Quintessence (physics)

In physics, quintessence is a hypothetical form of dark energy. It is a special kind of scalar field that is only weakly connected to gravity. Scientists think quintessence might explain why the universe is expanding.

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.

Recently, in 2021, some researchers suggested that observations of the Hubble tension might support certain types of quintessence models. This shows how scientists keep exploring 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 began in the Middle Ages when Aristotle added a fifth element to the four ancient elements. He thought this fifth element, called aether, was pure and made up the world above Earth.

In modern physics, quintessence is a special kind of energy in the universe. It is different from the other four parts, which are baryons, neutrinos, dark matter, and radiation. This new quintessence changes over time and space, unlike the steady 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 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 think dark energy might come from very small changes in space and time, called quantum fluctuations.

Studies with quintessence show it can change how matter comes together in the universe. When quintessence has certain properties, it can make it harder for matter to heat up and balance out.