In cosmology, quintessence is a natural form of energy distinct from any ordinary matter or radiation or even “dark matter.” Its bulk properties – energy density, pressure, and so forth – lead to novel behavior and unusual astrophysical phenomena.
The cosmological constant, or Einstein’s cosmological constant, is the energy density of space, or vacuum energy, that arises in Albert Einstein’s field equations of general relativity.
There is a missing energy problem in cosmology: the total energy density of the Universe, based on a wide range of observations, is much greater than the energy density contributed by all baryons, neutrinos, photons, and dark matter. The logical explanation is quintessence, a time-dependent, spatially inhomogeneous, negative pressure energy component that drives the cosmic expansion. ( In a spatially flat Universe). Quintessence serves as a bridge between the fundamental theory of nature, and the observable structure of the Universe. The existence of a new energy component is the answer to finding the missing energy. It resides in a soon-to-be-discovered cosmic scalar field.
The energy density in the matter is well below the critical energy density required to close the Universe. This makes the Universe spatially flat. The cosmic expansion of the Universe is accelerating.
Insertion of a Cosmological Constant to fill the gap between the matter and critical density required for a flat Universe, and drive accelerated expansion with its negative pressure, isn’t the answer.
Dark matter itself consists of slow-moving, or “cold,” particles that come together to form structures ranging from hundreds of thousands of times the mass of the Milky Way galaxy to clumps no more massive than the heft of a commercial airplane. Cold refers to the particles’ speed.
New physics is on the way once quintessence is validated.
Dark matter is five times as abundant as normal matter in the universe. But it continues to be an enigma because it is invisible and nearly always passes right through normal matter. It keeps spinning galaxies from flying apart.
Theorists claim that there exist two classes of dark matter: 1) those that congregate around individual members of a cluster of visible galaxies, and 2) those that encompass the clusters as a whole. An example of a hot dark matter particle is the neutrino. Neutrinos have very small masses and do not take part in two of the four fundamental forces, the electromagnetic interaction, and the strong interaction. They interact by the weak interaction and gravity, but due to the feeble strength of these forces, they are difficult to detect.
After the big bang, hot dark matter was the glue that held the newly formed clusters and superclusters of galaxies together.
