Theses and Dissertations at Montana State University (MSU)
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Item Numerical methods for rotating compact objects in modified gravity theories(Montana State University - Bozeman, College of Letters & Science, 2020) Sullivan, Andrew Patrick Kyung; Chairperson, Graduate Committee: Neil J. Cornish and Nicolas Yunes (co-chair); Nicolas Yunes was a co-author of the article, 'Slowly-rotating neutron stars in massive bigravity' in the journal 'Classical and quantum gravity' which is contained within this dissertation.; Nicolas Yunes, and Thomas Sotiriou were co-authors of the article, 'Numerical black hole solutions in modified gravity theories: spherical symmetry case' in the journal 'Physical review D' which is contained within this dissertation.; Nicolas Yunes, and Thomas Sotiriou were co-authors of the article, 'Numerical black hole solutions in modified gravity theories: axial symmetry case' submitted to the journal 'Physical review D' which is contained within this dissertation.Detailed observations of phenomena involving compact objects will provide us with a new avenue to test general relativity in the strong field regime. So as to not bias our analysis of these new experiments, we require knowledge of the spacetimes around these objects both within and beyond general relativity. Here I will describe work that applies two specific methods to solve the modified Einstein's equations that describe the exotic spacetimes beyond general relativity for neutron stars and black holes. The first method is a fourth-order Runge-Kutta-Fehlberg ordinary differential equation numerical integrator method. The second method is a relaxed Newton- Raphson method applied to a system of nonlinear partial differential equations. Using these methods, we solve for the spacetimes of slowly rotating neutron stars in massive bigravity and rotating black holes in scalar Gauss-Bonnet gravity in a theory independent methodology. We validate our numerical methods by applying them to compact objects in general relativity and using them to recover known perturbative solutions. We can then compare the fully nonlinear solutions to these perturbative solutions and comment on their differences. We then use these numerical solutions to calculate the physical observables of these systems and finally construct analytic fitted models that can be used in rapid computation methods that future experiments may use to constrain the free parameters in these theories.Item Testing general relativity through the computation of radiative terms and within the neutron star strong-field regime(Montana State University - Bozeman, College of Letters & Science, 2019) Saffer, Alexander; Chairperson, Graduate Committee: Nicolas Yunes; Kent Yagi and Nicolas Yunes were co-authors of the article, 'The gravitational wave stress-energy (pseudo)-tensor in modified gravity' in the journal 'Classical and quantum gravity' which is contained within this thesis.; Nicolas Yunes was a co-author of the article, 'Angular momentum loss for a binary system in Einstein-aether theory' in the journal 'Physical review D' which is contained within this thesis.; Hector O. Silva is an author and Nicolas Yunes is a co-author of the article, 'The exterior spacetime of relativistic stars in scalar-Gauss-Bonnet gravity' submitted to the journal 'Physical review D' which is contained within this thesis.The recent detection of coalescing black holes by the Laser Interferometer Gravitational-wave Observatory has brought forth the era of gravitational wave astronomy. Physicists are only now beginning to probe the mergers of compact objects that send ripples through space and time. These distortions carry with them the information from the system where they originated. The dynamics of black hole collisions and neutron star mergers are new and exciting events which were undetectable just a few years ago. Einstein's theory of General Relativity has done an excellent job of describing gravity and the information that can be extracted from gravitational systems. However, his theory contains several anomalies such as the inability to explain the inflation of the universe, the effects of dark matter and energy, the presence of singularities, as well as a failure to reconcile with quantum mechanics. Modified theories of gravity have been proposed to answer any remaining questions about gravitation while prescribing solutions to the problems General Relativity still has. The work within this thesis describes how we may study modified theories of gravity in the strong field regime through two different means. The first, is through the calculation of the rate of gravitational radiation from binary systems. This rate varies depending on the theory of gravity being studied. Comparing the theoretical predictions of these rates from alternative theories to astronomical observation will allow us to place better constraints on modified gravity and test General Relativity like never before. The second way is through the investigation of the spacetime surrounding a neutron star. Unlike black holes which emit no light, we are able to see neutron stars (more specifically pulsars) through their light curve as they rotate. The shape of the light curve is dictated by the theory of gravitation used to describe the spacetime around the neutron star. My goal of constructing such a spacetime for neutron stars in modified gravity allows for future scientists to study the light curves to be detected and place constraints on the particular theory.Item Constraining scalar-tensor theories of gravity through observations(Montana State University - Bozeman, College of Letters & Science, 2018) Anderson, David Sutton; Chairperson, Graduate Committee: Nicolas Yunes; Nicolas Yunes and Enrico Barausse were co-authors of the article, 'The effect of cosmological evolution on solar system constraints and on the scalarization of neutron stars in massless scalar-tensor theories' in the journal 'American physical society' which is contained within this thesis.; Nicolas Yunes was a co-author of the article, 'Solar system constraints on massless scalar-tensor gravity with positive coupling constant upon cosmological evolution of the scalar field' in the journal 'American physical society' which is contained within this thesis.; Nicolas Yunes was a co-author of the article, 'Scalar charges and scaling relations in massless