Is the cosmological constant the same thing as dark energy?

The cosmological constant is the simplest possible form of dark energy, a vacuum energy with a fixed density; 'dark energy' is the broader term that also includes dynamical models, so all cosmological constants are dark energy but not all dark energy need be a constant.

Why was adding the constant once called a blunder?

Einstein added it to force a static universe, the prevailing assumption at the time; had he trusted the unmodified equations he might have predicted cosmic expansion before it was observed, which is why he later regretted the move, though the term proved real after all.

Cosmological Constant

The cosmological constant is an extra term in the Einstein field equations, proportional to the metric, that behaves like a uniform energy density of empty space and drives an accelerated cosmic expansion.

Definition

The cosmological constant, denoted Lambda, is a constant term added to the Einstein field equations that contributes a uniform energy density and equal-magnitude negative pressure to the vacuum, equivalent to the simplest form of dark energy.

Scope

This topic covers the algebraic form of the cosmological-constant term and its consistency with energy-momentum conservation, its interpretation as vacuum energy with negative pressure, Einstein's original motivation for a static universe and his later retraction, its revival to explain the observed cosmic acceleration, and the cosmological-constant problem of its enormous discrepancy with quantum-field-theory estimates.

Core questions

Why can a constant term be added to the Einstein equations without violating conservation laws?
How does a cosmological constant produce accelerated rather than decelerated expansion?
Why is the observed value so much smaller than quantum field theory predicts?

Key concepts

Lambda term
Vacuum energy density
Negative pressure
Accelerated expansion
Dark energy
Cosmological-constant problem

Key theories

Vacuum energy interpretation: A cosmological constant is equivalent to a uniform vacuum energy density with pressure equal to minus that density, so it contributes a repulsive gravitational effect that accelerates the expansion of the universe.
Cosmological-constant problem: Quantum field theory predicts a vacuum energy many tens of orders of magnitude larger than the tiny observed value, an unexplained discrepancy that is one of the deepest puzzles connecting gravity and quantum physics.

Clinical relevance

Observations of distant supernovae, the cosmic microwave background, and large-scale structure are consistent with a small positive cosmological constant making up most of the present energy budget of the universe; whether it is a true constant or a dynamical dark energy is a central question of observational cosmology.

History

Einstein introduced Lambda in 1917 to obtain a static universe, then abandoned it after Hubble's discovery of cosmic expansion, reportedly calling it his biggest blunder; it returned decisively in 1998 when supernova surveys revealed that the expansion is accelerating, reviving the constant as the leading model of dark energy.

Debates

Constant versus dynamical dark energy: Whether cosmic acceleration is caused by a true cosmological constant or by a slowly evolving field (quintessence) is observationally open; distinguishing them requires precise measurement of how the dark-energy density changes, if at all, over cosmic time.

Key figures

Albert Einstein
Willem de Sitter
Sean Carroll

Seminal works

carroll2001
carroll2004

Frequently asked questions

Is the cosmological constant the same thing as dark energy?: The cosmological constant is the simplest possible form of dark energy, a vacuum energy with a fixed density; 'dark energy' is the broader term that also includes dynamical models, so all cosmological constants are dark energy but not all dark energy need be a constant.
Why was adding the constant once called a blunder?: Einstein added it to force a static universe, the prevailing assumption at the time; had he trusted the unmodified equations he might have predicted cosmic expansion before it was observed, which is why he later regretted the move, though the term proved real after all.