Theses and Dissertations from UMD
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Item Hunting Inflationary Fossils in Primordial Inhomogeneities(2023) Bodas, Arushi; Sundrum, Raman; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cosmological observables such as the Cosmic Microwave Background (CMB) allow us to probe the early universe at extremely high energies far beyond the reach of any particle collider on Earth. In the inflationary paradigm, small perturbations in the energy distribution across space can be directly linked to the quantum fluctuations of an "inflaton'' field that drives inflation. Using these perturbations, it is, therefore, possible to learn about physics at energies as high as 10^(13) GeV. In this thesis, we exploit this powerful connection and explore novel mechanisms to hunt for previously unexplored inflationary dynamics. During inflation, particles with masses larger than the inflationary Hubble scale (H) are produced due to an accelerating spacetime. If coupled to the inflaton, these particles could imprint distinct oscillatory features in higher moments of the density perturbations. Since H can be as high as 5*10^(13) GeV, these oscillatory features present a unique opportunity to directly detect very heavy particles with masses ~ H. In Chapter 2, we explore a mechanism that can boost spin-0 particle production by mining the kinetic energy of the inflaton. This leads to an enhancement of the oscillatory features, which can bring heavier particles with masses up to 60H within the reach of observations. In the final part of the thesis, spanning chapters 3 and 4, we explore the viability of gravitational wave backgrounds (GWB) as novel data sources for unexplored inflationary physics. It was recently shown that a GWB from a first-order phase transition must exhibit fluctuations, much like the CMB. Despite the close analogy, it is possible for fluctuations of the GWB to differ significantly in their detailed pattern from those of the CMB, which would imply the existence of a second light field during inflation in addition to the inflaton. Such a GWB could thus unlock a wealth of new information about multi-field inflation. In Chapter 3, we elaborate on this point with an example. We show that there may exist signals that cannot be extracted using standard cosmological probes such as the CMB and galaxy surveys, but can in principle be detected within GWB with upcoming and proposed gravitational wave experiments. Lastly, in Chapter 4, we focus on the detectability of GWB itself. We discuss a cosmological mechanism that can enhance the strength of the gravitational wave signal from phase transitions, thereby increasing their detection prospects significantly.Item Simulating Bursty and Continuous Reionization Using GPU Computing(2023) Hartley, Blake Teixeira; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Reionization is the process by which the neutral intergallactic medium of the early universe was ionized by the first galaxies, and took place somewhere between roughly redshift 30 and redshift 6, or from 100~Myr into the universe to 1~Gyr. The details of this transition are still not well understood, but observational constraints suggest that reionization happened faster than naive estimates would suggest. In this thesis, we investigate the theory that galaxies which form their stars in short bursts could complete reionization faster than galaxies which emit their photons continuously over their lifespans. We began investigating this theory with a semi-analytic model of the early universe. We used analytic methods to model the expansion of \HII (ionized hydrogen) regions around isolated galaxies, as well as the behavior of the remnant \HII regions after star formation ceases. We then compiled assortments of galaxies matching dark matter simulation profiles and associated each with an \HII region that could either grow continuously or grow quickly before entering a dormant period of recombination. These tests indicated that the remnants of bursty star formation had lower overall recombination rates than those of continuously expanding \HII regions, and that these remnants could allow for ionizing radiation from more distant sources to influence ionization earlier. We decided that the next step towards demonstrating the differences between continuous and bursty star formation would require the use of a more accurate model of the early universe. We chose a photon conserving ray tracing algorithm which follows the path of millions of rays from each galaxy and calculates the ionization rate at every point in a uniform 3D grid. The massive amount of computation required for such an algorithm led us to choose MPI as the framework for building our simulation. MPI allowed us to break the grid into 8 sub-volumes, each of which could be assigned to a node on a supercomputer. We then used CUDA to track the millions of rays, with each of the thousands of CUDA cores handling a single ray. Creating my own simulation library would afford us complete control over the distribution and time dependence of ionizing radiation emission, which is critical to isolating the effect of bursty star formation on reionization. Once we had completed, we conducted a suite of simulations across a selection of model parameters using this library. Every set of model parameters we selected corresponds to two models, one continuous and one bursty. This selection allowed us to isolate the effect of bursty star formation on the results of the simulations. We found that the effects we hoped to see were present in our simulations, and obtained simple estimates of the size of these effects.Item Holographic Cosmological Models and the AdS/CFT Correspondence(2023) Antonini, Stefano; Swingle, Brian; Jacobson, Theodore; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The formulation of a quantum theory of gravity is a central open problem in theoretical physics. In recent years, the development of holography---and in particular the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence---provided a new framework to investigate quantum gravity and led to consistent advancement. However, how to describe cosmology within holography remains an unanswered question whose solution could determine whether holography is able to capture physics in our universe. This dissertation describes a new proposal for embedding cosmological physics in the holographic paradigm. This is articulated in two different but related approaches, both involving time-symmetric