Scale Factor Dependent Equations of State for Curvature Inspired Dark   Energy, Phantom Barrier and Late Cosmic Acceleration
  Here, it is found that dark energy and dark matter emerge from the gravitational sector, if non-linear term of scalar curvature is added to Einstein-Hilbert lagrangian. An equation of state for dark energy, having the form $p_{\rm de} = - \rho_{\rm de} + f(a) (with $p_{\rm de}(\rho_{\rm de})$ being the pressure(density) for dark energy, $f(a)$ being a function of scale feactor $a(t)$ and $t$ being the cosmic time) is explored. Interestingly, this equation of state leads to a phantom barrier ${\rm w}_{\rm de} = p_{\rm de}/\rho_{\rm de} = - 1$ at $a = a_{\rm w}$. It is found that when $a < a_{\rm w}, {\rm w}_{\rm de} > - 1$ and ${\rm w}_{\rm de} < - 1$ for $a > a_{\rm w},$ showing a transition from non-phantom to phantom phase at $a = a_{\rm w} < a_0 (a_0$ being current scale factor of the universe). PACS no.: 98.80 Cq. 
S. K. Srivastava