3 research outputs found
On Energy Conditions and Stability in Effective Loop Quantum Cosmology
In isotropic loop quantum cosmology, non-perturbatively modified dynamics of
a minimally coupled scalar field violates weak, strong and dominant energy
conditions when they are stated in terms of equation of state parameter. The
violation of strong energy condition helps to have non-singular evolution by
evading singularity theorems thus leading to a generic inflationary phase.
However, the violation of weak and dominant energy conditions raises concern,
as in general relativity these conditions ensure causality of the system and
stability of vacuum via Hawking-Ellis conservation theorem. It is shown here
that the non-perturbatively modified kinetic term contributes negative pressure
but positive energy density. This crucial feature leads to violation of energy
conditions but ensures positivity of energy density, as scalar matter
Hamiltonian remains bounded from below. It is also shown that the modified
dynamics restricts group velocity for inhomogeneous modes to remain sub-luminal
thus ensuring causal propagation across spatial distances.Comment: 29 pages, revtex4; few clarifications, references added, to appear in
CQ
Background independence in a nutshell
We study how physical information can be extracted from a background
independent quantum system. We use an extremely simple `minimalist' system that
models a finite region of 3d euclidean quantum spacetime with a single
equilateral tetrahedron. We show that the physical information can be expressed
as a boundary amplitude. We illustrate how the notions of "evolution" in a
boundary proper-time and "vacuum" can be extracted from the background
independent dynamics.Comment: 19 pages, 19 figure
Spin Foam Models for Quantum Gravity
In this article we review the present status of the spin foam formulation of
non-perturbative (background independent) quantum gravity. The article is
divided in two parts. In the first part we present a general introduction to
the main ideas emphasizing their motivations from various perspectives.
Riemannian 3-dimensional gravity is used as a simple example to illustrate
conceptual issues and the main goals of the approach. The main features of the
various existing models for 4-dimensional gravity are also presented here. We
conclude with a discussion of important questions to be addressed in four
dimensions (gauge invariance, discretization independence, etc.).
In the second part we concentrate on the definition of the Barrett-Crane
model. We present the main results obtained in this framework from a critical
perspective. Finally we review the combinatorial formulation of spin foam
models based on the dual group field theory technology. We present the
Barrett-Crane model in this framework and review the finiteness results
obtained for both its Riemannian as well as its Lorentzian variants.Comment: Topical review, to appear in CQG. Typos corrected and new references
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