Fifth force in the context of "Fundamental interaction"

In physics, quintessence is a hypothetical form of dark energy, more precisely a scalar field minimally coupled to gravity, 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) and Wetterich (1988). 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. It has been proposed by some physicists to be a fifth fundamental force. 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.

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.

Ephraim Fischbach (born 1942) is an American physicist and a professor at Purdue University. He is best known for his attempts to find a fifth force of nature and his research relating to the detection of neutrinos. He has also done work relating to the prediction of solar flares and the detection of radiation by cell phones.

Fischbach studies variation in radioactive decay rates, suggesting that neutrino emission from the Sun reduces the rate of nuclear decay. He reanalysed the Eötvös experiment, which he saw as evidence for a fifth physical force. However, in 1992, he and Carrick Talmadge conducted an experiment which found no compelling evidence for a fifth force.