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We are now entering into an era of huge cosmological data sets, with observations testing the current paradigm to high accuracy. By combining different probes of cosmology, we can access new information about our Universe and investigate the systematic effects that plague our ability to measure cosmology accurately. In this thesis we use weak gravitational lensing as a tool to measure the large scale structure in the Universe. First, we present the theoretical framework in which this thesis is set. We then detail the methodology required to measure the cross-correlation between galaxy weak lensing and cosmic microwave background (CMB) weak lensing data sets. In particular we highlight the impact of the complicated nature of galaxy weak lensing data and investigate ways to improve the signal-to-noise of the cross-correlation measurement using simulations. We measure the CMB lensing/galaxy weak lensing power spectrum using the latest CMB lensing data from the Atacama Cosmology Telescope (ACT) and galaxy weak lensing data from the Kilo Degree Survey (KiDS), making a detection at 5σ significance. We compare our measured cross-spectrum to the cosmological parameters inferred from the Planck and KiDS experiments. We then present fits for the mat- ter density and the amplitude of matter fluctuations, marginalising over the galaxy weak lensing systematic biases. We forecast the expected signal-to-noise using the next data release from ACT and use simulated fields to investigate the impact on estimating cosmological parameters with that data set. Finally, we measure the masses of clusters selected from ACT CMB data via the Sunyaev-Zel’dovich effect using weak gravitational lensing data from KiDS, making a 20σ detection of the lensing signal. We present an estimate of the mass bias for a set of stacked clusters as well as the highest signal-to-noise individual clusters. We also describe tests of systematic effects and analysis choices.

The inflationary ΛCDM cosmology currently provides an accurate description of the Universe. It has been tested using several observational techniques over a wide redshift range, and it provides a good fit to most of them. In addition, it is a surprisingly economical model, requiring only six parameters to characterize the background cosmology and its fluctuations. In this model, the Universe is dominated by a cosmological constant Λ driving an accelerated expansion, and by cold dark matter. The strongest constraints on parameters to date come from observations of the temperature and polarization anisotropies of the cosmic microwave background measured by the Planck satellite. There are, however, indications of features in the Planck power spectra, possible differences with high redshift ground-based CMB experiments, and 'tensions' between Planck and low redshift measurements of the Hubble constant and weak gravitational lensing. In this thesis, we review possible tensions and extensions to the Planck cosmology, at both high and low redshifts. We begin with the high redshift analysis, using the Planck data to test models which introduce oscillatory features in the primordial power spectrum. We also study possible departures from slow roll inflation using the generalized slow-roll formalism, which allows for order unity deviations. Although we find models which give marginal improvements on the temperature or polarization power spectra, the combination of temperature and polarization is found to be consistent with a featureless power-law primordial spectrum. We then focus on measurements of the polarized CMB sky by the South Pole Telescope collaboration, who report tension between their measurements and the ΛCDM cosmology and with the cosmological parameters determined by Planck. We find evidence of a high χ2 in the SPTpol spectra which is unlikely to be cosmological. We report consistency between the Planck and SPTpol polarization spectra over the multipoles accessible to Planck (l ∼< 1500). We then investigate tension at low redshifts. We begin with weak gravitational lensing in which a number of surveys have suggested that the amplitude of the fluctuation spectra is lower than the Planck value. We review the small-angle approximations commonly used in galaxy weak lensing analyses and their effect on cosmological parameters. We find that these approximations are perfectly adequate for present and near future experiments. We find internal inconsistencies in the recent KiDS-450 analysis involving photometric redshifts and the KiDS covariance matrix at large scales. Finally, we investigate the difference between measurements of the present day expansion rate of the Universe. We apply a novel parameterization of the inverse distance ladder to determine the present date value of the Hubble parameter H0, which assumes General Relativity but makes no further assumptions about systematic errors or the nature of dark energy. Our analysis uses baryon acoustic oscillation data and Type Ia Supernovae to constrain the expansion history assuming a value of the sound horizon determined from the CMB. Our results are in tension with recent direct determinations of H0. We conclude that this tension, if real, cannot be solved by modifications of the ΛCDM model at late times. Instead, we would require a modification of the theory at early times which reduces the sound horizon. We conclude that at this time there is no compelling evidence that conflicts with the ΛCDM cosmology either at low or at high redshifts.

