In this thesis we present the XCS DR3 cluster candidate list. This represents the first major update of the XMM Cluster Survey since 2005. The candidate list comprises of 1365 entries with more than 300 detected counts distributed over 229 deg2. We note that a larger area (523 deg2) is available for the study of X-ray point sources and that the new XCS point source sample has more than 130,000 entries. After redshift follow-up and X-ray spectral analysis, these 1365 clusters will comprise the largest homogeneous sample of medium to high redshift X-ray clusters ever compiled. The future science applications of the XCS DR3 clusters include the study of the evolution of X-ray scaling relations and a measurement of cosmological parameters. In support of these science applications, we also present in this thesis detailed selection functions for the XCS. These selection functions allow us to quantify the number of clusters we didn't detect in our survey regions. We have taken two approaches to the determination of the selection function: the use of simple (circular & isothermal) β models and the use of ‘observations' of synthetic clusters from the CLEF N-body simulation. The β model work has allowed us to explore how the selection function depends on key cluster parameters such as luminosity, temperature, redshift, core size and profile shape. We have further explored how the selection function depends on the underlying cosmological model and applied our results to XCS cosmology forecasting (Sahlen et al. 2009). The CLEF work has allowed us to explore more complex cluster properties, such as core temperature, core shape, substructure and ellipticity. In summary, the combination of the cluster catalogues and selection functions presented herein will facilitate field leading science applications for many years to come.

We introduce a statistical exploration of the parameter space of the Munich semi-analytic model built upon the Millennium dark matter simulation. This is achieved by applying a Monte Carlo Markov Chain (MCMC) method to constrain the 6 free parameters that define the stellar mass function at redshift zero. The model is tested against three different observational data sets, including the galaxy K-band luminosity function, B −V colours, and the black hole-bulge mass relation, to obtain mean values, confidence limits and likelihood contours for the best fit model. We discuss how the model parameters affect each galaxy property and find that there are strong correlations between them. We analyze to what extent these are simply reflections of the observational constraints, or whether they can lead to improved understanding of the physics of galaxy formation. When all the observations are combined, the need to suppress dwarf galaxies requires the strength of the supernova feedback to be significantly higher in our best-fit solution than in previous work. We interpret this fact as an indication of the need to improve the treatment of low mass objects. As a possible solution, we introduce the process of satellite disruption, caused by tidal forces exerted by central galaxies on their merging companions. We apply similar MCMC sampling techniques to the new model, which allows us to discuss the impact of disruption on the basic physics of the model. The new best fit model has a likelihood four times better than before, reproducing reasonably all the observational constraints, as well as the metallicity of galaxies and predicting intra-cluster light. We interpret this as an indication of the need to include the new recipe. We point out the remaining limitations of the semi-analytic model and discuss possible improvements that might increase its predictive power in the future.

These are privileged times to be a cosmologist. Recent years have witnessed unprecedented progress in observational and computational techniques and we now are able to quantify cosmological properties with unprecedented accuracy. My work builds upon this observational accuracy by establishing a connection with viable theoretical models. I focus on two specifics eras of the universe's evolution, namely inflation and today's cosmic acceleration. In the context of single field inflationary models I illustrate the relation between the spectra of curvature and gravitational wave perturbations. I conclude that their mutual interdependence extends beyond the usual amplitude consistency relation and can be traced all the way to infinite order of accuracy. This yields an infinite hierarchy of consistency relations between these spectra and their derivatives. On a observational perspective, using WMAP's data, I explore the dependence of CMB constraints on inflation with the cosmological scale at which these are chosen to be presented. I develop a technique that allows for an appropriate choice of this scale and show that this way constraints may be improved by as much as 5 times. In the context of the particle physics motivated quintessence models I have looked at the ability of early universe probes - namely Big Bang Nucleosynthesis - for distinguishing between different dark energy proposals when combined with standard distance modulus or the Hubble rate techniques. I conclude that more yet more accurate measurements are required if observations are to successfully confirm or rule out these models as potential candidates against a cosmological constant. I also analyze possible effects that may mimic or underlie cosmic acceleration effects. I focus on a potential lack of knowledge of the precise values of particular cosmological parameters such as the curvature and matter content of the universe. I find that even a small uncertainty in any of this two quantities leads to significant bias on the reconstruction of dark energy properties, when typical probes like the distance luminosity and the Hubble rate are considered. I conclude that in order to disentangle between these effects a combination of distance and expansion history measurements is required.

