Domain walls, primordial black holes and inflation

Abstract

[eng] This thesis investigates primordial black holes and their observational consequences. In the first part, we examine the behavior of domain walls produced during inflation, and their subsequent collapse, leading to the formation of PBHs. The repulsive gravitational field of large spherical domain walls drives gravitational collapse with an unusual spacetime structure, which leads to the formation of a baby universe connected to the parent universe by a wormhole. In this cosmological setup we study the energy-momentum tensor of a quantum field to elucidate the behaviour of Hawking radiation in this peculiar geometry. We find that the energy momentum tensor remains finite everywhere during the evolution. No pathologies occur on the past event horizons of the Schwarzschild segment, and Hawking radiation emanates from the two future event horizons facing the parent and the baby universe, at the expected temperature. The trace anomaly changes abruptly at the end of inflation, causing transient fluxes of radiation towards the FLRW regions on both sides of the wall. We conclude that such quantum effects are small and therefore the dynamics of PBH formation is semiclassically stable, even when the PBH carries a baby universe.

In the second part of the thesis, we examine the observational consequences of primordial black holes produced by generic mechanisms, such as large curvature perturbations. More precisely, we investigate the merger rate of binaries and its impact on the stochastic gravitational wave background. Using a phenomenological model, we constrain the merger rate to redshifts around z ∼ 4 based on data from the LIGO/Virgo-O3 run. We discover distinctive features in the gravitational wave signal at intermediate frequencies for steep merger rates. Our findings have implications for future experiments such as the Einstein Telescope, LISA, and PTA, covering a wide frequency range. Additionally, we highlight the importance of direct observations of the merger rate to break degeneracies in the stochastic background, and provide constraints on the abundance of primordial black holes.

The structure of the thesis is as follows. In Chapter 1 we review some basics of de Sitter spacetime which models the inflationary epoch and the nucleation of domain walls by quantum tunneling. Moreover we discuss the dynamics of such domain walls in de Sitter and Schwarzschild backgrounds. The gravitational collapse of large supercritical domain walls and the construction of the resulting spacetime is explained in detail in Chapter 2. We dedicate Chapter 3 to calculate the stress-energy tensor for a massless scalar field in their spacetime. Finally, in Chapter 4 we study the observational consequences of such collapse by examine the history of different types of observers moving in the spacetime. In Chapter 5, we review the dynamics of the formation of primordial black hole binaries as well as the calculation of their merger rate. We also review the basics on the detection of the SBGW. Finally, in Chapter 6 we show how the SBGW allows as to constrain merger histories for PBH binaries.

[cat] En aquesta tesi investiguem els forats negres primordials les seves conseqüències observacionals. Primerament, examinem el comportament de parets de domini produïdes durant inflació i el seu subseqüent col·lapse, que condueix a la formació de forats negres primordials. El camp gravitatori repulsiu de parets de domini esfèriques grans, porta a una forma de col·lapse gravitatori amb una estructura d’espai-temps inusual, que condueix a la formació d’un univers nadó connectat a l’univers matern mitjançant un forat de cuc. En aquest escenari cosmològic, estudiem el tensor d’energia-moment d’un camp quàntic per aclarir el comportament de la radiació de Hawking en aquesta geometria peculiar. Trobem que la densitat d’energia, pressió i fluxos es comporten de manera suau i sense patologies durant tota l’evolució. En la segona part de la tesi, estudiem les conseqüències observacionals de forats negres primordials produïts per mecanismes genèrics, com pertorbacions de curvatura. En concret, investiguem el ritme de fusió de binàries i el seu impacte en el fons estocàstic d’ones gravitacionals. Utilitzant un model fenomenològic, restringim l’exponent en el ritme de fusió fins a redshifts d’aproximadament z ∼ 4 basant-nos en dades de l’observació LIGO/Virgo-O3. Descobrim característiques distintives en el senyal d’ones gravitacionals a freqüències intermèdies per a ritmes de fusió amb elevat pendent en funció del redshit. Els resultats obtinguts tenen implicacions per a futurs experiments com el Einstein Telescope, LISA i PTA, que cobreixen un ampli rang de freqüències. A més, destaquem la importància de les observacions directes del ritme de fusió per a trencar degeneracions en el fons estocàstic i proporcionar restriccions sobre l’abundància de forats negres primordials.