This thesis investigates the effect of rapid accretion and differential rotation on neutron star oscillations. The research is motivated by the fact that vibrating neutron stars are a promising source of gravitational waves. The first part of the thesis is a study of a nascent neutron star accreting supernova remnant material. We model an unstable r-mode oscillation that leads to the emission of gravitational waves, and the torques and heating associated with rapid accretion onto a star with a magnetic field. We consider the consequences for both gravitational wave emission and the rotation rate of the star. The main part of the thesis addresses differential rotation. This is likely to arise at times, such as the immediate aftermath of the supernova, when we expect strong vibrations. We focus on two factors unique to differentially rotating systems; dynamical shear instabilities, and the existence of a corotation band (a frequency band in which mode pattern speed matches the local angular velocity). Using a simple model, we find dynamical shear instabilities that arise where modes cross into the corotation band, if the degree of differential rotation exceeds a certain threshold. Recently, several authors have reported the discovery of dynamical instabilities in differentially rotating stars at low values of the ratio of kinetic to potential energy. We demonstrate that our instability mechanism explains all of the reported features of these instabilities. We also investigate the nature of oscillations within the corotation band. The band gives rise to a continuous spectrum whose collective physical perturbation exhibits complicated temporal behaviour. We also report the existence of modes within the continuous spectrum that appear physically indistinguishable from the discrete modes outside the band, despite the singular nature of their eigenfunctions.

Dynamically screened scalar field theories form an attractive collection of models that were introduced to drive the late-time expansion of our universe. A common consequence of these theories is a screening mechanism which leads to the suppression of the fifth-forces mediated by the scalar field in sufficiently dense environments. This enables the models within this class of theories to avoid conflict with the stringent results from local tests of gravity, without the need for any fine tuning. The prototypical example of a dynamically screened scalar field is the chameleon model, for which screening arises due to the mass of the scalar responding to the local density. It has been recently demonstrated that atom interferometry is a powerful technique for constraining such scalars, with near future experiments capable of probing a large portion of the model parameter space. The nature of screening however means that closing in on what remains of the chameleon parameter space is going to become increasingly more difficult. This work aims to address this issue by examining the intricacies of how the chameleon field responds to the configuration of an atom interferometry experiment, where it is found that the non-linearities governing the theory can ultimately be harnessed in order to improve the prospects of detection.

