Browsing by Subject "Cosmology"
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Item Aspects of cosmology and quantum gravity in an accelerating universe(2006) Krishnan, Chethan, 1978-; Fischler, WillyThe observation that we are living in a Universe that is expanding at an ever-increasing rate is a major challenge for any fundamental theory. The most obvious explanation for an accelerating Universe is a positive cosmological constant, but we do not really know how to do quantum field theory or string theory in spacetimes that are not asymptotically flat. In this thesis, we address various issues that arise in this general context. The problems we address include the stability and evolution of de Sitter-like compactifications, the possibility of defining a quantum theory in de Sitter space using quantum groups, and finally, the classical evolution of thin shells (boundaries of new phase bubbles) in an inhomogeneous Universe with positive Λ.Item Constraining fundamental physics with cosmology(2009-08) Flauger, Raphael Manfred; Weinberg, StevenIt is shown in three examples that future cosmological data may allow us to constrain fundamental physics in interesting ways. The first example illustrates that correlations in the polarization of the cosmic microwave background may allow us to put the strongest limit yet on the mass of a particle, the graviton, at a level of m . 10−30 eV. In the second example, it is shown that observations of the correlations of temperature anisotropies and polarization of the cosmic microwave background may reveal hints for the realization of a class of string theoretic inflationary models that go by the name of axion monodromy inflation, or, rule them out. If the evidence for inflation strengthens substantially, just the requirement that inflation occurred may be used to constrain models of fundamental physics. The third example shows that a class of string compactifications that are commonly used in the context of string phenomenology cannot support inflation and might thus be ruled out by cosmology. For completeness, a review of the physics underlying the cosmic microwave background radiation is included and some analytical results for the signatures of primordial gravitational waves in the cosmic microwave background are given.Item Cosmology and gravity in the brane world(Texas A&M University, 2005-11-01) Dent, James BlackmanThe cosmology in the Hubble expansion era of the Horava-Witten M-theory compactified on a Calabi-Yau threefold is studied in the reduction to five-dimensions where the effects of the Calabi-Yau manifold are summarized by the volume modulus, and all perturbative potentials are included. Matter on the branes are treated as first order perturbations of the static vacuum solution, and all equations in the bulk and all boundary conditions on both end branes are imposed. It is found that for a static volume modulus and a static fifth dimension, y, one can recover the four dimensional Robertson-Friedmann-Walker cosmology for relativistic matter on the branes, but not for non-relativistic matter. For relativistic matter, the Hubble parameter H becomes independent of y to first order in matter density, and if a consistent solution for nonrelativistic matter exists it would require H to be y dependent. These results hold also when an arbitrary number of 5-branes are included in the bulk. The five dimensional Horava-Witten model is compared with the Randall Sundrum phenomenology with a scalar field in the bulk where a bulk and brane potential are used so that the vacuum solutions can be rigorously obtained.(In the Appendix, the difficulty of obtaining approximate vacuum solutions for other potentials is discussed.) In this case nonrelativistic matter is accommodated by allowing the distance between the branes to vary. It is suggested that non-perturbative potentials for the vacuum solution of Horava-Witten theory are needed to remove the inconsistency that non-relativistic matter creates. Also considered is the problem of gravitational forces between point particles on the branes in a Randall-Sundrum (R-S) two brane model with S1/Z2 symmetry. Matter is assumed to produce a perturbation to the R-S vacuum metric and all the 5D Einstein equations are solved to linearized order (for arbitrary matter on both branes). We show that while the gauge condition hi5 = 0, i = 0, 1, 2, 3 can always be achieved without brane bending, the condition h55 = 0 leads to large brane bending. The static potential arising from the zero modes and the corrections due to the Kaluza-Klein (KK) modes are calculated. Gravitational forces on the Planck (y1 = 0) brane recover Newtonian physics with small KK corrections (in accord with other work). However, forces on the TeV (y2) brane due to particles on that brane are strongly