Inflation without quantum gravity

Understand

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.