Browsing by Subject "Cosmology"

Now showing 1 - 19 of 19
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    Bicep Array: Searching for Signals of Inflation From The South Pole
    (2022-01) Crumrine, Michael
    The $\Lambda$CDM cosmological model posits a universe that began with a big bang - like singularity, which contains mostly cold dark matter, and which is experiencing an accelerating expansion due to a dark energy component. This model has experienced resounding success and is consistently upheld by experiments across the globe. The model is incomplete however, it cannot describe how the specific initial conditions required to create the universe we see today came about. Inflation is an extension to the $\Lambda$CDM model which hypothesizes that the universe underwent a period of exponential expansion just after the big bang, sufficient to set up the required initial conditions. Most inflationary theories predict a stochastic gravitational wave background generated as a result of this expansion which would have imprinted a characteristic B-mode signal into the polarization pattern of the Cosmic Microwave Background. Detecting this primordial gravitational wave signal would provide direct evidence for inflation The \textsc{Bicep}/{\it Keck} program constitutes a series of polarization sensitive microwave telescopes situated at the geographic South Pole targeting the degree-angular scale $B$-modes and searching for a primordial signal. Over the last two decades this program has consistently reported the tightest constraints on this signal, with the most recent analysis of data through $2018$ providing an upper limit on the tensor-to-scalar ratio $r<0.036$ at $95\%$ confidence. {\sc Bicep} Array is the latest experiment in the series and replaces the {\it Keck Array}, expanding the frequency coverage to two new low-frequency bands and, once fully operational, increasing the detector count by over an order of magnitude. {\sc Bicep} Array is expected to achieve $\sigma(r) = 0.002 - 0.004$ depending on foreground complexity and the degree of lensing removal. In this dissertation I cover the design of this new experiment -- with a focus on the design and performance of the cryogenics down to $4$\,K -- and the first year's observations. I analyze the first year of new low frequency data in combination with the recently release BK18 results and find that the new data provides no improvement on $\sigma(r)$. However, it provides significant constraining power on galactic synchrotron radiation resulting in a factor of two decrease in the uncertainty on the amplitude of this foreground signal.
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    Calibration and design of the E and B EXperiment (EBEX) cryogenic receiver
    (2014-08) Zilic, Kyle Thomas
    I discuss the design, construction, and calibration of the E \& B EXperiment (EBEX), a balloon-borne telescope designed to measure the B-mode polarization anisotropy of the cosmic microwave background (CMB). EBEX observes the sky with 8 arcmin resolution in three frequency bands centered on 150, 250, and 410 GHz, with over 1,500 detectors. Polarimetry is performed through use of a continuously rotating achromatic half-wave plate with fixed wire-grid polarizer. The experiment was designed to detect the gravitational-lensed B-mode signal and detect or set an upper limit for the inflationary B-mode signal. In this thesis, I describe the design and structure of various subsystems of the EBEX receiver and predict their experimental performance. Several calibrating instrumental response experiments are described and the results reported and compared to predictions. A brief review of the 2012-2013 long duration balloon (LDB) flight from McMurdo Station, Antarctica, is provided and a summary of the receiver performance during flight characterized.
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    Coupled quantum systems in inflationary cosmology.
    (2010-08) Gumrukcuoglu, Ahmet Emir
    The studies presented in this thesis describe applications of quantum field theory in a time dependent background. Two distinct problems are addressed in the framework of inflationary cosmology. The strict predictions of inflation are mostly in agreement with the Cosmic Microwave Background observations. In the recent years, large scale anomalies in the data motivated a series of analyses leading to a detection of broken statistical isotropy. Assuming that this effect is sourced by early time cosmology, I discuss the phenomenology of inflationary models extended to anisotropic backgrounds. Due to lack of rotational invariance, these models generically involve a system of coupled quantum fields. This leads to a tensor-scalar correlation function, which is a characteristic signature of these models. Another open question in cosmology involves the transition from inflation to the Hot Big Bang cosmology. In the presence of supersymmetric flat directions, the formation of the thermal radiation may undergo a dramatic delay, provided that these directions decay only perturbatively. In the scope of a toy model and a realistic example, both involving two flat directions, I discuss the nonperturbative decay that rapidly depletes the flat directions. If realized, this process can dramatically affect the previous assumptions on the thermalization scale. Due to the vast number of degrees of freedom, this problem generically involves coupled quantum fields. The decay of the flat directions gets contributions from both the diagonal (nonadiabatic evolution of frequency eigenvalues) and nondiagonal (nonadiabatic evolution of frequency eigenstates) effects. An additional characteristic effect of coupled quantization is the rotation of light eigenstates to heavy ones, which do not get produced in a diagonal system.
