Thesis

Fundamental fields and strong gravity

• Call:

  IDPASC Portugal - PHD Programme 2015

• Academic Year:

  2015 / 2016

• Domains:

  General Relativity | Astrophysics

• Supervisor:

  Vitor Cardoso

• Co-Supervisor:

• Institution:

  Instituto Superior Técnico

• Host Institution:

  Instituto Superior Técnico

• Abstract:

  Einstein's theory of General Relativity (GR) celebrates its 100th anniversary in 2015 as perhaps the most elegant attempt by mankind to capture the laws of physics. Tested to exquisite precision in the weak-field regime, it displays its unique geometric structure in strong field regions. One century of peering into Einstein’s field equations has given us elegant and simple solutions, and shown how they behave when slightly displaced from equilibrium. We were rewarded with an ornate mathematical theory of black holes and their perturbations, and a machinery able to handle all weak-field phenomena. In the last few decades, the demand for an accurate knowledge of strong-field dynamical gravity grew considerably, driven by several unrelated developments at the observational, instrumental, technical and conceptual levels. Progress on novel techniques now allows us to scrutinize BHs and other compact objects, in particular the region within a few Schwarzschild radii, with radio and deep infrared interferometry, and enables us to measure BH spins more accurately than ever before using X-ray spectroscopy or continuum-fitting methods. These observations provide, for the first time since GR was conceived, detailed electromagnetic information about the strong-gravity regime, probing the nature of compact objects, and perhaps even tests of GR itself. But gravitation has many faces, going beyond astrophysics. Fundamental fields, either minimally or nonminimally coupled to curvature are essential for cosmological models, for explaining the nature of dark matter or to extend the Standard Model of particle physics. These theories include generalized scalar-tensor theories, the axiverse scenario and many others. In addition, scalar fields are often used as proxy for other, more complex interactions. The equivalence principle forces all these fields – no matter how small their coupling to standard model particles– to gravitate, and it is thus not surprising that some of the best constraints on these fields and theories arise in strong-field gravity contexts. Neutron star spontaneous scalarization and “condensation” of ultralight degrees of freedom in the vicinities of supermassive BHs are some of the smoking-gun effects of the presence of fundamental fields and/or modified theories of gravity.