Dark energy

Dark energy is the name for the unknown cause of the universe’s accelerated expansion. In the simplest cosmological model, it behaves like a constant energy density of empty space.

Cosmic expansion: The increase in distances between unbound regions of the universe over time. Dark energy is inferred from the changing rate of this expansion.
Type Ia supernova: A thermonuclear explosion of a white dwarf, used as a distance indicator in astronomy. Their measured brightness revealed that distant expansion was accelerating.
Redshift: The increase in a light wave’s wavelength as its source recedes or spacetime expands. Supernova redshifts show how cosmic expansion changed over time.
Quintessence: A hypothetical dynamical scalar field proposed to drive the universe’s accelerated expansion. Unlike a cosmological constant, its energy density can change over time.
Cosmological constant problem: The mismatch between quantum-field estimates of vacuum energy and the small value inferred cosmologically. It asks why the simplest dark-energy model has such a tiny density.
Friedmann equations: Equations in general relativity that describe how the universe’s expansion depends on its contents and geometry. They connect cosmic energy density and pressure to the expansion rate.
Supernova Cosmology Project: An international research collaboration that measured distant Type Ia supernovae to study cosmic expansion. Its observations helped establish the evidence for acceleration.
Luminosity distance: A distance measure inferred from an object’s intrinsic luminosity and observed brightness. It converts supernova brightness into a distance for expansion-history tests.
Phantom energy: A hypothetical form of dark energy with pressure more negative than its energy density. It predicts behavior beyond the cosmological-constant limit, including possible runaway expansion.
Hubble tension: The disagreement between measurements of the present expansion rate from the early and nearby universe. A resolution could require changes to the standard picture containing dark energy.

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