Models with extra dimensions have attracted much interest recently. The reason is that string theory, a serious candidate for the theory of unification of all interactions, predicts the existence of up to seven extra dimensions. These models have many attractive features and may turn out to provide the solution for many long standing problems in physics. One interesting and very attractive idea is that our visible universe is confined to a four-dimensional hypersurface in a higher-dimensional spacetime. This membrane like universe was dubbed brane-world.

Inflation is a period of the very early universe during which the universe undergoes an extremely rapid expansion. Vacuum fluctuations of any light scalar field are enlarged by the expansion and generate the primordial density perturbations of the universe on all scales. This thesis is about these perturbations generated during inflation and their observational consequences. After introducing inflationary cosmology we calculate the power spectra of primordial curvature and isocurvature perturbations from a general two field inflation model including next-to-leading order correction terms in a slow-roll expansion.

One of the aims of modern cosmology is to understand the evolution of the Universe from its genesis. Many of the observational tools used in mapping the evolution of the Universe center on the use of astrophysical objects whose intrinsic properties are well understood. These standard candles and rulers have, with measurements of their observed characteristics and knowledge of their intrinsic characteristics, allowed the observable Universe to be explored. In this work we adopt standard clocks to explore the evolution of the Universe. We record the age of a population of quiescent, passively evolving galaxies estimated at different epochs. From this chronology we are able to trace the evolution of the Universe through redshift and place constraints on ΛCDM cosmological models.

The first half of this Thesis investigates the observational features of dark energy dominated cosmologies. Using semi-analytic expressions for the Cold Dark IVlatter (CDM) halo mass function , we make predictions for halo abundance in models where t.he dark energy scalar field is coupled to CDM. "'e find that the mass function is a sensitive probe of coupled dark energy models, and that for certain forms of the dark energy potential, these models can offer a viable alternative to the standard ACDlVI paradigm. A generic prediction of string theory is the existence of multiple axion fields - giving rise to an axiverse. VVe couple such an axion to its corresponding modulus, extending the original axiverse scenario. A dynamical systems analysis is presented, which reveals how the axion and modulus fields leave observable signatures as both dark matter and dark energy. Among these signatures include multiple periods of accelerated expansion in the matter era, the presence of dark energy at early times, and future cosmological collapse. "'e use such phenomena to place constraints on the model parameter space, leaving open a small region that is observationally acceptable. Rather than appealing to fundamental theory to describe dark energy, we can reconstruct its properties in a model independent way. VIle introduce a parametrisation of the dark energy equation of state which uses the dark energy density as a time variable. Using our parametrisation, we place constraints on the time evolution of dark energy which are considerably tighter than those obtained using existing parametrisations. The second half of this Thesis examines the sensitivity of multi- field inflationary observables to perturbative reheating. Vve include in our analysis the local- type non- linearity parameter fi-?Lal , the spectral index of the primordial curvature perturbations nc;, and the tensor-to- scalar ratio TT. Vle demonstrate with concrete examples that if the curvature perturbation is still evolving after inflation has ended, the primordial observables can acquire substantial reheating dependent corrections. By way of a sudden decay calculation, which we verify with numerical simulations, we show that if both fields participate in the reheating phase, the corrections to the primordial observables can be within the sensitivity of CMB experiments such as Planck. Our analytic results are very general, encompassing the curvaton scenario, the inflating curvaton scenario, and any applicable multi- field model between these two regimes. Our results imply that the statistics of (, especially fi-fLal , can only be reliahly used as a probe of multi- field inflation if the physics of perturbative reheating is accounted for.

X-ray observations of the Galactic Centre (GC) can provide invaluable insight into the physical processes within this unique region. In this thesis we investigate the nature of the GC diffuse X-ray emission. We have utilised archival XMM-Newton data to create image mosaics of the central 100 pc region in both continuum bands spanning the 2–10 keV energy range and in a set of narrow bands encompassing various emission lines. We use these mosaics, along with complementary X-ray spectral information, to show that the bulk of the very-hot thermal emission (kT ~ 7:5 keV) can be explained in terms of a population of unresolved point sources, most likely magnetic CVs. Our analysis argues against the requirement for significant amounts of very-hot diffuse plasma. We also investigate a number of soft thermal features. We propose that the putative outflow from Sgr A* may result from the collimated winds of massive stars in the Central Cluster, or alternatively from outbursts on the central supermassive black hole. We show that the characteristics of the soft thermal emission to the north-east of Sgr A* are consistent with those of a mixed- morphology supernova remnant. The final feature considered is a looped structure to the south of the Galactic Plane. Our analysis supports the hypothesis that this is a superbubble deriving from the interaction of supernova shocks with the surrounding ISM. We also investigate the nature and properties of the intermediate-to-faint Galactic Bulge point source population based on a sample of 3610 X-ray source detections from the 2XMMi-DR3 catalogue. We conclude that the spectrally-soft sources are largely coronally-active systems, whereas the spectrally-hard sources are a more heterogeneous set, albeit the majority of which are accreting binaries.

