Trace anomaly driven inflation

TL;DR

This paper explores a trace anomaly-driven inflation model that avoids initial condition problems, using instantons and AdS/CFT techniques to analyze perturbations and observational constraints, highlighting the suppression of short-scale metric fluctuations.

Contribution

It introduces a novel inflation model based on trace anomaly without contrived initial conditions and analyzes its perturbations using AdS/CFT correspondence.

Findings

Quantum loops suppress short-scale metric perturbations.

The model can be nucleated by a four-sphere instanton larger than the Planck scale.

Short distance gravity modifications are likely unobservable in the CMB.

Abstract

This paper investigates Starobinsky's model of inflation driven by the trace anomaly of conformally coupled matter fields. This model does not suffer from the problem of contrived initial conditions that occurs in most models of inflation driven by a scalar field. The universe can be nucleated semi-classically by a cosmological instanton that is much larger than the Planck scale provided there are sufficiently many matter fields. There are two cosmological instantons: the four sphere and a new ``double bubble'' solution. This paper considers a universe nucleated by the four sphere. The AdS/CFT correspondence is used to calculate the correlation function for scalar and tensor metric perturbations during the ensuing de Sitter phase. The analytic structure of the scalar and tensor propagators is discussed in detail. Observational constraints on the model are discussed. Quantum loops of matter fields are shown to strongly suppress short scale metric perturbations, which implies that short distance modifications of gravity would probably not be observable in the cosmic microwave background. This is probably true for any model of inflation provided there are sufficiently many matter fields. This point is illustrated by a comparison of anomaly driven inflation in four dimensions and in a Randall-Sundrum brane-world model.