Gravitational collapse, compact objects and gravitational waves in General Relativity and modified gravity

Since the upgrade of the two LIGO Detectors it has been an exciting time for General Relativity (GR) research, as soon after they were upgraded to advanced status GW150914, the first gravitational wave signal from the inspiral and merger of two black holes was detected. The following consistent detections (9 further black hole binaries and one neutron star system) and the introduction of the European interferometer Virgo, as well as other planned detectors such as KAGRA and IndiGO mark the start of a new era of Gravitational Wave Physics and Astrophysics. This provides us with a window into the dynamics of strong gravity as well as the opportunity to test modified gravity and alternative theories in the strong regime, by confirming deviations from GR or constraining such theories. Why is General Relativity not enough though? It has passed all the tests so far with flying colors; in addition, the gravitational waves detected so far have demonstrated excellent agreement with GR’s predictions. Despite all these successful tests, there are still important questions related to Dark Energy and Dark Matter left unexplained. As a consequence, a clearer understanding of modified gravity theories is needed as well as a catalogue of gravitational waves resulting from these theories that could be used in the analysis of interferometer data and for stochastic background and continuous wave searches. In this Thesis we provide source modelling for GR and one of the most popular candidates for modified gravity, Scalar Tensor (ST) theories, as well as look for smoking-gun signatures. We analyse the formation of compact objects from core collapse simulations of stars in massive ST theories over the astrophysically plausible range of stellar progenitor masses and metallicities, as well as a large part of the parameter space of this class of modified theories of gravity. Next we test the robustness of our results by expanding the simulations to ST theories with self-interacting potentials. Finally, we study the recoil resulting from black holes mergers by varying the orbital eccentricity in an attempt to amplify the kicks

Amplification of superkicks in black-hole binaries through orbital eccentricity

We present new numerical-relativity simulations of eccentric merging black holes with initially anti-parallel spins lying in the orbital plane (the so-called superkick configuration). Binary eccentricity boosts the recoil of the merger remnant by up to 25 %. The increase in the energy flux is much more modest, and therefore this kick enhancement is mainly due to asymmetry in the binary dynamics. Our findings might have important consequences for the retention of stellar-mass black holes in star clusters and supermassive black holes in galactic hosts

Amplification of superkicks in black-hole binaries through orbital eccentricity

We present new numerical-relativity simulations of eccentric merging black holes with initially antiparallel spins lying in the orbital plane (the so-called superkick configuration). Binary eccentricity boosts the recoil of the merger remnant by up to 25%. The increase in the energy flux is much more modest, and therefore this kick enhancement is mainly due to asymmetry in the binary dynamics. Our findings might have important consequences for the retention of stellar-mass black holes in star clusters and supermassive black holes in galactic hosts

Stochastic gravitational wave background from supernovae in massive scalar-tensor gravity

In massive scalar-tensor gravity, core-collapse supernovae are strong sources of scalar-polarized gravitational waves. These can be detectable out to large distance. The dispersive nature of the propagation of waves in the massive scalar field mean the gravitational wave signals are long lived and many such signals can overlap to form a stochastic background. Using different models for the population of supernova events in the nearby universe, we compute predictions for the energy-density in the stochastic scalar-polarized gravitational wave background from core-collapse events in massive scalar-tensor gravity for theory parameters that facilitate strong scalarization. The resulting energy density is below the current constraints on a Gaussian stochastic gravitational wave background but large enough to be detectable with the current generation of detectors when they reach design sensitivity, indicating that it will soon be possible to place new constraints on the parameter space of massive scalar-tensor gravity.Comment: to match published version in Phys.Rev.

Inverse-chirp signals and spontaneous scalarisation with self-interacting potentials in stellar collapse

We study how the gravitational wave signal from stellar collapse in scalar-tensor gravity varies under the influence of scalar self-interaction. To this end, we extract the gravitational radiation from numerical simulations of stellar collapse for a range of potentials with higher-order terms in addition to the quadratic mass term. Our study includes collapse to neutron stars and black holes and we find the strong inverse-chirp signals obtained for the purely quadratic potential to be exceptionally robust under changes in the potential at higher orders; quartic and sextic terms in the potential lead to noticeable differences in the wave signal only if their contribution is amplified, implying a relative fine-tuning to within 5 or more orders of magnitude between the mass and self-interaction parameters.This work was supported by the European Union’s H2020 ERC Consolidator Grant “Matter and strong-field gravity: New frontiers in Einstein’s theory” grant agreement no. MaGRaTh–646597 funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 690904, the COST Action Grant No. CA16104, from STFC Consolidator Grant No. ST/P000673/1, the SDSC Comet and TACC Stampede2 clusters through NSFXSEDE Award Nos. PHY-090003, and Cambridge’s CSD3 system system through STFC capital grants ST/P002307/1 and ST/R002452/1, STFC operations grant ST/R00689X/1 and DiRAC Allocation ACTP186. R.R.-M. acknowledges support by a STFC studentship

Structure of Neutron Stars in Massive Scalar-Tensor Gravity

We compute families of spherically symmetric neutron-star models in two-derivative scalar-tensor theories of gravity with a massive scalar field. The numerical approach we present allows us to compute the resulting spacetimes out to infinite radius using a relaxation algorithm on a compactified grid. We discuss the structure of the weakly and strongly scalarized branches of neutron-star models thus obtained and their dependence on the linear and quadratic coupling parameters α0, β0 between the scalar and tensor sectors of the theory, as well as the scalar mass μ. For highly negative values of β0, we encounter configurations resembling a “gravitational atom”, consisting of a highly compact baryon star surrounded by a scalar cloud. A stability analysis based on binding-energy calculations suggests that these configurations are unstable and we expect them to migrate to models with radially decreasing baryon density and scalar field strength