Observations to date have shown that the Universe is probably flat and composed mainly of baryons (",4%), dark matter (",26%) and dark energy ("'70%). Even though these observations provide us with a wealth of information, they fail to explain the nature of more than 90% of our Universe or to include known particles such as neutrinos in the standard model of Cosmology. We attempt here to show how future experiments can, with 'all hemisphere' surveys of the sky, probe further properties of dark energy that might give us an answer as to what its nature is and also show a concrete signal of neutrinos in cosmology. A deep redshift survey would be able to produce an accurate measurement of the power spectrum at different redshifts, thus helping us to probe the evolution of the components of the universe, including dark energy (Abdalla & Rawlings, 2005). A high resolution imaging survey of the sky would allow us to measure the shear power spectrum accurately and provide results orthogonal to existing or future constraints (Rawlings et al., 2004). Both surveys will be available, in the radio part of the spectrum, with the next generation of radio telescopes/interferometers. Even though this next generation of instruments will only be available by the next decade, theoretical predictions on these models are currently required given that these projects are being designed now (Blake et al., 2004). We estimate the possible answers these surveys will provide during the next decade in the case of dark energy (Abdalla et al., 2006) and neutrinos (Abdalla & Rawlings, 2006).

In this thesis I study the spatial distribution of galaxies, haloes and dark matter particles using a suite of state-of-the-art cosmological N-body simulations of the growth of structure in the Universe. The subjects investigated are conceptually divided into three areas.

In this thesis we place constraints on models of the late-time acceleration of the universe. We put constraints on DGP braneworld models using supernovae, the cosmic microwave background shift parameter, and the baryon oscillation peak in the SDSS luminous red galaxy sample. The flat self-accelerating DGP model is marginally allowed by the combined likelihood, but the standard Lambda COM model fits the data better. The best fit of the flat non-selfaccelerating DGP model corresponds to the LCDM limit, which therefore gives a slightly better fit to the data, having oi less parameter. However, there is still a region of non-LCDM parameter space allowed by observations. We then use Bayesian statistics to compare a set of general relativistic dark energy models with a set of DGP models. Using Hubble rate data from differential iges of galaxies, combined with other background tests, we find that the flat self-accelerating DGP model is strongly disfavoured with respect to the flat LCDM model, and that braneworld models are generally disfavoured. We also show that all the models with more than two parameters are poorly constrained.

Recent observations arising from a number of sources suggest that the Universe is currently undergoing a period accelerated expansion. The standard approach to modeling this behaviour is to introduce a new component total energy density of the Universe. This so called dark energy drives the accelerated expansion by virtue of having negative pressure. To date, there exists no compelling explanation as to the physical nature of dark energy. Various models exist, however they are typically plagued by fine tuning issues and unnanaturally suppressed couplings to the Standard Model. An alternative explanation of the observed acceleration is to postulate that at low curvature (that is, at large scales), gravity is modified from its standard General Relativistic form. The aim of this approach is to construct a theory in which the late time acceleration of the Universe arises naturally as a consequence of modifying the Einstein equations. In this thesis, we will be concerned with the cosmological evolution of two classes of modified gravity models. We begin with a study of so called F(R) models, where an arbitrary function F(R) introduced into the gravitational action. We compile a set of simple consistency conditions that these models must satisfy in order to possess no dangerous instabilities, and in doing so constrain tje form that the function F(R) may take. We then construct a model which satisfies these conditions. In order for our F(R) function to be observationally viable, we find that it must mimic General Relativity very closely throughout the cosmological history. We discuss the genererality of this conclusion.

No description available.

