Far-infrared observations have detected dusty star-forming galaxies, a subset of galaxies which is extremely dust-extincted from the ultraviolet down to near-infrared colours. Recent studies show that this population of sources contributes significantly to the history of star formation, especially out to very high redshift. Recent surveys with the Herschel Space Observatory have uncovered around half a million of these sources, with the largest of these surveys, the H-ATLAS, covering 616 square degrees. One of the most exciting discoveries is the lensing nature of the brightest of these sources, where the gravitational potential of a foreground galaxy lenses and amplifies the signal. The applications of gravitational lensing range from studying individual sources down to unprecidented resolution at high redshift in sub-mm wavelengths with ALMA, to cosmological studies by analysing the distribution of groups of lenses. In this thesis, I explore the effect of applying a more inclusive selection criterion for lensed sources, and study the properties of the sources that are selected. Whereas the first attempts at finding lensed sources use a strict S500μm > 100 mJy flux density cut, the sample I study is selected with a flux cut at 80 mJy: The Herschel Bright Sources (HerBS) sample. A photometric redshift cut of zphot > 2 is also taken, as most lensing takes place out at higher redshift. This redshift is calculated by fitting a spectral template to the 250, 350 and 500 μm observations from the Herschel SPIRE instrument. I push down the selection flux in order to select more lensed sources from the sub-mm surveys, whilst potentially including several unlensed sources. These unlensed sources could be among the most intrinsically luminous and star-forming objects in the Universe. Only less than five of such objects are known to exist, while our HerBS sample could contain up to 35 of these sources, which could teach us about the upper-limits of star-formation and their contribution to forming the most massive galaxies in the Universe. I use 850 μm SCUBA-2 observations on the James Clerk Maxwell Telescope (JCMT) to remove blazar interlopers, which results in 209 sources in the HerBS sample, after removing 14 blazar sources. At the time I wrote the paper upon which Chapter 2 is based, 24 sources had a spectroscopic redshift. I use this sub-sample to fit a two-temperature modified blackbody, and find a cold-body temperature of 21.3 K, a warm-body temperature of 45.8 K, a mass ratio of 26.7 and a dustemissivity index of 1.8. These values do not challenge the current knowledge of sub-mm galaxies, but the quality of the fit suggests a large diversity among the galaxies in the sub-sample, and that they are poorly fitted by a single template. This diversity is also found by the spectroscopic observations with the IRAM 30m-telescope observations on eight of the highest-redshift (zphot > 4) sources of the HerBS sample. We found five spectroscopic redshifts, with one of the sources at the highest known HerBS redshift at zspec = 4.8. The spectrum fitted in Chapter 2 shows a poor agreement with the photometric data points. The spatial resolution of the SPIRE instrument on Herschel is not fine enough to resolve the structure of these high-redshift sources. Worse still, the beam width is so large, ranging from 18 to 36 arcseconds, that it is unsure whether we observe a single galaxy, or perhaps observe multiple galaxies together. The beam width of the SCUBA-2 instrument at 850 μm is only 13 arcseconds. In the case the sample would be dominated by blended sources, one would expect to resolve several of the sources into their individual components. This is not seen in any of the continuum images, although the blended sources might be blended on scales smaller than 13 arcseconds. The IRAM-observations of two sources have detected multiple, contradicting spectral lines, suggesting we might be observing multiple sources, instead of a single source, that are aligned along the line-of sight. Unfortunately, only single spectral lines have been observed per source, and we are awaiting more observations verifying the blending nature of these sources, which are still expected to lie at high redshift. The hypothesis that our sample consists for a significant portion out of blended sources is in contradiction with multi-wavelength observations. When I look at the positions of these sources at different wavelengths, I find that most sources have a counterpart in these multi-wavelength observations, also when chance-encounters are considered. Considering the high redshift nature of our sources, together with the possibility of lensing, these counterpart sources are most likely foreground, lensing galaxies. I compare the positions of the HerBS sources to both the Sloan Digital Sky Survey (SDSS), which covers 121 out of the 209 sources, and the VISTA Kilo-Degree Infrared Galaxy (VIKING) survey, which covers 98 HerBS sources. For the SDSS counterparts, I use the H-ATLAS catalogue of counterpart sources, which was done by using a statistical estimator. This statistical estimator assumes a certain angular distribution between the sources in the Herschel position, and the opticalor near-infrared observations. I expect the majority of my sources to be lensed, and therefore I adjust the original angular distribution by including the effect of gravitational lensing. The adjustment is based on 15 ALMA observations of lensed, bright H-ATLAS sources. The revised analysis finds 41 counterparts, instead of the 31 that were found by the initial analysis. This catalogue is not available for VIKING counterparts, and therefore I had to do the entire analysis for the VIKING counterparts, starting from the VIKING fields. I use the sextractor package to extract the potential counterparts, and then derive the necessary estimators for the statistical method. I find a significantly different angular distribution, even than the one derived from the 15 ALMA observations of lensed H-ATLAS sources. The angular distribution extends to much larger angular scales, potentially suggesting a stronger contribution to galaxy-cluster lensing, which produces larger angular offsets due to the larger masses and different mass profiles associated with galaxy clusters. In total, I find 60 counterparts with a reliability greater than 80% to the 98 HerBS sources covered by VIKING. Possibly, not all counterparts could be positively identified, as the analysis showed 88% of sources has a source within 10 arcseconds when taking chance encounters into account. This is mostly due to ambiguity between several nearby sources, which causes a low reliability of the counterpart identification, but it does allow us to state that an counterpart could be present. A cosmological model suggest that 76% of our sources are gravitationally lensed. This model assumes a certain distribution of halo masses, and lensing magnification based on mass density profiles. The validity of these models has been shown with the 15 ALMA observations of lensed H-ATLAS sources, and also agree with the SMA observations from Bussmann et al. (2013). The IRAM observations provide me with both line luminosities and line velocity widths. Larger galaxies are expected to be brighter, and have larger line velocity widths. The five sources with confirmed redshift (and therefore line luminosity) have a luminosity-to-velocity width ratio agreeing with a magnification of around 10, when compared to unlensed, and known lensed sources. I show that the SDSS is not deep enough to observe all the foreground galaxies, while the VIKING observations agree with the results from the simulation, with 60 sources actually cross-compared, and 88% of sources have a source nearby, when accounting for random chance.

