http://arxiv.org/abs/1407.4691
Possible observation of tensor modes from inflation is sometimes presented as the first evidence for quantum gravity. However, in the usual inflationary formalism, also the scalar modes involve quantised metric perturbations. We consider the issue in a semiclassical setup in which only matter is quantised, and spacetime is classical. We assume that the state collapses on a timelike hypersurface, and find that the spectrum of scalar perturbations depends on the hypersurface. For reasonable choices, we can recover the usual inflationary predictions for scalar perturbations in minimally coupled single-field models. In models where non-minimal coupling to gravity is important and the field value is sub-Planckian, we do not get a nearly scale-invariant spectrum of scalar perturbations. As gravitational waves are only produced at second order, the tensor-to-scalar ratio is negligible. We conclude that detection of inflationary gravitational waves could indeed be considered the first observational evidence for quantum gravity.
T. Markkanen, S. Rasanen and P. Wahlman
Fri, 18 Jul 14
4/76
Comments: 17 pages, 1 figure
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