The present thesis focuses on the behaviour of a particular type of Near-Unstable Cavities (NUCs), and their application to the sensitivity enhancement of current and future gravitational wave detectors. Advanced detectors use high power laser beams. A small fraction of the light energy is absorbed by the cavity mirrors and converted into heat. The operation of near-unstable cavities requires high-precision thermal control of the cavity mirrors, and thus a thermal model of the cavity mirror and its surroundings was built and is presented in this thesis. The model aids the development of mitigation strategies of thermal effects on detector sensitivity. Nearunstable cavities have been proposed as an enabling technology for future gravitational wave detectors, as their compact structure and large beam spot can reduce the thermal noise floor of the interferometer. Throughout my Ph.D., I designed and built an experiment to investigate the technical hurdles associated with near-unstable cavities. A near-unstable table-top cavity was built and accurate control achieved through length and alignment control systems. This experiment provides an insight into how far cavity parameters can be pushed towards geometrical instability. The work I carried out will aid the design of future ground-based gravitational wave detectors.

This thesis primarily considers evolved exoplanetary systems through the use of asteroseismology as a tool to investigate the fundamental properties of the host stars, and the inferred planetary parameters. Of particular interest are the masses of evolved stars, and investigating how the available observables may bias the recovered mass estimates. Accurate and precise stellar masses are of critical importance. Whilst most of this work considers ensemble analysis, where relevant individual systems are considered, including a binary star system with an M dwarf and an asteroseismic red giant primary star. Another system of note is a transiting gas giant orbiting a red giant host, that will be consumed by the expansion of the host star. The metallicity distributions of evolved exoplanet hosts, and a suitable ensemble of field stars are also investigated using spectroscopy. This is to determine if evolved giant planet exoplanet hosts display the same metallicity excess seen in main sequence giant planet hosts. We fail to find a statistically significant excess in metallicity. Finally the noise properties of evolved stars are considered, including predicting the noise properties from stellar parameters, and how the elevated noise levels in evolved stars impact the detectable of planets around them.

This thesis discusses the development of optical read-out techniques, including a simple shadow sensor and a more elaborate compact homodyne interferometer, known as EUCLID. Both of these sensors could be utilised as part of a seismic isolation and suspension system of a ground-based gravitational wave observatory, such as Advanced LIGO. As part of the University of Birmingham’s commitment to the upgrade of the Advanced LIGO, it was responsible for providing a large quantity of sensor and actuator units. This required the development and qualification of the shadow sensor, through to production and testing. While characterising production units, an excess noise issue was uncovered and eventually mitigated; demonstrating that even for a ‘simple’ shadow sensor, ensuring a large quantity of units meet the target sensitivity requirement of 300 pm/rt-Hz at 1 Hz, is not a trivial exercise. Over the duration of this research, I played a key role in the design and fabrication of a novel compact interferometer. The objective of this work was to demonstrate that the interferometric technique offers a significant improvement over the existing shadow sensors and could easily be deployed in current, or future, generations of gravitational wave observatories. Encouraging sensitivities of approximately 50 pm/rt-Hz at 1 Hz, over operating ranges of approximately 6 mm have been achieved, whilst maintaining around 1 degree of mirror tilt immunity. In addition, this design overcomes many of the drawbacks traditionally associated with interferometers.

Super massive black holes have been found in the centre of massive galaxies, which have also been observed to merge. Consequently, the central black holes should also form a binary and merge when their host galaxies collide. The galaxy merger rate can be calculated from astrophysical observables, such as the galaxy stellar mass function, pair fraction and merger time scale. Using galaxy - black hole relations it can be converted into a super massive black hole binary (SMBHB) merger rate, i.e. the population of SMBHBs in the Universe. Additionally, properties of individual SMBHBs, like their eccentricities and the densities of their stellar environment, also have an impact on the emission of gravitational waves (GWs). All the GWs emitted by the population of SMBHBs form a gravitational wave background (GWB), which is one of the prime targets for PTAs. Although, PTAs have not been able to detect such a background yet, stringent upper limits on the GWB have been produced. This thesis aims to investigate what we can learn from these upper limits as well as what could be learned from a detection.

A new generation of ground-based interferometric gravitational wave (GW) detectors is due to begin operation this year, with routine detections anticipated within the next decade. Compact binary coalescences (CBCs), comprising pairs of neutron stars and/or black holes, are among the most promising sources for these detectors. In this work, we focus on two aspects of the science effort in GW astronomy with CBCs. Firstly, an attractive prospect for GW astronomers, in the wake of a CBC de- tection, is to observe its electromagnetic counterpart using a conventional telescope. In the first part of this thesis, I investigate our prospects for timely electromagnetic follow-up of such events and the degree to which a galaxy catalogue might aid such observation campaigns. Secondly, an important aspect of the science effort for GW detections is to ef- ficiently estimate the parameters of the system from which a detected signal origi- nated. In the latter part of this thesis I describe a refinement on existing Bayesian inference techniques used for this purpose. I follow this description with a reference implementation and an application to parameter estimation for CBCs.

