# Hamiltonian analysis of the cuscuton

## TL;DR

This paper performs a Hamiltonian analysis of the cuscuton field, revealing its degrees of freedom and conditions for consistent evolution, especially in the homogeneous limit and under specific potential conditions.

## Contribution

It provides the first Hamiltonian formulation of the cuscuton, clarifying its dynamical degrees of freedom and behavior in different frames and conditions.

## Key findings

- Homogeneous limit exhibits singular behavior in Hamiltonian framework.
- Extra scalar degree of freedom appears in non-homogeneous frames.
- Conditions on the potential ensure a non-zero lapse function during evolution.

## Abstract

The cuscuton was introduced in the context of cosmology as a field with infinite speed of propagation. It has been claimed to resemble Ho\v{r}ava gravity in a certain limit, and it is a good candidate for an ether theory in which a time-dependent cosmological constant appears naturally. The analysis of its properties is usually performed in the Lagrangian framework, which makes issues like the counting of its dynamical degrees of freedom less clear-cut. Here we perform a Hamiltonian analysis of the theory. We show that the homogeneous limit with local degrees of freedom has singular behavior in the Hamiltonian framework. In other frames, it has an extra scalar degree of freedom. The homogeneous field has regular behavior only if defined a priori as a spatially constant field in a CMC foliation and contributing with a single global degree of freedom. Lastly, we find conditions on the cuscuton potential for the resulting lapse function to be non-zero throughout evolution.