Quintessence (physics)

In physics, quintessence is a hypothetical form of dark energy, more precisely a scalar field, postulated as an explanation of the observation of an accelerating rate of expansion of the universe, rather than due to a true cosmological constant. The first example of this scenario was proposed by Ratra and Peebles (1988). The concept was expanded to more general types of time-varying dark energy and the term "quintessence" was first introduced in a paper by Robert R. Caldwell, Rahul Dave and Paul Steinhardt. It has been proposed by some physicists to be a fifth fundamental force. Quintessence differs from the cosmological constant explanation of dark energy in that it is dynamic; that is, it changes over time, unlike the cosmological constant, which always stays constant. It is suggested that quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. Those working with this postulate believe that quintessence became repulsive about ten billion years ago, about 3.5 billion years after the Big Bang.

Quintessence is a scalar field with an equation of state where w_q, the ratio of pressure p_q and density ${\displaystyle \rho }$_q, is given by the potential energy ${\displaystyle V(Q)}$ and a kinetic term:

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