Dynamical solution to the problem of a small cosmological constant and late-time cosmic acceleration

Increasing evidence suggests that most of the energy density of the univers e consists of a dark energy component with negative pressure that causes th e cosmic expansion to accelerate. We address why this component comes to do minate the universe only recently. We present a class of theories based on an evolving scalar field where the explanation is based entirely on interna l dynamical properties of the solutions. In the theories we consider, the d ynamics causes the scalar field to lock automatically into a negative press ure state at the onset of matter domination such that the present epoch is the earliest possible time consistent with nucleosynthesis restrictions whe n it can start to dominate.