distorted by large R-S exponentials, making the model in disagreement with experiment if the TeV brane is the physical brane.Item Cosmology driven by physics beyond the standard model(2007-12) Žanić, Marija, 1972-; Paban, SoniaThis dissertation investigates several problems inspired by the interplay of cosmology and theories beyond the Standard Model of particle physics. The first part of this work is a study of time evolution of unstable dS[subscript p] x S[superscript q] configurations with flux in theories of gravity with a cosmological constant. We find that, depending on the flux, these configurations either evolve towards newly identified stable solutions with a smaller final effective cosmological constant, or tend toward decompactication of the internal sphere. In the second part, we investigate the problem of evolution of vacuum bubbles in inhomogeneous backgrounds. It is expected that the process of inflation will signifcantly smooth out spatial inhomogeneities. However, the initial conditions for inflation are often taken in the already homogeneous and isotropic FRW form, even though it is assumed that initial homogeneity is not necessary for the onset of inflation. We determine the effects of certain inhomogeneities, introduced in the curvature of the outside spacetime, on the propagation of bubbles, and how these effects differ depending on whether the perspective taken is that of the outside observer or an observer on the bubble. The last part of the dissertation presents a model for a novel component of the energy density of the universe. The observational limits on the present energy density allow for a component that redshifts like 1/a² and can contribute significantly to the total. We show that one possible origin for such a contribution is that the universe has a toroidal topology with "wound" scalar fields around its cycles.Item Cosmology with Bose-Einstein-condensed scalar field dark matter(2013-05) Li, Bohua, Ph. D.; Shapiro, Paul R.Despite the great successes of the Cold Dark Matter (CDM) model in explaining a wide range of observations of the global evolution and the formation of galaxies and large-scale structure in the universe, the origin and microscopic nature of this dark matter is still unknown. The most common form of CDM considered to-date is that of Weakly Interacting Massive Particles (WIMPs), but some of the cosmological predictions for this kind of CDM are in apparent conflict with observations (e.g. cuspy-cored halos and an overabundance of satellite dwarf galaxies). For these reasons, it is important to consider the consequences of different forms of CDM. We focus here on the hypothesis that the dark matter is comprised, instead, of ultralight bosons that form a Bose-Einstein Condensate (BEC), described by a complex scalar field. We start from the Klein-Gordon and Einstein field equations to describe the evolution of the Friedmann-Robertson-Walker (FRW) universe in the presence of this kind of dark matter. We find that, in addition to the phases of radiation-domination (RD), matter-domination (MD) and Lambda-domination (LD) familiar from the standard CDM model, there is an earlier phase of scalar-field-domination (SFD) which is special to this model. In addition, while WIMP CDM is non-relativistic at all times after it decouples, the equation of state of BEC-SFDM is found to be relativistic at early times, evolving from incompressible ($\bar{p} = \bar{\rho}$) to radiation-like ($\bar{p} = \bar{\rho}/3$), before it becomes non-relativistic and CDM-like at late times. The timing of the transitions between these phases and regimes is shown to yield fundamental constraints on the particle mass and self-interaction coupling strength. We also discuss progress on the description of structure formation in this model, which includes additional constraints on these parameters.Item Cosmology with high (z>1) redshift galaxy surveys(2010-08) Jeong, Donghui; Komatsu, Eiichiro; Bromm, Volker; Hill, Gary; Seljak, Uros; Shapiro, PaulGalaxy redshift surveys are powerful probes of cosmology. Yet, in order to fully exploit the information contained in galaxy surveys, we need to improve upon our understanding of the structure formation in the Universe. Galaxies are formed/observed at late times when the density field is no longer linear so that understanding non-linearities is essential. In this thesis, we show that, at high redshifts, we can accurately model the galaxy power spectrum in redshift space by using the standard cosmological perturbation theory. Going beyond the power spectrum, we can use the three-point function, or the bispectrum, to gain important information on the early universe as well as on the galaxy formation via measurements of primordial non-Gaussianity and galaxy bias. We show that the galaxy bispectrum