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\\( \\)Low density cosmic voids gravitationally lens the cosmic microwave background (CMB), leaving a negative imprint on the CMB convergence \\(\\kappa\\). This effect provides insight into the distribution of matter within voids, and can also be used to study the growth of structure. We measure this lensing imprint by cross-correlating the Planck CMB lensing convergence map with voids identified in the Dark Energy Survey Year 3 data set, covering approximately 4,200 deg\\(^2\\) of the sky. We use two distinct void-finding algorithms: a 2D void-finder which operates on the projected galaxy density field in thin redshift shells, and a new code, Voxel, which operates on the full 3D map of galaxy positions. We employ an optimal matched filtering method for cross-correlation, using the MICE N-body simulation both to establish the template for the matched filter and to calibrate detection significances. Using the DES Y3 photometric luminous red galaxy sample, we measure \\(A_\\kappa\\), the amplitude of the observed lensing signal relative to the simulation template, obtaining \\(A_\\kappa = 1.03 \\pm 0.22\\) (\\(4.6\\sigma\\) significance) for Voxel and \\(A_\\kappa = 1.02 \\pm 0.17\\) (\\(5.9\\sigma\\) significance) for 2D voids, both consistent with \\(\\Lambda\\)CDM expectations. We additionally invert the 2D void-finding process to identify superclusters in the projected density field, for which we measure \\(A_\\kappa = 0.87 \\pm 0.15\\) (\\(5.9\\sigma\\) significance). The leading source of noise in our measurements is Planck noise, implying that future data from the Atacama Cosmology Telescope (ACT), South Pole Telescope (SPT) and CMB-S4 will increase sensitivity and allow for more precise measurements.

We study a phenomenological class of models where dark matter converts to dark radiation in the low redshift epoch. This class of models, dubbed DMDR, characterizes the evolution of comoving dark matter density with two extra parameters, and may be able to help alleviate the observed discrepancies between early- and late-time probes of the universe. We investigate how the conversion affects key cosmological observables such as the CMB temperature and matter power spectra. Combining 3x2pt data from Year 1 of the Dark Energy Survey, {\\it Planck}-2018 CMB temperature and polarization data, supernovae (SN) Type Ia data from Pantheon, and baryon acoustic oscillation (BAO) data from BOSS DR12, MGS and 6dFGS, we place new constraints on the amount of dark matter that has converted to dark radiation and the rate of this conversion. The fraction of the dark matter that has converted since the beginning of the universe in units of the current amount of dark matter, \\(\\zeta\\), is constrained at 68\\% confidence level to be \\(<0.32\\) for DES-Y1 3x2pt data, \\(<0.030\\) for CMB+SN+BAO data, and \\(<0.037\\) for the combined dataset. The probability that the DES and CMB+SN+BAO datasets are concordant increases from 4\\% for the \\(\\Lambda\\)CDM model to 8\\% (less tension) for DMDR. The tension in \\(S_8 = \\sigma_8 \\sqrt{\\Omega_{\\rm m}/0.3}\\) between DES-Y1 3x2pt and CMB+SN+BAO is slightly reduced from \\(2.3\\sigma\\) to \\(1.9\\sigma\\). We find no reduction in the Hubble tension when the combined data is compared to distance-ladder measurements in the DMDR model. The maximum-posterior goodness-of-fit statistics of DMDR and \\(\\Lambda\\)CDM model are comparable, indicating no preference for the DMDR cosmology over \\(\\Lambda\\)CDM.

