Astronomy and Astrophysics – Astrophysics – General Relativity and Quantum Cosmology
Scientific paper
2002-10-24
Astronomy and Astrophysics
Astrophysics
General Relativity and Quantum Cosmology
19 pages, 8 figures. Invited contribution to ``Science & Ultimate Reality: From Quantum to Cosmos (Explorations Celebrating th
Scientific paper
Suppose the usual description of spacetime as a 4-dimensional manifold with a Lorentzian metric breaks down at Planck energies. Can we still construct sensible theoretical models of the universe? Are they testable? Do they lead to a consistent quantum cosmology? Is this cosmology different than the standard one? The answer is yes, to all these questions, assuming that quantum theory is still valid at this scale. I describe the basic features of such models, mainly quantum causal histories and spin foams. They are given by a partition function, similar to spin systems and lattice gauge theories. This suggests that we should treat this approach to quantum gravity as a problem in statistical physics, but with significant complications: there is no background and, in particular, no external time. Gravity and the familiar 3+1 manifold spacetime are to be derived as the low-energy continuum approximation to these models.
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