Spinning Black Holes with Scalar Hair and Horizonless Compact Objects within and beyond General Relativity

Understand

The last years have brought upon us a golden age of observational gravitational physics.

• The several observations by the LIGO/Virgo/KAGRA collaboration about gravitational waves and by the EHT collaboration about the shadow and lensing of light around the supermassive black hole in the centre of M87 will point the scientific community in the correct direction to find an answer to the Kerr hypothesis.
• In order to follow that direction, the systematic construction and analysis of the physical properties of solutions within General Relativity with additional fields or within modified theories of gravity is necessary.
• In this thesis, we shall provide such construction and analysis for compact objects within (complex-)Einstein-Klein-Gordon theory with various scalar potentials and within a particular scalar-tensor theory -- the shift-symmetric Horndesky theory.