Unbraiding the bounce: superluminality around the corner

Abstract

We study a particular realization of the cosmological bounce scenario proposed recently by Ijjas and Steinhardt in [1]. First, we find that their bouncing solution starts from a divergent sound speed and ends with its vanishing. Thus, the solution connects two strongly coupled configurations. These pathologies are separated from the bouncing regime by only a few Planck times. We then reveal the exact structure of the Lagrangian, which reproduces this bouncing solution. This reconstruction allowed us to consider other cosmological solutions of the theory and analyze the phase space. In particular, we find other bouncing solutions and solutions with superluminal sound speed. These stable superluminal states can be continuously transformed into the solution constructed by Ijjas and Steinhardt. We discuss the consequences of this feature for a possible UV-completion.

Emergent universe and Genesis from the DHOST cosmology

“…We illustrate that c 2 T is positive during the whole cosmological process, but it will exceed the speed of light squared, i.e., c 2 > 1 for a short time interval near the genesis event, triggering the superluminality problem. The existence of superluminality is a general feature in scalartensor theory beyond Horndeski [67], and here we shall argue that superluminality does not necessarily correspond to acasuality [69,70,[72][73][74]. Still, it is interesting to see whether we can develop a genesis model without the superluminality issue.…”

Superluminality in beyond Horndeski theory with extra scalar field

“…This problem has been addressed from various perspectives, e.g., in Refs. [21,22,23,24,25]. Superluminal propagation was argued to be troublesome, since it indicates that a Lorentz-covariant UV-completion is impossible for such a theory [26].…”

Cuscuton bounce

“…The stability and superluminal nature of these models has been a subject of interesting debates in literature. Authors in [5,17,22] have argued that it is possible to obtain a healthy bounce using Galilean action while [16,23] argue that when coupling to matter or other regions of phasespace are included, Galilean models have superluminal speed of sound 2 .…”