This thesis presents spectroscopic observations of z ≥ 7 galaxy candidates in the Hubble Ultra Deep Field, which were selected with HST WFC3 imaging, using the Lyman-Break technique. Four z-band (z ≈ 7) dropout galaxies were targeted with Gemini/GNIRS, one z-band dropout galaxy and three Y -band (z ≈ 8 − 9) dropout galaxies with VLT/XSHOOTER, and 22 z-band dropouts with VLT/FORS2, where 15 of the latter are strong candidates. No evidence of Lyman-α emission is found, and the upper limits on the Lyman-α flux and the broad-band magnitudes are used to constrain the rest-frame equivalent widths for this line emission. Amongst the targeted objects, observations were made of HUDF.YD3, a relatively bright Y -band dropout galaxy likely to be at z ≈ 8 − 9 on the basis of its colours in the HST ACS and WFC3 images. Lehnert et al. (2010) observed this galaxy using the VLT/SINFONI integral field spectrograph and claim that it exhibits Lyman-α emission at z = 8.55. In observations of this object described in this thesis, which were made with VLT/XSHOOTER and Subaru/MOIRCS, this line was not reproduced despite the expected signal in the combined MOIRCS & XSHOOTER data being 5σ. Hence it appears unlikely that the reported Lyman-α line emission at z > 8 is real. Accounting for incomplete spectral coverage, in total (across all spectro- graphs) 9.63 z-band dropouts and 1.15 Y -band dropouts are surveyed to a Lyman-α rest-frame Equivalent Width better than 75 ̊A. A model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z = 3−6.5 is inconsistent with these non-detections at z = 7−9 at a confidence level of ∼ 91%, which may indicate that a significant neutral HI fraction (χHI) in the intergalactic medium suppresses the Lyman-α line at z > 7. In particular, the lack of detection of Lyman-α emission in this spectroscopy is compared with results at lower redshift by Stark et al. (2010), who derive a mapping between Lyman-α fractions and χHI based on radiative transfer simulations by McQuinn et al. (2007). These results suggest a lower limit of χHI ~ 0.5.

This DPhil dissertation presents two new and independent samples of faint radio sources. The first sample is the 37 SXDS radio sources with flux densities at 1.4-GHz above 2 mJy, a spectroscopic completeness of 65% and a median redshift z_med ≈ 1.1. The second sample is the 47 TOOT00 radio sources with flux densities at 151 MHz above 100 mJy, a spectroscopic completeness of 85% and z_med ∼ 1.25. Optical, near- and mid-IR photometry, optical spectroscopy, and radio observations are used in the analysis and comparison of the two samples. The quasar fraction in the TOOT00 radio sources is 0.13 < f_q < 0.25 above the FRI/FRII break in radio luminosity, while use of 24 μm data reveals objects with significant but sometimes obscured accretion and gives quasar-mode fraction of 0.5 → 0.9 above the FRI/FRII break. The FRI/FRII divide seen at z < ∼ 0.5 is also observed at z ∼ 1 for FRII objects in the TOOT00 and SXDS samples, but examples of FRI radio sources above the FRI/FRII break do exist. The total number of the TOOT00 objects and their distribution are consistent with simulations based on extrapolations from previous work, while for the SXDS objects the results are only broadly similar. Based on that comparison, the redshift spikes seen at z ∼ 1.3 in TOOT00 and at z ∼ 0.65 & 2.7 in SXDS appear to be significant, and might be due to Large-Scale Structure. A V/Vmax test suggests the cosmic evolution of the TOOT00 and SXDS samples, is different. The TOOT00 radio sources are 2-times more luminous in host-galaxy starlight than the SXDS radio sources. The almost proportionality between radio luminosity at 1.4 GHz and 24 μm luminosity suggests that L_rad traces accretion luminosity and L[OII] ∝ L_rad^0.7 may reflect imperfections in the L[OII]-accretion luminosity scaling. Mid-IR 24 μm observations in the SXDS sample suggest that 30% of the light from the nucleus is absorbed by the torus and re-emitted in the mid-IR, while ∼ 1% of the light is scattered above and below the torus.