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    Design, implementation, and calibration of physics Half-Wave Plate polarimetry for the E and B Experiment
    (2014-10) Klein, Jeffrey Michael
    The E and B Experiment (EBEX) is a balloon-borne telescope designed to measure the polarization of the Cosmic Microwave Background (CMB) and dust foregrounds at 10' scales and three frequency bands of 150 GHz, 250 GHz, and 410 GHz in order to detect or constrain B-mode polarization. Results may provide evidence to support the theory of cosmological inflation, or constrain specific models.EBEX's polarization measurement capability is implemented via continuously-rotating Half-Wave Plate (HWP) polarimetry. We discuss the design and implementation of the polarimetry hardware for the E and B Experiment (EBEX). In order to achieve low-temperature rotation of our 15 cm, 635 g achromatic HWP stack, we implement a unique application of a Superconducting Magnetic Bearing (SMB), building off an earlier prototype. We discuss design constraints, detail our implementation, and present results of tests of power dissipation, rotation speed stability, dynamic stability, and operational lifetime. We find power dissipation of 15 mW in our LDB configuration, and achieve successful operation of the system in both a 2009 test flight and a 2012 Long Duration (LDB) flight.We design and carry out calibration tests to verify our ability to measure polarized signals. We develop a data analysis pipeline to extract polarization measurements from the chopped polarized signals we use in calibration; we verify and optimize the performance of this pipeline with a simulation. We find that a thorough understanding of the time constants of EBEX's bolometric sensors is essential to measure polarization. We develop methods to measure and remove the effects of these time constants. Tests of polarization rotation across our bands verify predictions of rotation due to our achromatic HWP 5-stack. Polarized beam scans allow us to set an absolute calibration for EBEX with a standard deviation of 1.5 degrees.
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    The E and B EXperiment: implementation and analysis of the 2009 engineering flight.
    (2011-06) Milligan, Michael Bryce
    The E and B EXperiment (EBEX) is a balloon-borne telescope designed to map the polarization of the cosmic microwave background (CMB) and emission from galactic dust at millimeter wavelengths from 150 to 410 GHz. The primary science objectives of EBEX are to: detect or constrain the primordial B-mode polarization of the CMB predicted by in ationary cosmology; measure the CMB B-mode signal induced by gravitational lensing; and characterize the polarized thermal emission from interstellar dust. EBEX will observe a 420 square degree patch of the sky at high galactic latitude with a telescope and camera that provide an 80 beam at three observing bands (150, 250, and 410 GHz) and a 6:2#14; diffraction limited field of view to two large-format bolometer array focal planes. Polarimetry is achieved via a continuously rotating half-wave plate (HWP), and the optical system is designed from the ground up for control of sidelobe response and polarization systematic errors. EBEX is intended to execute y or more Antarctic long duration balloon campaigns. In June 2009 EBEX completed a North American engineering flight launched from NASA's Columbia Scientific Ballooning Facility (CSBF) in Ft. Sumner, NM and operated in the stratosphere above 30 km altitude for #24; 10 hours. During flight EBEX must be largely autonomous as it conducts pointed, scheduled observations; tunes and operates 1432 TES bolometers via 28 embedded Digital frequency-domain multiplexing (DfMux) computers; logs over 3 GiB/hour of science and housekeeping data to onboard redundant disk storage arrays; manages and dispatches jobs over a fault-tolerant onboard Ethernet network; and feeds a complex real-time data processing infrastructure on the ground via satellite and line-of-sight (LOS) downlinks. In this thesis we review the EBEX instrument, present the optical design and the computational architecture for in-flight control and data handling, and the quick-look software stack. Finally we describe the 2009 North American test flight and present analysis of data collected at the end of that flight that characterizes scan-synchronous signals and the expected response to emission from thermal dust in our galaxy.