The Equivalence Principle is accepted as one of the most fundamental principles in modern Physics. However, theories towards the unification of the four forces typically predict violations of this principle. Testing it at a high sensitivity is expected to make a breakthrough in the current understanding of Physics. A space-based project, STEP (Satellite Test of the Equivalence Principle), aims at testing the principle to the level of 10\(^{−18}\). This corresponds to an improvement of the current limits, established by ground-based experiments, by approximately five orders of magnitudes. To achieve the sensitivity, imperfections in STEP test masses, such as density inhomogeneity and thermal distortion, could be a problem. This thesis presents preliminary work on the verification of STEP test masses. We have measured density inhomogeneities in materials intended to be used as STEP test masses (beryllium and niobium). In addition, we have developed a device to measure differential thermal expansion of samples that cannot be machined, by using a capacitive sensing method. It is shown that the device has a precision of approximately 0.3 % in the differential thermal expansion of beryllium. This device could in principle be applied for the measurements of the real STEP test masses in the final shape. Our analysis based on the results of our measurements and literature survey shows that it is feasible to obtain materials that satisfy STEP requirements.

The effective use of statistical techniques is one of the cornerstones of modern astrophysics. In this thesis we use sophisticated statistical methodology to expand our understanding of astrophysics. In particular, we focus on the physics of coalescing binary black holes, and the observation of these events using gravitational-wave astronomy. We use Fisher matrices to explore how much we expect to learn from gravitational-wave observations, and then use machine learning techniques, including random forests and Gaussian processes, to facilitate an otherwise intractable Bayesian comparison of real observations to our model. Finally, we develop a technique based on Gaussian processes for characterising stochastic variability in time series data.

