Loop quantum cosmology: a status report

By Abhay Ashtekar, Parampreet Singh. 2011

1,094 citations recorded by the source corpus.

Loop quantum cosmology (LQC) is the result of applying principles of loop quantum gravity (LQG) to cosmological settings. The distinguishing feature of LQC is the prominent role played by the quantum geometry effects of LQG. In particular, quantum geometry creates a brand new repulsive force which is totally negligible at low spacetime curvature but rises very rapidly in the Planck regime, overwhelming the classical gravitational attraction. In cosmological models, while Einstein's equations hold to an excellent degree of approximation at low curvature, they undergo major modifications in the Planck regime: for matter satisfying the usual energy conditions, any time a curvature invariant grows to the Planck scale, quantum geometry effects dilute it, thereby resolving singularities of general relativity. Quantum geometry corrections become more sophisticated as the models become richer. In particular, in anisotropic models, there are significant changes in the dynamics of shear potentials which tame their singular behavior in striking contrast to older results on anisotropies in bouncing models. Once singularities are resolved, the conceptual paradigm of cosmology changes and one has to revisit many of the standard issues—e.g. the ‘horizon problem’—from a new perspective. Such conceptual issues as well as potential observational consequences of the new Planck scale physics are being explored, especially within the inflationary paradigm. These considerations have given rise to a burst of activity in LQC in recent years, with contributions from quantum gravity experts, mathematical physicists and cosmologists. The goal of this review is to provide an overview of the current state of the art in LQC for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general and cosmologists who wish to apply LQC to probe modifications in the standard paradigm of the early universe. In this review, effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without loss of continuity.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/969b94c49bcf7220bece3275d06fa6a474ec1083

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Citations in the corpus

The pre-inflationary dynamics of loop quantum cosmology: confronting quantum gravity with observations (2012, methodology): “On the mathematical side, it turns out that the formal quantum constraint Ŝo again has the operator form 2∂2 φ + as in the V (φ) = 0 case but now the operator is no longer essentially self-adjoint [7].”
Introduction to Loop Quantum Cosmology (2011, background): “A recent review [24] emphasizes aspects that are only briefly mentioned here, such as the “simplified” or “solvable” LQC framework, the details of effective dynamics for FRW models with non-zero curvature and/or cosmological constant, and inflationary perturbation theory in LQC.”
The matter bounce scenario in loop quantum cosmology (2012, background): “V of [6] and Part III of [7].”
Emergent Friedmann dynamics with a quantum bounce from quantum gravity condensates (2016, background): “[26] A.”
Inflationary Cosmology in Modified Gravity Theories (2015, methodology): “Particularly, we consider inflation (i) derived by modification terms of gravity [26]; (ii) through the quantum anomaly [27]; and (iii) in R gravity in the framework of the so-called loop quantum cosmology (LQC) [29–33] to include quantum effects [28] (for reviews on LQC, see, for example, [34–40]).”
Minimal Length Scale Scenarios for Quantum Gravity (2012, background): “The semi-classical limit is clearly inappropriate when energy densities reach the Planckian regime, but the key feature of the bounce and removal of the singularity survives in the quantized case [56, 44, 58, 57].”
The continuum limit of loop quantum gravity - a framework for solving the theory (2014, background): “Cosmology rather involves coarse data, one might therefore hope that a derivation of cosmology can be obtained at low truncation order [65] (but sufficiently fine to determine the dynamical embedding maps essential for the understanding of the truncation).”
Extension of the quantum theory of cosmological perturbations to the Planck era (2012, methodology): “But the quantum state (and even the effective trajectory) remains well-defined throughout the entire evolution, including the Planck scale neighborhood of the bounce [13, 14, 31].”