Gravitational waves from the early Universe and their detection

In this thesis we study several aspects regarding the nature of gravitational waves and their detection. Gravitational waves in the present days are object of intensive theoretical studies and are interesting from the detection point of view. At present there are several detectors aiming their detection from many compact objects such as neutron stars and black holes. In connection with that, in this thesis we study graviton emission by primordial black holes in the early universe and predict their spectrum. After the Bing Bang the density of the primordial plasma which composed the universe was too high and the probability of gravitational collapse of matter into compact object, namely black holes could have been high. Once these objects are being formed they start interacting effectively between each other emitting gravitons by several mechanisms. We study graviton emission by light primordial black holes as a result of quantum and classical scattering between them, graviton emission by binary systems of primordial black holes and graviton evaporation through Hawking mechanism. The spectrum of emitted gravitons is quite wide starting from the lower part of few Hz and ending at very high frequencies above the GHz. The predicted spectrum could be reveled by planned space interferometers such as DECIGO in the lower part of it. The high frequency part of the spectrum is difficult to observe because there are no such a high sensitive detectors in order to detect it. Because of this difficulty we constructed a new way in order to observe it through the mechanism of graviton to photon oscillation. This mechanism is possible whenever a gravitational wave couples to a background magnetic field and as a result is generated a non trivial energy-momentum tensor which curves the space-time. Based on present day limits on the strength of large scale magnetic field we predict an isotropic electromagnetic background of extragalactic origin which could fill the universe. Such a background arises due to the fact that gravitons evaporated by primordial black holes could oscillate into photons in magnetic field. Moreover we show that this background can be the main component of the Cosmic X-ray Background for massive black holes or even explain it without requiring obscured AGNs. Keywords: Gravitational waves, Black Holes, Graviton to photon oscillation, Primordial magnetic fields.