We present a simple cosmological model in which a single , non-minimally coupled scalar field with a quartic potential is responsible for both inflation at early times and acceleration at late times .
Little or no fine tuning is needed to explain why the present density of dark energy is comparable to that of pressureless matter .
Dark energy is identified with the potential of the scalar field , which is sourced by the trace of the energy-momentum tensor .
This becomes significant when matter has decoupled from radiation and become fully non-relativistic , so that \phi \propto \rho _ { m } ^ { 1 / 3 } \propto \rho _ { m, 0 } ^ { 1 / 3 } ( a _ { 0 } / a ) \sim ( 10 ^ { -120 } ) ^ { 1 / 3 } % ( 10 ^ { 10 } ) \sim 10 ^ { -30 } and V \sim \phi ^ { 4 } \sim 10 ^ { -120 } in Planck units , as observed .