The Well-Tempered Cosmological Constant

TL;DR

This paper introduces a scalar-tensor gravity model that dynamically cancels a large cosmological constant while preserving standard cosmic evolution, including matter, radiation, and current acceleration.

Contribution

It develops a new model based on cubic Horndeski gravity that tempers self-tuning, allowing cancellation of the cosmological constant without disrupting matter or radiation dominance.

Findings

Achieves de Sitter solutions independent of the cosmological constant magnitude

Preserves matter and radiation dominated phases

Supports a standard cosmic history with current acceleration

Abstract

Self tuning is one of the few methods for dynamically cancelling a large cosmological constant and yet giving an accelerating universe. Its drawback is that it tends to screen all sources of energy density, including matter. We develop a model that tempers the self tuning so the dynamical scalar field still cancels an arbitrary cosmological constant, including the vacuum energy through any high energy phase transitions, without affecting the matter fields. The scalar-tensor gravitational action is simple, related to cubic Horndeski gravity, with a nonlinear derivative interaction plus a tadpole term. Applying shift symmetry and using the property of degeneracy of the field equations we find families of functions that admit de Sitter solutions with expansion rates that are independent of the magnitude of the cosmological constant and preserve radiation and matter dominated phases. That is, the method can deliver a standard cosmic history including current acceleration, despite the presence of a Planck scale cosmological constant.