### Browsing by Subject "Inflation"

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Item Bicep Array: Searching for Signals of Inflation From The South Pole(2022-01) Crumrine, MichaelShow more The $\Lambda$CDM cosmological model posits a universe that began with a big bang - like singularity, which contains mostly cold dark matter, and which is experiencing an accelerating expansion due to a dark energy component. This model has experienced resounding success and is consistently upheld by experiments across the globe. The model is incomplete however, it cannot describe how the specific initial conditions required to create the universe we see today came about. Inflation is an extension to the $\Lambda$CDM model which hypothesizes that the universe underwent a period of exponential expansion just after the big bang, sufficient to set up the required initial conditions. Most inflationary theories predict a stochastic gravitational wave background generated as a result of this expansion which would have imprinted a characteristic B-mode signal into the polarization pattern of the Cosmic Microwave Background. Detecting this primordial gravitational wave signal would provide direct evidence for inflation The \textsc{Bicep}/{\it Keck} program constitutes a series of polarization sensitive microwave telescopes situated at the geographic South Pole targeting the degree-angular scale $B$-modes and searching for a primordial signal. Over the last two decades this program has consistently reported the tightest constraints on this signal, with the most recent analysis of data through $2018$ providing an upper limit on the tensor-to-scalar ratio $r<0.036$ at $95\%$ confidence. {\sc Bicep} Array is the latest experiment in the series and replaces the {\it Keck Array}, expanding the frequency coverage to two new low-frequency bands and, once fully operational, increasing the detector count by over an order of magnitude. {\sc Bicep} Array is expected to achieve $\sigma(r) = 0.002 - 0.004$ depending on foreground complexity and the degree of lensing removal. In this dissertation I cover the design of this new experiment -- with a focus on the design and performance of the cryogenics down to $4$\,K -- and the first year's observations. I analyze the first year of new low frequency data in combination with the recently release BK18 results and find that the new data provides no improvement on $\sigma(r)$. However, it provides significant constraining power on galactic synchrotron radiation resulting in a factor of two decrease in the uncertainty on the amplitude of this foreground signal.Show more Item Calibration and design of the E and B EXperiment (EBEX) cryogenic receiver(2014-08) Zilic, Kyle ThomasShow more I discuss the design, construction, and calibration of the E \& B EXperiment (EBEX), a balloon-borne telescope designed to measure the B-mode polarization anisotropy of the cosmic microwave background (CMB). EBEX observes the sky with 8 arcmin resolution in three frequency bands centered on 150, 250, and 410 GHz, with over 1,500 detectors. Polarimetry is performed through use of a continuously rotating achromatic half-wave plate with fixed wire-grid polarizer. The experiment was designed to detect the gravitational-lensed B-mode signal and detect or set an upper limit for the inflationary B-mode signal. In this thesis, I describe the design and structure of various subsystems of the EBEX receiver and predict their experimental performance. Several calibrating instrumental response experiments are described and the results reported and compared to predictions. A brief review of the 2012-2013 long duration balloon (LDB) flight from McMurdo Station, Antarctica, is provided and a summary of the receiver performance during flight characterized.Show more Item Calibration of the E and B EXperiment (EBEX), a balloon-borne cosmic microwave background polarimeter.(2009-10) Polsgrove, Daniel EdwardShow more We discuss pre-flight calibration of the E and B EXperiment (EBEX), a balloon-borne telescope designed to measure the B-mode polarization anisotropy of the cosmic microwave background (CMB). EBEX will observe the sky with 8' resolution in each of three bands centered on 150, 250 and 410 GHz. Employing over 1,400 detectors and performing polarimetry through a continuously rotating half-wave plate with fixed wire-grid polarizer, we expect to detect the B-mode signal or set a new upper limit one order of magnitude below the current value. In this thesis we describe a set of ground-based experiments devised for calibrating instrumental response to incident millimeter-wave flux with varying spectral and polarization properties. We chronicle the design, construction and execution of these experiments, along with preliminary results from tests executed prior to our North American (NA) test flight which originated at the Columbia Scientific Ballooning Facility, Ft Sumner, NM in June 2009. A brief review of this inaugural flight is provided, as is a synopsis of our current plan for