The work presented here looks at several aspects of searching for continuous gravitational waves from pulsars, often referred to simply as continuous waves or CWs. This begins with an examination of noise in the current generation of laser interferometer gravitational wave detectors in the region below ~100 Hz. This frequency region is of particular interest with regards to CW detection as two prime sources for a first CW detection, the Crab and Vela pulsars, are expected to emit CWs in this frequency range. The Crab pulsar's frequency lies very close to a strong noise line due to the 60 Hz mains electricity in the LIGO detectors. The types of noise generally present in this region are discussed. Also presented are investigations into the noise features present in the LIGO S6 data and the Virgo VSR2 data using a program called Fscan. A particular noise feature present during VSR2 was discovered with the use of Fscan, which I report on and show how it degrades the sensitivity of searches for CWs from the Vela pulsar using this data. I next present search results for CWs from the Vela pulsar using VSR2 and VSR4 data. Whilst these searches did not find any evidence for gravitational waves being present in the data, they were able to place upper limits on the strength of gravitational wave emission from Vela lower than the upper limit set by the pulsars spin-down, making it only the second pulsar for which this milestone has been achieved. The lowest upper limit derived from these searches confines the spin-down energy lost from Vela due to gravitational waves as just 9% of Vela's total spin-down energy. The data from VSR2 and VSR4 are also examined, analysis of hardware injections in these datasets verify the calibration of the data and the search method. Similar results are also presented for a search for CWs from the Crab pulsar, where data from VSR2, VSR3, VSR4, S5 and S6 are combined to produce an upper limit on the gravitational wave (GW) amplitude lower than has been previously possible, representing 0.5% of the energy lost by the pulsar as seen through its spin-down. The same search method is also applied to analyse data for another 110 known pulsars, with five of these being gamma-ray pulsars that have been timed by the Fermi satellite. GWs from the pulsars timed by Fermi are expected at frequencies below 40 Hz, the LIGO detectors are not calibrated below these frequencies but the Virgo detector is. Hence the data used to search for GWs from these pulsars is the Virgo VSR4 data. The other 105 pulsars were analysed using out of date ephemerides obtained for the LIGO S5 run and the data analysed was from the LIGO S6 run, hence the results obtained for these pulsars are presented as an indication of what results can be expected with updated ephemerides only. For these 110 pulsars the spin-down limits were not able to be beaten, although there are a few pulsars for which this may be able to be achieved with an analysis combining all the possible datasets, in particular J1913+1011. The final part of this thesis reports extensions to the search method used for the analyses previously described. The first way in which this search method is extended is the use of a nested sampling algorithm to perform the parameter estimation stage of the analysis which was previously preformed using a MCMC. The nested sampling code also allows for model selection through the computation of the Bayesian evidence, I present results from characterisation tests of this nested sampling search code that demonstrate the equivalence of its results to those from the MCMC and grid based codes. The other extension to the search method looks at a new CW emission mechanism from a neutron star with a pinned superfluid core that is misaligned from the star's principle axes. This emission model predicts CWs at both the stars spin frequency f and twice its spin frequency 2f, providing an extra data channel with which to perform a search when compared to the triaxial rotator model which only emits at 2f. I present the development of a search for the emission from this new model, tests of the algorithm developed using simulated data, and results from a search using actual data from the VSR4 run for CWs from the Crab pulsar. The testing of the search algorithm shows that the posterior for the model is sufficiently complex to inhibit useful parameter estimation, but that the computation of the Bayesian evidence allows one to distinguish between this model and the triaxial rotator given a low SNR signal in the f data channel.

This thesis concerns the analysis of continuous gravitational waves from neutron stars with non-axisymmetric rotational motion using data from ground-based interferometric gravitational wave detectors, the development of a computationally efficient algorithm for analysis of this data and the use of this algorithm in follow up searches, which were previously too computationally expensive to consider.

The process of converting gas into stars underpins much of astrophysics, yet many fundamental questions surrounding this process remain unanswered. For example -- how sensitive is star formation to the local environmental conditions? How do massive and dense stellar clusters form, and how does this crowded environment influence the stars that form within it? How do the most massive stars form and is there an upper limit to the stellar initial mass function (IMF)? Answering questions such as these is crucial if we are to construct an end-to-end model of how stars form across the full range of conditions found throughout the Universe. The research described in this thesis presents a study that utilises a multi-scale approach to identifying and characterising the early precursors to young massive clusters and high-mass proto-stars, with a specific focus on the extreme environment in the inner few hundred parsecs of the Milky Way -- the Central Molecular Zone (CMZ). The primary sources of interest that are studied in detail belong to the Galactic centre dust ridge -- a group of six high-mass (M ~ 10^(4-5) Msun), dense (R ~ 1-3 pc, n > 10^(4) cm^(-3)), and quiescent molecular clouds. These properties make these clouds ideal candidates for representing the earliest stages of high-mass star and cluster formation. The research presented makes use of single-dish and interferometric far-infrared and (sub-)millimetre observations to study their global and small-scale properties. A comparison of the known young massive clusters (YMCs) and their likely progenitors (the dust ridge clouds) in the CMZ shows that the stellar content of YMCs is much more dense and centrally concentrated than the gas in the clouds. If these clouds are truly precursors to massive clusters, the resultant stellar population would have to undergo significant dynamical evolution to reach central densities that are typical of YMCs. This suggests that YMCs in the CMZ are unlikely to form monolithically. Extending this study to include YMCs in the Galactic disc again shows that the known population of YMC precursor clouds throughout the Galaxy are not sufficiently dense or central concentrated that they could form a cluster that then expands due to gas expulsion. The data also reveal an evolutionary trend, in which clouds contract and accrete gas towards their central regions along with concurrent star formation. This is argued to favour a conveyor-belt mode of YMC formation and is again not consistent with a monolithic formation event. High angular resolution observations of the dust ridge clouds with the Submillimeter Array are presented. They reveal an embedded population of compact and massive cores, ranging from ~ 50 - 2150 Msun within radii of ~ 0.1 - 0.25 pc. These are likely formation sites of high-mass stars and clusters, and are strong candidates for representing the initial conditions of extremely massive stars. Two of these cores are found to be young, high-mass proto-stars, while the remaining 13 are quiescent. Comparing these cores with high-mass proto-stars in the Galactic disc, along with models in which star formation is regulated by turbulence, shows that these cores are consistent with the idea that the critical density threshold for star formation is greater in the turbulent environment at the Galactic centre.