scalar-tensor theories' submitted to the journal 'Classical quantum gravity' which is contained within this thesis.; Paulo Freire and Nicolas Yunes were co-authors of the article, 'Binary pulsar constraints on massless scalar-tensor theories using bayesian statistics' submitted to the journal 'Classical quantum gravity' which is contained within this thesis.Scalar-tensor theories of gravity have been among the most popular and well-studied alternatives to Einstein's General Relativity. These theories of gravity contain an extra scalar degree of freedom that allows them to rectify some of the limitations of General Relativity but also fail some of the cornerstone tests of gravity that General Relativity passes with flying colors. Because of these conflicting features, it becomes necessary to investigate if scalar-tensor theories can pass current tests of gravity while still allowing for possible deviations from General Relativity in regimes that are not as highly constrained. In this thesis, we present the first self-consistent study of scalar-tensor theories in which we study the effects and constraints from Solar System observations, cosmological evolution of the universe, and the precise timing of binary pulsar systems. We constrain the free parameters of a certain class of massless-scalar-tensor theories first through cosmology and Solar System tests, in which we investigate the consistency between cosmological evolution scenarios and current Solar System observations. We then study strong field tests involving binary pulsar systems and investigate the various constraints that can be placed from measurements of the Keplerian and post-Keplerian parameters that determine the orbits.Item Detecting and characterizing gravitational waves with minimal assumptions(Montana State University - Bozeman, College of Letters & Science, 2018) Millhouse, Margaret Ann; Chairperson, Graduate Committee: Neil J. CornishAfter many years of preparation and anticipation, we are finally in the era of routine gravitational-wave detection. All of the detected signals so far have come from merging compact objects-- either black holes or neutron stars. These are signals for which we have very good waveform models, but there still exist other more poorly modeled sources as well as the possibility of completely new gravitational-wave sources. Because of this, it is important to have the ability to confidently detect gravitational-waves from a wide variety of sources. In this Thesis I will describe one particular algorithm used to detect and characterize gravitational-wave signals using Bayesian inference techniques, and minimal assumptions on the source of the gravitational wave. I will report on the methods and results of the implementation of this search in the first two observing runs of advanced LIGO. I will also discuss developments to this algorithm to improve waveform reconstruction, and target certain signals without using full waveform templates.Item Eccentric compact binaries: modeling the inspiral and gravitational wave emission(Montana State University - Bozeman, College of Letters & Science, 2018) Loutrel, Nicholas Peter; Chairperson, Graduate Committee: Neil J. CornishThe modeling of the inpsiral and subsequent gravitational wave emission from black hole binary systems has been a long outstanding problem in astrophysics and relativity. Astrophysical models predict that most binaries will have low orbital eccentricity by the time the gravitational wave emission enters the detection band of ground based detectors, and significant success has been made by restricting attention to the this limit, where the binary's orbit is approximately circular. However, exotic formation channels in globular clusters and galactic nuclei predict a small, but non-negligible, fraction of systems will enter the detection band of ground-based detectors with significant orbital eccentricity. In this thesis, we present new methods of modeling eccentric binaries under the influence of gravitational wave emission, focusing on two regimes: (i) the early inspiral of highly eccentric binaries, and (ii) the late inspiral of generic eccentric binaries.Item Superfluid effects on thermal evolution and rotational dynamics of neutron stars(Montana State University - Bozeman, College of Letters & Science, 2001) Larson, Michelle BeauvaisItem Superfluid hydrodynamics in neutron stars(Montana State University - Bozeman, College of Letters & Science, 1991) Mendell, Gregory AllenItem Measuring surface temperature of isolated neutron stars and related problems(Montana State University - Bozeman, College of Letters & Science, 2001) Teter, Marcus AltonItem Thermal evolution of neutron stars(Montana State University - Bozeman, College of Letters & Science, 1995) Qin, LetaoItem Cooling of neutron stars with quark core(Montana State University - Bozeman, College of Letters & Science, 2012) Beisenkhanova, Neilya; Chairperson, Graduate Committee: Sachiko TsurutaOrdinary neutron stars can undergo two possible scenarios of cooling: with conventional 'standard' neutrino emission processes or with faster 'non-standard' processes. For both of these scenarios various mechanisms have been proposed. As possible nonstandard options, previous detailed studies already explored direct URCA processes involving hyperon-mixed matter and pion condensates. In the current research we explore another possible non-standard scenario - quark cooling where a hybrid star with a quark core undergoes direct URCA cooling. We used the exact evolutionary code originally constructed by Nomoto and Tsuruta (1987) which was modified for quark cooling. We chose a model with a medium equation of state TNI 6, where transition from neutron to quark matter takes place at a critical density of four times the nuclear density. Our results show that low mass stars undergo standard cooling while heavier stars, with mass larger than about 1.45 mass compared to the sun, possess a central core where nonstandard accelerating quark cooling is in operation but it can be suppressed significantly due to density-dependent superfluid property. We showed that our quark cooling scenario can be consistent with the observational data on neutron star temperatures. An important result is that we obtained more realistic cooling behavior than obtained earlier, by adopting a density-dependent superfluid energy gap model, instead of constant gaps employed earlier.