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Quantum Gravity (quantum + gravity)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

Nonlocal quantum gravity and the size of the universe

FORTSCHRITTE DER PHYSIK/PROGRESS OF PHYSICS, Issue 6-7 2004
M. Reuter
Motivated by the conjecture that the cosmological constant problem is solved by strong quantum effects in the infrared we use the exact flow equation of Quantum Einstein Gravity to determine the renormalization group behavior of a class of nonlocal effective actions. They consist of the Einstein-Hilbert term and a general nonlinear function Fk(V) of the Euclidean spacetime volume V. For the V + V ln V -invariant the renormalization group running enormously suppresses the value of the renormalized curvature which results from Planck-size parameters specified at the Planck scale. One obtains very large, i.e., almost flat universes without finetuning the cosmological constant. A critical infrared fixed point is found where gravity is scale invariant. [source]

Semiclassical quantum gravity: statistics of combinatorial Riemannian geometries

ANNALEN DER PHYSIK, Issue 8 2005
L. Bombelli
Abstract This paper is a contribution to the development of a framework, to be used in the context of semiclassical canonical quantum gravity, in which to frame questions about the correspondence between discrete spacetime structures at "quantum scales" and continuum, classical geometries at large scales. Such a correspondence can be meaningfully established when one has a "semiclassical" state in the underlying quantum gravity theory, and the uncertainties in the correspondence arise both from quantum fluctuations in this state and from the kinematical procedure of matching a smooth geometry to a discrete one. We focus on the latter type of uncertainty, and suggest the use of statistical geometry as a way to quantify it. With a cell complex as an example of discrete structure, we discuss how to construct quantities that define a smooth geometry, and how to estimate the associated uncertainties. We also comment briefly on how to combine our results with uncertainties in the underlying quantum state, and on their use when considering phenomenological aspects of quantum gravity. [source]

Cosmology: a matter of all and nothing

ASTRONOMY & GEOPHYSICS, Issue 4 2002
John D Barrow
John D Barrow gave the Gerald Whitrow Lecture for 2002. He reviews modern ideas about the Big Bang and the constants of Nature. Abstract The modern picture of the expanding Big Bang universe is described. Implications of the expansion for the evolution of life are highlighted, together with the new features contributed by the inflationary universe theory. Observational tests of inflation are described along with some of the possibilities introduced by new theories of strings and quantum gravity. These theories allow the numbers of dimensions of space and of time to be larger than the three and one we experience and permit the observed "constants" of Nature to vary slowly in time. We describe recent astronomical evidence that is consistent with small variations of the fine-structure constant and discuss some of its far-reaching implications. [source]