Inphysics,quintessence is ahypothetical form ofdark energy, more precisely ascalar field minimally coupled to gravity, postulated as an explanation of the observation of anaccelerating rate of expansion of the universe. The first example of this scenario was proposed byRatra andPeebles (1988)^[1] andWetterich (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 byRobert R. Caldwell, Rahul Dave andPaul Steinhardt.^[4] It has been proposed by some physicists to be afifth fundamental force.^[5][6][7][8] Quintessence differs from thecosmological 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 theBig Bang.^[9]

A group of researchers argued in 2021 that observations of theHubble tension may imply that only quintessence models with a nonzerocoupling constant are viable.^[10]

The name comes fromquinta essentia (fifth element). So called in Latin starting from the Middle Ages, this was the (first) element added byAristotle to the other four ancientclassical elements because he thought it was the essence of the celestial world. Aristotle posited it to be a pure, fine, andprimigenial element which he referred to asaether in his textOn the Heavens. 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 theancient 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 masslessphotons. 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]

Quintessence (Q) is ascalar field with anequation of state wherew_q, the ratio of pressurep_q and density${\displaystyle \rho }$_q, is given by the potential energy${\displaystyle V(Q)}$ and akinetic term:

${\displaystyle w_q=\frac{p_q}{{\rho }_q}=\frac{\frac{1}{2}{\dot{Q}}^2-V(Q)}{\frac{1}{2}{\dot{Q}}^2+V(Q)}}$

Hence, quintessence is dynamic, and generally has a density andw_q parameter that varies with time. Specifically,w_q parameter can vary within the range [-1,1]. By contrast, a cosmological constant is static, with a fixedenergy density andw_q = −1.

Many models of quintessence have atracker behavior, which according to Ratra and Peebles (1988) andPaul Steinhardtet al. (1999) partly solves thecosmological constant problem.^[11] In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density untilmatter-radiation equality, which triggers quintessence to start having characteristics similar to dark energy, eventually dominating the universe. This naturally sets the lowscale of the dark energy.^[12] When comparing the predictedexpansion 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 theirequation 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]

Some special cases of quintessence arephantom dark energy, in whichw_q < −1,^[16] and k-essence (short for kinetic quintessence), which has a non-standard form ofkinetic energy. If this type of energy were to exist, it would cause aBig 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 models, compared with cosmological constant models, imply a highdegeneracy.^{[clarification needed][18]} It has been suggested that dark energy might originate fromquantum fluctuations ofspacetime, 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]

• Aether (classical element)
• Phantom dark energy
• Quintom

• Christof, Wetterich (1987-09-24). "Cosmology and the fate of dilatation symmetry".Nuclear Physics B.302 (4):668–696.arXiv:1711.03844.Bibcode:1988NuPhB.302..668W.doi:10.1016/0550-3213(88)90193-9.S2CID 118970077.
• Ostriker, J. P.; Steinhardt, P. (January 2001). "The Quintessential Universe".Scientific American.284 (1):46–53.Bibcode:2001SciAm.284a..46O.doi:10.1038/scientificamerican0101-46.PMID 11132422.
• Krauss, Lawrence M. (2000).Quintessence: The Search for Missing Mass in the Universe.Basic Books.ISBN 978-0-465-03741-4.

• Dark energy

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