- #1

marcus

Science Advisor

Gold Member

Dearly Missed

- 24,757

- 788

**New Bojowald paper---BB non-singularity continued**

classical GR breaks down at time zero because of infinite density and curvature

quantizing GR removes the singularity---when quantized the Friedmann equations go smoothly back in time, showing a bounce at time zero

Martin Bojowald was the first to get this result, today he posted a new paper:

"Quantum Gravity and the Big Bang"

http://arxiv.org/astro-ph/0309478 [Broken]

6 pages, 2 figures

Here's the abstract and a few sentences from the beginning:

Martin Bojowald (New Address: Max-Planck-Institut fuer Gravitationsphysik, Albert-Einstein-Institut, Am Muehlenberg 1, D-14476 Golm, Germany)

Abstract. Quantum gravity has matured over the last decade to a theory which can tell in a precise and explicit way how cosmological singularities of general relativity are removed. A branch of the universe “before” the classical big bang is obtained which is connected to ours by quantum evolution through a region around the singularity where the classical space-time dissolves. We discuss the basic mechanism as well as applications ranging to new phenomenological scenarios of the early universe expansion, such as an inflationary period.

1 Introduction

When the big bang is approached, the volume becomes smaller and smaller and one enters a regime of large energy densities. Classically, those conditions will become so severe that a singularity is reached; the theory simply breaks down. For a long time, the expectation has been that somewhere along the way quantum gravity takes over and introduces new effects, e.g. a discrete structure, which prevent the singularity to develop. This presumably happens at scales the size of the Planck length l

_{P}, i.e. when the universe has about a volume l

_{P}

^{3}.

Since at the classical singularity space itself becomes singular and gravitational interactions are huge, such a quantum theory of gravity must be background independent and non-perturbative. A theory satisfying these conditions is in fact available in the form of loop quantum gravity/quantum geometry (see [1, 2] for reviews). One of its early successes was the derivation of discrete spectra of geometric operators like area and volume [3, 4, 5]. Thus, the spatial geometry is discrete in a precise sense. Furthermore, matter Hamiltonians exist as well-defined operators in the theory which implies that ultraviolet divergences are cured in the fundamental formulation [6, 7].

Both properties must be expected to have important consequences for cosmology. The discreteness leads to a new basic formulation valid at small volume, and since gravity couples to the matter Hamiltonian, its source term is modified at small scales when the good ultraviolet behavior is taken into account. It is possible to introduce both effects into a cosmological model in a systematic way, which allows us to test the cosmological consequences of quantum gravity (reviewed in [8, 9])."

The first time this came out was Bojowald's

"Absence of a Singularity in Loop Quantum Cosmology"

http://arxiv.org/gr-qc/0102069 [Broken]

Last edited by a moderator: