Bouncing Universes in String-inspired Gravity

TL;DR

The Big Bang singularity remains a challenge in canonical GR and inflationary scenarios do not by themselves resolve it; the paper proposes ghost-free, asymptotically free nonlocal gravity actions of the form $F(R,BoxR,...)$ to realize a non-singular bouncing cosmology in flat FRW spacetimes.By analyzing both $F(R)$ and more general $F(R,BoxR,...)$ theories, the authors identify ghost-free constraints and derive a consistent Newtonian limit, showing how gravity weakens at short distances while reducing to Einstein gravity at large scales.The work provides explicit bouncing constructions, including exact and approximate cosine-hyperbolic (cosh) bounces, and demonstrates how a smooth transition to standard FRW evolution with radiation-like or matter-like fluids can occur after the bounce.A concrete ghost-free example with nonlocal higher-derivative terms yields a finite short-distance Newtonian potential and a controllable bounce, highlighting the potential of string-inspired nonlocal gravity to address the Big Bang singularity with observationally testable implications for perturbations and gravitational waves.

Abstract

We consider the effects on cosmology of higher-derivative modifications of (effective) gravity that make it asymptotically free without introducing ghosts. The weakening of gravity at short distances allows pressure to prevent the singularity, producing a solution with contraction preceding expansion.