 Phantom energy

A fluid with $w<-1$ grows in density during expansion and can produce a finite-time Big Rip singularity.

Phantom energy is a theoretical concept in cosmology associated with the accelerated expansion of the universe. It is a type of dark energy with an equation of state parameter \( w \) that is less than -1. This means that the pressure exerted by phantom energy is more negative than its energy density. In standard cosmological models, dark energy typically has an equation of state parameter \( w \) around -1, corresponding to the cosmological constant.