Thesis

Applications of modified gravity in high energy astroparticle physics and cosmology

• Call:

  PT-CERN Call 2022/1

• Academic Year:

  2022

• Domain:

  Astroparticle Physics

• Supervisor:

  Francisco Lobo

• Co-Supervisor:

• Institution:

  FCUL (Universidade de Lisboa)

• Host Institution:

  IA - Instituto de Astrofísica e Ciências do Espaço

• Abstract:

  From the experimental side, General Relativity (GR) has been successfully tested directly in the Solar System in its weak-field, slow motion regime. Binary pulsars, most notably the Hulse-Taylor system PSR 1913 C 16, allow for indirect tests beyond the Solar System. However, strong gravity tests are still scarse and gravity is tested poorly at the scale of galaxies and clusters, where Newtonian gravity breaks . This has led to the development of several modified theories of gravity to replace galactic dark matter. In fact, the discovery that the present expansion of the universe appears to be accelerated indicates that gravity may not be described by GR at large scales. To explain the cosmic acceleration, one needs to introduce an exotic negative pressure fluid, denoted dark energy, which constitutes approximately 70% of the energy content of the universe. Dark energy is thus an ad hoc solution to the problem of the present acceleration of the universe, and alternatives such as modified gravity have been explored. In fact, a plethora of modifications of gravity have been proposed as reliable alternatives to dark matter and dark energy. In the high-energy astroparticle physics regime, modified gravity could shed light on several outstanding problems in particle physics, such as the nature of dark matter, since its thermal production in the early universe may give rise to a relic density of the same order of magnitude of the present dark matter density. Particularly relevant in these scenarios could be the role played by modified gravity, as these theories predict a thermal evolution of the universe different with respect to one based on GR. More precisely, modified gravity predicts a modification/amplification of the expansion rate of the universe with respect to standard cosmology so that the thermal relics decouple with larger relic abundances. Consequently, the correct value of the relic abundance comes out from larger annihilation cross-sections.