Since the discovery that the large-scale dynamics of galaxy clusters are dominated by dark matter, cosmologists have aspired to measure the spatial distribution of dark matter and identify its nature. Gravitational lensing, especially employing the Hubble Space Telescope (HST) has emerged as the tool-of-choice for mapping dark matter. Standing on the shoulders of the pioneering 1990's, this thesis is the first homogeneous lensing study of clusters with HST. We measure the mass and structure of an objectively-selected sample of X-ray luminous clusters at a single epoch (z ~ 0.2). We present observations often clusters (L(_x)≥8x10(^44)[0.1=2.4 keV] ergs(^-1)) and use the numerous gravitationally-lensed features in these data to constrain a detailed model of the central regions (r ~ 500 kpc) of each cluster. Our models provide an unprecedented view of cluster morphology, revealing that 60% of the sample contain significant substructure. Chandra X-ray observations confirm this is a signature of dynamical immaturity, and show that the mean temperature of the intra-cluster medium of the morphologically complex clusters is ~ 25% higher than then regular siblings. This offset results in a critical, and previously unexplored, systematic uncertainty in the use of clusters to normalise the mass power spectrum. We also use the detailed morphology of the clusters to constrain the nature of dark matter. We then exploit the clusters as gravitational telescopes, using ground-based near-infrared imaging to construct a sample of 60 gravitationally magnified Extremely Red Objects (EROs), a population that is believed to harbour important clues on the formation epoch and mechanism of massive galaxies. This unique sample overcomes the faintness of EROs (R ≥ 23, K ≥ 18) to uncover a wealth of morphological, photometric and spectroscopic evidence of diversity in both passively evolving and dusty active EROs. Coupled with our deep number counts (to K ~ 22), these observations provide important new constraints on competing theoretical models of galaxy formation.

Modern observational Cosmology is highly developed and the observable Universe on a large scale is currently well described by the standard LCDM model. The model has a small number of free parameters and describes how the Universe evolved from a hot, dense and homogeneous state with primordial perturbations that are Gaussian, adiabatic and close to scale-invariant. The inflation paradigm extends the LCDM model and interprets the initial conditions as natural consequences of a hypothesized exponential expansion. However, generic inflationary models make a prediction that has not yet been observed: the existence of a background of primordial gravitational waves (PGWs), which would leave an imprint of B-mode polarization in the Cosmic Microwave Background (CMB). Measuring the degree-scale B-mode polarization therefore emerges as one of the most promising methods to detect or set limits on PGWs. For the past decade, the BICEP/Keck collaboration has been operating a series of telescopes at the Amundsen-Scott South Pole Station optimized for B-mode observation. These telescopes are compact refracting polarimeters mapping about 2% of the sky under the exceptionally stable and transparent atmosphere of the Geographical South Pole. They observe at a broad range of frequencies to separate the cosmological signals from polarized Galactic synchrotron and thermal dust emission which dominate at the two ends of the microwave regime. In this dissertation, we discuss the BICEP/Keck experimental progress in two major areas. We first present the "BK18 analysis" utilizing data collected up to the 2018 observing season, in conjunction with selected WMAP and Planck polarization maps. The analysis particularly exploits new data from (1) the 3-year BICEP3 map, the current deepest CMB polarization measurement at the foreground-minimum 95 GHz; and (2) the Keck 220 GHz map which has a higher signal-to-noise ratio on the dust foreground than the Planck 353 GHz map. The likelihood analysis of the BB auto- and cross-spectra of the maps reduces the experimental uncertainty on the tensor-to-scalar ratio r to sigma(r)=0.009, and the inference of r from our baseline model is tightened to r=0.014+0.010-0.011 and r