Cosmological analyses with type Ia supernovae (SNe Ia) often assume a single empirical relation between color and luminosity (\\(\\)) and do not account for varying host-galaxy dust properties. However, from studies of dust in large samples of galaxies, it is known that dust attenuation can vary significantly. Here we take advantage of state-of-the-art modeling of galaxy properties to characterize dust parameters (dust attenuation \\(A_V\\), and a parameter describing the dust law slope \\(R_V\\)) for the Dark Energy Survey (DES) SN Ia host galaxies using the publicly available BAGPIPES code. Utilizing optical and infrared data of the hosts alone, we find three key aspects of host dust that impact SN Ia cosmology: 1) there exists a large range (\\(1-6\\)) of host \\(R_V\\) 2) high stellar mass hosts have \\(R_V\\) on average \\(0.7\\) lower than that of low-mass hosts 3) there is a significant (\\(>3\\)) correlation between the Hubble diagram residuals of red SNe Ia that when corrected for reduces scatter by \\(13\\%\\) and the significance of the ``mass step'' to \\(1\\). These represent independent confirmations of recent predictions based on dust that attempted to explain the puzzling ``mass step'' and intrinsic scatter (\\(_ int\\)) in SN Ia analyses. We also find that red-sequence galaxies have both lower and more peaked dust law slope distributions on average in comparison to non red-sequence galaxies. We find that the SN Ia \\(\\) and \\(_ int\\) both differ by \\(>3\\) when determined separately for red-sequence galaxy and all other galaxy hosts. The agreement between fitted host-\\(R_V\\) and SN Ia \\(\\) \\& \\(_ int\\) suggests that host dust properties play a major role in SN Ia color-luminosity standardization and supports the claim that SN Ia intrinsic scatter is driven by \\(R_V\\) variation.

The orientation of triaxial galaxy clusters with respect to the line-of-sight is expected to be one of the prime sources of scatter and potential bias in optical observables (e.g., richness and weak-lensing signal) of galaxy clusters. In this work, we use the observed shape of the central Brightest Cluster Galaxy (BCG) as proxy for the orientation along the line-of-sight for clusters selected via the Sunyaev-Zel'dovich (SZ) effect from the South Pole Telescope (SPT) and Atacama Cosmology Telescope (ACT) surveys, matched to optically selected clusters from the Dark Energy Survey Year 3 (DES). We construct two samples of clusters that are designed to be identical in SZ mass estimate and redshift but with the roundest vs. the most elliptical BCGs, which we expect to correspond to BCGs (and clusters) with major axes aligned along the line-of-sight vs. in the plane of the sky, respectively. We find that the optical richness of round-BCG clusters is \\(\\sim 10\\)\\% larger than that of elliptical-BCG clusters, in agreement with the expectation from projection effects and presenting the first such detection in data. The density profiles, however, are not in agreement with the expectation from projection effects: the 1-halo term (below \\(6~h^{-1}\\rm{Mpc}\\)) of both the weak-lensing and galaxy density profiles are the same for the subsamples, contrary to previous studies based on X-ray selected clusters. In the 2-halo regime (above \\(6~h^{-1}\\rm{Mpc}\\)), we find a significant excess of the elliptical-BCG cluster profiles compared to the round-BCG cluster profiles, which is the opposite of the expectation from numerical simulations. We hypothesize that the intrinsic shape of the BCG reflects not just the orientation angle, but also intrinsic properties of the cluster which can affect both the SZ signal and the amplitude of the 2-halo term.

A typical Bayesian inference on the values of some parameters of interest \\(q\\) from some data \\(D\\) involves running a Markov Chain (MC) to sample from the posterior \\(p(q,n | D) L(D | q,n) p(q) p(n),\\) where \\(n\\) are some nuisance parameters. In many cases, the nuisance parameters are high-dimensional, and their prior \\(p(n)\\) is itself defined only by a set of samples that have been drawn from some other MC. Two problems arise: first, the MC for the posterior will typically require evaluation of \\(p(n)\\) at arbitrary values of \\(n,\\) ıe\\ one needs to provide a density estimator over the full \\(n\\) space from the provided samples. Second, the high dimensionality of \\(n\\) hinders both the density estimation and the efficiency of the MC for the posterior. We describe a solution to this problem: a linear compression of the \\(n\\) space into a much lower-dimensional space \\(u\\) which projects away directions in \\(n\\) space that cannot appreciably alter \\(L.\\) The algorithm for doing so is a slight modification to principal components analysis, and is less restrictive on \\(p(n)\\) than other proposed solutions to this issue. We demonstrate this ``mode projection'' technique using the analysis of 2-point correlation functions of weak lensing fields and galaxy density in the Dark Energy Survey, where \\(n\\) is a binned representation of the redshift distribution \\(n(z)\\) of the galaxies.

