Browsing by Subject "Early universe"
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Item Constraining the end of reionization with ly [alpha] spectroscopy(2019-07-29) Jung, Intae; Finkelstein, Steven L.; Dickinson, Mark; Bromm, Volker; Gebhardt, Karl; Casey, Caitlin MThe reionization of the intergalactic medium (IGM) marks the time in the early universe when the first stars and galaxies began to affect the universe around them, as during this last major phase transition high-energy ultraviolet photons from these objects ionized the gas in the IGM, and it remains ionized to the present day. Studying reionization is a key frontier in observational cosmology, as it can therefore provide key insights into the formation and evolution of galaxies in the early universe. As Lyα photons are resonantly scattered by neutral hydrogen in the IGM, an analysis of this line can be used to trace the existence of neutral hydrogen in the IGM at different points in the history of the universe (i.e., when the IGM becomes neutral, we should stop seeing these photons, as they are likely scattered out of our line-of-sight). The work in this dissertation focuses on completing a spectroscopic survey of galaxies in the early universe, to measure the Lyα equivalent width (EW) distribution into the epoch of reionization and investigate the evolution of the IGM during reionization, pinning down the late time-evolution of reionization. To measure the Lyα EW distribution in the reionization era, we utilize deep spectroscopic observations of candidate galaxies from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) using both the DEIMOS (optical) and MOSFIRE (near-infrared; NIR) spectrographs on the Keck telescopes. Our large spectroscopic dataset compiled with both Keck telescope observations represent the deepest and most complete spectroscopic survey for galaxies in the epoch of reionization. We study the Lyα emission strength through constraining the Lyα equivalent width (EW) distribution with our spectroscopic dataset by constructing detailed simulations of mock emission lines, accounting for the observational conditions (e.g., exposure time, wavelength coverage, and sky emission) and galaxy photometric redshift probability distribution functions. The measurements of the EW distribution with the detected Lyα emission lines from our DEIMOS and MOSFIRE observations provide additional evidence that the Lyα EW distribution declines at z > 6, suggesting an increasing fraction of neutral hydrogen in the IGM. Thanks to plenty of high-quality observational data from space telescopes such as the Hubble Space Telescope and the Spitzer Space Telescope, observational studies of the evolution of galaxies in the early universe have been performed over the past decade. Those studies have revealed statistical trends of the star formation history and the evolution of those galaxies over the cosmic time. However, since the star formation happens in a complex way, an analysis of the integrated properties of galaxies is not enough to grasp the physical processes governing the star formation and the growth of galaxies. The advent of a spatially resolved study of each individual galaxy has provided us an excellent approach to explore star formation processes inside a galaxy and to examine the inside-out / outside-in growth scenarios of galaxies. We perform a spatially resolved study of galaxies in the early universe at z ≳ 4 using the CANDELS Survey and HAWK-I UDS GOODS (HUGS) survey data. We estimate stellar mass, star formation rate, and dust extinction for galaxy inner and outer regions via spatially resolved spectral energy distribution fitting based on a Markov Chain Monte Carlo algorithm. By comparing specific star formation rates (sSFRs) between inner and outer parts of the galaxies we find that the majority of galaxies with high central mass densities show evidence for a preferentially lower sSFR in their centers than in their outer regions, indicative of reduced sSFRs in their central regionsItem Gardening the landscape and bushwhacking through the swampland : exploring the consequences of quantum gravity for cosmic inflation(2022-02-07) Rosati, Robert James; Paban, Sonia; Distler, Jacques; Kilic, Can; Boylan-Kolchin, MichaelThis dissertation consists of five chapters. The first broadly and briefly orients the reader through an introduction to inflationary cosmology, and why we might expect multi-field inflation to take place. The next four chapters correspond to distinct lines of research conducted during my time as a graduate student. Chapter two is based on work conducted with my advisor Sonia Paban, studying the landscape of possible multi-field inflationary models through a random-matrix generated potential [1]. Chapter three is based on work with Diederik Roest and Perseas Christodoulidis, studying universality and prior dependence in multi-field inflation [2]. Chapters four and five are based on work with Sonia Paban and Vikas Aragam, studying inflation in potentials compatible with quantum gravity and rapidly turning trajectories [3, 4].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 The kinetic Sunyaev-Zel’dovich effect as a probe of the physics of cosmic reionization : the effect of self-regulated reionization(2014-12) Park, Hyunbae; Shapiro, Paul R.We calculate the angular power spectrum of the cosmic microwave background temperature fluctuations induced by the kinetic Sunyaev-Zel'dovich (kSZ) effect from the epoch of reionization (EOR). We use detailed N-body+radiative-transfer simulations to follow inhomogeneous reionization of the intergalactic medium. For the first time, we take into account the "self-regulation" of reionization: star formation in low-mass dwarf galaxies or minihalos is suppressed if these halos form in the regions that were already ionized or Lyman-Werner dissociated. Some previous work suggested that the amplitude of the kSZ power spectrum from the EOR can be described by a two-parameter family: the epoch of half-ionization and the duration of reionization. However, we argue that this picture applies only to simple forms of the reionization history which are roughly symmetric about the half-ionization epoch. In self-regulated reionization, the universe begins to be ionized early, maintains a low level of ionization for an extended period, and then finishes reionization as soon as high-mass atomically cooling halos dominate. While inclusion of self-regulation affects the amplitude of the kSZ power spectrum only