Modelizations of a network of non-abelian cosmic strings.

The thesis treats theoretically and through numerical simulations some features of cosmic strings. These are topological defects which may form during phase transition in cosmological epochs. These structures can be compatible with experimental observations such us CMB. This characteristic differentiate them with respect to monopoles and domain walls. Neglecting interactions curvature effects an appropriate description of cosmic strings’ dynamics is furnished by Nambu action. We deal with a (1, 3)-dimensional space-time and this allows us to treat these objects as relativistic strings. We derive equations describing motion of cosmic strings in the Minkowski and FRW space-time and we reason on the possibility that a scaling solution manifests. The search for this particular in- stance is motivated by some features of the action used to describe cosmic strings. Nambu action indeed hints the possibility for this system to lodge a single scale. Some typical feature of the non Abelian case are presented. Par- ticularly we point out the possibility of forming complex structures which may not occur in the Abelian scenario. This property at least theoretically may void some results derived assuming Abelianity. In literature some pathological aspects of scaling solution has been highlighted. Two articles try to furnish numerical predictions on non Abelian cosmic strings networks. Their results are not in complete agreement and this motivates our interest in a new detailed discussion regarding the topic. The goal of our speculations is to provide a modeling for a network of non Abelian cosmic strings. We critically examined the results proposed in one of this articles trying to perceive what may cause effects which could affect numerical simulation. We repeated the simulation, measuring some new observables in order to clarify statistical properties of the network. We also examined a different which poses emphasis on the dynamics of the strings. In a non Abelian theory the interconnection of two strings can produce configurations containing vertices where three strings are connected. This problem is treated with a theoretical approach. We attempted to define the most natural action to describe these objects and then we derived the equations of motion describing the evolution of these structures

Emerging chromo-natural inflation

The shift-symmetric coupling of a pseudo-scalar particle driving inflation to gauge fields provides a unique way of probing cosmic inflation. We show for an SU(2) gauge group how a classical isotropic background gauge field develops from the standard quantum mechanical vacuum in the far past. Over the course of inflation, the theory dynamically evolves from an approximately abelian regime into an inherently non-abelian regime, with distinct predictions for the scalar and tensor power spectra. The latter regime closely resembles a setup known as chromo-natural inflation, although our main focus here is on a new part of the parameter space which has received little attention so far. For single-field slow roll inflation models, large scales may exit the horizon in the abelian regime, ensuring agreement with the observations of the anisotropies in the cosmic microwave background, whereas smaller scales experience the non-abelian effects. This results in a strong enhancement of the stochastic gravitational wave background at small scales, e.g. at frequencies accessible with ground-based interferometers. For the scalar power spectrum, a similar enhancement arises due to non-linear contributions.Comment: 53 pages, 6 appendixe

Probing primordial black holes at high redshift with future gravitational wave detector

We analyze the detection prospects for potential Primordial Black Hole Binary (PBHB) populations buried in the Stellar-Origin Black Hole Binary (SOBHB) population inferred by the LVK collaboration. We consider different PBHB population scenarios and several future Gravitational Wave (GW) detectors. To separate the PBHB component from the SOBHB one, we exploit the prediction that the PBHB merger rate does not decline as fast as the SOBHB one at high redshift. However, only a tiny fraction of PBHB events may be resolved individually, and the sub-threshold events may yield an undetectable Stochastic GW Background (SGWB). For this reason, we determine the statistical significance of the PBHB contributions in the number of resolvable events seen in future Earth-based detectors and the SGWB measured at LISA. We find that the synergy between these probes will consistently help assess whether or not a sizeable PBHB population is present.Comment: 31 pages, 8 figure

Measuring parity violation in the Stochastic Gravitational Wave Background with the LISA-Taiji network

Parity violation is a powerful observable to distinguish a cosmological background of Gravitational Waves (GWs) from an astrophysical one. Planar single GW interferometers, both on ground and in space, are unable to measure the net circular polarization of an isotropic Stochastic Gravitational Wave Background (SGWB). In this paper, we explore the possibility of detecting circular polarization of an isotropic SGWB by cross-correlating two space-based detectors planned to be launched around 2034: LISA and Taiji. We compute the response of such a network to chirality and we perform a Fisher forecast analysis on the $I$ and $V$ Stokes parameters for the SGWB. We find that a clear measurement of chirality can be claimed for a maximally chiral signal with $h^2 \, \Omega_{\rm GW} \simeq 10^{-12}$.Comment: 22 pages, 4 figure