Dark energy
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Dark energy | |
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| Type | Cosmological component |
| Field | Cosmology; Gravitation; Theoretical physics |
| Core idea | Cause of the accelerated expansion of the universe |
| Assumptions | Large-scale dynamics require a repulsive gravitational component |
| Status | Observationally supported; physically unexplained |
| Related | Cosmological constant; Vacuum energy; General relativity; Expansion of the universe |
Dark energy is a hypothesized component of the universe introduced to explain the observed accelerated expansion of cosmic space. It constitutes the dominant contribution to the universe’s total energy budget, yet its physical nature remains unknown.
Dark energy is inferred from astronomical observations rather than detected directly.
Definition
Dark energy refers to whatever physical mechanism drives the acceleration of the universe’s expansion. In cosmological models, it is characterized by a roughly uniform energy density that does not dilute significantly as space expands.
Its defining property is negative effective pressure.
Observational evidence
Evidence for dark energy arises from multiple independent observations, including:
Distance–redshift measurements of Type Ia supernovae
Cosmic microwave background anisotropies
Large-scale structure surveys
Together, these observations indicate accelerated expansion beginning several billion years ago.
Cosmological constant
The simplest model of dark energy is the cosmological constant, representing a constant energy density intrinsic to spacetime. This model fits current data well with minimal parameters.
However, its theoretical interpretation is problematic.
Equation of state
Dark energy is often parameterized by an equation-of-state parameter relating pressure to energy density. Observations are consistent with a value corresponding to a cosmological constant, though small deviations remain possible.
Constraining this parameter is a major goal of observational cosmology.
Alternatives
Proposed alternatives to a cosmological constant include:
Dynamical scalar fields
Modifications of general relativity at large scales
Emergent or effective phenomena
None have been confirmed.
Relationship to vacuum energy
Dark energy is frequently associated with vacuum energy, but naïve quantum field theory estimates exceed observed values by many orders of magnitude. Reconciling this discrepancy is the cosmological constant problem.
This mismatch represents a major theoretical challenge.
Impact on cosmic fate
Dark energy influences the long-term evolution of the universe. Depending on its properties, the universe may approach perpetual accelerated expansion, altered expansion dynamics, or more exotic outcomes.
Current data favor continued acceleration.
Misconceptions
Dark energy is not a form of ordinary matter and does not clump into structures like galaxies. It does not exert forces locally in the same way as matter or radiation.
It is also not “dark” due to obscuration; the term reflects ignorance of its nature.
Limits and uncertainty
While the existence of dark energy is strongly supported, its microphysical origin is unknown. Current observations constrain its behavior but do not identify its underlying mechanism.
Future surveys aim to refine these constraints.
Status
Dark energy is a central component of the standard cosmological model. It is empirically established as a phenomenon but remains one of the deepest unsolved problems in fundamental physics.
Its resolution is expected to reshape understanding of gravity and spacetime.