Session J14: Cosmology II: Early Universe and Inflation

Sensitivity of Inflationary Predictions to Pre-inflationary Phases

How sensitive are the predictions of inflation to pre-inflationary conditions when the number of efolds of inflation is not too large? In an attempt to address this question, we consider a simple model where the inflationary era is preceded by an era dominated by a radiation fluid that is coupled to the inflaton only gravitationally. We show that there is a natural generalized Bunch-Davies vacuum state for perturbations to the coupled inflaton-gravity-fluid system at early times. With this choice of initial state the model predicts interesting deviations from the standard power spectrum of single field slow-roll inflation. However, the deviations are too small to be observable in near future observations.   

Inflation : Long Wavelength Modes and Non-Gaussianities

For our work, we assume that the whole inflation process can be separated into two main regimes; one described by a field $\Phi$ (the inflaton field) interacting weakly with $\Sigma$ (in the hidden sector), and another one, at energies $\Lambda_{2\to1}$ and lower, where the field $\Sigma$ has been integrated out and we are in a single field $\Phi$ model with a small sound speed $c_s$. During the two-field regime, coupling between short and long wavelength can generate non-equilateral non-Gaussianities. Upon imposing some conditions, such a generation of non-Gaussianities is not allowed in a single field regime. Our objective is to include the contribution of these long and short wavelength couplings in the single effective field model. For that, we derived an effective Lagrangian and studied the influence of each interaction term on the physics of inflation. We saw that both terms would lead to a similar sound speed and non-Gaussianities. Open questions remain concerning the effect of the splitting, which we are currently investigating. One could expect a different background evolution of the theory, or additional terms in the effective Lagrangian for the fluctuations $\mathcal{L}_{\mathrm{eff}}$. Another possibility would be to also consider a modified initial state.   

The Early Universe $f^{2}FF$ Model of Primordial Magnetic Field at Natural Inflation

We study the simple gauge invariant model ${f^2}FF$ as a way to generate primordial magnetic fields (PMF) in Natural Inflation (NI). We compute both magnetic and electric spectra generated by the ${f^2}FF$ model in NI for different values of model parameters and find that both de Sitter and power law expansion lead to the same results at sufficiently large number of e-foldings, as expected. We also find that the necessary scale invariance property of the PMF cannot be obtained in NI in first order of slow roll limits under the constraints derived from the recent BICEP2 results. Furthermore, if these constraints are relaxed to achieve scale invariance, then the model suffers from backreaction problems for almost all values of model parameters. We show that there is a narrow range of the height of the potential $\Lambda $ around ${\Lambda _{\min }} \approx 0.00460{M_{{\rm{Pl}}}}$ and of the commoving wave number k around $k_{\min }\sim 5.5 \times 10^{ - 4}$, at which the problem of backreaction might be avoided. The value of ${\Lambda _{\min }}$ lies within the range of $\Lambda $ compatible with the BICEP2 results. However, the relatively short range of $k$ presents a serious challenge to the viability of this model.   

Gravitational Collapse in the Primordial Plasma

Given sufficiently large inflationary perturbations, primordial black holes can form from the plasma of the very early universe. Such large perturbations tend towards spherical symmetry. Naive theoretical estimates suggest that for a black hole to form, when a perturbation enters the horizon, it requires approximately 30\% more energy than one would expect for the background FRW evolution, although this has been shown to depend on the profile of the perturbation. We present an overview of a comprehensive formalism for the description of primordial black hole formation under spherical symmetry, which has been optimized for numerical evolution. Numerically, we demonstrate how perturbations can grow and form black holes in the early universe. We suggest a better estimator for whether or not a black hole will form from a given inflationary configuration.   

