Generating scale-invariant tensor perturbations in the non-inflationary universe

TL;DR

This paper demonstrates that nearly scale-invariant tensor perturbations can be generated in a non-inflationary, contracting universe through scalar-tensor gravity modifications, challenging the inflation-centric view of early universe tensor modes.

Contribution

It introduces a model where a contracting phase with modified gravity produces observable tensor perturbations, showing duality with inflation models via conformal transformations.

Findings

Scale-invariant tensor perturbations can be generated during contraction.

The model allows the universe to bounce and approach general relativity.

Tensor perturbations may have amplitudes consistent with BICEP2 results.

Abstract

It is believed that the recent detection of large tensor perturbations strongly favors the inflation scenario in the early universe. This common sense depends on the assumption that Einstein's general relativity is valid at the early universe. In this paper we show that nearly scale-invariant primordial tensor perturbations can be generated during a contracting phase before the radiation dominated epoch if the theory of gravity is modified by the scalar-tensor theory at that time. The scale-invariance protects the tensor perturbations from suppressing at large scales and they may have significant amplitudes to fit BICEP2's result. We construct a model to achieve this purpose and show that the universe can bounce to the hot big bang after long time contraction, and at almost the same time the theory of gravity approaches to general relativity through stabilizing the scalar field. Theoretically, such models are dual to inflation models if we change to the frame in which the theory of gravity is general relativity. Dual models are related by the conformal transformations. With this study we reinforce the point that only the conformal invariant quantities such as the scalar and tensor perturbations are physical. How did the background evolve before the radiation time depends on the frame and has no physical meaning. It is impossible to distinguish different pictures by later time cosmological probes.