a comprehensive calibration strategy to be implemented in conjunction with a future long duration balloon (LDB) flight over Antarctica.Show more Item Constraining Inflation Models with the BICEP/Keck B-mode Experiments(2023-08) Lau, KennyShow more Modern observational Cosmology is highly developed and the observable Universe on a large scale is currently well described by the standard LCDM model. The model has a small number of free parameters and describes how the Universe evolved from a hot, dense and homogeneous state with primordial perturbations that are Gaussian, adiabatic and close to scale-invariant. The inflation paradigm extends the LCDM model and interprets the initial conditions as natural consequences of a hypothesized exponential expansion. However, generic inflationary models make a prediction that has not yet been observed: the existence of a background of primordial gravitational waves (PGWs), which would leave an imprint of B-mode polarization in the Cosmic Microwave Background (CMB). Measuring the degree-scale B-mode polarization therefore emerges as one of the most promising methods to detect or set limits on PGWs. For the past decade, the BICEP/Keck collaboration has been operating a series of telescopes at the Amundsen-Scott South Pole Station optimized for B-mode observation. These telescopes are compact refracting polarimeters mapping about 2% of the sky under the exceptionally stable and transparent atmosphere of the Geographical South Pole. They observe at a broad range of frequencies to separate the cosmological signals from polarized Galactic synchrotron and thermal dust emission which dominate at the two ends of the microwave regime. In this dissertation, we discuss the BICEP/Keck experimental progress in two major areas. We first present the "BK18 analysis" utilizing data collected up to the 2018 observing season, in conjunction with selected WMAP and Planck polarization maps. The analysis particularly exploits new data from (1) the 3-year BICEP3 map, the current deepest CMB polarization measurement at the foreground-minimum 95 GHz; and (2) the Keck 220 GHz map which has a higher signal-to-noise ratio on the dust foreground than the Planck 353 GHz map. The likelihood analysis of the BB auto- and cross-spectra of the maps reduces the experimental uncertainty on the tensor-to-scalar ratio r to sigma(r)=0.009, and the inference of r from our baseline model is tightened to r=0.014+0.010-0.011 and r<0.036 at 95% confidence, the most powerful constraints on PGWs to date. These B-mode spectra hence provide unprecedented power to discriminate among popular classes of inflation theories in the r-ns plane — the natural inflation models and the monomial power law inflation models are now strongly disfavored by the data. We subsequently discuss the instrument development of BICEP Array, a BICEP3-style multiple-frequency (30/40/95/150/220/270 GHz) telescope succeeding Keck from 2020. We focus on the construction of a novel mount and the corresponding pointing model for the telescope. The performance of both are validated by the first robust detection of cosmological polarization signals at 40 GHz in our observation field. Preliminary forecasts show that the constraint can be improved to sigma(r) < 0.003 using the upcoming BICEP3, BICEP Array and SPT-3G data up to 2027.Show more Item Coupled quantum systems in inflationary cosmology.(2010-08) Gumrukcuoglu, Ahmet EmirShow more The studies presented in this thesis describe applications of quantum field theory in a time dependent background. Two distinct problems are addressed in the framework of inflationary cosmology. The strict predictions of inflation are mostly in agreement with the Cosmic Microwave Background observations. In the recent years, large scale anomalies in the data motivated a series of analyses leading to a detection of broken statistical isotropy. Assuming that this effect is sourced by early time cosmology, I discuss the phenomenology of inflationary models extended to anisotropic backgrounds. Due to lack of rotational invariance, these models generically involve a system of coupled quantum fields. This leads to a tensor-scalar correlation function, which is a characteristic signature of these models. Another open question in cosmology involves the transition from inflation to the Hot Big Bang cosmology. In the presence of supersymmetric flat directions, the formation of the thermal radiation may undergo a dramatic delay, provided that these directions decay only perturbatively. In the scope of a toy model and a realistic example, both involving two flat directions, I discuss the nonperturbative decay that rapidly depletes the flat directions. If realized, this process can dramatically affect the previous assumptions on the thermalization scale. Due to the vast number of degrees of freedom, this problem generically involves coupled quantum fields. The decay of the flat directions gets contributions from both the diagonal (nonadiabatic evolution of frequency eigenvalues) and