Cosmology for Scalar Fields with Negative Potentials and w <-1
  We study the cosmology of canonically normalized scalar fields that lead to an equation of state parameter of w_\phi=p_\phi/\rho_\phi<-1 without violating the weak energy condition: rho=\Sigma_i\rho_i \geq 0 and \rho_i+p_i\geq 0. This kind of behavior requires a negative scalar potential V, widely predicted in particle physics. We show that the energy density \rho_\phi=E_k+V takes negative values with an equation of state with w_\phi < -1. However, the net effect of the $\phi$ field on the scale factor is to decelerate it giving a total equation of state parameter w=p/\rho> w_b=p_b/\rho_b, where \rho_b stands for any kind of energy density with -1\leq w_b \leq 1. The fact that \rho_\phi<0 allows, at least in principle, to have a small cosmological constant or quintessence today as the cancellation of high energy scales such as the electroweak or susy breaking scale. While V is negative |\rho_\phi| is smaller than the sum of all other energy densities regardless of the functional form of the potential V. We show that the existence of a negative potential leads, inevitable, to a collapsing universe, i.e. to a would be "big crunch". In this picture we would still be living in the expanding universe. 
A. de la Macorra and G. German