It has been suggested that dark energy might originate from quantum fluctuations of spacetime, and are limited by the event horizon of the universe. Holographic dark energy models, compared with cosmological constant models, imply a high degeneracy. If this type of energy were to exist, it would cause a big rip 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. Some special cases of quintessence are phantom energy, in which w q < −1, and k-essence (short for kinetic quintessence), which has a non-standard form of kinetic energy. 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, whereas it has been shown that at most a two-parameter model can optimally be constrained by mid-term future data (horizon 2015–2020). This naturally sets the low scale of the dark energy. 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. (1999) partly solves the cosmological constant problem. Many models of quintessence have a tracker behavior, which according to Ratra and Peebles (1988) and Paul Steinhardt et al. By contrast, a cosmological constant is static, with a fixed energy density and w q = −1. Hence, quintessence is dynamic, and generally has a density and w q parameter that varies with time. Quintessence ( Q) is a scalar field with an equation of state where w q, the ratio of pressure p q and density ρ 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. Of course, the other four components are not the ancient Greek classical elements, but rather " baryons, neutrinos, dark matter, radiation." Although neutrinos are sometimes considered radiation, the term "radiation" in this context is only used to refer to massless photons. Similarly, modern quintessence would be the fifth known "dynamical, time-dependent, and spatially inhomogeneous" contribution to the overall mass–energy content of the universe. Later scholars identified this element with aether. Aristotle posited to be a pure, fine, and primigenial 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. The name comes from quinta essentia (fifth element). Ī 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. 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 can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. 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. It has been proposed by some physicists to be a fifth fundamental force. Caldwell, Rahul Dave and Paul Steinhardt. 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. The first example of this scenario was proposed by Ratra and Peebles (1988) and Wetterich (1988). 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.
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