Cosmology
Dark Energy
Mysterious force accelerating universe expansion — comprises 68% of universe's content
Dark energy is the mysterious form of energy causing universe to expand at accelerating rate. Discovered 1998 (Riess, Perlmutter, Schmidt — Nobel Prize 2011) via Type Ia supernovae appearing dimmer than expected at high redshift. Constitutes ~68% of universe's energy content (vs 27% dark matter, 5% baryons). Nature unknown — leading candidates: cosmological constant (Λ; vacuum energy), quintessence (scalar field), modified gravity. Dark energy density nearly constant; will dominate universe in future.
- Universe fraction~68% (today)
- Discovered1998 (Type Ia SN observations)
- Nobel Prize2011 (Riess, Perlmutter, Schmidt)
- Equation of statew = -1.0 ± 0.1 (cosmological constant)
- Density~10⁻²⁷ kg/m³
- EffectAccelerated expansion, future universe dominated
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Why dark energy matters
- Cosmology. ~68% of universe; dominant energy.
- Universe future. Determines fate (heat death, big rip).
- Particle physics. Beyond standard model search.
- General relativity. Modified gravity alternatives.
- Cosmological constant problem. 120-order-magnitude mystery.
- Type Ia surveys. Constrain DE properties.
- Multiverse arguments. Anthropic selection for tiny Λ.
Common misconceptions
- DE is dark matter. Different — DM clusters; DE drives expansion.
- DE is detected. Inferred from acceleration; not directly observed.
- DE is constant. Cosmological constant: yes; quintessence: no.
- DE explains everything cosmological. Many open questions.
- DE is energy. Yes — but unusual — has negative pressure.
- DE makes universe accelerate gravitationally. Repulsive due to negative pressure in GR.
Frequently asked questions
How was dark energy discovered?
Two independent teams (Riess et al.; Perlmutter et al.) measured Type Ia SN at high redshift. Found: distant SN appear ~25% dimmer than expected for decelerating universe. Implied: universe is accelerating. Required new energy form — dark energy. Confirmed by CMB, BAO. Major surprise — Universe expansion was thought to be slowing.
What's the equation of state?
w = pressure/energy density. Cosmological constant: w = -1 exactly. Other models: w varies (quintessence). Current data: w = -1.0 ± 0.1. If w < -1: phantom DE → big rip. If w > -1: DE may eventually decay. Tightly constrained but not perfectly known.
What's the cosmological constant?
Λ — energy density of vacuum. Predicted by quantum field theory. Calculated value: 10¹²⁰ × observed. Largest disagreement in physics. Current Λ from observations small but non-zero. Mystery: why is observed value tiny but non-zero? Anthropic argument: only universes with small Λ allow life — observer selection effect.
What's quintessence?
Alternative model. Scalar field with time-varying energy. w may be different from -1; vary over time. Could explain DE without exact cosmological constant. No specific particle predicted by quintessence — phenomenological. Discrimination from cosmological constant requires precision DE measurements.
How does DE affect universe?
Dominates at large scales. Accelerated expansion. Galaxies recede from each other faster. Cosmic web filaments stretch as voids expand. Eventually: dominant energy form. Future: universe expands forever; isolated galaxy local groups; "heat death." DE energy density nearly constant — not diluted by expansion.
Has DE always been important?
No. In early universe: matter dominated; DE negligible. ~5 Gyr ago: DE became dominant. Today: ~68% of energy. Future: increasing fraction. Time-varying contribution made cosmology different at different epochs. Early-universe physics doesn't include DE significantly.
What's the future of universe?
Depends on DE. If w = -1 (cosmological constant): heat death — all matter spreads out, cools, isolated regions. If w < -1: big rip — gravitational structures torn apart. If w > -1 evolves: complex. Current data favor cosmological constant → heat death scenario.