GRAVITATIONAL WAVES FROM PRIMORDIAL BLACK HOLE FORMATION IN ALTERNATIVE THEORIES OF GRAVITY

ations of general relativity. Primordial black holes provide important information about the early Universe complementary to large-scale cosmological observations. In the standard picture, primordial black holes are formed, during the epoch of radiation domination at early times, by (non-standard) enhancements in the power spectrum of primordial perturbations generated during inflation over some scale ranges. An alternative mechanism for describing the generation of primordial black holes is currently under active development. It is based on non-standard gravitational effects at sub-millimetre scales in the early Universe, for which we expect deviations from general relativity to become significant. In this approach to the primordial black hole formation, the power spectrum of primordial fluctuations is assumed to be of the standard, nearly scale-invariant form set by inflation, while the evolution of the fluctuations after inflation results in enhancements of the power spectrum, providing a new mechanism for generating primordial black holes. In this project we study the latter scenario: a non-inflationary mechanism for producing primordial black holes. The aim of this work is to investigate the physical consequences of this scenario in order to be able to set constraints on Alternative Theories of Gravity. In particular, we study the production of scalar induced gravitational waves from primordial black hole formation in this newly proposed framework for the production of primordial black holes. The study of the production of scalar induced gravitational waves, which is the main original contribution of this thesis work, could help to better understand and constrain the non-inflationary mechanism proposed in a work in progress by Akrami, Patil, Vardanyan et al., to which I have been exposed, for producing primordial black holes and, as a consequence, modific