nondiagonal (nonadiabatic evolution of frequency eigenstates) effects. An additional characteristic effect of coupled quantization is the rotation of light eigenstates to heavy ones, which do not get produced in a diagonal system.Show more Item The evolution and decay of supersymmetric flat directions in the early universe and their role in thermalizing the universe(2008-12) Sexton, Matthew G.Show more I study the post-inflation oscillation and decay of light coherent scalar field condensates that may develop during an inflationary phase of the universe. In particular, the light scalars studied are a composition of the scalar particles of a supersymmetric theory which correspond to the flat directions of the theory's scalar potential. Some toy models that possess supersymmetric flat directions are presented and numerical solutions for the evolution of the scalar fields are obtained. Both analytic and numeric results suggest that such condensates, if they existed in the early universe, can decay through a rapid and nonperturbative process long before these condensates could significantly affect the thermalization of the universe.Show more Item Gauge Field Amplification during Axion Inflation(2018-08) UNAL, CANERShow more This thesis studies the interaction of different fields during inflation and resultant phenomenology at different scales. It particularly focuses on one of the most well motivated inflationary models, called Axion inflation. Axions are pseudo-scalars that possess the shift symmetry at least at the approximate level, which protects their potential from quantum corrections and elevates them as a strong inflaton candidate. However, in the particle inventory of UV complete theories, axion particles are abundant, which motivates studying axions as inflaton or spectator field during inflation. In this work, we study a chiral shift symmetric dimension-five operator arising naturally in any axion theory. Due to this coupling, the gauge field'Äôs dispersion relation is modified and one helicity of the gauge field is produced abundantly as a function of a dimensionless parameter proportional to speed of the axion. This breaks the parity conservation. Furthermore, this amplified gauge quanta inversely decays (ie. sources back) to scalar and tensor degrees of freedom via two-to-one way; hence, the sourced perturbations obey non-Gaussian statistics. These sourced modes leave unique imprints on cosmological observables such as : Chiral gravitational wave (GW) background, large tensor non-Gaussianity, non-zero TB and EB correlators, detectable GW background at interferometer scales and the production of primordial black holes.Show more Item No-Scale Inflation(2016-08) Garcia Garcia, MarcosShow more Supersymmetry is the most natural framework for physics above the TeV scale, and the corresponding framework for early-Universe cosmology, including inflation, is supergravity. No-scale supergravity emerges from generic string compactifications and yields a non-negative potential, and is therefore a plausible framework for constructing models of inflation. No-scale inflation yields naturally predictions similar to those of the Starobinsky model based on $R + R^2$ gravity, with a tilted spectrum of scalar perturbations: $n_s∼0.96$, and small values of the tensor-to-scalar perturbation ratio $r < 0.1$, as favored by Planck and other data on the cosmic microwave background (CMB). In this thesis we introduce a novel no-scale inflationary model that averts the stabilization problem of supergravity models; to study it we develop a multi-field formalism applicable to supergravity models. We discuss the low-energy phenomenology of generic no-scale models and its connection to the lifetime of the inflaton. We use our results to analyze the constraints on these models imposed by CMB measurements, which through the calculation of the number of e-folds $N_*$ , we relate to constraints on the inflaton decay rate and other parameters of specific no-scale inflationary models. Finally, we revisit gravitino production following inflation, including thermal and non-thermal effects, and discuss the potential implications of upper limits on the gravitino abundance for no-scale models of inflation. Our results may provide insights into the embedding of inflation within string theory as well as its links to collider physics.Show more Item Phenomenology of Axion-Gauge Interactions in the Early Universe(2021-05) Papageorgiou, AlexandrosShow more The present thesis explores various effects that arise as a consequence of axion-gauge couplings in the early universe. Axions (or Axion-Like Particles) are pseudo-scalar particles that enjoy an approximate shift symmetry which protects the flatness of their potential from obtaining large radiative corrections. This property, as well as the fact that axions are abundantly predicted by high energy theories such as Supergravity and String Theory, makes the study of the phenomenology of axions in the early universe particularly interesting. Assuming