The composition and evolution of galaxies have been an elemental but long-standing mystery in Astronomy. In the last century, the advent of telescopes across the electromagnetic spectrum has revolutionized our perception of galaxies from a mere assembly of stars to a complex ecosystem. Both observational and theoretical studies have pointed towards the existence of a gaseous medium beyond the stellar component of galaxies, aka, the circumgalactic medium (CGM).The CGM is a multi-phase gas surrounding the stellar disk of a galaxy, filling up its dark-matter halo. The CGM is simultaneously the fuel tank, waste dump, and recycle hub of galaxies. It is expected to harbor the baryons, metals, and feedback that are missing from the stellar disk. I have studied the two extreme phases of the CGM to investigate how the feeding (accretion) and the feedback (outflow) at the galactic scale govern the evolution of the Milky Way and similar nearby galaxies.The ≥106 K hot CGM, despite being challenging to detect, is a treasure trove of galaxy evolution. By probing the hot CGM of the Milky Way (MW) using X-ray absorption lines of multiple metal ions, I have discovered a super-virial 107 K phase coexisting with the well-known virialized 106 K phase, featuring non-solar abundance ratios of light elements, α-enhancement, and non-thermal line broadening. I have also detected this super-virial phase of MW CGM in X-ray emission analyses. Detection of these surprising properties of the CGM along multiple directions in the sky suggests a strong connection between the hot CGM and past Galactic outflow(s).Observations of MW-like galaxies complement our observations of the Milky Way. I have discovered the hot CGM emission of an MW-mass galaxy NGC 3221 that is extended (~150-200 kpc) and is massive enough to account for its missing baryons. The CGM is not isothermal, with the CGM within 100 kpc of NGC 3221 being super-virial, and fainter along the minor axis than the global average. These results, at par with my findings in the Milky Way, compel us to rethink the impact of galactic feedback on the hot CGM of star-forming galaxies without an active nucleus.The accretion rate of neutral hydrogen (HI) to the disk of nearby galaxies as observed with radio interferometers is insufficient to explain the star formation rate in these galaxies. It indicates that the rest of the circumgalactic hydrogen might be diffuse and extended, beyond the scope of interferometers. I look for 21-cm emission from the diffuse neutral CGM of two MW-mass galaxies NGC 891 and NGC 4565 using a single-dish radio telescope because it can probe all spatial scales. By designing a novel approach complementary to the mapping, and by comparing the measurements between single-dish and interferometers, we have detected the diffuse, extended (>100 kpc), massive (108 M☉), neutral CGM at an unprecedented sensitivity of 1016 cm-2. The scale height and mass distribution of this HI are consistent with the bathtub model of star formation, attesting to the relation between HI accretion and star formation.We report the measurement of the thermal Sunyaev-Zel'dovich (SZ) Effect in the CGM of 9.8 ≤ log(M*/M☉) ≤ 11.3 galaxies at zWISE and SuperCosmos with the Compton-y maps derived from the combined data of Planck and Atacama Cosmology Telescope. We revise the data analyses method (stacking, aperture photometry, volume integral) compared to previous studies. We detect the tSZ effect down to 1010.4 M☉ galaxies at >99.9% confidence, and there is >80% evidence of tSZ effect down to 1010.1 M☉ galaxies.

A precise determination of the light-element abundances produced during the epoch of Big Bang Nucleosynthesis (BBN) is an important probe of the early universe. The Deuterium-Hydrogen abundance ratio (D/H) is of particular importance due to its sensitivity to the cosmological baryon density, its use as a constraint of non-standard BBN scenarios and the fact that Deuterium is not created outside of BBN, but destroyed inside of stars at a predictable rate, so any Deuterium observed in interstellar or intergalactic gas is primordial. Due to the difficulty of measurements and relative rarity of appropriate conditions required to observe D in quasar absorption systems, there are only 15 published measurements of the primordial D/H in literature. Here we examine the primordial Deuterium-to-Hydrogen abundance ratio using high-resolution optical spectroscopy of diffuse gas towards two quasars. We measure \DI\ towards the bright Quasar J1201$+$0116 at moderate redshift ($z=2.98$) in a Lyman-Limit system (N=$10^{17.41}\cmm$) which exhibits very low metal content indicating pristine conditions ([Si/H]=$-3.3 \pm 0.8$). This value, $10^5$D/H=$2.50\pm 0.18$ is in complete agreement with D/H from the CMB and theory. We measure \DI\ in one component of DLA towards quasar J0744$+$2059 (N=$10^{20.8} \cmm$) and measure D/H to be $ 2.359 \pm 0.095$ \textemdash $2\sigma$\ lower than D/H from theory and the CMB. Additionally, we present measurements on the \lyaf\ towards 25 high-resolution QSO. The statistics we report are suitable for comparison to simulations, which are the primary means of extracting physical information from the Intergalactic Medium.