This thesis is an account of work carried out at the Institute for Gravitational Research (IGR), in the University of Glasgow between October 2010 and March 2014. The research presented contributes to the design of ultra-low mechanical loss suspensions for use in gravitational wave detectors and other kinds of interferometry. This thesis focuses on measuring the parameters of mechanical loss in fused silica suspensions that will limit the sensitivity of advanced gravitational wave detectors and other kinds of interferometers where fused silica suspensions are used. These investigations were carried out under the supervision of Dr Giles Hammond and Professor Sheila Rowan. Chapter 1 gives an introduction to gravitational wave astronomy and provides an insight into which astrophysical objects are able to emit gravitational radiation. The chapter goes on to describe current and future detection techniques that are used and planned in order to make the first direct detection of gravitational waves, noting some results and limits which have been achieved to date. The chapter also introduces the different noise sources that will limit the sensitivity of gravitational wave detectors. The information detailed in this chapter has all been derived from previously published literature. Chapter 2 introduces the theory of thermal noise and derives the relationship between the mechanical loss and thermal noise in fused silica suspensions via the fluctuation dissipation theorem. Discussion covers how this limits the sensitivity of a gravitational wave detector. The chapter includes the theory of loss mechanisms present in fused silica. Again the information contained in this chapter has all been derived from previously published literature. Chapter 3 contains details and results of an experiment, where the expansion coefficient of fused silica fibres was measured under varying amounts of stress. Results demonstrate that the effective thermal expansion co-efficient of a fused silica fibre can be nulled by placing the fibre under a particular level of stress. This nulling of the effective thermal expansion coefficient should lower the thermoelastic noise contribution in silica suspensions, essential for allowing second generation gravitational wave detectors to reach their target thermal noise sensitivity of below 10^{-19}m/sqrt{Hz} at 10Hz. The experimental work in this chapter was conceived by Professor James Faller with a prototype demonstrated by Dr Stuart Reid. The set-up was then revised and modified by the author and Dr Giles Hammond to achieve the results presented in this thesis. Throughout this experiment Colin Craig helped with the machining of the invar set-up and Dr Kirill Tokmakov with suspending the silica fibres placed under large amounts of stress. Experimental measurements and analysis were carried out by the author. Chapter 4 describes an experiment in which a fused silica fibre was held under tension and the harmonic violin mode losses over a range of frequencies were measured. The fibre was then cut and cantilever modes of the fibre measured. The contributions from excess losses were calculated and shown not to limit the experiment. A theoretical dilution factor was determined along with the modal strain distribution of the violin and cantilever modes from finite element analysis (FEA). The FEA was aided by Dr Rahul Kumar and Dr Alan Cumming. The data measured was then compiled with a loss model to give information about the loss contributions of fused silica such as thermoelastic loss, surface loss and weld loss. Designing of the silica pendulum system used in this experiment was helped by Russell Jones and the machining of the silica mass holders for CO2 welding was done by Steven Craig. Construction of the silica pendulum system was undertaken by Dr Giles Hammond and the author, who carried out the experimental measurements. Analysis of the data presented in this chapter was aided by Dr Matthew Pitkin who contributed a Markov Chain Monte Carlo regression fitting code. Chapter 5 repeats the above experiment; where the author used a modified fused silica fibre to measure violin mode losses. The modified silica fibre allowed loss measurements to be made at a much lower frequency than in the previous violin mode set-up. In an attempt to study the nonlinear thermoelastic loss in more detail. The stress on the silica was also varied to observe the nulling of the effective thermal expansion coefficient directly through measurements of the mechanical loss. This experiment used many of the components described in chapter 4 and so the same people are acknowledged for their contribution. Construction of the silica pendulums used was carried out by Dr Giles Hammond, Dr Kirill Tokmakov and the author. Chapter 6 focuses on measuring the mechanical loss of 20-30micron diameter fused silica fibres, for use in the Hannover AEI 10m prototype interferometer. This chapter illustrates the problems faced when trying to measure the mechanical losses of thin fibres. The mechanical loss data was then compiled with a loss and finite element model to give information about the loss contributions of fused silica such as thermoelastic, surface and weld loss in thin silica fibres. This experiment was constructed initially by Dr Stuart Reid with some of the welding being performed by Dr Kirill Tokmakov. All of the experimental measurements and analysis were the work of the author. Chapter 7 details the conclusions that can be drawn from the various experiments in previous chapters. The results will be be applicable across many areas of research where low mechanical thermal noise is required. More generally the results can be used as a basis for research that requires mechanical systems at room temperatures for example systems needed to produce stable optical cavities. A further important discovery emerging from this thesis is the ability to cancel and reverse the effective thermal expansion coefficient of fused silica by placing the silica under stress. This process allows stressed silica to be used in systems were low thermal expansion coefficient materials are needed. Thus fused silica can be used as an alternative to composite materials such as invar.

The torsion balance is one of the key pieces of apparatus used in experimental searches for weak forces. In the search for an understanding of a Unified Theory, physicists have suggested a number of signatures that are detectable in laboratory measurements. This thesis describes the development of a new torsion balance facility, relocated from the BIPM (Bureau International des Poids et Mesures) [1], which has excellent environmental stability and benefits from a new compact interferometric readout for measuring angular motion which has been characterised and installed onto the torsion balance. The interferometer has sensitivities of 5 \( \times\) 10\( {-11}\) radians\(\char{cmti10}{0x2f}\)\(\sqrt{Hz}\) between 10\( {-1}\) Hz and 10 Hz, an angular range of over \(\pm\)1\( \circ\) and significantly reduces sensitivity to ground tilt. With the new facility the first experiment searching for temporal variations in the Newtonian gravitational constant has been undertaken with a null result for \( \delta\)\(\char{cmti10}{0x47}\)\(\char{cmti10}{0x2f}\)\(\char{cmti10}{0x47}\)\(_0\) for both sidereal and half sidereal signals at magnitudes greater than 5\( \times\)10\( {-6}\). These results have been used to set an upper limit on some of the parameters within the Standard Model Extension framework [2]. The thesis also reports on the design and manufacture of prototype test masses with a high electron-spin density of approximately 10\( {24}\) and negligible external magnetic field \( \leq\) 10\( {-4}\)\(\char{cmr10}{0x54}\). These test masses can be used within the facility to potentially make it sensitive enough to conduct future spin-coupling experiments.

As the global population urbanises, the benefits derived from contact with nature increasingly depend upon the presence of diverse urban ecological communities. These may be threatened by changes in land-cover and the intensification of land-use. A key question is how to design and manage cities to retain desirable species, habitats and processes. Addressing this question is challenging, due to the dominant role of humans in shaping spatially and temporally complex urban landscapes. Earlier research identified ecological patterns along urban–rural gradients, often using simplified measures of built form and disturbance. The central theme within this thesis is that we require a more mechanistic understanding of the processes that created today‘s ecological patterns, which recognises the interactions between social and ecological sub-systems. Using bats (Chiroptera) as a case study group, I identified a broadly negative association between bat activity and built density. Urban tree networks appeared beneficial for one species, and further work revealed that their role in facilitating movement depended upon the size of gaps in tree lines and their illumination level. Resilience analyses were used to map diverse dependencies between the functioning of urban bat habitats and human social factors; illustrating the value of a more mechanistic systems-based approach.

