Cuscuton bounce

Abstract

In general relativity producing a regular bounce entails violation of Null Energy Condition for a dynamical source in the model. That generically indicates existence of ghosts or other instabilities. However, in cuscuton modification of gravity, the correspondence between a background bounce and violation of Null Energy Condition for dynamical sources is broken. Cuscuton action, modifies equations of motion in Infra Red limit allowing the background to go through a regular bounce phase. At the same time, since it does not contain any dynamical degrees of freedom, it does not lead to ghosts or other instabilities. Here, we present a toy scenario of a regular bouncing cosmology and prove this claim. Our model is presented as a proof of concept at this point and does not aim to explain observations in late time cosmology.

Consistency of tachyacoustic cosmology with de Sitter swampland conjectures

“…This has the form of a "Cuscuton" theory [38][39][40][41], with a kinetic term linear in the field velocity. We keep the constant C as an parameter in Eq.…”

Cuscuton gravity as a classically stable limiting curvature theory

“…Cosmological perturbations in cuscuton gravity have already been explored in Refs. [20,25,29]. Let us revisit the issue of the perturbations in a bouncing phase, highlighting some important subtleties.…”

“…The most basic cuscuton theory has a Lagrangian density of the form −∂ µ φ∂ µ φ − V (φ) and has been shown to possess many interesting properties. Indeed, the theory has stable cosmological perturbations [20,25], is free from caustic instabilities [26], appears as the extreme relativistic limit of a 5-dimensional brane theory [27] and as the ultraviolet (UV) limit of an anti-Dirac-Born-Infeld theory [28], and has non-singular bouncing solutions [29] (see also Ref. [30]), besides other phenomenological applications [31].…”

Accelerating universe with a stable extra dimension in cuscuton gravity

“…First, the presence of cuscutons can violate energy conditions assumed in the no-hair theorem [30,31]. Remarkably, even if the energy condition is violated, there is no instabilities in cuscuton gravity because the cuscuton is non-dynamical [25][26][27]. Second, the cuscuton has a nontrivial coupling with the vector field, which also violate the assumption in the proof of no-hair theorem [14,15].…”

“…Aspects of symmetry in cuscuton gravity is studied in [23]. In particular, it has been shown that cuscuton gravity is useful in cosmology [24,25], for instance, a healthy bouncing solution without instabilities were realized [26]. Moreover, any inflation models could be reconciled with observations by virtue of cuscutons [27].…”