Scalar-Tensor Gravity and Quintessence

TL;DR

This paper explores scalar-tensor theories with ultra-light scalar fields that could explain cosmic acceleration and dark energy, while addressing their potential effects on gravity and cosmological observations.

Contribution

It introduces a model with dynamical scalar-matter coupling in scalar-tensor theories, featuring dual attractor mechanisms for cosmic acceleration and compatibility with current tests of gravity.

Findings

Predicts significant effects on cosmic microwave background spectra.

Suggests the scalar field can drive accelerated expansion.

Shows late-time behavior is insensitive to initial conditions.

Abstract

Scalar fields with inverse power-law effective potentials may provide a negative pressure component to the energy density of the universe today, as required by cosmological observations. In order to be cosmologically relevant today, the scalar field should have a mass $m_\phi = O(10^{-33} {\mathrm eV})$, thus potentially inducing sizable violations of the equivalence principle and space-time variations of the coupling constants. Scalar-tensor theories of gravity provide a framework for accommodating phenomenologically acceptable ultra-light scalar fields. We discuss non-minimally coupled scalar-tensor theories in which the scalar-matter coupling is a dynamical quantity. Two attractor mechanisms are operative at the same time: one towards the tracker solution, which accounts for the accelerated expansion of the Universe, and one towards general relativity, which makes the ultra-light scalar field phenomenologically safe today. As in usual tracker-field models, the late-time behavior is largely independent on the initial conditions. Strong distortions in the cosmic microwave background anisotropy spectra as well as in the matter power spectrum are expected.