The first direct detection of gravitational waves last year was the beginning of a new field of astronomy. While we have already learned a great deal from the signals sensed by the LIGO interferometers in their first observation run, research is already underway to improve upon the sensitivity of the state of the art detectors. Novel mirror designs, new interferometer topologies and larger, more advanced detectors are all being considered as future improvements, and these topics form the focus of this thesis. A reduction in the thermal noise arising from the mirrors within gravitational wave detectors will enhance sensitivity near their most sensitive frequencies, and this can potentially be achieved through the use of waveguide mirrors employing gratings. It has been shown that the thermal noise is reduced in waveguide mirrors compared to standard dielectric mirrors whilst retaining the required reflectivity, but an open question regarding their suitability remains due to the potential for increased technical noise coupling created by the substructure. We place an upper limit on this coupling with a suspended cavity experiment, showing that this approach to the design of grating mirrors has promise. While the use of higher classical laser input initially increases interferometer sensitivity, eventually the Michelson interferometer topology employed in existing detectors reaches the standard quantum limit preventing further enhancement. Efforts are being made to test the suitability of so-called quantum non-demolition (QND) technologies able to surpass this limit, one of which involves the use of a new interferometer topology altogether. An experiment to demonstrate a reduction in quantum radiation pressure noise in a QND-compatible Sagnac speed meter topology is underway in Glasgow, and we introduce novel techniques to control this suspended, audio-band interferometer to inform the technical design of future detectors wishing to measure beyond the standard quantum limit. In particular, the problem of controlling the interferometer at low frequencies is discussed. Due to the nature of the speed meter topology, the response of the interferometer vanishes towards zero frequency, while the interferometer's noise does not. This creates a control problem at low frequencies where test mass perturbations arising from, for example, seismic and electronic noise, can lead to loss of interferometer sensitivity over the course of minutes to hours. We present a solution involving the blending of signals from different readout ports of the interferometer, facilitating measurements with almost arbitrary integration times. The longer, larger Einstein Telescope facility planned as part of the next generation of detectors will push the Michelson interferometer topology to the limit. The low frequency interferometer will utilise optomechanical interactions to enhance its sensitivity at low frequencies, and the control problems associated with this technique have not been investigated in detail. Following the approach taken in the current generation of detectors we show that the interferometer can be controlled without adversely affecting its sensitivity to gravitational waves, paving the way for a future technical design.

The Silicon Photomultiplier (SIPM) is a novel photon sensing device, with potential applications in particle physics, astronomy and general gamma-ray spectroscopy. Various SiPM designs were evaluated for these roles by coupling them to a range of scintillators, including LSO, NaI(Tl) and CsI(Tl). It was found that a LSO-SiPM gamma-ray detector provide sufficiently good energy resolution (11% at 511keV) in a very compact package to be of interest in PET imaging. SiPMs awere also found to provide competitive spectra to PIN diodes of comparable area (1cm2) when coupled to CsI(Tl), raising interest in gamma-ray astronomy where a CsI(Tl)-SiPM detector could be used as an imaging element. Good performance with CsI(Tl) encouraged investigation of SiPMs in the gamma-ray spectroscopy role, specifically radioisotope identification with portable instruments. The ScintiSphere concept was exploited to allow a larger, more sensitive detector to be built using SiPMs without sacrificing spectral quality. An 8cc sphere of CsI(Tl) coupled to a 1cm2 SiPM array was found to give a noise floor of 35keV and a resolution of 9% at 662keV, closely comparable to a 0.8cc square crystal, despite the factor of 10 improvement in sensitive volume. Neutron detection with a SiPM array is achieved using LiI(Eu), which was found to resolve thermal neutron captures to 12% FWHM and achieve excellent gamma-ray rejection. In this application, SiPMs are preferable to PIN diodes as they are immune to direct gamma-ray interactions which could degrade gamma-ray rejection. When packaged in a compact moderator of 15mm depth and volume 58cc of HDPE, a compact neutron counter was built and found to be slightly more efficient than a 3” He3 tube packaged in 16mm of HDPE. Finally, an instrument is proposed that exploits the intrinsic advantages of SiPMs. The use of SiPMs with LaBr(Ce) is explored and simulations are carried out of a directional spectrometer, predicted to locate a Cs137 source of 1mC at 2m to within 15° in 15 seconds. In conclusion, it is found that SiPMs should be of great interest in these fields due to providing comparable performance to existing systems whilst providing additional benefits such as low power electronics, high gain and immunity to direct X-ray interactions.