arXiv subjects

Alina Mierna

Publications and source records attributed to Alina Mierna.

5 recordsLinked to original sources

arXiv2026

Exploring Gravitational Wave Signatures Due to Primordial Non-gaussianity and Large Scale Structure Using SKAO

This chapter explores theoretical and observational strategies to use the stochastic gravitational-wave background detectable by Square Kilometre Array Observatory (SKAO) as a probe of precision cosmology. We detail the critical phenomenon of scalar-induced gravitational waves, demonstrating their unique features and their sensitivity to primordial non-Gaussianity on scales much smaller than those probed by the cosmic microwave background and large-scale structure. We investigate the phenomenology of parity violation in the early Universe through the chirality imprinted in the stochastic gravitational-wave background, demonstrating that a parity-odd primordial trispectrum can generate a detectable scale-dependent helicity. On the observational side, we point out that the standard gravitational-wave angular auto-correlation analysis is significantly limited by astrophysical shot noise. We show that cross-correlating the gravitational wave signal with independent large-scale structure tracers enhances the signal-to-noise ratio and allows us to isolate the astrophysical and cosmological components in the background. These results can be achieved only thanks to the enhanced sensitivity of SKAO, extensive sky coverage, and high angular resolution, which together make such targets observationally feasible.

astro-ph.CO↗

arXiv2025

Quantum production of gravitational waves after inflation

A variety of mechanisms in the early Universe lead to the generation of gravitational waves (GWs). We introduce here a novel source of GWs generated by vacuum fluctuations after inflation. Given that gravitons are minimally coupled particles, their quantum creation takes place during inflation, but is absent in an unperturbed Universe during the radiation-dominated epoch, since they behave as conformally coupled particles. However, the presence of inhomogeneities breaks the conformal flatness of the metric, allowing scalar metric perturbations to induce the quantum production of gravitons. We compute the resulting GW spectrum from this mechanism for different models of the primordial scalar power spectrum. We find that this GW signal peaks around the GHz frequency range, distinguishing it from other astrophysical and cosmological backgrounds and underscoring the need for detectors sensitive to these high frequencies.

gr-qc↗

arXiv2024

Non-linear effects on the Cosmological Gravitational Wave Background anisotropies

The Cosmological Gravitational Wave Background (CGWB) anisotropies contain valuable information about the physics of the early universe. Given that General Relativity is intrinsically nonlinear, it is important to look beyond first-order contributions in cosmological perturbations. In this work, we present a non-perturbative approach for the computation of CGWB anisotropies at large scales, providing the extension of the initial conditions and the Sachs-Wolfe effect for the CGWB, which encodes the full non-linearity of the scalar metric perturbations. We also derive the non-perturbative expression for three-point correlation of the gravitational wave energy density perturbation in the case of an inflationary CGWB with a scale-invariant power spectrum and negligible primordial non-Gaussianity. We show that, under such conditions, the gravitational wave energy density perturbations are lognormally distributed, leading to an interesting effect such as intermittency.

astro-ph.CO↗

arXiv2024

Inflationary Initial Conditions for the Cosmological Gravitational Wave Background

The initial conditions on the anisotropies of the stochastic gravitational-wave background of cosmological origin (CGWB) largely depend on the mechanism that generates the gravitational waves. Since the CGWB is expected to be non-thermal, the computation of the initial conditions could be more challenging w.r.t. the Cosmic Microwave Background (CMB), whose interactions with other particles in the early Universe lead to a blackbody spectrum. In this paper, we show that the initial conditions for the cosmological background generated by quantum fluctuations of the metric during inflation deviate from adiabaticity. These primordial gravitational waves are indeed generated by quantum fluctuations of two independent degrees of freedom (the two polarization states of the gravitons). Furthermore, the CGWB plays a negligible role in the Einstein's equations, because its energy density is subdominant w.r.t. ordinary matter. Therefore, the only possible way to compute the initial conditions for inflationary gravitons is to perturb the energy-momentum tensor of the gravitational field defined in terms of the small-scale tensor perturbation of the metric. This new and self-consistent approach shows that a large, non-adiabatic initial condition is present even during the single-field inflation. Such a contribution enhances the total angular power spectrum of the CGWB compared to the standard adiabatic case, increasing also the sensitivity of the anisotropies to the presence of relativistic and decoupled particles in the early Universe. In this work we have also proved that our findings are quite general and apply to both single-field inflation and other scenarios in which the CGWB is generated by the quantum fluctuations of the metric, like the curvaton.

astro-ph.CO↗

arXiv2023

Adiabatic or Non-Adiabatic? Unraveling the Nature of Initial Conditions in the Cosmological Gravitational Wave Background

The non-thermal nature of the cosmological gravitational wave background (CGWB) makes it difficult to define the initial condition for the graviton distribution, which determines the initial contribution to the CGWB anisotropies. Specifically, unlike Cosmic Microwave Background (CMB) photons, primordial gravitational waves (GWs) are not necessarily subject to adiabatic initial conditions. For the CGWB generated by quantum fluctuations of the metric during inflation the initial conditions are not adiabatic. The violation of adiabaticity arises from the presence of independent tensor perturbations during inflation, which behave as two extra fields that affect the standard single-clock argument. The non-adiabatic initial contribution enhances the total CGWB angular power spectrum compared to the standard adiabatic case. Given the high degree of correlation between the CGWB and Cosmic Microwave Background (CMB) anisotropies, we present the constrained realization maps of the CGWB built using the high-resolution CMB Planck maps for different initial conditions.

astro-ph.CO↗