So, there was no longer a need for a cosmological constant. The problem with the cosmological constant, Steinhardt says, is that it is indeed constant. It yields the same force throughout time. Observational evidence indicates that whatever this force is that is accelerating the universe, it hasn't been constant over time. There had to be periods when the force was negligible, otherwise stars and planets and chipmunks never would have formed.
Quintessence encompasses a wide class of possibilities.
It is a dynamic, time-evolving and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion. Vacuum energy is the potential energy in an absolute vacuum, devoid of matter or radiation. Quintessence is a quantum field with both kinetic and potential energy. Depending in the ratio of the two energies and the pressure they exert, quintessence can either attract or repel. Quintessence became a force to be reckoned with about 10 billion years ago, according to the theory.