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    Extending the Reach of Gravitational Wave Detectors and Probing the Isotropic Stochastic Background
    (2021-08) Ormiston, Rich
    Beginning with the first detection of gravitational waves in 2015 by the Advanced Laser Interferometer Gravitational Wave Observatory (aLIGO) a new era of astrophysics emerged. Within 5 years, aLIGO has detected 50 mergers from binary black hole (BBH) and binary neutron star (BNS) systems and kick started the field of multi-messenger astrophysics with the measurement of an electromagnetic counterpart to the BNS merger GW170817. A detection on the horizon for LIGO is that of the stochastic gravitational wave background (SGWB). The detection of such a background would have far reaching consequences in astrophysics and cosmology as these measurements can probe the first fractions of a second after the Big Bang, revealing insights and parameters of proposed and possibly undiscovered cosmological models. Even a SGWB formed by BBH and BNS mergers near to us would provide valuable information about star formation rates, the formation of large scale structure, as well as the populations of these compact objects. There are two main topics in this dissertation: detector characterization/data quality methods and characterization of the SGWB. The first chapter provides a background into General Relativity and derives import dynamics relevant to LIGO that are heavily used throughout the thesis. Chapter 2 delves into detector characterization and understanding the noise which enters into the detector output data stream. This is accomplished through the development of a coherence calculation package, \texttt{STAMP-PEM}, which creates a hash table lookup for quick followup analysis and a user friendly API. In Chapter 3 I discuss the sky-averaged SGWB, search methods and present the most recent results combined over aLIGO's first three observing runs. Chapter 4 will extend the isotropic SGWB search to Cosmic Explorer sensitivities and attempt a novel solution to subtract the foreground compact binary coalescences (CBCs) in the $f-t$ space. This mock data analysis sets a benchmark for future search methodologies and sensitivities. Finally in Chapters 5 and 6 I discuss data quality filtering methods. The former will employ a new deep learning architecture known as \texttt{DeepClean} to identify and subtract noise couplings of arbitrary order without introducing artifacts or phase misalignment of the output signal. The final chapter is dedicated to the construction of analytic filters used for removing linear, nonlinear and non-stationary noise. These analytic filters are useful as they are lightweight and can brute force search through the auxiliary channel combinatorics and aid in identifying relevant physical couplings.
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    The First Supernovae
    (2013-09) Chen, Ke-Jung
    One of the frontiers in modern cosmology is understanding the end of the cosmic dark age, when the first luminous objects (e.g., stars, supernovae (SNe), and galaxies) re-shaped the primordial Universe into the present one of much complexity. In this dissertation, I use numerical simulations to study the evolution of the first supernovae and their cosmological consequences. To push the model frontiers of the first SNe, I apply new numerical approaches to advance models of the first SNe. The goal of my dissertation is to provide a better understanding of the first SNe that may be observed by the large telescopes of the future.
