Dissertation committee:
Β. Σπανός (Αναπλ. Καθηγητής Τμ. Φυσικής ΕΚΠΑ)
Γ. Διαμάντης (Αναπλ. Καθηγητής Τμ. Φυσικής ΕΚΠΑ)
A. Δέδε (Καθηγητή Τμ. Φυσικής Παν/μιο Ιωαννίνων)
Κ. Σφέτσος (Καθηγητής Τμ. Φυσικής ΕΚΠΑ)
Ν. Τετράδης ( Καθηγητής Τμ. Φυσικής ΕΚΠΑ)
Α. Κεχαγιάς (Καθηγητής ΕΜΠ, ΣΕΜΦΕ )
Ν. Μαυρόματος (Καθηγητής ΕΜΠ, ΣΕΜΦΕ )
Summary:
The recent data for Cosmic Microwave Background released by Planck collaboration provides us with new constraints in the study of inflation. Moreover, the recent observation of black hole merges by LIGO/VIRGO opens a new window in the study of Primordial Black Holes (PBHs). In particular, the production of PBHs, and as a result, an amount of Dark Matter (DM) in the Universe, can be explained in the context of inflation. This idea is adopted by numerous theoretical researches during the last few years. In addition to the important detection of LIGO/VIRGO, the future space-based experiment, such as LISA, will provide us with important results for the detection of Gravitational Waves (GWs). The generation of GWs can also be studied in the framework of inflation.
In this thesis we study the inflationary Cosmology and especially models which are embedded in supergravity theory (SUGRA). Specifically, we study inflationary models which are capable to explain the generation of both PBHs and GWs. These models should have an enhancement in the scalar power spectrum. In order to achieve this reinforcement, we use three different mechanisms. The first one is characterized by an inflection point in the effective scalar potential. The second concerns step-like feature in the potential. Finally, the last mechanism is based on the waterfall regime in the study of hybrid models.
We implement these three techniques in SUGRA theories in order to derive specific models which matches with the observable constraints. In the case of the inflection point we study no-scale theory. Specifically, we modify well-known superpotentials, which reduce to Starobinsky-like effective scalar potentials. Thus, we derive scalar potentials which, on the one hand, explain the production of PBHs and, on the other hand, conserve the transformation laws, which yield from the parameterization of the coset SU(2,1)/SU(2)$\times$U(1). Moreover, we generate PBHs by modifying the kinetic term of the Lagrangian (or the K\"ahler potential) and we keep the superpotentials unmodified.
For steep step-like feature we study the superconformal $\alpha$-attractors, in order to provide a different SUGRA theory with underlying symmetries. The scalar power spectrum, which is evaluated from this mechanism, has an amplification and an additional oscillation pattern in small scales of scalar power spectrum. We apply this mechanism in a model derived from superconformal $\alpha$-attractors and we present models, which are consistent with the observable constraints.
Finally, for hybrid model we use SUGRA corrections, in order to be in accordance with the observable constraints. Specifically, we study the effect of SUGRA corrections in order to observe acceptable values for the observable constraints. This model can explain the whole DM for some choices of parameters. It can also satisfy the GW signal from NANOGrav collaboration at the cost of significantly lower PBH abundance.
In all models we present the evaluation of scalar power spectrum, the abundance of PBHs and the energy density of GWs and we find essential results. We analyze the issue of fine-tuning of the parameters and how it can be decreased through these mechanisms.
Keywords:
cosmic inflation, primordial black holes, gravitational waves, supergravity, cosmology
