Cosmological constant

The cosmological constant is a constant term in Einstein’s field equations, equivalent to uniform vacuum energy density. A positive value can drive accelerated expansion of the universe.

Einstein field equations: Equations relating spacetime curvature to the distribution of matter and energy. The cosmological constant enters these equations as an additional geometric term.
Albert Einstein: The physicist who developed general relativity and introduced the cosmological constant into its equations. He added the term in 1917 while seeking a static cosmological model.
Lambda-CDM model: The standard cosmological model, combining a cosmological constant with cold dark matter. Its lambda is the constant used to account for late-time accelerated expansion.
Quintessence: A model of dark energy driven by a slowly evolving scalar field. Unlike the cosmological constant, its energy density can change over time.
Vacuum catastrophe: The enormous discrepancy between naive quantum estimates of vacuum energy and its observed gravitational effect. It is the sharpest form of the problem of why the constant is so small.
Friedmann equations: Equations describing how homogeneous and isotropic universes expand over time. They show how the cosmological constant affects cosmic expansion.
Einstein static universe: A model universe balanced between gravitational attraction and cosmological repulsion. It was the original setting for Einstein’s cosmological constant.
Cosmic microwave background: Relic radiation released when the early universe became transparent to light. Its measured patterns constrain the universe’s matter content and cosmological constant.
Phantom energy: A hypothetical dark-energy component with pressure more negative than its energy density. Its equation of state differs from the constant’s fixed vacuum-like value.
Fine-tuning problem: The puzzle of why a theory’s parameters must take unusually precise values to match observation. The observed constant appears extraordinarily small compared with natural theoretical scales.

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