STEREO’s imager, HI-1A, monitors nearly 500,000 stars down to 4th magnitude as it orbits the Sun. The resulting light curves offer unique characteristics and repeatability, which could make significant contributions to stellar variability surveys and the hunt for transiting planets. The HI-1A trend removal pipeline (TRP) was constructed to counteract systematic and observationsal effects and thereby increase the potential of a transit search. This pipeline uses a non-linear iterative filter to provide an adaptable noise-reduction process, without effecting transit-like signals. The TRP reduces the point-to-point scatter by up to 50 % for the brightest targets (R ≤ 6) and 25 % for the faintest (R ≥ 9). The correlated noise on transit timescales was found to be negligible for most targets and only 20 % of the total noise for the brightest stars, which amount to < 3 % of the sample. In an automated search for planet candidates, several transit signals were detected in the HI-1A light curves. Further testing suggests that none of these were veritable planet candidates. However the results show that Neptune-sized planets are certainly detectable and that a more effective reduction of solar-noise will lead to a successful planet search in the future.

This thesis focuses on the application of asteroseismology to red giants observed with Kepler alongside searching for solar g-modes using the Birmingham Solar Oscillations Network (BiSON). In the case of the Sun, solar gravity modes are highly sought after because they can shed light on the inner rotation profile of the Sun. This thesis contains work showing how the low frequency regime of BiSON data has been cleaned enabling the search to be made in BiSON data without instrumental artefacts. Moving onwards along the stars evolution, thanks to space mission such as Kepler and CoRoT tens of thousands of red giant stars have been observed allowing huge ensemble investigations. The ability to use high-quality, long datasets as constraints to shorter and noiser datasets has been investigated through fitting the background power of 6000 Kepler red giants. Red giants also offer the opportunity to study the inclination angle distribution of stars to confirm that the distribution conforms to the expected isotropy used in many simulations. This can be extended to inferring the obliquity through asteroseismology, as applied to a red-giant, M-dwarf eclipsing binary. This offering a means to probe obliquity distributions in in a different regime to that using traditional spectroscopic techniques.

The winds of hot massive stars and hot giant planets grant us insight into the mechanisms by which the interstellar medium is enriched and the history behind planetary system formation. This thesis comprises a series of studies investigating the magnetospheric dynamics and emission properties of both these astronomical bodies. An analytic study of thermal radio and sub-mm emission from the winds of massive stars investigates the contribution from acceleration and wind clumping. The results show strong variation of the spectral index, corresponding to the wind acceleration region and clumping of the wind. This shows a strong dependence of the emission on the wind velocity and clumping profile. By performing simulations of a magnetic rotating massive star with a non-zero dipole obliquity, it has been shown that the predicted radio and sub mm observable light curves and continuum spectra are highly dependent on the magnetic confinement of the stellar wind close to the surface, and that understanding the observer inclination and magnetic dipole obliquity are vital for determining the stellar mass-loss rate from radio observations. Hot Jupiter exoplanets are expected to produce strong radio emission in the MHz range but have not been detected. To explain the absence of observational results, simulations of the interactions between a solar type star and hot Jupiter were conducted and used to calculate the efficiency of radio emission generation within the planet's magnetosphere. Results show that it is completely inhibited by the planet's expanding atmosphere. Finally, the first simulations of wind-wind interactions between a solar type star and a short period hot Jupiter exoplanet that resolves accretion onto the surface of the star are presented. The accretion point, rate and periodicity are quantified, with the results indicating that material accreting onto the star perturbs surface density and temperature in a periodic manner, in agreement with observations.

Following the first detection of gravitational waves from a binary coalescence, the study of the formation and evolution of these gravitational-wave sources and the recovery and analysis of any detected event will be crucial for the newly realised field of observational gravitational wave astrophysics. This thesis covers a wide range of these topics including simulating the dense environments where compact binaries are likely to form, focusing on binaries containing an intermediate mass black hole (IMBH). It is shown that such binaries do form, are able to merge within a ∼ 100 Myr simulation, and that the careful treatment of the orbital evolution (including post-Newtonian effects) implemented here was crucial for correctly describing the binary evolution. The later part of the thesis covers the analysis of the gravitational waves emitted by such a binary, and shows it is possible to identify the IMBH with high confidence, together with most other parameters of the binary, despite the short-duration signals and assumed uncertainties in the available waveform models. Finally a method for rapid parameter estimation of gravitational wave sources is presented and shown to recover source parameters with comparable accuracy using only a small fraction ∼ 0.1% of the computational resources required by conventional methods.

The revolutionary discoveries of the last few years have opened a new era of astronomy. With the detection of gravitational-waves, we now have the opportunity of investigating new phenomena, such as mergers of black-holes. Furthermore, multi-messenger observations now allow us to combine information from different channels, providing insight into the physics involved. With this rapid evolution and growth of the field, many challenges need to be faced. In this thesis we propose three data analysis strategies to efficiently study the coalescences of compact binaries. First we propose an algorithm to reduce the computational cost of Bayesian inference on gravitational-wave signals. Second we prove that machine-learning signal classification could enhance the significance of gravitational-wave candidates in unmodelled searches for transients. Finally we develop a tool, saprEMo, to predict the number of electromagnetic events, which according to a specific emission model, should be present in a particular survey.