Inflation without quantum gravity

Abstract

It is sometimes argued that observation of tensor modes from inflation would provide 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 spacelike 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 would indeed be needed to have observational evidence of quantisation of gravity.

18 pages, 1 figure. v2: Published version. Clarified text, fixed typos, added references, updated discussion of BICEP2 results

References in corpus (19)

• The Standard Model Higgs boson as the inflaton
• A Joint Analysis of BICEP2/Keck Array and Planck Data
• Toward an Understanding of Foreground Emission in the BICEP2 Region
• Inflation with Non-Minimal Coupling: Metric vs. Palatini Formulations
• Relativistic cosmology and large-scale structure
• Higgs inflation at the critical point
• On the one loop corrections to inflation and the CMB anisotropies
• Higgs-Palatini Inflation and Unitarity
• A joint analysis of Planck and BICEP2 B modes including dust polarization uncertainty
• On the One Loop Corrections to Inflation II: The Consistency Relation
• On the Decoherence of Primordial Fluctuations During Inflation
• Fingerprints of Galactic Loop I on the Cosmic Microwave Background
• Quantum radiative corrections to slow-roll inflation
• Can topological defects mimic the BICEP2 B-mode signal?
• Can primordial magnetic fields be the origin of the BICEP2 data?
• Can Self-Ordering Scalar Fields explain the BICEP2 B-mode signal?
• The initial value problem for ordinary differential equations with infinitely many derivatives
• From B Modes to Quantum Gravity and Unification of Forces
• The Formulation of Quantum Field Theory in Curved Spacetime