Quintom scenario

The Quintom scenario (derived from the words quintessence and phantom, as in phantom energy) is a hypothetical model of dark energy.

Equation of State
In this scenario, the equation of state $$p = w\rho$$ of the dark energy, relating its pressure and energy density, can cross the boundary $$w = -1$$ associated with the cosmological constant. The boundary separates the phantom-energy-like behavior with $$w < -1$$ from the quintessence-like behavior with $$w > -1$$. A no-go theorem shows that this behavior requires at least two degrees of freedom for dark energy models involving ideal gases or scalar fields.

The Quintom scenario was applied in 2008 to produce a model of inflationary cosmology with a Big Bounce instead of a Big Bang singularity.