This thesis presents the results from the Chandra Deep Group Survey, a survey dedicated to find high redshift groups of galaxies in deepest observations available in the Chandra archive. The catalogue compiled from this survey contains 26 groups and 36 clusters with available redshifts, with largest redshift being 1.3. This sample has been used to investigate the evolution of the thermal state of the gas at the centre of groups and compare this evolution to that of clusters. Different parameters have been used to quantify the strength of cool cores and it has been showed that groups and clusters have similar evolution of their cool core properties. Both classes of systems have a wide spread in the cool core strength at low redshifts, which then narrows at high redshifts showing a lack of strong cool core systems.

itqquad Then to the Heav'n itself I cried,qquad Asking, `What Lamp had destiny to guideqquad Her little Children stumbling in the Dark?'qquad And - `A blind understandingMissing data. Heav'n replied. qquadqquadqquad Rubaiyat of Omar Khayyam It seems that not everybody in Persia in the eleventh century was as convinced as the astrologers that the movements of the heavens controlled the destiny of Man. Nevertheless, for many centuries before and since, kings and emperors rewarded handsomely those astronomer/astrologers who could give them advice based on the movements of the planets and other celestial bodies. (There may be some astronomers today who would wish for similar generous patronage). Since the advent of modern celestial mechanics with the work of Isaac Newton, orbital motion has been studied for its own sake, and in the last thirty years, for the purposes of sending artificial satellites and manned craft into space. Yet for 300 years, one of the most important questions posed by celestial mechanics remains unanswered: are the motions of the planets in the Solar System stable? Could planets collide or even escape? Countless workers since Newton's time have sought Lamps to the destiny of the Solar System, but our Understanding is still obscured by many blind-spots. This thesis does not claim to give any definitive answers to these questions. It does indicate how to obtain quantitative estimates of the likelihood of certain events occurring. Simple statistical methods are applied to the results of numerical experiments and give probabilities of planetary orbits crossing or bodies escaping dynamical systems altogether. In Chapter 1 a general review of the problem of the Solar System's stability is given along with brief descriptions of methods and definitions of stability which have been used in the past. This thesis studies the stability of real and fictitious dynamical systems not necessarily associated with the Solar System. It investigates one particular definition of stability, namely hierarchical stability, using special perturbation methods. The definitions of hierarchical systems, hierarchical stability and empirical stability parameters are reviewed in Chapter 2. These will form the basis for subsequent numerical experiments. One further definition of stability - Hill stability is an important condition for hierarchical stability. It has been studied in a mathematically rigorous way in the problem of three massive bodies in mutually perturbed orbits. This analysis as well as some new numerical results are given in Chapter 3. Numerical integration experiments were carried out, with the aid of a mainframe computer, to study the period of time for which various three-body systems remain stable. Several hundred fictitious systems with different masses and starting conditions were studied. In each case, all three bodies' orbits lay in the same plane. In some systems, all the bodies orbited in the same direction (direct); for other systems, one body orbited in the opposite direction from the other two (retrograde). The results of these experiments are presented in Chapter 4 (for retrograde systems) and Chapter 5 (for direct systems). The results are grouped in such a way that analytical curves may be fitted to the data. This allows predictions of stability lifetimes for similar systems without the need for lengthy numerical integration experiments. Systems whose masses, initial positions and initial velocities fall into certain ranges are always stable. These regions of hierarchical stability are mapped out and compared with corresponding regions of Hill stability. In the case of direct systems, commensurabilities give rise to large fluctuations in stability lifetimes, if the initial conditions are varied slightly. Additional statistical methods are described in Chapter 5 to cope with this effect. In Chapter 6, the results of Chapters 4 and 5 are compared with real three-body systems within the Solar System. Possible origins of the Solar System are discussed in the light of the results.

