Quintessence (physics)

In physics, quintessence is a hypothetical form of dark energy. It is thought to be a special kind of scalar field that is only weakly connected to gravity. Scientists suggest that quintessence might explain why the universe is expanding at an increasing rate.

The idea of quintessence was first suggested in 1988 by Ratra, Peebles, and Wetterich. The term “quintessence” was introduced in 1998 by Robert R. Caldwell, Rahul Dave, and Paul Steinhardt. Unlike the cosmological constant, which is a fixed value, quintessence can change over time. This dynamic nature means it can be either pulling things together or pushing them apart, depending on its energy.

Recently, in 2021, some researchers suggested that observations of the Hubble tension might support the idea that only certain types of quintessence models, those with a nonzero coupling constant, could be correct. This shows how scientists continue to explore and test ideas about the universe’s expansion and its mysterious dark energy.

Terminology

The term "quintessence" comes from the Latin phrase quinta essentia, meaning "fifth element." This idea started in the Middle Ages when Aristotle added a fifth element to the four ancient classical elements. He believed this fifth element, called aether, was pure and fine, making up the celestial world.

In modern physics, quintessence is like a fifth kind of energy in the universe. It is different from the other four parts, which are baryons, neutrinos, dark matter, and radiation. This modern quintessence changes over time and space, unlike the steady and unchanging cosmological constant.

Scalar field

Quintessence is a type of scalar field that scientists think might explain why the universe is expanding faster and faster. Unlike a fixed value called the cosmological constant, quintessence changes over time. This means its effects on the universe aren’t the same always. Scientists study it to better understand the mysterious forces shaping our cosmos.

Tracker behavior

Many models of quintessence show a special behavior called "tracker" behavior. This means the quintessence field changes in a way that closely follows the density of radiation in the early universe. Later, when matter and radiation densities became equal, quintessence began to act more like dark energy and eventually became the most important energy in the universe. This helps explain why dark energy has such a low value.

These models need four parameters to fully describe how quintessence behaves, but scientists believe future data may only allow them to measure two of these parameters well.

Specific models

Some special types of quintessence are called phantom dark energy. Scientists have suggested that dark energy might come from tiny changes in the fabric of space and time, known as quantum fluctuations.

Studies using quintessence in simulations have shown that it can affect how matter comes together in the universe. The results suggest that when the properties of quintessence are certain ways, it makes it more difficult for matter to heat up and balance out.