Quintessence (physics)

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 which he referred to as aether in his text On 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 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.

Scalar field

Quintessence (Q) is a scalar field with an equation of state where w**q, the ratio of pressure p**q and density \rhoq, is given by the potential energy V(Q) and a kinetic term: :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 and w**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 fixed energy density and w**q = −1.

Tracker behavior

Many models of quintessence have a tracker behavior, which according to Ratra and Peebles (1988) and Paul Steinhardt et al. (1999) partly solves the cosmological constant problem.{{cite journal | article-number=123504 | article-number=023526 | article-number=043509

Specific models

Some special cases of quintessence are phantom dark energy, in which w**q {{cite journal | article-number= 123520

Holographic dark energy

Holographic dark energy models, compared with cosmological constant models, imply a high degeneracy. It has been suggested that dark energy might originate from quantum fluctuations of spacetime, and is limited by the event horizon of the universe.

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.

References

References

1. Wetterich, C.. (1988-06-13). "Cosmology and the fate of dilatation symmetry". Nuclear Physics B.
2. Doran, Michael. (2001-10-01). "Quintessence and the Separation of Cosmic Microwave Background Peaks". The Astrophysical Journal.
3. (1998). "Cosmological Imprint of an Energy Component with General Equation-of-State". Physical Review Letters.
4. Carroll, S. M.. (1998). "Quintessence and the Rest of the World: Suppressing Long-Range Interactions". Physical Review Letters.
5. Wetterich, C.. "Quintessence – a fifth force from variation of the fundamental scale". Heidelberg University.
6. (2002). "Changing α With Time: Implications For Fifth-Force-Type Experiments And Quintessence". Physical Review Letters.
7. (2012-07-23). "Natural Quintessence in String Theory". Journal of Cosmology and Astroparticle Physics.
8. Wanjek, Christopher. "Quintessence, accelerating the Universe?".
9. (16 September 2021). "Does Hubble Tension Signal a Breakdown in FLRW Cosmology?". Classical and Quantum Gravity.
10. (2015). "Holographic Dark Energy with Cosmological Constant". Journal of Cosmology and Astroparticle Physics.
11. Gao. (2013). "Explaining Holographic Dark Energy". Galaxies.
12. (2015). "Holographic thermalization and gravitational collapse in the spacetime dominated by quintessence dark energy". Physical Review D.