that axions are present and cosmologically relevant in the early universe, it is a natural question to ask what effects may arise from couplings of axions to other fields such as boson or fermions. There is a unique shift symmetric, five-dimensional axion-gauge coupling which is expected in any axion theory. The gauge fields that are studied in the present work are either Abelian U(1) or non-Abelian SU(2) gauge fields. In both cases, the presence of the axion-gauge interaction modifies the dispersion relation of the various gauge field perturbations and under certain conditions one helicity of the perturbation degrees of freedom may become tachyonically unstable. These unstable perturbations are produced exponentially rapidly and their abundance can backreact on scalar and tensor perturbations leading to modified predictions for inflationary models compared to the predictions that are neglecting such contributions. An additional effect of these enhanced perturbations, is the production of a primordial lepton asymmetry which could in principle account for the matter-antimatter asymmetry observed today. Finally such enhanced perturbations could play a role in theories of Quintessential Dark Energy. In such theories, the Dark Energy component is a scalar field in a slow-roll configuration. The possibility that the Quintessence field is an axion field with a steep potential is explored. In this case slow-roll is maintained as a consequence of the axion producing the unstable perturbations at the expense of its own kinetic energy.Show more Item Primordial black hole formation in the inflationary Universe(2022-08) Mahbub, RafidShow more In this work, we consider the formation of primordial black holes (PBHs) using the theory of cosmic inflation. We construct an inflaton potential using $\alpha$-attractors, possessing a plateau-like region where the inflaton enters into a non-attractor phase, giving rise to the phenomenon of ultra slow-roll (USR). Exploiting the fact that the inflaton experiences a dramatic deceleration during USR, a sharp enhancement in the curvature perturbation $\mathcal{P}_\zeta$ can be shown to be characteristic of such models, subsequently allowing for copious production of PBHs in the post-inflationary, radiation epoch of the Universe. The parameters in our model are highly fine-tuned such that an $\mathcal{O}(10^7)$ enhancement in the power spectrum is produced for modes with comoving wavenumber $k\sim 10^{14}\:\text{Mpc}^{-1}$, allowing for abundant PBH formation of mass $M\sim 10^{17}\:\text{g}$. In the mass range $10^{17}-10^{19}\:\text{g}$, PBHs can constitute the entirety of cold dark matter (CDM) after pre-existing constraints in the range have been called into question. The formation fraction of PBHs $\beta(M)$ is the most important quantity that cosmologists compute. It relates the mass-energy contained in PBHs to that of the expanding background. The computation of $\beta(M)$ sometimes controversial due to the different methodologies one adopts for performing the calculations. In essence, this is the Press-Schechter vs. peaks theory debate. In this work, we address both issues. Firstly, we improve upon the simplest rendition of Press-Schechter -- which, with its simplicity set aside -- does not take into account the inherently nonlinear relationship between the primordial curvature perturbations and the post-inflationary overdensity perturbations. We refine the simple Press-Schechter formalism by taking into account such nonlinearities. One can then show that this unavoidably generates non-Gaussianities in the probability distribution of overdensities, with the suppression of PBH abundance arising as a consequence. Using this, we calculate the first two, nontrivial higher order moments, namely the skewness and kurtosis, which are subsequently used in a nonlinear redefinition of the PBH formation fraction. Furthermore, we perform the same calculations using optimized peaks theory, combining a more accurate calculation of PBH mass. Taking into account the detailed shape and profile of curvature perturbations, along with an accurate mass calculation, we show that the proper consideration of the effects of curvature generally predict PBHs to be more massive, by around a factor of 10, compared to na\"{i}ve estimates. Finally, we study the effects of enhanced effects of quantum diffusion in the USR region through the stochastic inflationary formalism. We aim at providing a generic framework with which such quantum diffusion effects can be numerically modeled without resorting to slow-roll approximations, commenting on how to skirt around the inherently non-Markovian nature of the noise terms arising in stochastic inflation. We show that, once quantum diffusion effects are taken into account, there is additional enhancement in $\mathcal{P}_\zeta$ during the USR phase, on top of the usual amplification, which can have important consequences not only for PBH formation, but also for inflationary model building and parameter fine-tuning.Show more