Partial UV Completion of $P(X)$ from a Curved Field Space

TL;DR

This paper proposes a multi-field UV completion for shift-symmetric $k$-essence models with curved field space, resolving caustic formation issues and analyzing the effective field theory limits in cosmological contexts.

Contribution

It introduces a curved field space approach with heavy fields to prevent caustics in $k$-essence models and studies the EFT reduction in cosmological scenarios.

Findings

Heavy fields resolve caustic formation in $k$-essence.

EFT reduction is valid in the infinite curvature limit.

Breakdown of EFT occurs at vanishing sound speed.

Abstract

The $k$-essence theory is a prototypical class of scalar-field models that already gives rich phenomenology and has been a target of extensive studies in cosmology. General forms of shift-symmetric $k$-essence are known to suffer from formation of caustics in a planar-symmetric configuration, with the only exceptions of canonical and DBI-/cuscuton-type kinetic terms. With this in mind, we seek for multi-field caustic-free completions of a general class of shift-symmetric $k$-essence models in this paper. The field space in UV theories is naturally curved, and we introduce the scale of the curvature as the parameter that controls the mass of the heavy field(s) that would be integrated out in the process of EFT reduction. By numerical methods, we demonstrate that the introduction of a heavy field indeed resolves the caustic problem by invoking its motion near the would-be caustic formation. We further study the cosmological application of the model. By expanding the equations with respect to the curvature scale of the field space, we prove that the EFT reduction is successfully done by taking the limit of infinite curvature, both for the background and perturbation, with gravity included. The next leading-order computation is consistently conducted and shows that the EFT reduction breaks down in the limit of vanishing sound speed of the perturbation.