This thesis presents four optical spectroscopic studies of absorption by matter in the diffuse interstellar medium on scales ranging from 10 AU to 10 kpc. The observations investigate two current problems in interstellar medium (ISM) research: small-scale structure (SSS), and the diffuse interstellar bands (DIBs). Very high spectral resolution observations of interstellar Na I, Ca I, Ca II, K I and CH absorption towards kappa Vel are presented. Combined with observations over the last 15 years taken from the literature, the small-scale structure in front of this star is probed on scales of ~ 10 AU. The high resolution and signal-to-noise of the new observations allow detailed modelling of the absorption profiles and the identification of multiple absorption components. For the two narrowest components, the line profile models are used to constrain the temperature, depletion, electron density and total number density within the structures. Diffuse interstellar bands are used as probes of SSS in long-slit observations of lines-of-sight towards three binary/multiple star systems: HD 168075/6, HD 176269/70 and four members of the mu Sgr system, one of which is identified as a member for the first time. The results show clear variations in DIB absorption in the HD 168075/6 and mu Sgr systems over scales of ~ 50,000-200,000 AU, and demonstrate the efficacy of medium-resolution observations of DIBs for identification of small-scale structure in the ISM. Multi-object spectroscopy of 452 stars in the omega Cen globular cluster is also presented, which probes interstellar absorption by Na I, Ca II and DIBs in two dimensions, on scales of ~ 1 pc. The first detections of diffuse interstellar bands in the M33 galaxy are reported. Multi-object spectroscopy of 43 stars is used to derive spectral types and reddenings, and measure DIB absorption across the disk of the galaxy (~ 10 kpc). Very strong DIB absorption per unit E(B-V) is found for one star in the sample, towards which a total of seven DIBs are detected.

The solar wind provides a unique natural "laboratory" in which plasma turbulence may be investigated in-situ. Turbulence is statistically reproducible. Thus, in this thesis we investigate plasma turbulence in the solar wind through the statistical study of magnetic field observations. In particular, we investigate single-satellite time-series of the magnetic field. We discuss hydrodynamic turbulence, and make parallels between hydrodynamic and magnetohydrodynamic turbulence. In hydrodynamic turbulence a unique scaling relation may be determined from dimensional analysis. Importantly, one may not derive a unique scaling relation for magnetohydrodynamic turbulence from dimensional analysis. Therefore, comparison of observations and turbulence models are key to determining the underlying physics for specific plasma parameters. The inertial range is a range of scales over which energy cascades from large to small temporal-spatial scales. This thesis will predominantly be focused on the anisotropy and scaling of the inertial range within the solar wind. We investigate how sampling a solenoid field, i.e. . B = 0, with a single satellite produces an apparent nonaxisymmetry with respect to the background magnetic field. We also investigate how time-series discontinuities produced by non-turbulent structures alter the statistical analysis of various anisotropy measures. We will find that the commonly held picture of the solar wind, that specific temporal-spatial scales have a distinct physical origin, is an over simplified model. We will show that non-turbulent structures must be removed from the observations in order to analysis the statistics of the turbulence accurately. The work in this thesis helps to constrain theories of plasma turbulence where there is a background magnetic field with a greater magnitude than the root-mean-square magnitude of the turbulent fluctuations.

Close binaries containing compact objects come in many different forms, but have one thing in common: their evolution involves at least one common envelope phase and angular momentum losses. However, many aspects of these two fundamental processes are still poorly understood. Ample observational input holds the key to improving our understanding, as only then can theoretical models be properly calibrated and tested. Close binaries containing a white dwarf are perhaps the best-suited class of objects to provide such input, due to their ubiquity. White dwarf binaries that additionally display eclipses are of added interest, as accurate and model-independent determinations of fundamental stellar parameters, such as the masses and radii of the binary components, can only be obtained in such systems. In this thesis, I present a study of eclipsing white dwarf binaries. I identify SDSSJ 0110+1326, SDSSJ 0303+0054, SDSSJ 1210+3347, SDSSJ 1435+3733 and SDSSJ 1548+4057 as new eclipsing, detached, post-common-envelope, white dwarf +M-dwarf binaries. I use follow-up photometric and spectroscopic observations, as well as a light curve fitting technique to measure their orbital periods, and derive the masses, radii and radial velocities of the binary components. These five systems have been identified as part of the first dedicated search for eclipsing post-common-envelope binaries and almost double the existing population. The measurements of the stellar parameters, and others obtained from similar systems, are of great value both for the calibration of the common envelope equations and for testing the mass-radius relations of white dwarfs and low-mass main sequence stars. I also identify HS 2325+8205 as a new eclipsing and very frequently outbursting dwarf nova. Combined constraints from photometric and spectroscopic observations are used to infer the binary and stellar parameters. The combination of eclipses, frequent outbursts, brightness range and high declination make HS 2325+8205 an ideal laboratory for detailed studies of accretion discs and accretion processes in close binaries. Finally I study the cataclysmic variable V455And, in an attempt to verify the presence of non-radial pulsations in the white dwarf primary. This is achieved by analysing ten-years worth of photometric observations using time-series analysis techniques and Fourier transforms. The results are indeed consistent with white dwarf pulsations, although a very complex behaviour of the power spectra is revealed, most likely a result of the rapid rotation of the accreting white dwarf primary.

