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Smoothing and flattening the universe through slow contraction versus inflation
Anna Ijjas, Paul J. Steinhardt, David Garfinkle +1
In a systematic study, we use an equivalent pair of improved numerical relativity codes based on a tetrad-formulation of the classical Einstein-scalar field equations to examine wh…
Gauge/frame invariant variables for the numerical relativity study of cosmological spacetimes
Anna Ijjas
To numerically evolve the full Einstein equations (or modifications thereof), simulations of cosmological spacetimes must rely on a particular formulation of the field equations co…
Initial conditions problem in cosmological inflation revisited
David Garfinkle, Anna Ijjas, Paul J. Steinhardt
We present first results from a novel numerical relativity code based on a tetrad formulation of the Einstein-scalar field equations combined with recently introduced gauge/frame i…
Slow Contraction and the Weyl Curvature Hypothesis
Anna Ijjas
Using the power of numerical relativity, we show that, beginning from generic initial conditions that are far from flat, homogeneous and isotropic and have a large Weyl curvature,…
The robustness of slow contraction and the shape of the scalar field potential
Timo Kist, Anna Ijjas
We use numerical relativity simulations to explore the conditions for a canonical scalar field minimally coupled to Einstein gravity to generate an extended phase of slow contr…
Numerical Relativity as a New Tool for Fundamental Cosmology
Anna Ijjas
Advances in our understanding of the origin, evolution and structure of the universe have long been driven by cosmological perturbation theory, model building and effective field t…
Dynamical attractors in contracting spacetimes dominated by kinetically coupled scalar fields
Anna Ijjas, Frans Pretorius, Paul J. Steinhardt +1
We present non-perturbative numerical relativity simulations of slowly contracting spacetimes in which the scalar field driving slow contraction is coupled to a second scalar field…
Entropy, Black holes, and the New Cyclic Universe
Anna Ijjas, Paul J. Steinhardt
We track the evolution of entropy and black holes in a cyclic universe that undergoes repeated intervals of expansion followed by slow contraction and a smooth (non-singular) bounc…
The Effects of Multiple Modes and Reduced Symmetry on the Rapidity and Robustness of Slow Contraction
Anna Ijjas, Frans Pretorius, Paul J. Steinhardt +1
We demonstrate that the rapidity and robustness of slow contraction in homogenizing and flattening the universe found in simulations in which the initial conditions were restricted…
Ultralocality and Slow Contraction
Anna Ijjas, Andrew P. Sullivan, Frans Pretorius +2
We study the detailed process by which slow contraction smooths and flattens the universe using an improved numerical relativity code that accepts initial conditions with non-pertu…
Nearly scale-invariant curvature modes from entropy perturbations during graceful exit
Anna Ijjas, Roman Kolevatov
In this Letter, we describe how a spectrum of entropic perturbations generated during a period of slow contraction can source a nearly scale-invariant spectrum of curvature perturb…
Sourcing curvature modes with entropy perturbations in non-singular bouncing cosmologies
Anna Ijjas, Roman Kolevatov
The observed temperature fluctuations in the cosmic microwave background can be traced back to primordial curvature modes that are sourced by adiabatic and/or entropic matter pertu…
Robustness of slow contraction to cosmic initial conditions
Anna Ijjas, William G. Cook, Frans Pretorius +2
We present numerical relativity simulations of cosmological scenarios in which the universe is smoothed and flattened by undergoing a phase of slow contraction and test their sensi…
Supersmoothing through Slow Contraction
William G. Cook, Iryna A. Glushchenko, Anna Ijjas +2
Performing a fully non-perturbative analysis using the tools of numerical general relativity, we demonstrate that a period of slow contraction is a `supersmoothing' cosmological ph…
A new kind of cyclic universe
Anna Ijjas, Paul J. Steinhardt
Combining intervals of ekpyrotic (ultra-slow) contraction with a (non-singular) classical bounce naturally leads to a novel cyclic theory of the universe in which the Hubble parame…
Stability and the Gauge Problem in Non-Perturbative Cosmology
Anna Ijjas, Frans Pretorius, Paul J. Steinhardt
In this paper, we describe the first steps towards fully non-perturbative cosmology. We explain why the conventional methods used by cosmologists based on the ADM formulation are g…
Space-time slicing in Horndeski theories and its implications for non-singular bouncing solutions
Anna Ijjas
In this paper, we show how the proper choice of gauge is critical in analyzing the stability of non-singular cosmological bounce solutions based on Horndeski theories. We show that…
Fully stable cosmological solutions with a non-singular classical bounce
Anna Ijjas, Paul J. Steinhardt
We recently showed how it is possible to use a cubic Galileon action to construct classical cosmological solutions that enter a contracting null energy condition (NEC) violating ph…
Classically stable non-singular cosmological bounces
Anna Ijjas, Paul J. Steinhardt
One of the fundamental questions of theoretical cosmology is whether the universe can undergo a non-singular bounce, i.e., smoothly transit from a period of contraction to a period…
NEC violation in mimetic cosmology revisited
Anna Ijjas, Justin Ripley, Paul J. Steinhardt
In the context of Einstein gravity, if the null energy condition (NEC) is satisfied, the energy density in expanding space-times always decreases while in contracting space-times t…