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    Investigating the role of the baryon-dark matter transition in galaxy-scale gravitational lenses with ramifications for galaxy structure and cosmology
    (2020-06) Gomer, Matthew
    Gravitational lensing is a powerful tool to study the structure of galaxies and cosmology, however the constraints from lensing are subject to degeneracies and cannot provide a unique solution. The lens model informs the ultimate choice of solution, and so it is critical that the lens model accurately reflects galaxy structure. Probably the most commonly-used lens model is a power-law ellipse+shear model. We show that this ellipse+shear model is unable to statistically explain the angular distribution of quad image systems. Considering additional complications to the azimuthal structure of a lens, we show that the observed angular distribution cannot be explained by $\Lambda$CDM substructure, but can be explained by a transition region between two mass components representing baryons and dark matter. The combination of offset centers, misaligned position angles, and Fourier components introduces enough asymmetry in a lens to explain the observed population. Because lensing is used to measure $H_0$, it is important to know the potential biasing effects that simplifying assumptions implicit to modeling can create. We therefore study the effect of the radial profile assumption (that the profile is a power law) and the azimuthal shape assumption (that the lens is ellipse+shear) on the recovery of $H_0$. To do so, we create mock lenses which are more complicated than the model, then fit their images with the model. For the radial structure, we find that when two-component lenses are fit with a power law, they return biased values of $H_0$. Worse, the bias does not match the analytical prediction, making it more difficult to account for. Stellar kinematic information, which in practice is used to inform the solution by providing a measure of mass, does not correctly inform the unbiased value of $H_0$ because the power-law model is inaccurate. For the azimuthal structure, different types of shape complications have different effects, but the recovered value of $H_0$ can be biased substantially, especially if the profiles are offset from one another. Finally, we discover that the image distance ratios of observed quads are statistically different from mock quads, indicating additional complications to the structure of lenses which have not yet been accounted for.
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    Lowering Backgrounds and Thresholds in the Search for Light Dark Matter with SuperCDMS
    (2024-01) Nelson, Jack
    Cosmological observations have produced a wealth of evidence which demonstrates that the majority of thematter content in our Universe is “dark”. The identity of this dark matter remains elusive, as none of the members of the standard model of particle physics accurately describe its properties. This has prompted the scientific community to launch a broad search, spanning decades in time, mass and sensitivity, with the goal of detecting and identifying this source of new physics. The next generation of the Super Cryogenic Dark Matter Search (SuperCDMS) is currently under construction deep underground at SNOLAB. The experiment aims to expand the search for dark matter to lower masses (≲ 10 GeV/c2) and greater sensitivities using silicon and germanium detectors by minimizing experimental backgrounds and operating detectors with superb energy resolution. SuperCDMS will accomplish its low projected background in part by deploying a robust shield to protect its detectors from environmental radiation. This dissertation presents the results of simulations which demonstrate the success of the shield design at stopping radiogenic neutrons. The shield will be able to reduce these environmental sources to the point where coherent scattering from solar neutrinos are expected to dominate the nuclear recoil backgrounds. In order to search for such light dark matter masses, SuperCDMS uses sensitive transition edge sensors to measure small energy depositions in the detectors. The ultimate energy resolution of these devices, expected to be < 1 eV, has not yet been realized. This dissertation describes the analysis of a dark matter search performed at the University of Massachusetts Amherst with a prototype detector which uses SuperCDMS style sensors to achieve a baseline energy resolution of 2.3 eV. The results of this search demonstrate sensitivity to dark matter candidates with masses as low as ∼ 25 MeV.
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    Mapping Mass Distributions in Clusters of Galaxies using Strong Gravitational Lensing
    (2022-12) Ghosh, Agniva
    Over the last three decades, multiple imaged strong gravitational lensing systems have become an indispensable tool for determining the mass and mass-related properties of cosmic objects such as galaxies and clusters of galaxies. Lens reconstructions techniques play a crucial role in exploring the spatial organization of dark matter and its dynamics within galaxy clusters: Deviations from mass-follows-light, signatures of dark matter self-interactions, lensing effects of line-of-sight mass features and dark matter subhalos within the clusters using their lensing effects. In this work we have studied the free-form lens reconstruction method, Grale, that relies exclusively on the strongly lensed multiple image data. We used synthetic galaxy clusters to test Grale's efficiency and compatibility with future observations using 150-1000 strongly lensed multiple images. We found that with an increasing number of input images, Grale produces improved reconstructed mass distributions, with the fraction of the lens plane recovered at better than 10% accuracy increasing from 40-50% for ~150 images to 65% for ~1000 images.The goal of lensing reconstructions is to produce maps with properly quantified accuracy and precision. One of the ways to accomplish that is to compare reconstructions that use different lens inversion methods with differing modeling philosophies and assumptions leading to different reconstruction results, even if they use similar lensing data. The dispersion between various methods' reconstructions is probably the best estimation of systematic uncertainties in the cluster mass maps. We used Grale to obtain reconstructions of the real clusters Abell 370 and Abell 1689 using the latest available strong lensing data from the Hubble Space Telescope. We compared our results with existing reconstructions obtained by different parametric, free-form or hybrid methods. In our mass models for both the clusters, we found signatures of deviations from mass-follows-light scenarios which are also recovered by several other reconstruction methods. We conclude that on spatial scales of about ~100 kpc and above, most, and probably all mass reconstructions agree. The discrepancies between reconstructions start on scales below ~100 kpc.
