajectory of a test particle in a specific gravitational field, with the aim of understanding possible signatures of such a background.

When considering the strong field regime, the geodesic equation is solved to either constrain the signatures due to an hidden companion or study the luminosity of matter accreted by a Schwarzschild black hole. In the context of stars at the Galactic Center, we investigate the possibility that dark matter in the form of an ultralight field clusters around Sagittarius A* creating a cloud, and we use the available data for star S2, one of the closest to the Galactic Center, to constrain the mass of such a cloud. Moreover, we also explore the possibility that a fifth force exists in the Universe and appears as a Yukawa-like modification to the Newtonian potential in the weak field limit. By means of S2 data, the intensity of such a fifth force is constrained at the Galactic Center.