The nature of dark matter remains one of the biggest questions in physics today. Weakly Interacting Massive Particles (WIMPs) are a particularly well motivated candidate for the missing matter that makes up 85% of the mass of the Universe. The most promising method for an unambiguous proof of the existence of WIMPs is via detection of the predicted directional anisotropy. The DRIFT detector at the Boulby Underground Laboratory in the UK is the world's first large scale directionally sensitive dark matter detector. This thesis presents work focussing on the ability of DRIFT to be competitive with non-directional detectors in exploring new spin-dependent WIMP interaction phase-space. Experimental efforts towards this are discussed, including the first calibration measurements of spin-dependent target gases in DRIFT, and development and implementation of an automated gas mixing system required for spin-dependent gas mixture operation. This thesis presents the first long-term study of backgrounds in DRIFT in which current limiting backgrounds are identified and studied, providing information crucial to future background reduction strategies. Developments of the WIMP analysis procedure are presented that result in an improved sensitivity to WIMP-mimicking neutron-induced nuclear recoils by a factor of 2.4. Data from the first runs with spin-dependent sensitive CS2-CF4 gas mixtures are presented with improved analysis methods. This thesis presents the first blind analysis results from a directionally sensitive dark matter detector with upper limits on the SD WIMP-proton interaction cross-section with a minimum of 0.93 pb for a 100 GeV WIMP.

Work contributing to experimental limits on WIMP dark matter scattering in a liquid xenon target is presented. The ZEPLIN-III detector at Boulby completed 83 days’ continuous operation in 2008 and, following a hardware upgrade, a further 319 days in 2010–11. Inelastic dark matter hypotheses to explain the DAMA/LIBRA modulation as scattering from iodine are excluded with ≥ 87% confidence using 2008 data. Upper limits on the elastic WIMP-nucleon cross section are set from the combined exposure, with a minimum of 3.9 × 10⁻⁸ pb for 50 GeVc⁻² WIMP mass at 90% CL, in addition to competitive limits on the WIMP-neutron spin-dependent cross section. The detector responses to scintillation and ionisation—throughout the fiducial volume and over the duration of the run — are characterised via calibration with ⁵⁷Co and ¹³⁷Cs sources, as well as sidebands in search data. Signal spectra are modeled using these responses and an in situ measurement of the energy-dependent light and charge yields of nuclear recoils. Analysis software, event selection and background estimation are all described. Confidence intervals based on sparse regions of parameter space (Yellin limits) and the likelihood ratios are implemented for ZEPLIN-III and discussed in the context of rare-event searches with significant background uncertainty.

Forthcoming wide field weak lensing surveys, such as DES and Euclid, present the possibility of using lensing as a tool for precision cosmology. This means exciting times are ahead for cosmological constraints for different gravity and dark energy models, but also presents possible new challenges in modelling, both non-standard physics and the lensing itself. In this thesis I look at how well DES and Euclid will be able to discriminate between different cosmological models and utilise lensing’s combination of geometry and growth information to break degeneracies between models that fit geometrical probes, but may fail to fit the observed growth. I have focussed mainly on the non-linear structure growth regime, as these scales present the greatest lensing signal, and therefore greatest discriminatory power. I present the predicted discriminatory power for modified gravities models, DGP and f(R), including non-linear scales for DES and Euclid. Using the requirement that modified gravities must tend to general relativity on small scales, we use the fitting formula proposed by Hu & Sawicki to calculate the non-linear power spectrum for our lensing predictions. I demonstrate the improved discriminatory power of weak lensing for these models when non-linear scales are included, and show that not allowing for the GR asymptote at small scales can lead to an overestimation in the strength of the constraints. I then parameterise the non-linear power spectrum to include the growth factor, and demonstrate that even including these extra parameters there is still more power in a full non-linear analysis than just using linear scales. I then present non-linear weak lensing predictions for coupled dark energy models using the CoDECS simulations. I obtain predictions for the discriminatory power of DES and Euclid in distinguishing between ΛCDM and coupled dark energy models; I show that using the non-linear lensing signal we could discriminate between ΛCDM and exponential constant coupling models with β0 ≥ 0.1 at 99.994% confidence level with a DES-like survey, and β0 ≥ 0.05 at 99.99994% confidence level with Euclid. I also demonstrate that estimating the coupled dark energy models’ non-linear power spectrum, using the ΛCDM Halofit fitting formula, results in biases in the shear correlation function that exceed the survey errors. I then present weak lensing predictions for DES and Euclid, and the CMB temperature power spectrum expected for Planck for fast transition adiabatic unified dark matter models. I demonstrate that in order to constrain the parameters in this model a high and low redshift observational probe is required. I show that for a ΛCDM fiducial, Planck could constrain zt > 5 at a 95% confidence level, and DES and Euclid could constrain the maximum time the transition can take to < 5 × 10−6/H0 at a 95% confidence level. Finally I look at a full general relativistic model of lensing. I adopt the use of a Lemaitre-Tolman-Bondi model, with and without pressure, to model an overdensity in an expanding background in a continuous spacetime. I use this to examine how the modelling of intermediate scales affects lensing quantities, and whether, as has been suggested recently, the cosmological constant has a direct effect on the lensing observables.