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    Measurement of nuclear recoils in the CDMS II Dark Matter Search
    (2014-12) Fallows, Scott Mathew
    The Cryogenic Dark Matter Search (CDMS) experiment is designed to directly detect elastic scatters of weakly-interacting massive dark matter particles (WIMPs), on target nuclei in semiconductor crystals composed of Si and Ge. These scatters would occur very rarely, in an overwhelming background composed primarily of electron recoils from photons and electrons, as well as a smaller but non-negligible background of WIMP-like nuclear recoils from neutrons. The CDMS~II generation of detectors simultaneously measure ionization and athermal phonon signals from each scatter, allowing discrimination against virtually all electron recoils in the detector bulk. Pulse-shape timing analysis allows discrimination against nearly all remaining electron recoils taking place near detector surfaces. Along with carefully limited neutron backgrounds, this experimental program allowed for ``background-free'' operation of CDMS~II at Soudan, with less than one background event expected in each WIMP-search analysis. As a result, exclusionary upper-limits on WIMP-nucleon interaction cross section were placed over a wide range of candidate WIMP masses, ruling out large new regions of parameter space.These results, like any others, are subject to a variety of systematic effects that may alter their final interpretations. A primary focus of this dissertation will be difficulties in precisely calibrating the energy scale for nuclear recoil events like those from WIMPs.Nuclear recoils have suppressed ionization signals relative to electron recoils of the same recoil energy, so the response of the detectors is calibrated differently for each recoil type. The overall normalization and linearity of the energy scale for electron recoils in CDMS~II detectors is clearly established by peaks of known gamma energy in the ionization spectrum of calibration data from a $^{133}$Ba source. This electron-equivalent (keV$_mathrm{ee}$) energy scale enables calibration of the total phonon signal (keV$_mathrm{t}$) by enforcing unity yield for electron recoils, in aggregate. Subtracting an event's Luke phonon contribution from its calibrated total phonon energy (keV$_mathrm{t}$), as measured by the ionization signal, results in a valid measure of the true recoil energy (keV$_mathrm{r}$) for both electron and nuclear recoils.I discuss systematic uncertainties affecting the reconstruction of this recoil energy, the primary analysis variable, and use several methods to constrain their magnitude. I present the resulting adjusted WIMP limits and discuss their impact in the context of current and projected constraints on the parameter space for WIMP interactions.
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    Model-free analysis of quadruply imaged gravitationally lensed systems.
    (2015-04) Woldesenbet, Addishiwot
    Gravitational lensing has proven to be a very valuable tool as a probe to better understand our universe. Parametric modeling of one multiple image gravitational lens system at a time is a common practice in the field of lensing. Instead of individual lens modeling, an alternative approach is to use symmetries in different spaces to make conclusions about families of lenses. The latter method is the focus of this thesis. Three types of lenses are defined based on whether they do or do not obey two-fold and double mirror symmetries. The analysis concentrates on quadruply imaged systems, or quads, and uses only the relative polar angles of quads around the center of the lens. The analysis is statistical in nature, and model-free because its conclusions relate to whole classes of models, instead of specific models. The work done here is twofold. Firstly, exploratory analysis is done to check for possible existence of degeneracies. Type I lenses which obey both symmetries mentioned above are found to form a nearly invariant surface in the 3D space of relative image angles. In the same space, lenses that break the double mirror symmetry, grouped as Type II, form two distinct surfaces. In addition, degeneracy in this class of lenses is discovered. A preliminary study of the last group of lenses, Type III, that break both symmetries, is done. Secondly, quad distributions in the 3D space from each of the three families were compared to observed galaxy-lens quads. Three quarters of observed quads were inconsistent with the distribution of quads of Type I lenses. Type II lenses reproduce most individual lens systems but fail to reproduce the population properties of observed quads. Preliminary exploration of Type III lenses shows a very promising agreement with observations. Examples of Type IIIs are lenses with substructure (with clump masses larger than those responsible for flux ratio anomalies in quads), and lenses with luminous or dark nearby perturber galaxies, or line of sight structures.