The Dark Energy Survey (DES) is an ongoing cosmological survey intended to study the properties of the accelerated expansion of the Universe. In this dissertation, I present work of mine that has advanced the progress of DES.First is an introduction, which explores the physics of the cosmos, as well as how DES intends to probe it. Attention is given to developing the theoretical framework cosmologists use to describe the Universe, and to explaining observational evidence which has furnished our current conception of the cosmos. Emphasis is placed on the dark sector – dark matter and dark energy – the content of the Universe not explained by the Standard Model of particle physics. As its name suggests, the Dark Energy Survey has been specially designed to measure the properties of dark energy. DES will use a combination of galaxy cluster, weak gravitational lensing, angular clustering, and supernovae measurements to derive its state of the art constraints, each of which is discussed in the text. The work described in this dissertation includes science measurements directly related to the first three of these probes.The dissertation presents my contributions to the readout and control system of the Dark Energy Camera (DECam); the name of this software is SISPI. SISPI uses client-server and publish-subscribe communication patterns to coordinate and command actions among the many hardware components of DECam – the survey instrument for DES, a 570 megapixel CCD camera, mounted at prime focus of the Blanco 4-m Telescope. The SISPI work I discuss includes coding applications for DECam’s filter changer mechanism and hexapod, as well as developing the Scripts Editor, a GUI application for DECam users to edit and export observing sequence SISPI can load and execute.Next, the dissertation describes the processing of early DES data, which I contributed. This furnished the data products used in the first-completed DES science analysis, and contributed to improving the collaboration-wide treatment of the data. The science measurements themselves are also detailed. We verified DES’s capabilities for performing weak lensing analyses by measuring the masses of four galaxy clusters, finding consistency with previous measurements, and utilized DECam’s wide field-of-view for a photometric study of filament-like structures in the fields.Finally, my recent work with Balrog is presented. Balrog is a simulation toolkit for embedding fake objects into real survey images in order to correct for systematic biases. We have used Balrog to extend DES galaxy clustering measurements down to fainter limits than previously possible, finding results consistent with higher-resolution space-based data. The methodology used in this analysis generalizes beyond galaxy clustering alone, and promises to be useful in future imaging survey measurements.

I present an investigation into the local warm-hot gaseous environment of the Milky Way as observed through highly ionized metal absorption lines in ultraviolet and X-ray spectra. These X-ray lines (primarily OVII) had been reported at redshifts consistent with zero in previous studies of background quasars; however, it has been unclear whether this gas exists close to the Galaxy (within a few tens of kpc) or extends far out into intergalactic space, thereby comprising most of the mass in the local universe. Additionally, highly-ionized OVI high-velocity clouds (HVCs), some of which are associated with the ubiquitous extended neutral hydrogen HVCs seen around the Galaxy, had been extensively studied. However, the distance to the OVI HVCs, and their relation to the X-ray lines, remained undetermined. With three of the highest-quality Chandra grating spectra of extragalactic sources to date, a large number of z=0 absorption lines are detected; the FUSE spectra of these same objects show low- and high-velocity OVI absorption. Using advanced curve-of-growth and ionization balance analysis, limits are placed on the velocity dispersion, temperature, and density of the warm-hot gas along these lines of sight. In none of these cases can the absorption be placed conclusively at Galactic or extragalactic distances. However, in two of the three cases (Mrk 421 and Mrk 279), the observed OVI UV absorption components are found to be inconsistent with the X-ray absorber, indicating that the X-ray absorption is either extragalactic or traces a previously undiscovered Galactic component. The third sightline (PKS 2155-304) exhibits absorption with properties more similar to Mrk 421 than Mrk 279; thus, there may be more than one physical process contributing to the observed absorption along any given sightline. While the X-ray components of this research exclusively employ Chandra data, the XMM-Newton mission can in principle be used for the same purpose. XMM's effectiveness in observations of WHIM lines is quantitatively analyzed in the context of two recently detected intervening WHIM systems toward Mrk 421. The XMM grating spectrograph is found to be inferior to Chandra/LETG due to lower resolution and narrow detector features that hinder the detection of unresolved lines.