is more sensitive to primordial non-Gaussianities than previously recognized, making high-redshift galaxy surveys a particularly potent probe of the physics of inflation. Weak lensing offers yet another way of probing cosmology. By cross correlating the angular position of galaxies with the shear measurement from galaxy lensing or CMB lensing, we also show that one can obtain the information on cosmological distance scale, the galaxy bias, and the primordial non Gaussianity from weak lensing method.Item Dark matter detection with polarized detectors(2012-08) Chiang, Chi-Ting; Komatsu, Eiichiro; Gebhardt, KarlWe consider the prospects to use polarized dark-matter detectors to discriminate between various dark-matter models. If WIMPs are fermions and participate in parity-violating interactions with ordinary matter, then the recoil-direction and recoil-energy distributions of nuclei in detectors will depend on the orientation of the initial nuclear spin with respect to the velocity of the detector through the Galactic halo. If, however, WIMPS are scalars, the only possible polarization-dependent interactions are extremely velocity-suppressed and, therefore, unobservable. Since the amplitude of this polarization modulation is fixed by the detector speed through the halo, in units of the speed of light, exposures several times larger than those of current experiments will be required to be probe this effect.Item Hints of Universality from Inflection Point Inflation(2013-07-25) Downes, Sean DonovanThis work aims to understand how cosmic inflation embeds into larger models of particle physics and string theory. Our work operates within a weakened version of the Landscape paradigm, wherein it is assumed that the set of possible Lagrangians is vast enough to admit the notion of a generic model. By focusing on slow-roll inflation, we examine the roles of both the scalar potential and the space of couplings which determine its precise form. In particular, we focus on the structural properties of the scalar potential, and find a surprising result: inflection point inflation emerges as an important ?and under certain assumptions, dominant ? possibility in the context of generic scalar potentials. We begin by a systematic coarse graining over the set of possible inflection point inflation models using V.I. Arnold?s ADE classification of singularities. Similar to du Val?s pioneering work on surface singularities, these determine structural classes for inflection point inflation which depened on a distinct number of control parameters. We consider both single and multifield inflation, and show how the various structural classes embed within each other. We also show how such control parameters influence the larger physical models in to which inflation is embedded. These techniques are then applied to both MSSM inflation and KKLT-type models of string cosmology. In the former case, we find that the scale of inflation can be entirely encoded within the super- potential of supersymmetric quantum field theories. We show how this relieves the fine-tuning required in such models by upwards of twelve orders of magnitude. Moreover, unnatural tuning between SUSY breaking and SUSY preserving sectors is eliminated without the explicit need for any hidden sector dynamics. In the later case, we discuss how structural stability vastly generalizes ? and addresses ? the Kallosh-Linde problem. Implications for the spectrum of SUSY breaking soft terms are then discussed, with an emphasis on how they may assist in constraining the inflationary scalar potential. We then pivot to a general discussion of the FLRW-scalar phase space, and show how inflection points induce caustics ? or dynamical fixed points ? amongst the space of possible trajectories. These fixed points are then used to argue that for uninformative priors on the space of couplings, the likelihood of inflection point inflation scales with the inverse cube of the number of e-foldings. We point out the geometric origin for the known ambiguity in the Liouville measure, and demonstrate of inflection point inflation ameliorates this problem. Finally we investigate the effect of the fixed point structure on the spectrum of density perturbations. We show how an anomaly in the Cosmic Mircowave Background data ? low power at large scales ? can be explained as a by product of the fixed point dynamics.Item The hunt for the first supernovae : the source density and observability of pair instability supernovae from the first stars(2012-05) Hummel, Jacob Alexander; Bromm, VolkerTheoretical models predict that some of the first stars ended their lives as extremely energetic pair-instability supernovae (PISNe). With energies approaching 10⁵³ ergs, these supernovae are expected to be within the detection limits of the