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    No-Scale Inflation
    (2016-08) Garcia Garcia, Marcos
    Supersymmetry is the most natural framework for physics above the TeV scale, and the corresponding framework for early-Universe cosmology, including inflation, is supergravity. No-scale supergravity emerges from generic string compactifications and yields a non-negative potential, and is therefore a plausible framework for constructing models of inflation. No-scale inflation yields naturally predictions similar to those of the Starobinsky model based on $R + R^2$ gravity, with a tilted spectrum of scalar perturbations: $n_s∼0.96$, and small values of the tensor-to-scalar perturbation ratio $r < 0.1$, as favored by Planck and other data on the cosmic microwave background (CMB). In this thesis we introduce a novel no-scale inflationary model that averts the stabilization problem of supergravity models; to study it we develop a multi-field formalism applicable to supergravity models. We discuss the low-energy phenomenology of generic no-scale models and its connection to the lifetime of the inflaton. We use our results to analyze the constraints on these models imposed by CMB measurements, which through the calculation of the number of e-folds $N_*$ , we relate to constraints on the inflaton decay rate and other parameters of specific no-scale inflationary models. Finally, we revisit gravitino production following inflation, including thermal and non-thermal effects, and discuss the potential implications of upper limits on the gravitino abundance for no-scale models of inflation. Our results may provide insights into the embedding of inflation within string theory as well as its links to collider physics.
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    Phenomenology of Axion-Gauge Interactions in the Early Universe
    (2021-05) Papageorgiou, Alexandros
    The present thesis explores various effects that arise as a consequence of axion-gauge couplings in the early universe. Axions (or Axion-Like Particles) are pseudo-scalar particles that enjoy an approximate shift symmetry which protects the flatness of their potential from obtaining large radiative corrections. This property, as well as the fact that axions are abundantly predicted by high energy theories such as Supergravity and String Theory, makes the study of the phenomenology of axions in the early universe particularly interesting. Assuming that axions are present and cosmologically relevant in the early universe, it is a natural question to ask what effects may arise from couplings of axions to other fields such as boson or fermions. There is a unique shift symmetric, five-dimensional axion-gauge coupling which is expected in any axion theory. The gauge fields that are studied in the present work are either Abelian U(1) or non-Abelian SU(2) gauge fields. In both cases, the presence of the axion-gauge interaction modifies the dispersion relation of the various gauge field perturbations and under certain conditions one helicity of the perturbation degrees of freedom may become tachyonically unstable. These unstable perturbations are produced exponentially rapidly and their abundance can backreact on scalar and tensor perturbations leading to modified predictions for inflationary models compared to the predictions that are neglecting such contributions. An additional effect of these enhanced perturbations, is the production of a primordial lepton asymmetry which could in principle account for the matter-antimatter asymmetry observed today. Finally such enhanced perturbations could play a role in theories of Quintessential Dark Energy. In such theories, the Dark Energy component is a scalar field in a slow-roll configuration. The possibility that the Quintessence field is an axion field with a steep potential is explored. In this case slow-roll is maintained as a consequence of the axion producing the unstable perturbations at the expense of its own kinetic energy.
