As this NASA chart shows, dark energy makes up around 70% or more of the universe, about which we know very little. The actual structure of this dark energy remains a mystery. It is known to be extremely uniform, not particularly dense, and not to interact with any of the fundamental forces besides gravity.
It’s difficult to conceive laboratory studies to discover it because it’s not very dense—roughly 1029 grams per cubic centimeter. Because dark energy uniformly fills otherwise empty space, it can have a considerable effect on the universe, amounting to 70% of all energy. The cosmological constant and quintessence are the two most popular hypotheses.
- Cosmological Constant
Dark energy is merely the “expense of having space,” according to the most basic interpretation: A volume of space, in other words, has some underlying, fundamental energy. The cosmological constant is also known as Lambda (thus the Lambda-CDM model) after the Greek letter Λ which is used to numerically describe it. Because the equation E=mc^{2} connects energy and mass, Einstein’s theory of general relativity implies that it will have a gravitational influence. Because it is the energy density of an empty vacuum, it is frequently referred to as vacuum energy. In reality, most particle physics models predict vacuum fluctuations that produce exactly this kind of energy. Cosmologists calculate the cosmological constant to be on the order of 10−29g/cm³ or roughly 10−120 in Planck units.
Because the cosmological constant exerts a negative pressure equal to its energy density, the universe’s expansion accelerates. Classical thermodynamics explains why a cosmological constant has negative pressure: energy must be lost from inside a container to do work on the container. Work equivalent to a change in energy −p dV is required for a change in volume dV, where p is the pressure.
However, because energy is equal to V, where (rho) is the energy density of the cosmological constant, the quantity of energy in a box of vacuum energy actually grows as the volume expands (dV is positive). As a result, p is negative, and in reality, p =−p.
One important unsolved issue is that most quantum field theories anticipate a massive cosmological constant derived from quantum vacuum energy that is up to 120 orders of magnitude too enormous. This would have to be almost, but not quite, canceled by a similarly huge term of the opposite sign. Some supersymmetric theories call for an exact zero cosmological constant, which isn’t feasible. The current empirical consensus entails extending observable information to prove predictions and perfectly alright ideas until a more efficient resolution is discovered. The anthropic principle, which states that if things were otherwise, humans would not be around to witness anything, maybe the most logical intellectual approach. In technical terms, this entails comparing theories to macroscopic observations. Sadly, many of these “deeper” answers remain unclear since the known error margin in the constant indicates the future of the universe more than its current condition.
Another issue that arises when the cosmic constant is included in the standard model is the creation of a solution with areas of discontinuities at low matter density. As one goes back to the origin of the Universe, the discontinuity also influences the historical sign of the vacuum energy, which changes from the current negative pressure to attractiveness. When a term for vacuum energy is included, this finding should be considered a flaw in the standard model.
Despite its flaws, the cosmological constant is the most cost-effective solution to the issue of cosmic acceleration in several ways. A single number effectively explains dozens of new observations. As a result, the cosmological constant is a fundamental aspect of the current standard model of cosmology, the Lambda-CDM model.
- Quintessence
When dark energy is buffeted by baryonic particles, it can transform into dark matter, resulting in particle-like excitations in a dynamical field known as quintessence. Dark matter is made up of particles that do not consume, reflect, or radiate light, making it impossible to discover electromagnetic radiation. The substance that cannot be seen directly is referred to as dark matter.
Quintessence is unlike the cosmological constant in that it can change throughout space and time. It must be very light, with a large Compton wavelength, to avoid clumping and forming structures like matter.
There is no proof of quintessence yet, but it has also not been ruled out. The cosmological constant predicts a slightly slower acceleration of the universe’s expansion than this model. Breach of Einstein’s equivalence principle and variations of the fundamental constants in space or time, according to some scientists, is the clearest indication of quintessence. The standard model and string theory anticipate scalar fields, but there’s a difficulty that’s similar to the cosmological constant issue: Scalar fields should develop enormous masses, according to renormalization theory.
The cosmic coincidence issue involves defining why the cosmic acceleration began at the specific time it did. Structures such as planets would never have had the tendency to develop if cosmic acceleration started sooner in the universe, and life, at least as we know it, would never have had a possibility to exist. This is seen by advocates of the anthropic principle as evidence for their claims. Many quintessence models, on the other hand, have a so-called tracker behavior that resolves this difficulty. In these models, the quintessence field has a density that roughly tracks that of the radiation field until matter-radiation equality is attained, at which point quintessence begins to act like dark energy, ultimately overtaking the universe. This automatically establishes the dark energy’s low energy scale.
Phantom energy, in which the energy density of quintessence actually rises with time, and kinetic quintessence, which possesses a non-standard form of kinetic energy, are two unique examples of quintessence. They can have peculiar characteristics: A Big Rip, for example, can be caused by phantom energy.
Reference:
- https://www.newworldencyclopedia.org/entry/Dark_energy
- https://home.cern/science/physics/dark-matter
