Purely Kinetic Coupled Gravity

TL;DR

This paper explores a new class of gravity theories where kinetic terms coupled to gravity drive cosmic acceleration, leading to a quasistable phase that mimics dark energy without a potential.

Contribution

It formulates the most general purely kinetic scalar-tensor action with second order equations, extending Galileon gravity with novel kinetic couplings to Einstein tensor.

Findings

Identifies a simple form involving Einstein tensor coupling with kinetic terms.

Classifies dynamical attractor solutions including a loitering phase.

Demonstrates the model can mimic late-time cosmic acceleration.

Abstract

Cosmic acceleration can be achieved not only with a sufficiently flat scalar field potential but through kinetic terms coupled to gravity. These derivative couplings impose a shift symmetry on the scalar field, aiding naturalness. We write the most general purely kinetic action not exceeding mass dimension six and obeying second order field equations. The result reduces to a simple form involving a coupling of the Einstein tensor with the kinetic term and can be interpreted as adding a new term to Galileon gravity in curved spacetime. We examine the cosmological implications of the effective dark energy and classify the dynamical attractor solutions, finding a quasistable loitering phase mimicking late time acceleration by a cosmological constant.