We present the statistical methods that have been developed to analyse the OzDES reverberation mapping sample. To perform this statistical analysis we have created a suite of customisable simulations that mimic the characteristics of each source in the OzDES sample. These characteristics include: the variability in the photometric and spectroscopic lightcurves, the measurement uncertainties, and the observational cadence. By simulating the sources in the OzDES sample that contain the CIV emission line, we developed a set of criteria that rank the reliability of a recovered time lag depending on the agreement between different recovery methods, the magnitude of the uncertainties, and the rate at which false positives were found in the simulations. These criteria were applied to simulated light curves and these results used to estimate the quality of the resulting Radius-Luminosity relation.We grade the results using three quality levels (gold, silver and bronze). The input slope of the R-L relation was recovered within \\(1\\sigma\\) for each of the three quality samples, with the gold standard having the lowest dispersion with a recovered a R-L relation slope of \\(0.454\\pm 0.016\\) with an input slope of 0.47. Future work will apply these methods to the entire OzDES sample of 771 AGN.

In order to place constraints on cosmology through optical surveys of galaxy clusters, one must first understand the properties of those clusters. To this end, we introduce the Mass Analysis Tool for Chandra (MATCha), a pipeline which uses a parallellized algorithm to analyze archival Chandra data. MATCha simultaneously calculates X-ray temperatures and luminosities and performs centering measurements for hundreds of potential galaxy clusters using archival X-ray exposures. We run MATCha on the redMaPPer SDSS DR8 cluster catalog and use MATCha's output X-ray temperatures and luminosities to analyze the galaxy cluster temperature-richness, luminosity-richness, luminosity-temperature, and temperature-luminosity scaling relations. We detect 447 clusters and determine 246 r2500 temperatures across all redshifts. Within 0.1 < z < 0.35 we find that r2500 Tx scales with optical richness as ln(kB Tx / 1.0 keV) = (0.52 \\pm 0.05) ln({\\lambda}/70) + (1.85 \\pm 0.03) with intrinsic scatter of 0.27 \\pm 0.02 (1 {\\sigma}). We investigate the distribution of offsets between the X-ray center and redMaPPer center within 0.1 < z < 0.35, finding that 68.3 \\pm 6.5% of clusters are well-centered. However, we find a broad tail of large offsets in this distribution, and we explore some of the causes of redMaPPer miscentering.

Periodically variable quasars have been suggested as close binary supermassive black holes. We present a systematic search for periodic light curves in 625 spectroscopically confirmed quasars with a median redshift of 1.8 in a 4.6 deg\\(^2\\) overlapping region of the Dark Energy Survey Supernova (DES-SN) fields and the Sloan Digital Sky Survey Stripe 82 (SDSS-S82). Our sample has a unique 20-year long multi-color (\\(griz\\)) light curve enabled by combining DES-SN Y6 observations with archival SDSS-S82 data. The deep imaging allows us to search for periodic light curves in less luminous quasars (down to \\(r{\\sim}\\)23.5 mag) powered by less massive black holes (with masses \\(\\gtrsim10^{8.5}M_{\\odot}\\)) at high redshift for the first time. We find five candidates with significant (at \\(>\\)99.74% single-frequency significance in at least two bands with a global p-value of \\(\\sim\\)7\\(\\times10^{-4}\\)--3\\(\\times10^{-3}\\) accounting for the look-elsewhere effect) periodicity with observed periods of \\(\\sim\\)3--5 years (i.e., 1--2 years in rest frame) having \\(\\sim\\)4--6 cycles spanned by the observations. If all five candidates are periodically variable quasars, this translates into a detection rate of \\({\\sim}0.8^{+0.5}_{-0.3}\\)% or \\({\\sim}1.1^{+0.7}_{-0.5}\\) quasar per deg\\(^2\\). Our detection rate is 4--80 times larger than those found by previous searches using shallower surveys over larger areas. This discrepancy is likely caused by differences in the quasar populations probed and the survey data qualities. We discuss implications on the future direct detection of low-frequency gravitational waves. Continued photometric monitoring will further assess the robustness and characteristics of these candidate periodic quasars to determine their physical origins.

