Loop quantum cosmology (LQC) 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 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, 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 (QGD) away from the Planck
regime, there is a dramatic difference once
densities and curvatures enter the Planck
scale.
In LQC the big bang 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, 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 represents a very active
field in physics, with about three hundred
papers on the subject published in the literature.
There has also recently been work by Carlo
Rovelli, et al. on relating LQC to the spinfoam-based
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.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.
== See also ==
Big Bounce – A hypothetical cosmological
model for the origin of the known universe
Loop quantum gravity – Theory of quantum
gravity, merging quantum mechanics and general
relativity
Cyclic model