upcoming James Webb Space Telescope (JWST), allowing observational constraints to be placed on the properties of the first stars. We estimate the source density of PISNe using a semi-analytic halo mass function based approach, accounting for the effects of feedback from star formation on the PISN rate using cosmological simulations. We estimate an upper limit of ~0.2 PISNe per JWST field of view at any given time. Feedback can reduce this rate significantly, e.g., lowering it to as little as one PISN per 4000 JWST fields of view for the most pessimistic explosion models. We also find that the main obstacle to observing PISNe from the first stars is their scarcity, not their faintness; exposures longer than a few times 10⁴ s will do little to increase the number of PISNe found. Given this we suggest a mosaic style search strategy for detecting PISNe from the first stars. Even rather high redshift PISNe are unlikely to be missed by moderate exposures, and a large number of pointings will be required to ensure a detection.Item The imprint of the ionized intergalactic medium on the temperature anisotropy of the cosmic microwave background and the mutual-impact of reionization and small-scale structure(2015-08) Park, Hyunbae; Shapiro, Paul R.; Komatsu, Eiichiro; Finkelstein, Steven; Milosavljevic, Milos; Kumar, PawanIonized intergalactic medium (IGM) is an important component in cosmic history. After recombination, the universe went though a dark age until the first stars formed. Since the formation of the first stars, the ionized gas, on one hand, played an important role in the history of the universe and, on the other hand, left its imprints on observables that current and future experiments can measure. In this dissertation, we discuss both of each aspects about ionized gas. First, we discuss the mutual-impact of reionization and the IGM in small-scale structures. While reionization took place preferentially from densest regions of the universe, IGM in average density regions is expected to have been ionized externally by galaxies formed in denser regions. Until ionized by external radiation, the IGM is expected to have grown numerous small-scale structures. We simulate how the hydrodynamic feedback on the small-scale structures and its impact on recombination. Then, we also discuss our result on how recombination can impact on the global progress of the reionization. Compared to previous works, we improve on the resolution of simulation. Previous studies took into account only the structures that can form in photoionized gas down to 10⁸ M [sun symbol] in mass. Here, we present a study that resolves halos down to 10⁴ M [sun symbol] to account for structures that were able to form before the reionization heats the gas. Second, we discuss the kinetic Sunyaev-Zel'dovich effect on the Cosmic Microwave Background (CMB) : temperature fluctuations via the Doppler shift induced by the line-of-sight (LOS) component of the momentum of electrons in the ionized IGM. For the EoR contribution to the signal, we calculate the expected signal from simulations of cosmic reionization that, for the first time, includes the effect of "self-regulation" of reionization: star formation in low-mass galaxies (10⁸ M [sun symbol] [less than or equal to] M [subscript halo] [less than or equal to] 10⁹ M [sun symbol]) and minihalos (10⁵ M [sun symbol] [less than or equal to] M [subscript halo] [less than or equal to] 10⁸ M [sun symbol]) is suppressed if these halos form in regions that are already ionized or Lyman-Werner dissociated. For the post-reionization signal, we revisit the currently used model for non-linear transverse momentum power spectrum with a particular emphasis on the connected term that has been neglected in the literature.Item Inferencing neutrino mass hierarchy from cosmology(2011-05) Leu, Richard Hsueh-Yee; Dicus, Duane A.; Bohm, Arno; Chiu, Charles; Komatsu, Eiichiro; Sitz, GregThe observation of solar and atmospheric neutrino oscillations place bounds on the mass differences. However, these probes are insensitive to the absolute mass. To date, cosmology has provided the best bounds on the total neutrino mass. These bounds are based on a degenerate mass model. With the increasing precision of cosmological data, we investigate the effect of the neutrino mass hierarchy. The precision of the parameter estimates stems from precise observations of the cosmic microwave background. However, the effect of neutrino mass hierarchy on this observation is smaller than the cosmic variance. Therefore, we rely on the measurement of the matter power spectrum for hierarchy effects. We propose the use of importance sampling rather than the commonly used Markov chain Monte Carlo. Importance sampling takes advantage of the microwave background's statistical insensitivity