Big Bang-Big Crunch cosmologies with negative cosmological constant $\Lambda$.In the first approach, the cosmological universe is given by a four-dimensional end-of-the-world brane moving in a five-dimensional AdS black hole spacetime. The proposed holographic dual description is given by a boundary conformal field theory. Under specific conditions, gravity is localized on the brane and effectively four-dimensional: an observer living on the brane is unaware of the existence of the extra dimension. In this dissertation, I show how these conditions can be met in an AdS-Reissner-Nordstr\"om background while retaining a holographic dual description. The second approach focuses on spatially flat $\Lambda<0$ cosmologies which analytically continue to Euclidean wormholes connecting two asymptotic AdS boundaries. The proposed dual theory is given by two holographic 3D CFTs coupled by non-holographic 4D degrees of freedom on a strip. A different analytic continuation of the Euclidean wormhole leads to a Lorentzian traversable wormhole. After discussing the general features of these holographic cosmologies, I describe how the traversable wormhole can be reconstructed from the dual theory and how the existence of the former constrains the latter. Finally, I show that these $\Lambda<0$ cosmologies can undergo phases of accelerated expansion and match observational data for the scale factor evolution. The results presented in this dissertation should be regarded as the initial steps on a new line of research which will hopefully lead to a description of quantum gravity in a cosmological universe via holography. Achieving this goal would render holography a viable candidate to describe quantum gravity in our universe.Item DETERMINING THE NEUTRINO LIFETIME FROM COSMOLOGY(2021) Dev, Abhish; Chacko, Zackaria ZC; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Neutrinos are the most mysterious particles in the standard model. Many of theirfundamental properties such as their masses, lifetimes, and nature (Dirac or Majorana) are yet to be pinned down by experiments. Currently, the strongest bound on neutrino masses comes from cosmology. This bound is obtained by scrutinizing the gravitational effect of the cosmic neutrinos on the evolution of structure in our universe. However, this bound assumes that the neutrinos from the Big Bang have survived until the present day. In this dissertation, the unstable neutrino scenario is studied in light of current and near-future cosmological experiments. We show that the current cosmological bound on the neutrino masses can be relaxed significantly in an unstable neutrino scenario. We further show that near-future experiments offer the possibility of independently measuring both the masses of the neutrinos and their lifetimes. We consider an elusive scenario in which the cosmic neutrinos decay into invisible radiation after becoming non-relativistic. The Boltzmann equations that govern the cosmological evolution of density perturbations in the case of unstable neutrinos are derived and solved numerically to determine the effects on the matter power spectrum and lensing of the cosmic microwave background (CMB). A Markov-Chain Monte-Carlo (MCMC) analysis is done on the current cosmological data and mock future data to obtain its sensitivity to the neutrino masses and lifetimes. We show that the effect of the neutrino masses on large scale structures is dampened by the decay of neutrinos, which leads to a parameter degeneracy between the neutrino masses and lifetimes inferred from the cosmological data. This degeneracy allows for a significant relaxation of the current cosmological upper bound on the sum of neutrino masses from about 0.2 eV in the stable neutrino case to 0.9 eV in the unstable neutrino scenario. This window is important for terrestrial experiments such as KATRIN which are seeking to independently measure the neutrino masses in the laboratory. We further show that near-future large scale structure measurements from the Euclid satellite, when combined with cosmic microwave background data from Planck, may allow an independent determination of both the lifetimes of the neutrinos and the sum of their masses. These parameters can be independently determined because the Euclid data will cover a range of redshifts, allowing the growth of structure over time to be tracked. If neutrinos are stable on the timescale of the age of the universe, we show that these observations can improve the lower limit on the lifetimes of the neutrinos by seven orders of magnitude, from O(10) years to 2 × 108 years(95%C.L.), without significantly affecting the measurement of the neutrino masses. On the other hand, if neutrinos decay after becoming non-relativistic but on timescales less than O(100) million years, these observations may allow for, not just the first measurement of the sum of neutrino masses, but also the determination of the neutrino lifetime from cosmology.Item GENERALIZED NATURAL INFLATION AND THE QUEST FOR COSMIC SYMMETRY BREAKING PATTERNS(2018) Riquelme, Simon; Chacko, Zackaria; Wentworth, Richard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a two-field model that generalizes Natural Inflation, in which the inflaton is the pseudo-Goldstone boson of an approximate symmetry that is spontaneously broken, and the radial mode is dynamical. Within this model, which we designate as ``Generalized Natural Inflation'', we analyze how the dynamics fundamentally depends on the mass of the radial mode and determine the size of the non-Gaussianities arising from such a scenario. We also motivate ongoing research within the coset construction formalism, that aims to clarify how the spontaneous symmetry breaking pattern of spacetime, gauge, and internal symmetries may allow us to get a deeper understanding, and an actual algebraic classification in the spirit of the so-called ``zoology of condensed matter'', of different possible ``cosmic states'', some of which may be quite relevant for model-independent statements about different phases in the evolution of our universe. The outcome of these investigations will be reported elsewhere.Item Non-Perturbative Methods in Quantum Field Theory and Quantum Gravity(2016) de la Fuente, Anton; Sundrum, Raman; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis considers non-perturbative methods in quantum field theory with applications to gravity and cosmology. In particular, there are chapters on black hole holography, inflationary model building, and the conformal bootstrap.