Abstract
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 theory. In this review, numerical relativity is introduced as a powerful new complementary tool for fundamental cosmology. To illustrate its power, applications of numerical relativity are discussed to studying the robustness of slow contraction and inflation in homogenizing, isotropizing, and flattening the universe beginning from generic unsmooth initial conditions. In particular, it is described how recent numerical relativity studies of slow contraction have revealed a novel, non-linear smoothing mechanism based on ultralocality that challenges the conventional view on what is required to explain the large-scale homogeneity and isotropy of the observable universe.
Highlights
Fundamental cosmology—the study of the most basic questions underlying the origin, structure and evolution of our universe [1]—owes much of its current success to applying the techniques and tools of high-energy/particle physics to cosmological theory development. This approach rests on the fact that, from the formation of the first elements until the onset of dark energy domination, the relativistic description of our large-scale universe is remarkably simple in two distinct ways: First, even though our spacetime geometry as observed by cosmic microwave background (CMB) experiments [2,3] is in the non-perturbative regime of Newtonian gravity and genuinely relativistic, the essential non-linearity is encapsulated in a single simple dynamic variable, the scale factor a(t) of the flat Friedmann-Robertson-Walker (FRW) line element
For many decades, sophisticated computer simulations have been a common and heavily used tool in cosmology. Whether it comes to understanding the CMB or structure formation, computation has been indispensable in evaluating existing theoretical models in light of observational data
In presenting the very basics of numerical relativity, we focus on two aspects as they relate to fundamental cosmology:
Summary
Fundamental cosmology—the study of the most basic questions underlying the origin, structure and evolution of our universe [1]—owes much of its current success to applying the techniques and tools of high-energy/particle physics to cosmological theory development This approach rests on the fact that, from the formation of the first elements until the onset of dark energy domination, the relativistic description of our large-scale universe is remarkably simple in two distinct ways: First, even though our spacetime geometry as observed by cosmic microwave background (CMB) experiments [2,3] is in the non-perturbative regime of Newtonian gravity and genuinely relativistic, the essential non-linearity is encapsulated in a single simple dynamic variable, the scale factor a(t) of the flat Friedmann-Robertson-Walker (FRW) line element. Perhaps the biggest open question in fundamental cosmology, the cosmic singularity problem, calls for modifications of Einstein gravity and/or forms of stress-energy that, in general, lead to non-perturbative effects. The paper concludes by comparing to numerical relativity studies of inflation [9–13] and commenting on future research directions
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