On the 14th of november, at 11:00 , in the Saló d'Actes of the Eduard Boscà Library, Burjassot, will take place the PhD thesis defense of Davide Guerra that has been supervised by Pablo Cerdá Durán i Jose A. Font Roda, professors of that department
Abstract:
The detection of gravitational waves from binary neutron star mergers has inaugurated a revolutionary era in astrophysics, opening ground-breaking opportunities to study the physics of matter at extreme densities and temperatures. Multimessenger observations of such events have not only provided new constraints on the neutron star equation of state but have also deepened our understanding of fundamental nuclear processes and compact object astrophysics. However,accurately interpreting these observations requires robust theoretical models and computational simulations in full general relativity that include realistic representations of nuclear equations of state and a comprehensive treatment of thermal effects. This thesis presents a numerical exploration of the dynamics of binary neutron star mergers paying particular attention and emphasis on how the inclusion of finite-temperature effects on the equation-of-state representation affects the gravitational-wave emission and long-term, post-merger evolution of neutron star remnants. To do so, using state-of-the-art numerical relativity codes we perform a meticulous comparison between fully tabulated finite-temperature equation of state models and hybrid approximations, identifying critical deviations in density and temperature profiles that significantly impact merger dynamics and gravitational-wave signals. Our simulations high-light distinct differences in gravitational-wave frequency evolution related to the thermal modeling in the equation of state, demonstrating that deviations from quasi-universal relations previously reported in the literature become significant at late post-merger phases. Furthermore, we introduce refined definitions for the Brunt–Väisälä frequency adapted to relativistic simulations, identifying novel convective instabilities and spiraling fluid structures within the bulk of the remnant that persist beyond 100 ms after merger, which differ substantially between tabulated and hybrid treatments of the equation of state. Moreover, in the context of phase transitions in dense nuclear matter, this thesis puts forward an enhanced Thermodynamically Adaptive Slope Piecewise Polytropic equation of state formulation. This new model accurately captures the non-monotonic behavior and first-order phase transitions characteristic of exotic matter phases, significantly improving thermodynamic consistency and fidelity compared to
traditional approximations. Utilizing this new formulation, we construct initial data sets for numerical relativity simulations that closely replicate the detailed microphysical behavior of tabulated equations of state. Additionally, this thesis briefly reports on a number of additional side projects in which I have also collaborated and succinctly documents my active contributions to the Virgo Collaboration in which I developed advanced noise characterization tools, actively participated in real-time gravitational-wave candidate verification, and engaged in outreach activities aimed at addressing Virgo’s visibility issues. The findings and developments reported in this thesis contribute to advance our theoretical understanding of binary neutron star merger dynamics, the modeling of finite-temperature equations of state, and gravitational-wave data analysis. These results not only contribute to the interpretation of current gravitational-wave observations but also prepare the groundwork for future multimessenger
astronomy and gravitational-wave astrophysics.
