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Quintessence is a hypothetical form of dark energy.
It is a scalar field and a postulated explanation for the accelerated expansion of the universe.
Bharat Ratra
Jim Peebles
Christof Wetterich
The first example of this scenario was proposed by Ratra and Peebles in 1988 and by Wetterich also in '88.
The concept was expanded to more types of time-varying dark energy.
The term 'Quintessence' was introduced in 1998. It has also been proposed by some physicists as a fifth fundamental force.
The idea behind Quintessence is different than the cosmological constant idea, insofar as it is dynamic. That is, it changes over time. The cosmological constant, by definition, does not change.
Quintessence can be either attractive or repulsive, depending on the ratio of its kinetic to its potential energy.
Many who work with this postulate say that Quintessence became repulsive about 10 billion years ago or about 3.5 billion years after the Big Bang.
Phantom energy
Phantom energy is a hypothetical form of dark energy that posseses negative kinetic energy. Phantom energy predicts expansion of the universe more than that predicted by the cosmological constant, leading to a Big Rip. The idea of phantom energy is often dismissed, as it would suggest an unstable vacuum, with negative mass particles bursting into existence.
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