Crossing the phantom divide

TL;DR

Recent cosmological data suggests dark energy's equation of state crosses -1, challenging physics; this paper reviews models explaining this crossing, including two-field, interacting, and modified gravity theories.

Contribution

This paper reviews theoretical approaches to explain the crossing of the dark energy equation of state parameter w through -1.

Findings

Observational data indicates w crosses -1 at low redshift.

Two-field, interacting, and modified gravity models can realize the crossing.

The crossing behavior poses a significant challenge to fundamental physics.

Abstract

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. First we show the key observations which imply the crossing behavior. And then we concentrate on the theoretical progresses on the dark energy models which can realize the crossing -1 phenomenon. We discuss three kinds of dark energy models: 1. two-field models (quintom-like), 2. interacting models (dark energy interacts with dark matter), and 3. the models in frame of modified gravity theory (concentrating on brane world).