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    Phenomenology of particle production during inflation
    (2013-09) Namba, Ryo
    This thesis is devoted to the study on particle production during the era of primordial inflation and its phenomenological impacts. The simplest models of inflation typically assume only one dynamical degree of freedom, inflaton, that is responsible for all the inflationary dynamics and predictions. Yet, it is a natural expectation that the inflaton should be coupled to some other fields, in need of successful reheating of the universe after inflation. We first consider the models in which the inflaton is coupled to a U(1) gauge field. For a pseudo-scalar inflaton, its natural coupling induces tachyonic growth of the gauge quanta, which then inverse-decay to the inflaton perturbations. This imprints non-Gaussianity in the cosmic microwave background (CMB) anisotropies. This non-Gaussianity has a nearly equilateral shape, and the fact that we have not observed it with Planck provides a bound on the axion decay constant, which is in the range naturally obtained in UV completed theories. The produced gauge quanta also source gravitational waves (GWs). Future GW interferometer experiments can improve over the CMB non-Gaussianity limits. We then study a different model characterized by a scalar inflaton coupled to gauge fields via a dilation-like interaction. This coupling can result in a nearly scale-invariant spectrum for the gauge field. Also in this case, the produced gauge quanta source inflaton perturbations, but the resulting non-Gaussianity now has a shape peaked for squeezed triangles, and which exhibits a peculiar angular dependence, that, if detected, would be a smoking gun of the higher-spin fields involved. In the above two models, the GW signals are always subdominant at the CMB scales, due to the non-Gaussianity bounds from the scalar perturbations (namely, from the perturbations generated by the inflaton quanta produced by the gauge fields). We study the radically different situation in which some field other than the inflaton produces the gauge quanta, and these quanta have no direct coupling (apart from the unavoidable gravitational interaction) to the inflaton. We study whether this production can result in a detectable GW signal at CMB scales, without conflicting with the bounds from non-Gaussianity of the scalar perturbations. We study two possibilities: (i) gauge quanta production due to a sudden variation of their mass, and (ii) gauge quanta production from a rolling pseudo scalar. In case (i), we find that GW signals are unlikely to be detectable, due to the suppressed quadrupole moment of non-relativistic quanta. In case (ii), we instead find that GWs from particle production can actually exceed the usual inflationary vacuum fluctuations. The observable B-mode polarization can be obtained for any choice of inflaton potential, and the amplitude of the signal is not necessarily correlated with the scale of inflation.
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    Probing large-scale structure with radio observations.
    (2009-06) Brown, Shea D.
    This thesis focuses on detecting magnetized relativistic plasma in the intergalactic medium (IGM) of filamentary large-scale structure (LSS) by observing the synchrotron emission emitted by structure formation shocks. Little is known about the IGM beyond the largest clusters of galaxies, and synchrotron emission holds enormous promise as a means of probing magnetic fields and relativistic particle populations in these low density regions. I'll first report on observations taken at the Very Large Array and the Westerbork Synthesis Radio Telescope of the diffuse radio source 0809+39. I use these observations to demonstrate that 0809+39 is likely the first &ldquoradio relic&rdquo discovered that is not associated with a rich X-ray emitting cluster of galaxies. I then demonstrate that an unconventional reprocessing of the NVSS polarization survey could reveal structures on scales from 15&rsquo to hundreds of degrees, far larger than the nominal shortest-baseline scale. This has yielded hundreds of new diffuse sources as well as the identification of a new nearby galactic loop. These observations also highlight the major obstacle that diffuse galactic foreground emission poses for any search for large-scale, low surface-brightness extragalactic emission. I therefore explore the cross-correlation of diffuse radio emission with optical tracers of LSS as a means of statistically detecting the presence of magnetic fields in the low-density regions of the cosmic web. This initial study with the Bonn 1.4&simGHz radio survey yields an upper limit of 0.2&sim&muG for large-scale filament magnetic fields. Finally, I report on new Green Bank Telescope and Westerbork Synthesis Radio Telescope observations of the famous Coma cluster of galaxies. Major findings include an extension to the Coma cluster radio relic source 1253+275 which makes its total extent &sim2~Mpc, as well as a sharp edge, or &ldquofront&rdquo, on the Western side of the radio halo which shows a strong correlation with merger activity associated with an infalling sub-cluster. This front is just interior to a temperature jump derived from XMM-Newton observations, and may be related to shocked infalling gas.