modestly (~10%), it can change the duration of reionization by a factor of more than two. We conclude that the simple two-parameter family does not capture the effect of a physical, yet complex, reionization history caused by self-regulation. When added to the post-reionization kSZ contribution, our prediction for the total kSZ power spectrum is below the current upper bound from the South Pole Telescope. Therefore, the current upper bound on the kSZ effect from the EOR is consistent with our understanding of the physics of reionization.Item Maximal X-ray feedback in the pre-reionization universe(2023-08-04) Jeon, Junehyoung; Bromm, Volker; Finkelstein, Steven LX-ray feedback in the pre-reionization Universe provided one of the major energy sources for reionization and the thermal evolution of the early intergalactic medium. However, X-ray sources at high redshift have remained largely inaccessible to observations. One alternative approach to study the overall effect of X-ray feedback in the early Universe is a full cosmological simulation. Toward this goal, in this paper we create an analytic model of X-ray feedback from accretion onto supermassive black holes (SMBHs), to be used as a sub-grid model in future cosmological simulations. Our analytic model provides a relation between the mass of a dark matter halo and the SMBH it hosts, where the efficiency is governed by an energy balance argument between thermal feedback and the confining gravitational potential of the halo. To calibrate the model, we couple the halo-level recipe with the Press-Schechter halo mass function and derive global mass and energy densities. We then compare our model to various observational constraints, such as the resulting soft X-ray and IR cosmic radiation backgrounds, to test our choice of model parameters. We in particular derive model parameters that do not violate any constraints, while providing maximal X-ray feedback prior to reionization. In addition, we consider the contribution of SMBH X-ray sources to reionization and the global 21 cm absorption signal.Item New insights into primordial star formation(2011-08) Stacy, Athena Ranice; Bromm, Volker; Milosavljevic, Milos; Wheeler, John C.; Evans, Neal J.; Dinerstein, Harriet; Loeb, AbrahamThe formation of the first stars, also known as Population III (Pop III), marked a pivotal point in the universe's evolution from relative smoothness and homogeneity to its current highly structured state. In this dissertation we study key aspects of Pop III star formation. We utilize three-dimensional cosmological simulations to follow the evolution of gas and DM from z ~100 until the first minihalo forms. Once the gas infalls toward the center of the minihalo and condenses, we implement the 'sink particle' method to represent regions that will form a star, and we follow the evolution of the metal-free, star-forming gas for many free-fall times. A disk forms around the initial Pop III star and fragments to form secondary stars with a range of masses (1 - 50 [solar mass]). This is markedly different from the previous paradigm of one single, massive star forming per minihalo. Using a ray-tracing technique, we also examine the effect of radiative feedback on protostellar growth and disk fragmentation. This feedback will not prevent the formation of secondary stars within the disk, but will reduce the final mass reached by the largest Pop III star. Measuring the angular momentum of the gas that falls onto the sink regions, we also find that the more massive Pop III stars accrete sufficient angular momentum to rotate at nearly break-up speeds, and can potentially end their lives as collapsar gamma-ray bursts or hypernovae. We furthermore numerically examine the recently discovered relative streaming motions between dark matter and baryons, originating from the era of recombination. Relative streaming will slightly delay the redshift at which Pop III stars first form, but will otherwise have little impact on Pop III star formation and the history of reionization. We finally evaluate the possible effect of a cosmic ray (CR) background generated by the supernova deaths of massive Pop III stars. A sufficiently large CR background could indirectly enhance the H₂ cooling within the affected minihalos. The resulting lower temperatures would lead to a reduced characteristic stellar mass (~ 10 [solar mass]), providing another possible pathway to form low-mass Pop III stars.Item Probing the nature of dark matter with the first stars and black holes(2022-06-14) Liu, Boyuan (Ph. D. in astronomy); Bromm, Volker; Ferrara, Andrea; Boylan-Kolchin, Michael; Offner, Stella S. R.; Finkelstein, Steven L.We use semi-analytical models and cosmological hydrodynamic simulations to derive the imprints of dark matter physics on the first generation of stars, black holes and galaxies, which, combined with observational data, place constraints on the properties of dark matter. Inspired by the recent detection of gravitational waves, we further investigate the gravitational waves of the first star remnants as a promising probe for early star/structure formation and dark matter physics. Our results indicate that although the first stars only make up a tiny fraction (~ 10⁻⁵) of all stars (ever formed) in the Universe, a much higher fraction (~ 10⁻³ -0.1) of massive compact object mergers can originate from the first stars, which carry valuable information of the early Universe. Finally, we explore the effects of primordial black holes, which can make up all or part of dark matter, on first star formation, and discuss their implications on using gravitational waves as a direct probe of dark matter.Item Understanding the signatures of single-field inflation in cosmological probes(2013-08) Ganc, Jonathan Gabriel; Paban, Sonia; Komatsu, EiichiroI will investigate the primordial squeezed limit bispectrum as produced by inflation in single-field models. Previous results have argued that generically, single-field inflation produces a negligible bispectrum. However, more careful evaluation yields a more ambiguous result. I will discuss an alternate method for calculating the squeezed limit bispectrum for a general single-field inflation model. I will also explore slow-roll inflation with a non-standard initial state, where we find an enhanced squeezed-limit. I will discuss the detectability of such models in various cosmological observables such as the Cosmic Microwave Background (CMB), Large Scale Structure, and mu-distortion of the CMB.