Browsing by Subject "Astrophysics"

We describe the measurement of muon neutrino disappearance due to

neutrino oscillation using the Tokai-2-Kamiokande (T2K) experiment's Run 1-4 (6.57×1020 POT)

data set. We analyze the data using the conventional

Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing

matrix for the three Standard Model neutrinos. The output of the

analysis is a measurement of the parameters sin2θ₂₃, Δm2₃₂ for the normal hierarchy and sin2θ₂₃, Δm2₁₃ for

the inverted hierarchy. The best-fit oscillation

parameters for the normal hierarchy are found to be

(sin2θ₂₃, Δm2₃₂) = ( 0.514, 2.51×10-3 eV2/c4}). The 90% 1D confidence interval -- determined for both parameters

using the Feldman-Cousins procedure -- is for the normal hierarchy

0.428 < sin2θ₂₃ < 0.598 and

2.34×10-3 eV2/c4 < Δm2₃₂ < 2.68\times10^{-3} eV2/c4.

For the inverted hierarchy, the best-fit oscillation parameters are

(sin2θ₂₃, Δm2₁₃) = (0.511, 2.48×10-3 eV2/c4. The 90\% 1D Feldman-Cousins confidence intervals for the inverted hierarchy are 2.31×10-3 eV2/c4 < \Delta m^2_{13} < 2.64×10-3 eV2/c4.

The question of ``What is Dark Matter?" has been a focus of cosmological research since the turn of the 20th century. Though the composition of Dark Matter is unknown, the existence of Dark Matter is crucial to the modern theory of cosmology. We focus on a theory of Dark Matter referred to as \textit{Scalar Field Wave Dark Matter} (SF$\psi$DM), which has received an increasing amount of interest from the research community since the late 2000s. SF$\psi$DM is a peculiar theory in which Dark Matter is composed of ultralight bosonic particles. As a result, SF$\psi$DM has an astronomically large deBroglie wavelength, generating complicated wave dynamics on the largest cosmological scales.

This thesis focuses on describing the status of SF$\psi$DM theory, SF$\psi$DM halos, and how SF$\psi$DM halos are affected by the wave-like features of the scalar field. In particular, we offer an analysis of galactic rotation curves and how they relate to SF$\psi$DM excited states. This analysis yields a novel model for an observed galactic trend referred to as the Baryonic Tully-Fisher Relation. Furthering this model, we formulate an eigenfunction decomposition which can be used to describe superpositions of excited states.

In this thesis we describe a class of Bayesian semiparametric models, known as Levy Adaptive Regression Kernels (LARK); a novel method for posterior computation for those models; and the applications of these models in astronomy, in particular to the analysis of the photon fluence time series of gamma-ray bursts. Gamma-ray bursts are bursts of photons which arrive in a varying number of overlapping pulses with a distinctive "fast-rise, exponential decay" shape in the time domain. LARK models allow us to do inference both on the number of pulses, but also on the parameters which describe the pulses, such as incident time, or decay rate.

In Chapter 2, we describe a novel method to aid posterior computation in infinitely-divisible models, of which LARK models are a special case, when the posterior is evaluated through Markov chain Monte Carlo. This is applied in Chapter 3, where time series representing the photon fluence in a single energy channel is analyzed using LARK methods.

Due to the effect of the discriminators on BATSE and other instruments, it is important to model the gamma-ray bursts in the incident space. Chapter 4 describes the first to model bursts in the incident photon space, instead of after they have been distorted by the discriminators; since to model photons as they enter the detector is to model both the energy and the arrival time of the incident photon, this model is also the first to jointly model the time and energy domains.

We examine the Einstein equation coupled to the Klein-Gordon equation for a complex-valued scalar field. These two equations together are known as the Einstein-Klein-Gordon system. In the low-field, non-relativistic limit, the Einstein-Klein-Gordon system reduces to the Poisson-Schrödinger system. We describe the simplest solutions of these systems in spherical symmetry, the spherically symmetric static states, and some scaling properties they obey. We also describe some approximate analytic solutions for these states.

The EKG system underlies a theory of wave dark matter, also known as scalar field dark matter (SFDM), boson star dark matter, and Bose-Einstein condensate (BEC) dark matter. We discuss a possible connection between the theory of wave dark matter and the baryonic Tully-Fisher relation, which is a scaling relation observed to hold for disk galaxies in the universe across many decades in mass. We show how fixing boundary conditions at the edge of the spherically symmetric static states implies Tully-Fisher-like relations for the states. We also catalog other ``scaling conditions'' one can impose on the static states and show that they do not lead to Tully-Fisher-like relations--barring one exception which is already known and which has nothing to do with the specifics of wave dark matter.

Weak gravitational lensing (a deflection of light from distant galaxies due to the gravitational potential of intervening mass, WL in short) is one of the most exciting probes in cosmology as it is sensitive both to the growth of the large-scale structure and the expansion history of the Universe. We can measure the coherent distortion of galaxy shapes (which we call cosmic shear) and infer the matter distribution. With next-generation imaging surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time (Rubin LSST) and the Nancy G. Roman Space Telescope (Roman), there is immense new promise in understanding the fundamental nature of dark matter and dark energy. With current data like the Dark Energy Survey (DES), we see apparent cosmological tensions between experiments that may either be real and indicate new physics, or new systematics we do not yet understand. LSST and Roman will increase the number of galaxies we have observed by an order of magnitude, leading to improved constraints on our cosmological model by up to 300%. These experiments will have the potential to prove if these cosmological tensions are real. However, our control of systematic uncertainties must also improve by similar levels to achieve the promise of what these missions can deliver.To achieve these scientific outcomes, the challenges of determining galaxy shapes and redshifts and modeling the impact of astrophysical effects must be solved. My PhD has focused on enabling WL science in DES and Roman. I co-led the shear analysis team in DES to produce the largest WL galaxy sample, as well as the final DES cosmological analysis of cosmic shear. For Roman, I have led two papers achieving shear-related systematics with image simulations.