In 1998 astronomical observations of distant stars exploding at the ends of their lives led to the discovery that the expansion of the Universe is accelerating. This is likely to be caused by an intrinsic part of Einstein’s General Theory of Relativity known as the cosmological constant, but naturalness issues and the need to improve observational tests have motivated the study of alternative models of the Universe. The research in this thesis is part of ongoing efforts to pin down the cause of late-time acceleration by better understanding these alternatives and their signatures in cosmological observations. One such alternative is known as interacting dark energy and would be caused by additional matter in the Universe, as yet unknown to particle physics. This would interact with another unknown particle called dark matter that has been part of the standard model of cosmology since the 1970’s. The first part of this thesis contains a review of works on interacting dark energy and investigates a particular version of the model which had not been studied in detail before, placing recent observational constraints on its parameters. Another alternative to the cosmological constant is known as modified gravity, where General Relativity is extended by the addition of new degrees of freedom. Theories of modified gravity are mathematically related to some models of interacting dark energy and can appear very similar in cosmological observations. The second part of this thesis investigates the extent to which the two can be distinguished using current observational data.

Hidden sector or dark sector states appear in many extensions to the Standard Model (SM), to provide particle mediators for dark matter in the universe or to explain astrophysical observations such as the positron excess in the cosmic microwave background radiation flux. A hidden or dark sector can be introduced with an additional U(1)d dark gauge symmetry. The discovery of the Higgs boson in 2012 during Run 1 by the Large Hadron Collider (ATLAS and CMS) opens a new and rich experimental program for Beyond Standard Model physics (BSM) based on the Higgs Portal. This exotic discovery route uses couplings to the dark sector at the Higgs level, which were not experimentally accessible before. This thesis presents the searches of possible exotic decays: H → ZdZ(d) → 4` where Zd is a dark vector boson. It had been initiated in the Run 1 period of the LHC using the ATLAS detector at CERN. The results showed (tantalizingly) two signal events where none were expected, so that in the strict criteria of High Energy Physics, the result was not yet statistically significant. The Run 1 analysis for a 8 TeV collision energy is further developed in Run 2 with a 13 TeV collision energy, to expand the search area, take advantage of higher statistics, a higher Higgs production cross section, and substantially better performance of the ATLAS detector. In this work, the search is further broadened and includes allowing the mass of the originating boson (the dark Higgs S) to vary from the SM value. This allows the search for the dark vector boson to also explore higher or lighter masses than the SM Higgs boson. This extended search is efficient and could include a more general class of models, with the mass constraint of the SM Higgs portal lifted. This thesis reviews the analysis results from Run 1 and Run 2, and presents its iteration in the full Run 2 search by focusing on its new channel where the additional scalar S (with mS 6= mH) decays to 4` via two dark vector boson states Zd . The case where the Higgs decays to 4` via two Zd (H → ZdZd → 4`) and also called high mass channel, has been just unblinded. Nineteen data events are observed where 14 were predicted. In overall, the data are consistent with the Monte Carlo prediction. No evidence of deviation from the Standard Model expectations are observed. / College of Engineering, Science and Technology / Ph. D. (Physics)