to hierarchy. We present forecasted bounds due to Planck and the proposed CMBPol. We also discuss the needed precision for future galaxy surveys in detecting the effect of neutrino mass hierarchy.Item Inflation : connecting theory to observation(2012-08) Meyers, Joel Ray, 1983-; Weinberg, Steven, 1933-; Distler, Jacques; Fischler, Willy; Komatsu, Eiichiro; Paban, SoniaThe inflationary paradigm has become widely accepted as an accurate framework in which to describe the physics of the early universe, due both to the conceptual advantages of the idea and the agreement of its predictions with observational data. However, it remains to be determined which of the many detailed theories of inflation correctly describe the universe in which we live. Any such theory faces the challenge of making accurate predictions which agree with observation while also fitting consistently into a theory of high energy physics. Within this challenge there exists the great opportunity to constrain speculative models of fundamental physics. Inflation thereby provides an observational window into theories conventionally thought to be unreachable by experiment. Measurements of anisotropies in the cosmic microwave background radiation and the distribution of large scale structure have proved to be invaluable tools to probe inflation. There has been recent interest in examining the deviations from gaussianity in the statistics of the observed fluctuations. These higher order statistics, if conclusively discovered, stand to teach us a great deal about inflation. Forthcoming data including improved measurements of the cosmic microwave background temperature and polarization will provide additional means to investigate the inflationary era. It is important to understand precisely what impact inflation has had on the universe we observe and thus understand precisely what observation can tell us about inflation and how it may fit into a fundamental theory of physics. We will show the conditions under which the cosmological correlation functions generated during inflation are conserved, and thus identify the conditions which allow us to use observations today to learn about inflation. We first prove a general result which applies only to the leading approximation of the correlation functions, and then we discuss how to treat the additional complications that come with subleading corrections. Next, we will discuss the observational implications of achieving the conditions for conservation for a particular class of inflationary models. Lastly, we discuss one example of how observations can be used to probe non-inflationary physics beyond the standard cosmological model.Item Lorentz-violating dark matter(2009-05-15) Mondragon, Antonio RichardObservations from the 1930s until the present have established the existence of dark matter with an abundance that is much larger than that of luminous matter. Because none of the known particles of nature have the correct properties to be identified as the dark matter, various exotic candidates have been proposed. The neutralino of supersymmetric theories is the most promising example. Such cold dark matter candidates, however, lead to a conflict between the standard simulations of the evolution of cosmic structure and observations. Simulations predict excessive structure formation on small scales, including density cusps at the centers of galaxies, that is not observed. This conflict still persists in early 2007, and it has not yet been convincingly resolved by attempted explanations that invoke astrophysical phenomena, which would destroy or broaden all small scale structure. We have investigated another candidate that is perhaps more exotic: Lorentz-violating dark matter, which was originally motivated by an unconventional fundamental theory, but which in this dissertation is defined as matter which has a nonzero minimum velocity. Furthermore, the present investigation evolved into the broader goal of exploring the properties of Lorentz-violating matter and the astrophysical consequences ? a subject which to our knowledge has not been previously studied. Our preliminary investigations indicated that this form of matter might have less tendency to form small-scale structure. These preliminary calculations certainly established that Lorentz-violating matter which always moves at an appreciable fraction of the speed of light will bind less strongly. However, the much more thorough set of studies reported here lead to the conclusion that, although the binding energy is reduced, the small-scale structure problem is not solved by Lorentz-violating dark matter. On the other hand, when we compare the predictions of Lorentz-violating dynamics with those of classical special relativity and general relativity, we find that differences might be observable in the orbital motions of galaxies in a cluster. For example, galaxies ? which are composed almost entirely of dark matter ? observed to have enlarged orbits about the cluster center of mass may be an indication of Lorentz violation.Item Low Energy Nuclear Recoil Response in Xenon Gas for Low Mass Dark Matter WIMP Search(2014-04-16) Sofka, Clement JamesOver 80 years of astrophysical observations suggest that the observable luminous matter makes up ? 