Loop
quantum cosmology (LQC)
[612.01236Loop Quantum Cosmology: A brief review">][002.05703Critical evaluation of common claims in loop quantum cosmology">][612.04551Testing loop quantum cosmology">][703.10274Loop quantum cosmology and singularities">][906.01501Effective Field Theory of Loop Quantum Cosmology">] is a
finite,
symmetry-reduced model of
loop quantum gravity (
LQG) that predicts a "quantum bridge" between contracting and expanding
cosmological branches.
The distinguishing feature of LQC is the prominent role played by the
quantum geometry effects of loop quantum gravity (LQG). In particular,
quantum geometry creates a brand new repulsive force which is totally negligible at low space-time curvature but rises very rapidly in the
Planck regime, overwhelming the classical gravitational attraction and thereby resolving
singularities of general relativity. Once singularities are resolved, the conceptual paradigm of
cosmology
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosophe ...
changes and one has to revisit many of the standard issues—e.g., the "
horizon problem"—from a new perspective.
Since LQG is based on a specific quantum theory of
Riemannian geometry
Riemannian geometry is the branch of differential geometry that studies Riemannian manifolds, smooth manifolds with a ''Riemannian metric'', i.e. with an inner product on the tangent space at each point that varies smoothly from point to po ...
,
geometric observables display a fundamental discreteness that play a key role in
quantum dynamics: While predictions of LQC are very close to those of quantum
geometrodynamics
In theoretical physics, geometrodynamics is an attempt to describe spacetime and associated phenomena completely in terms of geometry. Technically, its goal is to unify the fundamental forces and reformulate general relativity as a configurati ...
(QGD) away from the
Planck regime, there is a dramatic difference once densities and curvatures enter the
Planck scale. In LQC the
Big Bang
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from t ...
is replaced by a
quantum bounce.
Study of LQC has led to many successes, including the emergence of a possible mechanism for
cosmic inflation, resolution of
gravitational singularities
A gravitational singularity, spacetime singularity or simply singularity is a condition in which gravity is so intense that spacetime itself breaks down catastrophically. As such, a singularity is by definition no longer part of the regular ...
, as well as the development of effective semi-classical
Hamiltonians.
This subfield originated in 1999 by
Martin Bojowald, and further developed in particular by
Abhay Ashtekar and
Jerzy Lewandowski, as well as
Tomasz Pawłowski and
Parampreet Singh, et al. In late 2012 LQC represented a very active field in
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which rel ...
, with about three hundred papers on the subject published in the literature. There has also recently been work by
Carlo Rovelli
Carlo Rovelli (born May 3, 1956) is an Italian theoretical physicist and writer who has worked in Italy, the United States and, since 2000, in France. He is also currently a Distinguished Visiting Research Chair at the Perimeter Institute, and c ...
, et al. on relating LQC to
spinfoam cosmology.
However, the results obtained in LQC are subject to the usual restriction that a truncated classical theory, then quantized, might not display the true behaviour of the full theory due to artificial suppression of degrees of freedom that might have large quantum fluctuations in the full theory. It has been argued that singularity avoidance in LQC are by mechanisms only available in these restrictive models and that singularity avoidance in the full theory can still be obtained but by a more subtle feature of LQG.
Big bounce in loop quantum cosmology
Due to the quantum geometry, the Big Bang is replaced by a big bounce without any assumptions on the matter content or any fine tuning. An important feature of loop quantum cosmology is the effective
space-time description of the underlying quantum evolution.
The effective dynamics approach has been extensively used in loop quantum cosmology to describe physics at the Planck scale and the very early universe. Rigorous numerical simulations have confirmed the validity of the effective dynamics, which provides an excellent approximation to the full loop quantum dynamics.
It has been shown that only when the states have very large quantum fluctuations at late times, which means that they do not lead to macroscopic universes as described by general relativity, that the effective dynamics has departures from the quantum dynamics near bounce and the subsequent evolution. In such a case, the effective dynamics overestimates the density at the bounce, but still captures the qualitative aspects extremely well.
Scale-invariant loop quantum cosmology
If the underlying spacetime geometry with matter has a
scale invariance, which has been proposed to resolve the
problem of time,
Immirzi ambiguity and hierarchy problem of fundamental couplings, then the resulting loop quantum geometry has no definitive discrete gaps or a minimum size.
Consequently, in scale-invariant LQC, the Big Bang is shown not to be replaced by a quantum bounce.
See also
*
*
*
*
Canonical quantum gravity
*
Dilaton
In particle physics, the hypothetical dilaton particle is a particle of a scalar field \varphi that appears in theories with Dimension (mathematics and physics)#Additional dimensions, extra dimensions when the volume of the compactified dimensions ...
References
{{reflist
External links
Loop quantum cosmology on arxiv.orgQuantum Nature of The Big Bang in Loop Quantum CosmologyGravity and the QuantumLoop Quantum Cosmology, Martin BojowaldAbhay Ashtekar, Parampreet Singh "Loop Quantum Cosmology: A Status Report"
Physical cosmology
Loop quantum gravity