In alchemy, among the classical elements, quintessence (meaning "fifth element", along with earth, air, fire, and water) was another term for aether; it is the substance of which the heavenly bodies were supposed to be composed.

More loosely, quintessence is the purest form of a particular substance. This is seen in the Hamlet soliloquy: "The beauty of the world, the paragon of animals and yet to me what is this quintessence of dust?"

Modern cosmology

In a new usage in physics, quintessence is a hypothetical form of energy postulated to exist as a possible explanation of observations of an accelerating universe. Quintessence is a type of vacuum energy defined as having an equation of state (relating its pressure p_q and density ρ_q) of p_q = wρ_q. If w < - 1 / 3, the quintessence acts as a repulsive field, pushing space apart, and it is this property which makes quintessence a possible candidate for dark energy. In general, the parameter w can vary with cosmological time, although some theorists refer to quintessence with a time-variable w as kinessence to distinguish it from more stolid forms of energy that have an equation of state in which w remains constant. Also, the term quintessence is generally not used to describe any hypothesised energy that has an equation of state in which w > - 1 / 3. A non-zero cosmological constant can be seen as a special case where w = - 1 and does not vary in time. Another special case is phantom energy , in which w < - 1.

External links

• Dark Energy dominates the Universe http://www.astro.uni-bonn.de/~webiaef/outreach/posters/darkenergy/ (outreach poster from University of Bonn)

Classical Elements Western Air Fire | Aether | Water Earth See also: Quintessence Chinese Metal (金) | Wood (木) | Earth (土) | Water (水) | Fire (火)