5% of the total energy density in the Universe. The remaining ~ 95% comes from matter and energy that has not been observed directly. Discovering these "dark" sources of matter/energy is the single most important concern in the modern quest for understanding Nature. We live in an epoch that is almost certainly characterized by a at, expanding Universe. Coupling this with the wealth of astrophysical surveys, we are able to probe the vastness of space, and develop theories of space-time evolution, going back in time several billions of years. The evidence suggests that the Universe began in a Big Bang, underwent a brief moment of Inflation, then cooled and began forming the structures (atoms, molecules, stars, galaxies, etc.) we observe plainly today. An integral part of this consistent story of the Universe's birth and cosmic evolution is the existence of cold dark matter in the form of Weakly Interacting Massive Particles (WIMPs) and dark energy. Initial cosmological considerations suggested that WIMPs were some type of Standard Model (SM) particle, but even the best-case estimates lead to matter energy densities that come up well short without a significant modification of the underlying theory of gravity. The best proposed WIMP candidate has surfaced from efforts motivated by particle physics. A new type of WIMP arises out of Supersymmetry (SUSY). The Lightest Supersymmetric Particle (LSP), a neutralino, seems to fit perfectly into both particle physics and cosmology. First estimates from a Minimal Supersymmetric Standard Model (MSSM) placed the WIMP in the mass range of O(10) - O(10^(3)) GeV/c^(2). However, there is mounting evidence in recent years that suggests the existence of a low mass WIMP as a suitable dark matter candidate. Some of the most sensitive detectors to low mass WIMPs employ noble liquids as a target medium. Groups using noble liquid detectors are currently limited to the detection of relatively higher mass WIMPs because of detector threshold limits, background effects, or a lack of fundamental understanding of very low energy nuclear recoils (< 3 keVnr). This work is aimed at studying these very low nuclear recoil energies in xenon to improve noble element detector sensitivities and develop a fundamental understanding of nuclear stopping power theories originally studied by Lindhard et al. in the 1960's. We present the nuclear recoil results from measurements using a nearly mono-energetic beam of neutrons aimed at high-pressure gaseous xenon (HPXe) in a time projection chamber (TPC). This work demonstrates the viability of future low mass dark matter WIMP and other rare event searches (e.g. Neutrinoless Double Beta Decay, 0 ) using high pressure noble gases.Item Making the Dark Matter Connection Between Particle Physics and Cosmology(2012-10-19) Krislock, Abram MichaelDark matter has been shown to be extremely abundant in our universe. It comprises about 23 percent of the energy density of the entire universe, which is more than five times greater than the regular matter we already know about. Dark matter cannot be explained within the Standard Model of particle physics. However, models which extend the Standard Model, such as supersymmetry, can explain dark matter. This dissertation investigates the signals of some supersymmetry models in the context of collider physics. If dark matter particles or other supersymmetry particles are produced at some collider experiment, such as the Large Hadron Collider, it is important to know how we can find and measure the signatures and properties of these particles. This dissertation provides some measurement techniques for that exact purpose. These measurement techniques are also very general, making them useful for examining other models of particle physics as well. Lastly, if the supersymmetry model can be understood well enough from collider data, the connection back to cosmology can be made. Namely, it is possible to determine (from LHC data and using a standard cosmological calculation) the abundance of dark matter in the universe. Comparing this collider value with the value already measured will be a crucial step in understanding dark matter. This dissertation provides simulated results of this dark matter abundance calculation for a number of supersymmetry model points.Item Plato's mythological project in the