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Cosmology
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
pq and density ρq) of
pq = wρq. In general, the
parameter w can vary with cosmological time. 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. A non-zero cosmological constant can be seen as a special case where w = - 1
and does not vary in time.
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Elements
Among the classical elements, quintessence (meaning "fifth
element", along with earth, air, fire, and water) was another term for aether.
Alchemy
In alchemy, quintessence is the purest form of a particular element (as one would conceive them today). 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?"
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