The search for exoplanets has gone from the realm of speculation to being one of the most prolific topics of modern astronomy in the space of just 20 years. In particular, the geometric alignment of transiting exoplanets provides the added opportunity to measure a host of properties of these systems, including studies of planetary atmospheres. The vast majority of known transiting exoplanets to date were found using dedicated ground-based surveys such as the SuperWASP project. Such enterprises comprise of multiple small telescopes designed to perform high-precision photometry over a wide field of view and rely on efficiently compensating for several noise contributions. An analysis of the sources of noise in the SuperWASP light curves was performed, focussing on systematic e↵ects fixed in detector space. A study of a set of detector maps produced from the average of the fractional residuals of the light curves in CCD coordinates has revealed that the current flat-fielding strategy is introducing a component of red noise into the light curves due to the wavelengthdependent nature of the CCDs. The possibility of using such maps as a basis for an additional decorrelation step in the software pipeline is discussed. The next phase in planetary discoveries from ground-based surveys consists of the search for smaller planets and those in longer orbits around their host stars. This process involves an observing strategy that focuses on intensive coverage of particular locations of the sky. We develop simulation software to aid the choice of observed fields for the SuperWASP and Next Generation Transit Survey (NGTS) projects in order to maximise the chances of finding planets at those locations. Moreover, this simulation can be used for comparative studies of the planet finding probability for several design choices and has been used to justify the necessity to commission the NGTS instrument at ESO’s Paranal Observatory in order to benefit from one of the World’s premier sites. The increasing number of known transiting planets has triggered a new phase of exoplanet exploration, in which the properties of the atmospheres of these planets are being explored using techniques such as transmission spectroscopy. This process consists of measuring an enhanced transit depth at particular wavelengths due to the presence of opacity sources in the atmospheres of exoplanets. We use the multiband photometer ULTRACAM to attempt a similar measurement via the technique of transmission photometry for the highly inflated planets WASP-15b and WASP- 17b. The data are found to be dominated by systematic errors and a detailed study of the possible sources is performed.

One of the most surprising aspects of the exoplanet population is the existence of Jupiter sized planets orbiting close to their parent stars. It is currently uncertain how these planets reached such small separations, and they are thought to be markers for the dominant migration mechanism. The Wide Angle Search for Planets (WASP) project is ideally suited for studying these planets, as it has detected the largest number of hot Jupiters to date. I have inverted the observed sample of WASP planets to calculate the underlying population of hot Jupiters through a quantitative study of the selection biases in the WASP project. To achieve this, I synthesised transiting systems and inserted them into WASP data to calculate the probability of detection. The observed population ofWASP planets is then corrected through application of this probability to determine the underlying population. I find a clear pile up in the underlying population at orbital periods between 3 to 5 days, and apply a joint constraint with the underlying population measurement from the Kepler project to propose a new model for the underlying population of giant planets. I propose a model consisting of a rising power law win period with index 1:0 - 0:3, with a Gaussian excess at 3:7 - 0:1 days to model the period pile up. The observed period pile up places crucial constraints on models of hot Jupiter migration. The Next Generation Transit Survey (NGTS) is a new transiting exoplanet survey designed to find Neptunes and super Earths around nearby stars. These stars will be bright, allowing the characterisation of the bulk densities and atmospheric compositions by current and next generation instruments. These planets are numerous but they cause shallow transits, so we must achieve a higher level of precision than has been previously achieved by a wide angle ground based survey. To test the design characteristics and that the required high levels of precision would be achieved, prototype instruments were constructed on La Palma and Geneva. We found that we were able to reach < 0:1% precision on an ensemble of bright stars on the typical transit timescales, indicating that the instrument will be capable of detecting smaller planets. We use Monte Carlo simulations coupled with a detectability analysis to predict that NGTS will detect 200 Neptunes and 30 super Earths with the next generation instrument ESPRESSO.