Abstract
The advent of gravitationalwave (GW) astronomy has presented us with a completely new means for observing the Universe, allowing us to probe its structure and evolution like never before. In this thesis, we explore three distinct but complementary avenues for using GW observations to gain new insights into cosmology and fundamental physics.In chapter 1, we study the astrophysical GW background (AGWB): the cumulative GW signal arising from a large number of compact binary coalescences (CBCs) throughout the Universe. Since these compact binaries reside in galaxies, the AGWB contains anisotropies (i.e., intensity fluctuations on the sky) that trace out the largescale structure of the cosmic matter distribution. Despite their intrinsic interest as a source of novel cosmological information, these anisotropies have been neglected in most studies of the AGWB until quite recently. Applying tools and concepts from other cosmological observables such as galaxy surveys and the cosmic microwave background (CMB), we investigate the angular power spectrum of the AGWB, with the goal of developing predictions that can be confronted with current and future directional AGWB searches. Our key result is a simulated fullsky map of the AGWB anisotropies that we construct using data from the Millennium 푁 body simulation. We find that these anisotropies are much larger in amplitude than in earlyUniverse observables such as the CMB, and that the lowest few multipoles of the angular power spectrum are likely to be observed by thirdgeneration GW observatories. We also highlight the issue of shot noise due to the relatively low rate of CBCs in our frequency band of interest, and develop an optimal dataanalysis strategy for estimating the true angular power spectrum in the presence of this shot noise.
In chapter 2, we investigate the nonlinear GW memory effect, a fascinating prediction of general relativity in the dynamical, nonlinear regime, in which essentially all GW emission is accompanied by a hereditary, monotonicallyincreasing GW strain sourced by the energy of the escaping gravitons. Essentially all of the literature on this effect has focused on the memory signals associated with CBCs; we broaden this scope by calculating, for the first time, the nonlinear memory emitted by cusps and kinks on cosmic string loops, which are among the most promising cosmological sources of GWs. Working in the NambuGoto approximation, we obtain simple analytical waveforms for the memory emitted by cusps and kinks, as well as the ‘memory of the memory’ and other higherorder effects. Summing over all of these contributions, we show that, surprisingly, the combined cusp memory signal diverges for sufficiently large loops, indicating a breakdown in the validity of the weakfield description of the cusp. We trace this divergence back to the highfrequency behaviour of the original cusp waveform, which gives rise to a transPlanckian energy flux in the direction of the cusp’s motion. We then present one tentative possible solution to this divergence, in which the portion of the string surrounding the cusp collapses to form a primordial black hole (PBH). We investigate the observational predictions of this scenario, and show that these PBHs could act as a ‘smoking gun’ signature of cosmic strings.
Finally, in chapter 3 we develop a powerful new method for GW detection based on precision measurements of the orbits of binary systems. In the presence of a stochastic GW background (GWB) the trajectories of the binary’s components are perturbed, giving rise to a random walk in the system’s orbital parameters over time. By searching for this stochastic orbital evolution, we can infer the presence or absence of a GWB, turning the binary into a dynamical GW detector. We develop here a novel FokkerPlanck formalism for calculating the expected evolution in all six orbital elements. We then apply this formalism to two observational probes: timing of binary millisecond pulsars, and laser ranging of the Moon and artificial satellites. We use a Fisherforecasting approach to estimate the sensitivity of each of these probes to the GWB, and show that present data are already sensitive enough to place the strongest constraints to date in the μHz frequency band. This band lies between the frequencies probed by pulsar timing arrays and by future spacebased interferometers such as LISA, and is therefore an extremely attractive observational target, which could contain numerous cosmological GW signals. As an example, we consider the GWB sourced by a cosmological firstorder phase transition (FOPT), and show that the binary resonance searches we propose (in particular, with lunar laser ranging) will be sensitive to a region of the FOPT parameter space that no other current or nearfuture GW experiment can reach.
Date of Award  1 Mar 2022 

Original language  English 
Awarding Institution 

Supervisor  Mairi Sakellariadou (Supervisor) 