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This thesis paper describes QUaD, a millimetric polarimeter designed to precisely measure the temperature and polarization anisotropics in the cosmic microwave background (CMB) radiation. QUaD was built and integrated between 2000 and 2004, and commissioned at the south pole in the austral summer of 2004/2005. First light was achieved in February 2005, and QUaD acquired a full austral winter of data during that year. Modifications to the instrument were performed in the austral summer of 2005/2006, and QUaD is currently taking a second season of data. A third season is planned. QUaD is composed of a cryogenically cooled receiver mounted on a Cassegrain telescope using a 2.6 m primary mirror. The detectors are polarization sensitive Neutron Transmutation Doped germanium bolometers which are coupled to the optics via single- moded corrugated feed horns. These detectors are split between two frequency bands, 100 and 150 GHz, and the optics allow angular resolutions of 6.0 and 4.0 arcminutes in these two bands. The performance of the instrument has been characterized using extensive laboratory data, a battery of tests performed during commissioning, and specially designed measurements during the observing season. QUaD uses constant elevation azimuth scans with 15 degree lengths, allowing reconstruction of all 6 CMB power spectra in the range 50 < &pound; < 2000. Customized software and analysis methods are required to transform data time series from the telescope to refined data products. QUaD data have provided the most accurate measurements of the &pound;-mode CMB power spectrum to date, as well as measurements of T-mode power competitive with the best high angular resolution experiments. These measurements provide a wealth of information, and both confirm our model for the early Universe as well as constrain cosmological parameters. Furthermore, QUaD measures the formation of structure after recombination, and can constrain the physics of inflation.

We study the coherent temperature and polarization patterns produced in homogeneous but anisotropic cosmological models. We show a range of results for all Bianchi types with a Friedman-Robertson-Walker limit (i.e. Types I, V, VIIo, VII and IX) to illustrate the possible behaviours that can obtained. We show that localized features in the temperature pattern, perhaps similar to the cold spot observed in the Wilkinson Microwave Anisotropy Probe (WMAP) data, can be generated in models with negative spatial curvature, i.e. Bianchi types V and VII. In Bianchi VII/j, however, rotation of the polarization angle as light propagates along geodesies can convert E modes into B modes but in Bianchi V this does not happen. We also discuss general properties of the radiation fields. We then consider two possible ways to characterize patterns in CMB temperature maps generated by these anisotropic Bianchi cosmological models. The Pixel Distribution Histogram (PDH) is used as the basis of a discussion of what we mean by 'Gaussianity'. The other method uses multipole vectors, which are de rived from the perspective of polynomial defined on the 2-sphere. Although the PDH study gives us limited information to characterize the statistical properties of Bianchi universes, the method using multipole vectors demonstrates a very strong correlation between the multipoles and suggests this could be useful for diagnosing the presence of global asymmetry in the cosmic microwave background.

X-ray surveys of three Canada-France Redshift Survey (CFRS) fields using XMM-Newton are presented, with the aim of studying the Active Galactic Nuclei (AGN) and galaxy cluster populations in these fields. The X-ray sources detected in these surveys resolve 51% of the X-ray background (XRB) in the 0.5 10 keV X-ray band. The relation between the X-ray and sub-mm extra-galactic backgrounds is investigated using a combination of X-ray data and sub-mm data. The X-ray properties of the sub-mm sources and visa versa indicate that the XRB is domi nated by accretion onto super-massive black holes, while the sub-mm background is dominated by dust-obscured star formation. X-ray sources are identified with optical objects using the Canada-France Deep Fields (CFDF) survey, which covers the majority of two fields. The redshift dis tribution of the AGN shows a clear peak at z 0.7. The 2-point angular correlation function, W(6), is calculated for the identified AGN but no significant clustering is detected. However, the results are consis tent with X-ray selected AGN being good tracers of the normal, inactive galaxy population. The environments of moderate luminosity AGN at z 0.5 are investigated, using the clustering amplitude measure Bgq and close pair counts. When compared to a control sample of equivalent inactive galaxies no difference is found between the respective environments. Minor mergers with low mass companions is therefore the most likely mechanism by which these AGN are fuelled. A new method for finding high redshift, optically selected, galaxy clusters is presented and is compared to X-ray selection. It is found that most optically selected clusters may have lower than expected X-ray luminosities suggesting that they are dynamically young compared to X-ray selected clusters.