Timaeus(2011-05) Zawislanski, Andrew Peter; White, Stephen A. (Stephen Augustus); Mourelatos, Alexander P.In the Timaeus Plato sets forth his cosmological system, and near the beginning of the dialogue he carefully qualifies his claims by saying that his account of the cosmos is not absolutely true, but only no less likely than any other account. Rather than being an offhand remark, this statement is key to understanding Plato's aim in constructing his cosmological myth. Plato's epistemological position prevents him from making strong assertions about physical objects and phenomena, but does allow him to make assertions of truth in morality and metaphysics. Thus while the Timaeus is ostensibly an account of the physical universe, for Plato its true value is in using the physical universe as a mythological symbol for moral and metaphysical truth. Plato's account is no less likely than those of other ancient cosmologists because multiple accounts can fit with the observed phenomena. However, his account, while no more likely, is superior to those of others in that it avoids impiety and, by qualifying its claims about the physical universe, is not threatened by future observations.Item Prospects for directly detecting the first supernovae, and their impact on early star formation(2016-05) Hummel, Jacob Alexander; Bromm, Volker; Milosavljevic, Milos; Wheeler, J. Craig; Finkelstein, Steven; Yoshida, NaokiThe formation of the first stars in the Universe marked a pivotal moment in cosmic history, initiating the transition from the simple initial conditions of the big bang to the complex structures we see today. Ionizing radiation produced by these so-called Population III stars began the process of reionization, and the supernovae marking their deaths initiated the process of chemical enrichment. We assess the prospects for direct detection of the first supernovae should they happen to end their lives as extremely energetic pair-instability supernovae, which should be within the detection limits of the upcoming James Webb Space Telescope. Using a combination of semi-analytic models and cosmological simulations to estimate their source density, we find that the primary obstacle to observing such events is their scarcity, not their faintness. The first supernovae and the compact remnants they leave behind also produce significant amounts of high-energy X-rays and cosmic rays able to travel through the predominantly neutral intergalactic medium and build up a cosmic background. To better understand how these violent explosions impact subsequent episodes of metal-free star formation, we employ ab-initio, cosmological hydrodynamics simulations to model the formation of stars in a minihalo at z = 20-30 under the influence of both an X-ray and cosmic ray background. The presence of an ionizing background---whether X-rays or cosmic rays---serves to expedite the collapse of gas to high densities by enhancing molecular hydrogen cooling, thus allowing stars to form at substantially earlier epochs in strongly irradiated minihalos. The mass of the stars thus formed however appears to be quite robust, maintaining a characteristic mass of order a few tens of solar masses even as the strength of the ionizing background varies by several orders of magnitude. Finally, we describe the novel software developed to enable this research. These tools for manipulating and analyzing simulation data have been released as the open-source GAdget DataFrame Library: gadfly.Item String Phenomenology in the Era of LHC(2010-10-12) Maxin, James A.The low-energy supersymmetry phenomenology for specific classes of string compactifications is investigated given that the low-energy physics may provide a clue as to the structure of the fundamental theory at high energy scales. The one-parameter model (OPM), a highly constrained subset of minimal Supergravity where all the soft-supersymmetry breaking terms may be fixed in terms of the gaugino mass, is studied, in addition to a three-family Pati-Salam model constructed from intersecting D6-branes. Furthermore, the phenomenology of gravity mediated supersymmetry breaking F-theory SU(5) and SO(10) models, as well as F-SU(5) models with vector- like particles, are examined. We determine the viable parameter space that satisfies all the latest experimental constraints, including the most recent WMAP relic neutralino abundance observations, and find it to be consistent with the CDMS II and other concurrent direct-detection experiments. Moreover, we compute the gamma-ray flux and cross-sections of neutralino annihilations into gamma-rays and compare to the published Fermi-LAT satellite telescope measurements. In F-theory SU(5) and SO(10) models, we predict the exact small deviation of the gaugino mass relation at two-loop level near the electroweak