Binary supermassive black holes (BSBHs) are expected to be a generic byproduct from hierarchical galaxy formation. The final coalescence of BSBHs is thought to be the loudest gravitational wave (GW) siren, yet no confirmed BSBH is known in the GW-dominated regime. While periodic quasars have been proposed as BSBH candidates, the physical origin of the periodicity has been largely uncertain. Here we report discovery of a periodicity (P=1607\\(\\)7 days) at 99.95% significance (with a global p-value of ~\\(10^-3\\) accounting for the look elsewhere effect) in the optical light curves of a redshift 1.53 quasar, SDSS J025214.67-002813.7. Combining archival Sloan Digital Sky Survey data with new, sensitive imaging from the Dark Energy Survey, the total ~20-yr time baseline spans ~4.6 cycles of the observed 4.4-yr (restframe 1.7-yr) periodicity. The light curves are best fit by a bursty model predicted by hydrodynamic simulations of circumbinary accretion disks. The periodicity is likely caused by accretion rate modulation by a milli-parsec BSBH emitting GWs, dynamically coupled to the circumbinary accretion disk. A bursty hydrodynamic variability model is statistically preferred over a smooth, sinusoidal model expected from relativistic Doppler boost, a kinematic effect proposed for PG1302-102. Furthermore, the frequency dependence of the variability amplitudes disfavors Doppler boost, lending independent support to the circumbinary accretion variability hypothesis. Given our detection rate of one BSBH candidate from circumbinary accretion variability out of 625 quasars, it suggests that future large, sensitive synoptic surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time may be able to detect hundreds to thousands of candidate BSBHs from circumbinary accretion with direct implications for Laser Interferometer Space Antenna.

\\( \\)Low density cosmic voids gravitationally lens the cosmic microwave background (CMB), leaving a negative imprint on the CMB convergence \\(\\kappa\\). This effect provides insight into the distribution of matter within voids, and can also be used to study the growth of structure. We measure this lensing imprint by cross-correlating the Planck CMB lensing convergence map with voids identified in the Dark Energy Survey Year 3 data set, covering approximately 4,200 deg\\(^2\\) of the sky. We use two distinct void-finding algorithms: a 2D void-finder which operates on the projected galaxy density field in thin redshift shells, and a new code, Voxel, which operates on the full 3D map of galaxy positions. We employ an optimal matched filtering method for cross-correlation, using the MICE N-body simulation both to establish the template for the matched filter and to calibrate detection significances. Using the DES Y3 photometric luminous red galaxy sample, we measure \\(A_\\kappa\\), the amplitude of the observed lensing signal relative to the simulation template, obtaining \\(A_\\kappa = 1.03 \\pm 0.22\\) (\\(4.6\\sigma\\) significance) for Voxel and \\(A_\\kappa = 1.02 \\pm 0.17\\) (\\(5.9\\sigma\\) significance) for 2D voids, both consistent with \\(\\Lambda\\)CDM expectations. We additionally invert the 2D void-finding process to identify superclusters in the projected density field, for which we measure \\(A_\\kappa = 0.87 \\pm 0.15\\) (\\(5.9\\sigma\\) significance). The leading source of noise in our measurements is Planck noise, implying that future data from the Atacama Cosmology Telescope (ACT), South Pole Telescope (SPT) and CMB-S4 will increase sensitivity and allow for more precise measurements.