Complex Scalar Field Dark Matter and Cosmological B-Modes from Inflation

As an alternative to the WIMP CDM model, we consider dark matter comprised of ultralight bosons, described by a classical complex scalar field, for which particle number per unit comoving volume is conserved. When the homogeneous background universe evolves in the presence of this type of scalar field dark matter (SFDM), the equation of state of SFDM is relativistic at early times, evolving from stiff ($\bar p = \bar\rho$) to radiationlike ($\bar p = \bar\rho/3$), before it becomes nonrelativistic and CDM-like at late times ($\bar p = 0$). Thus, before the familiar radiation-dominated phase, there is an earlier phase of stiff-matter-domination. The timing of the transition between these phases determined by SFDM model parameters, particle mass $m$ and self-interaction coupling strength $\lambda$, is constrained by cosmological observables, particularly $N_{\rm{eff}}$, the effective number of neutrino species during BBN, and cosmological tensor fluctuations from inflation, which leave an imprint on CMB B-modes. Primordial tensor modes that reenter the horizon during the stiff phase contribute significantly to the total energy density of the universe as gravitational waves, increasing the expansion rate of the early universe. This effect yields constraints on SFDM model parameters.   

Chiral Imprint of a Cosmic Gauge Field on Primordial Gravitational Waves

A cosmological gauge field with isotropic stress-energy introduces parity violation into the behavior of gravitational waves. We show that a primordial spectrum of inflationary gravitational waves develops a preferred handedness, left- or right-circularly polarized, depending on the abundance and coupling of the gauge field during the radiation era. A modest abundance of the gauge field would induce parity-violating correlations of the cosmic microwave background temperature and polarization patterns that could be detected by current and future experiments.   

Observational signatures of loop quantum cosmology

An important feature of singularity resolution in loop quantum cosmology (LQC) is the occurrence of the quantum bounce when the spacetime curvature becomes Planckian leading the pre-inflationary evolution of the universe to be greatly modified. Due to the modified dynamics in the pre-inflationary era the initial conditions for both the background and cosmological perturbations are different from those in the standard inflationary scenario. We find that such modifications can lead to observational signatures on the cosmic microwave background (CMB) anisotropy spectrum, and provide a new window to explore the CMB anomalies. In this talk we describe these initial conditions, discuss their consequences on the inflationary power spectrum, and compare our results with data from recent CMB experiments.   

General Study of Perturbations in Bouncing and Cyclic Models

Perturbations are important in both understanding and evaluating the importance of bounces and turnarounds in models that predict a cyclic evolution of our Universe. Moreover, tracking these perturbations through the entirety of the cycle is important as it provides an outlet for a qualitative comparison with Cosmic Microwave Background (CMB) observations. However, tracking these perturbations through each cycle proves difficult as the physics to describe bounces and turnarounds is not well established. Therefore, we first studied general anaytical and numerical techniques in order to understand the evolution of fluctuations in simple cosmological models where physics is better understood. In our research, we first developed analytical techniques from background solutions to establish a solid foundation for describing super-Hubble fluctuations in our early Universe. These analytical solutions were developed for both bounces and turnarounds allowing us to numerically verify and then further investigate the consequences of these solutions in models such as bounce inflation and cyclic inflation.   

A Critical Examination of the Models Proposed to Account for Baryon-Antibaryon Segregation Following the Quark-Hadron Transition

The major concern of the Standard Cosmological Model (SCM) is to account for the continuing existence of the universe in spite of the Standard Particle Model (SPM). According to the SPM below the quark-hadron temperature ($\approx 150 \pm 50$ MeV) the rate of baryon-antibaryon pair creation from $\gamma$ radiation is in equilibrium with rate of pair annihilation. At freeze-out ($\approx 20 \pm 10$ MeV) the rate of pair creation ceases. Henceforth only annihilation occurs below this temperature, resulting in a terminal pair ratio B$_+$/$\gamma$ = B$_-$/$\gamma$ $\approx$ 10$^{-18}$, insufficient to account for the present universe which would require a pair ratio minimum of at least B$_+$/$\gamma$ = B$_-$/$\gamma \approx$ 10$^{-10}$. The present universe could not exist according to the SPM unless a mechanism was devised to segregation baryons from antibaryon before freeze-out. The SPM can be tweaked to accommodate the first two conditions but all of the mechanisms proposed over the past sixty years for the third condition failed. \textit{All baryon-number excursions devised were found to be \textbf{reversible.}} The major concern of the SCM is to account for the continuing existence of the universe in spite of the SPM. The present universe could not exist according to the SPM unless a mechanism was devised to segregation baryons from antibaryon before freeze-out. It is the examination of these possible mechanisms that is subject of this work.