Thesis

Modified Gravity in Cosmology and Astroparticle Physics

• Call:

  PT-CERN Call 2021/2

• Academic Year:

  2021

• Domain:

  Astroparticle Physics

• Supervisor:

  Francisco Lobo

• Co-Supervisor:

  Pedro Avelino

• Institution:

  FCUL (Universidade de Lisboa)

• Host Institution:

  IA - Instituto de Astrofísica e Ciências do Espaço

• Abstract:

  General Relativity (GR) is facing many theoretical and experimental challenges and this might be indicative of a need for new gravitational physics. Renormalization approaches to GR clearly showed that counterterms must be introduced, which alter the theory significantly and transform the general relativistic second order field equations to higher order. This implies that extra degrees of freedom,besides the spin-two massless graviton, need to be introduced. In a more general approach, when one tries to explore gravity from the high-energy regime and obtain low-energy physics, one does not recover GR. Adopting string theory as a full theory of quantum gravity, one gets a low energy limit that does not reproduce GR but instead provides a scalar-tensor theory of gravity. Indeed, dilaton fields and their couplings to the spacetime curvature are unavoidable features of string and scalar-tensor theories. It seems that attempts to fully quantize gravity introduce significant deviations from GR and extra degrees of freedom. In high-energy astroparticle physics, modified gravity could shed light on several outstanding problems, e.g. the nature of dark matter as its production in the early universe may give rise to a relic density of the same order of magnitude of the present dark matter density. Modified gravity theories and GR predict different thermal evolution of the universe. Fundamental issues in modified gravity, and particle physics, relate to the origin of the matter-antimatter asymmetry and Leptogenesis. Other issues to be explored are mixing fields, vacuum fluctuations, neutrino oscillations, WIMPS, gravitational waves and the absolute value of the neutrino mass, which could be important tools to probe modified gravity. The ultimate goal of the proposed research program is to devise viable modified gravity models that pass local tests, explain the dynamics of the Universe and be consistent with constraints from high-energy astroparticle physics.