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. The first example of this scenario was proposed by
Ratra and
Peebles (1988)[1] and
Wetterich (1988).[2][3] The concept was expanded to more general types of time-varying dark energy, and the term "quintessence" was first introduced in a 1998 paper by
Robert R. Caldwell, Rahul Dave and
Paul Steinhardt.[4] It has been proposed by some physicists to be a
fifth fundamental force.[5][6][7][8] 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, by definition, does not change. 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.[9]
A group of researchers argued in 2021 that observations of the
Hubble tension may imply that only quintessence models with a nonzero
coupling constant are viable.[10]
Terminology
The name comes from quinta essentia (fifth element). So called in Latin starting from the Middle Ages, this was the (first) element added by
Aristotle to the other four ancient
classical elements because he thought it was the essence of the celestial world. Aristotle posited it to be a pure, fine, and
primigenial element. Later scholars identified this element with
aether. Similarly, modern quintessence would be the fifth known "dynamical, time-dependent, and spatially inhomogeneous" contribution to the overall mass–energy content of the universe.
Of course, the other four components are not the
ancient Greek classical elements, but rather "
baryons,
neutrinos,
dark matter, [and]
radiation." Although neutrinos are sometimes considered radiation, the term "radiation" in this context is only used to refer to massless
photons. Spatial curvature of the cosmos (which has not been detected) is excluded because it is non-dynamical and homogeneous; the cosmological constant would not be considered a fifth component in this sense, because it is non-dynamical, homogeneous, and time-independent.[4]
Scalar field
Quintessence (Q) is a
scalar field with an
equation of state where wq, the ratio of pressure pq and density q, is given by the potential energy and a kinetic term:
Hence, quintessence is dynamic, and generally has a density and wq parameter that varies with time. By contrast, a cosmological constant is static, with a fixed
energy density and wq = −1.
Tracker behavior
Many models of quintessence have a tracker behavior, which according to Ratra and Peebles (1988) and
Paul Steinhardtet al. (1999) partly solves the
cosmological constant problem.[11] In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until
matter-radiation equality, which triggers quintessence to start having characteristics similar to dark energy, eventually dominating the universe. This naturally sets the low
scale of the dark energy.[12] When comparing the predicted
expansion rate of the universe as given by the tracker solutions with cosmological data, a main feature of tracker solutions is that one needs four parameters to properly describe the behavior of their
equation of state,[13][14] whereas it has been shown that at most a two-parameter model can optimally be constrained by mid-term future data (horizon 2015–2020).[15]
Specific models
Some special cases of quintessence are
phantom energy, in which wq < −1,[16] and k-essence (short for kinetic quintessence), which has a non-standard form of
kinetic energy. If this type of energy were to exist, it would cause a
big rip[17] in the universe due to the growing energy density of dark energy, which would cause the expansion of the universe to increase at a faster-than-exponential rate.
Holographic dark energy
Holographic dark energy models, compared with cosmological constant models, imply a high
degeneracy.[clarification needed][18] It has been suggested that dark energy might originate from
quantum fluctuations of
spacetime, and is limited by the event horizon of the universe.[19]
Studies with quintessence dark energy found that it dominates gravitational collapse in a spacetime simulation, based on the holographic thermalization. These results show that the smaller the state parameter of quintessence is, the harder it is for the plasma to thermalize.[20]
Quintom scenario
In 2004, when scientists fitted the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary (w = –1) from above to below. A proven
no-go theorem indicates this situation, called the
Quintom scenario, requires at least two degrees of freedom for dark energy models involving ideal gases or scalar fields.[21]