In order to understand the effects of low keV radiation upon small scales, a number of detector designs have been developed to investigate the ways energy is deposited. This research was conducted in order to investigate a number of different detector designs, looking in particular at their properties as small scale dosimeters exposed to photon radiation with an energy of 5-50 keV. In addition to this, Monte Carlo models were constructed of the different detector designs in order to ascertain the trends in energy absorption within the detectors. An important part of the research was investigating the dose enhancement effects produced when the low Z elements present in human tissues are in proximity to higher Z metallic elements within this energy range. This included looking at dose enhancement due to the photoelectric effect, with a photon energy of 5-50 keV and through the absorption of thermal neutrons. The reason for studying the dose enhancement was twofold - looking at the increase in energy absorption for elements that are currently being investigated for medical applications as well as elements that are present in dosemeters alongside the tissue equivalent elements. By comparing the results produced using the Monte Carlo codes MCNP4C and EGSnrc, simulations were produced for a variety of different detector designs, both solid state and gasfilled. These models were then compared with experimental results and were found to be able to predict trends in the behaviour of some of the detector designs.

The aim of this thesis is to study stellar evolution and asteroseimology of red-giant stars mainly from a modelling point of view, in particular the impact on core-convective-burning stars of adopting different mixing schemes. Thanks to NASA space telescope Kepler, asteroseismology of thousands of giants provided us new information related to their internal structure, that can be used for finding constraints on their cores. I used several stellar evolution codes (MESA, BaSTI, and PARSEC) to investigate the effect of different mixing schemes in the helium-core-burning stars. Comparing them with observed stars, I concluded that standard stellar models, largely used in literature, cannot describe the combined observed distribution of luminosity and period spacing. I then proposed as solution a penetrative convection model with moderate overshooting parameter. Additional tests on Kepler's open clusters (NGC6791 and NGC6819) and secondary clump stars, allowed me to revised to my mixing model.

The main focus of the thesis is the study of low-degree low-frequency solar p modes from the analysis of high-resolution power spectra generated from 20 years of high-quality data collected by the Birmingham Solar Oscillations Network (BiSON) Resonant Scattering Spectrometers (RSS). To that end we present a novel model of the RSS and its observations that allows for the determination of a significant improvement in calibration for ground-based Sun-as-a-star Doppler velocity observations. We show that the previously neglected multiple scattering in the RSS vapour cell is significant and demonstrate its impact on the spatial weighting to the solar disk, combining the new instrumental weighting with a detailed treatment of terrestrial atmospheric effects and a model of the solar surface velocity field. The resulting simulation allows for the development of a new and successful correction for differential atmospheric extinction generating up to a 25% increase in the signal-to-noise ratio at low frequencies (0.8 to 1.3 mHz). The improvement in signal to noise allows for the detection of low-frequency p modes with small associated errors in frequency and together with the fitting of mode structure, produces estimates of mode linewidth and power. Over the frequency range 972 to 1850 microHz we find the exponent of the frequency-linewidth dependence to be 7.5(0.4).

First, we investigated the ν max scaling relation, a widely-used equation that states that the frequency of maximum amplitude in a power spectrum scales with a combination of surface gravity and effective temperature. We tested how well the oscillations of cool main-sequence and sub-giant stars follow this relation, using a ensemble of asteroseismic targets observed by Kepler. We then tested seismic scaling relations in a small group of 10 bright red-giant stars observed by Kepler. These giants, some of the brightest observed in the Kepler field, have precise values of parallaxes. We compared the measured distances with inferences made using asteroseismic parameters. We also combined high-quality spectroscopic data with seismic constraints to determine their evolutionary phase. We compared the observed surface abundances of lithium and carbon with models that account for additional mixing processes in redgiants. Finally, we analyzed a group of 13 stars observed by Kepler, and use asteroseismic tools to extract modelindependent information about their internal regions. Our objective is to detect the so-called acoustic glitches, characterized as departures from the uniform frequency spacings predicted by the asymptotic relation. Such departures originate in regions where there is an abrupt change in the stratification of the star.