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Browsing by Subject "Gravitational waves"

Now showing 1 - 6 of 6
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    Cross-correlation searches for persistent gravitational waves with Advanced LIGO and noise studies for current and future ground-based gravitational-wave detectors.
    (2018-05) Meyers, Patrick
    Over the last three years, the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) has detected signals from colliding black holes and a signal from colliding neutron stars. These detections ushered in a new era of gravitational-wave (GW) astrophysics and multimessenger astronomy that allows us to probe new regions of the universe. One of the next frontiers for gravitational-wave astronomy is the detection and characterization of the stochastic GW background (SGWB). A measurement of the SGWB from unresolved compact binary systems could come as Advanced LIGO reaches design sensitivity, and future detectors will be important for digging beyond that astrophysical background towards trying to measure signals from relic gravitational waves produced in the early universe. In this dissertation, I present cross-correlation-based searches for a SGWB and other persistent sources of GWs. I introduce and use a new method for setting limits on the strain amplitude of a potential source of GWs in the directions of Scorpius X-1, the galactic center, and Supernova 1987a in the frequency band from 20-1726 Hz. I also set limits on persistent, broadband point sources of GWs across the whole sky. Finally, I show how we can implement data analysis techniques to improve the Advanced LIGO detector sensitivity to persistent sources of GWs. Improving sensitivity of current detectors and planning for future detectors is vital to the effort to measure and understand the SGWB. This will requires a better understanding of the noise sources that limit sensitivity, especially at lower frequencies. To this end, I outline a method for estimating and modeling correlated magnetic noise between spatially separated GW detectors. I also present results from a 3D seismometer array deployed at the Homestake Mine, aimed at characterizing seismic and Newtonian noise for future GW detectors. I estimate the fundamental Rayleigh-wave eigenfunction, and then use it in a seismic radiometer algorithm to separate different components of the seismic field that contribute differently to the Newtonian noise. Finally, I present estimates of the Newtonian noise as a function of depth in the frequency band from 0.5-5 Hz based on results from the seismic radiometer.
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    Gauge Field Amplification during Axion Inflation
    (2018-08) UNAL, CANER
    This thesis studies the interaction of different fields during inflation and resultant phenomenology at different scales. It particularly focuses on one of the most well motivated inflationary models, called Axion inflation. Axions are pseudo-scalars that possess the shift symmetry at least at the approximate level, which protects their potential from quantum corrections and elevates them as a strong inflaton candidate. However, in the particle inventory of UV complete theories, axion particles are abundant, which motivates studying axions as inflaton or spectator field during inflation. In this work, we study a chiral shift symmetric dimension-five operator arising naturally in any axion theory. Due to this coupling, the gauge field'Äôs dispersion relation is modified and one helicity of the gauge field is produced abundantly as a function of a dimensionless parameter proportional to speed of the axion. This breaks the parity conservation. Furthermore, this amplified gauge quanta inversely decays (ie. sources back) to scalar and tensor degrees of freedom via two-to-one way; hence, the sourced perturbations obey non-Gaussian statistics. These sourced modes leave unique imprints on cosmological observables such as : Chiral gravitational wave (GW) background, large tensor non-Gaussianity, non-zero TB and EB correlators, detectable GW background at interferometer scales and the production of primordial black holes.
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    Gravitational Waves As Tools For Astrophysics And Cosmology
    (2021-05) Banagiri, Narayana Sri Sharan
    The discovery of gravitational waves by LIGO and Virgo have unveiled a sector of the Universe previously hidden from us. Gravitational waves allow us to detect and observe the dynamics of phenomena usually hidden from electromagnetism probes like binary black hole mergers, providing new tools to study astrophysics and cosmology in the process. As the number of detections increase, the statistics of the mergers are starting to inform us about their progenitor distributions. This dissertation consists of three parts. First, analyses to detect potential gravitational waves from a post-merger remnant of the binary neutron star merger GW170817 are described, with particular emphasis on the STAMP pipeline-based search targeting gravitational waves from a long-lived remnant. Bayesian parameter estimation techniques for the poorly modeled post-merger signals are then described. A novel likelihood formalism is developed to account for the inaccuracies in models, focusing in particular on the phase evolution of the waveform. In the second part, techniques are developed, using hierarchical Bayesian modeling to measure N-point correlations of the distributions of black hole mergers, with a focus on two-point correlations. These methods allow us to use black hole mergers as a tool to measure the angular distribution and the large-scale structure of the matter in the Universe. The two-point correlation method is validated with simulations for the angular structure of the mergers in the Universe. Finally, Bayesian methods are devised to probe anisotropies in the angular distribution of the stochastic gravitational-wave backgrounds and foregrounds in the LISA band. A novel decomposition using \cg coefficients in the spherical harmonic basis is developed that allows us to infer the anisotropy of arbitrary distributions of gravitational-wave power. This method is employed and tested using different kinds of simulations, including that of the galactic gravitational-wave foreground from galactic white dwarf binaries.
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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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    Searches for stochastic gravitational waves and long gravitational wave transients in LIGO S5 data
    (2013-12) Kandhasamy, Shivaraj
    Incoherent superposition of gravitational waves from a large number of unresolved sources gives rise to the stochastic gravitational-wave background. This background could be of cosmological origin, produced by early universe events such as inflation. It could also be of astrophysical origin, produced by a large number of astrophysical objects such as binary neutron stars and black holes. Detection of the stochastic gravitational wave background would therefore provide information both about the state of the universe at its earliest moments and about its evolution at later times. Long gravitational-wave transients are gravitational waves whose times scales range from minutes to weeks. Such long gravitational-wave transients are predicted by a variety of astrophysical models, including stellar core collapse and accretion onto newly formed proto-neutron stars and black holes. Detection of long transient gravitational waves would provide clues about various dynamical process occurring in these astrophysical objects. In this thesis, we describe methods to search for stochastic and long transient gravitational waves in interferometric gravitational-wave detector data and present results obtained by using the Laser Interferometer Gravitational-wave Observatory (LIGO) data acquired during its fifth science run.
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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.