scale, which can be tested at the colliders. More- over, in F-SU(5), we predict the precise deviations from the mSUGRA gaugino mass relations due to the presence of the vector-like particles, also testable at the colliders. The compilation of all these results form a comprehensive collection of predictions with which to evaluate these string models alongside anticipated experimental dis- coveries in the coming decade.Item Toward a theory of observation(2014-08) Carney, Daniel Joseph, Jr.; Fischler, Willy; Paban, SoniaQuantum mechanics is usually formulated in terms of a single Hilbert space and observables are defined as operators on this space. Attempts to describe entire spacetimes and their resident matter in this way often encounter paradoxes. For example, it has been argued that an observer falling into a black hole may be able to witness deviations from unitary, violations of semi-classical quantum field theory, and the like. This thesis argues that the essential problem is the insistence on the use of a single, global Hilbert space, because in general it may be that a physical observer cannot causally probe all of the information described by this space due to the presence of horizons. Instead, one could try to define unitary quantum physics directly in terms of the information causally accessible to particular observers. This thesis makes steps toward a systematization of this idea. Given an observer on a timelike worldline, I construct coordinates which (in good cases) cover precisely the set of events to which she can send and then receive a signal. These coordinates have spatial sections parametrized by her proper time, and the metric manifestly encodes the equivalence principle in the sense that it is flat along her worldline. To describe the quantum theory of fields according to these observers, I define Hilbert spaces in terms of field configurations on these spatial sections and show how to implement unitary time-evolution along proper time. I explain how to compare the observations of a pair of observers, and how to obtain the description according to some particular observer given some a priori global description. In this sense, the program outlined here constructs a manifestly unitary description of the events which the observer can causally probe. I give a number of explicit examples of the coordinates, and show how the quantum theory works for a uniformly accelerated observer in flat spacetime and for an inertial (co-moving) observer in an inflating universe.Item Toward an understanding of the large scale structure of the universe with galaxy surveys(2011-12) Shoji, Masatoshi; Komatsu, Eiichiro; Gebhardt, Karl; Hill, Gary; Hui, Lam; Shapiro, PaulLarge-scale structures we see in the universe, such as galaxies, galaxy clusters and structures beyond the scale of clusters, result from gravitational instability of almost isotropic and homogeneous density distribution in the early universe. The degree of the initial anisotropy of the universe and the subsequent growth of gravitational instability, coupled with the expansion rate of the universe, determine the scale and abundance of the structures formed in the universe at later times. A galaxy survey directly observes a distribution of structures in the sky using galaxies as a tracer of the underlying density distribution, and yields constraints on cosmological models when compared to a physical theory of structure formation based on a given cosmological model. Among many cosmological and astronomical phenomena to be understood from a galaxy survey, the nature of the observed accelerated expansion of the universe is the most profound problem in the modern physics. Motivated by various planned and on-going galaxy surveys, including our own Hobby-Ebery Telescope Dark Energy eXperiment (HETDEX), we show the way to fully exploit the data from a galaxy survey. We improve a model of structure formation to include the effect of baryonic pressure and the free-streaming of massive neutrinos at a mildly non-linear regime. Future galaxy surveys are to reach the level of accuracy, where the effect of massive neutrinos on the observed power spectrum is no longer negligible. Proper understanding of these effects gives a way to measure the absolute masses of neutrinos: one of the most fundamental particles, which, by itself, will be a major development in the field of particle physics. Yet, most of the space (~80%) observed by galaxy surveys is occupied by voids. An ellipticity probability distribution function of voids offers yet another way of probing cosmology. Especially, a distribution of ellipticities in the redshift space provides a unique way to measure a growth rate of the structure in the universe apart from other cosmological parameters when combined with the galaxy power spectrum.