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    Simulations of Narrow-Angle Tail Radio Galaxy Evolution and Shock Interactions
    (2019-08) O'Neill, Brian
    We present the results of three-dimensional magnetohydrodynamic (MHD) simulations of radio galaxy (RG) outflows interacting with active galaxy cluster environments. The goal of these simulations was to analyze the consequences of these interactions such that observations of cluster radio emission can provide a more effective proxy for understanding the dynamical states of intracluster media (ICM). We conducted the first high-resolution numerical study dedicated to the evolution of a narrow-angle tail (NAT) RG utilizing 3D MHD and energy-dependent transport of cosmic ray electrons (CRe). We verify existing theoretical models of the jet bending process and, in an appendix, extend them into an empirical formalism that allows us to model the jet trajectories that develop from arbitrary jet-wind orientations. We observe that the early development of our NAT consists of an extended formation period, during which the evolving RG goes through a transient phase where it is more reminiscent of a more gently bent wide-angle tail RG. Once the jets are fully bent, we note the jet trajectories do not remain static, but episodically "flap" in response to instabilities that develop in the surrounding ICM wind. This occasional disruption of the jets enhances magnetic fields via dynamical stretching, and is critical to the state of the plasma in the tails that develops. Synthetic observations show that once plasma released early in the NAT's evolution has faded, the observed morphology of the NAT is very nearly steady-state with a roughly constant brightness distribution and a steady, self-similar, curved integrated spectrum. We then used the evolved NAT we created to run multiple simulations to explore the interactions between the tail plasma and shock fronts characteristic of those generated by a major cluster merger. Our primary motivation in conducting these simulations is to explore the possibility that CRe populations recently released from a NAT could be seed relativistic electrons necessary to explain observations of radio relics. We analyze the character the shock front develops within our simulated tails with a theoretical treatment based on a Riemann problem analysis. We note the development of vortical motions induced by the shock passage through the low-density plasma of the tails, and the resulting magnetic field amplification over and above amplification from shock compression. We examine synthetic observations of our shocked tails and compare to common properties of emission from radio relics.
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    Unmodeled searches for long-lasting gravitational-wave signals with LIGO and studies of underground seismic noise for future gravitational-wave detectors
    (2016-07) Prestegard, Tanner
    The Laser Interferometer Gravitational-wave Observatory (LIGO) has recently reported the first two direct detections of gravitational waves, confirming yet another prediction of general relativity and providing an arena for testing gravity in the strong-field, high-velocity regime. These detections herald the beginning of the era of gravitational-wave astronomy, in which frequent gravitational-wave observations will provide a means for probing currently inaccessible regions of the universe and periods in its history. In this dissertation, I focus on detecting and characterizing a stochastic gravitational-wave background and long-lasting gravitational-wave transients, which will be important components of this era for the information they can yield about the beginnings of the universe and the objects contained within it. I implement a method for estimating the parameters of a model of a stochastic gravitational-wave background and apply it to a model based on core-collapse supernovae. Using the expected sensitivities of Advanced LIGO and the Einstein Telescope, I estimate the detectability of such a background and compare the results to simulations of core-collapse events. I also develop an unmodeled all-sky search for long-lasting gravitational-wave transients and apply it to LIGO S5 and S6 data, setting the first upper limits on signals lasting between 10-500 s in the 40-1000 Hz band. Finally, I describe the implementation of a 3D array of seismometers in and around the Homestake Mine, with the goal of mitigating seismic and Newtonian noise for future generations of gravitational-wave detectors. A seismic radiometer algorithm is developed and applied to simulated and real data; I demonstrate its ability to separate various components of the seismic field and map their directional dependence. In order to improve models of surface seismic waves in the radiometer algorithm, I use measurements of Rayleigh waves from the Homestake array and perform parameter estimation to fit a biexponential model of the Rayleigh wave eigenfunctions.