We explore a model of dark matter called wave dark matter (also known as scalar field dark matter and boson stars) which has recently been motivated by a new geometric perspective by Bray. Wave dark matter describes dark matter as a scalar field which satisfies the Einstein-Klein-Gordon equations. These equations rely on a fundamental constant Upsilon (also known as the ``mass term'' of the Klein-Gordon equation). Specifically, in this dissertation, we study spherically symmetric wave dark matter and compare these results with observations of dwarf spheroidal galaxies as a first attempt to compare the implications of the theory of wave dark matter with actual observations of dark matter. This includes finding a first estimate of the fundamental constant Upsilon.

In the introductory Chapter 1, we present some preliminary background material to define and motivate the study of wave dark matter and describe some of the properties of dwarf spheroidal galaxies.

In Chapter 2, we present several different ways of describing a spherically symmetric spacetime and the resulting metrics. We then focus our discussion on an especially useful form of the metric of a spherically symmetric spacetime in polar-areal coordinates and its properties. In particular, we show how the metric component functions chosen are extremely compatible with notions in Newtonian mechanics. We also show the monotonicity of the Hawking mass in these coordinates. Finally, we discuss how these coordinates and the metric can be used to solve the spherically symmetric Einstein-Klein-Gordon equations.

In Chapter 3, we explore spherically symmetric solutions to the Einstein-Klein-Gordon equations, the defining equations of wave dark matter, where the scalar field is of the form f(t,r) = exp(i omega t) F(r) for some constant omega in R and complex-valued function F(r). We show that the corresponding metric is static if and only if F(r) = h(r)exp(i a) for some constant a in R and real-valued function h(r). We describe the behavior of the resulting solutions, which are called spherically symmetric static states of wave dark matter. We also describe how, in the low field limit, the parameters defining these static states are related and show that these relationships imply important properties of the static states.

In Chapter 4, we compare the wave dark matter model to observations to obtain a working value of Upsilon. Specifically, we compare the mass profiles of spherically symmetric static states of wave dark matter to the Burkert mass profiles that have been shown by Salucci et al. to predict well the velocity dispersion profiles of the eight classical dwarf spheroidal galaxies. We show that a reasonable working value for the fundamental constant in the wave dark matter model is Upsilon = 50 yr^(-1). We also show that under precise assumptions the value of Upsilon can be bounded above by 1000 yr^(-1).

In order to study non-static solutions of the spherically symmetric Einstein-Klein-Gordon equations, we need to be able to evolve these equations through time numerically. Chapter 5 is concerned with presenting the numerical scheme we will use to solve the spherically symmetric Einstein-Klein-Gordon equations in our future work. We will discuss how to appropriately implement the boundary conditions into the scheme as well as some artificial dissipation. We will also discuss the accuracy and stability of the scheme. Finally, we will present some examples that show the scheme in action.

In Chapter 6, we summarize our results. Finally, Appendix A contains a derivation of the Einstein-Klein-Gordon equations from its corresponding action.

As light travels from distant galaxies it is coherently bent by the gravitational potential of the mass distribution between the source and the observer. This weak gravitational lensing creates correlations between galaxy shapes that can be measured as a function of redshift in a tomographic cosmic shear measurement. Cosmic shear is a powerful probe of cosmological models, as different expansion scenarios for our Universe produce different weak lensing signals as a function of redshift. Comparison of cosmic shear survey data to theoretical models is a precise test of the current standard model of cosmology; $\Lambda$CDM. The upcoming Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory will produce sub-percent level measurements of cosmological parameters by optically imaging billions of galaxies to unprecedented depth. In anticipation of the LSST analysis, we examine three of the most recent cosmic shear datasets: the first year data from the Dark Energy Survey (DES-Y1), the 1,000 deg$^{2}$ dataset from the Kilo-Degree Survey (KiDS-1000), and the first year data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-Y1). In this work we re-analyze the cosmic shear results from each of the previous surveys using a unified pipeline from the LSST Dark Energy Science Collaboration (DESC). We then measure each of the datasets statistical consistencies, finding each of the surveys to be in agreement. We additionally assess the robustness of the results to analysis choices, and find the cosmology constraints to be robust to two different small-scale treatment methods. Finally we produce a combined cosmology constraint from the three datasets under our unified pipeline. Our combined analysis gives a $1.6-1.9\%$ constraint on the $S_{8}\equiv \sigma_{8}\sqrt{\Omega_{\rm m}/0.3}$, given different approximation methods for the cross-covariance of the overlapping HSC-Y1 and KiDS-1000 footprint. This constraint is as precise as the most recent cosmic shear survey measurement, DES-Y3. This work paves the way for a cosmology analysis with the upcoming LSST-Y1 dataset by DESC by serving as the first test of DESC pipelines on real data, and demonstrating its capabilities to reproduce Stage-III results. It additionally provides guidance to the LSST-Y1 analysis by examining the effects of prior choice and scale cuts on cosmology measurements.