Crossing the phantom divide

Understand

The cosmic acceleration is one of the most significant cosmological discoveries over the last century.

• Following the more accurate data a more dramatic result appears: the recent analysis of the observation data (especially from SNe Ia) indicate that the time varying dark energy gives a better fit than a cosmological constant, and in particular, the equation of state parameter $w$ (defined as the ratio of pressure to energy density) crosses -1 at some low redshift region.
• This crossing behavior is a serious challenge to fundamental physics.
• In this article, we review a number of approaches which try to explain this remarkable crossing behavior.