Dark Energy
Dark energy is a form of energy, uniformly permeating all of space, inferred to exist because it is understood to be driving the observed accelerating expansion of the universe — currently estimated to account for roughly 68% of the universe's total mass-energy content, making it, by this measure, the single largest component of the cosmos, despite remaining almost entirely unexplained at a fundamental physical level.
Curious
For most of the 20th century, cosmologists expected the universe's expansion to be gradually slowing down over time, as the mutual gravitational attraction between all the matter in the universe should act as a brake on the expansion set in motion by the Big Bang. In 1998, two independent research teams, studying distant Type Ia supernovae as standard candles (see the Codex's supernovae entry), found the opposite: the expansion is actually accelerating, not slowing down — a genuinely startling result that neither team had originally set out to find.
Whatever is driving this acceleration — now called dark energy — behaves unlike ordinary matter or radiation: rather than diluting as the universe expands (as matter density naturally does, spread over an ever-larger volume), dark energy's effective density appears to remain roughly constant even as space itself grows, acting something like a uniform, gentle outward pressure permeating all of space, gradually overcoming gravity's attractive pull on the largest cosmic scales.
Exploring
| Component | Approximate share of total mass-energy |
|---|---|
| Dark energy | ~68% |
| Dark matter | ~27% |
| Ordinary matter | ~5% |
Deep Dive
The simplest and most widely favoured theoretical explanation for dark energy is Einstein's cosmological constant (Λ) — a term Einstein originally introduced into his general relativity field equations in 1917 for an entirely different reason (to allow for a static, unchanging universe, before Hubble's 1929 discovery of cosmic expansion made this static assumption unnecessary), and which Einstein later reportedly regarded as one of his biggest blunders once cosmic expansion was established. Remarkably, the cosmological constant framework — representing a constant energy density inherent to space itself, mathematically equivalent to modern quantum field theory's concept of vacuum energy — turns out to fit the observed accelerating expansion data reasonably well, decades after Einstein first (for different reasons) introduced the mathematical term.
The cosmological constant problem: A serious, unresolved theoretical tension exists between the cosmological constant's observationally required value (extremely small but nonzero) and naive theoretical estimates of vacuum energy density from quantum field theory, which predict a value vastly larger — by as much as 120 orders of magnitude in the most extreme estimates — than what is actually observed. This enormous, unexplained discrepancy (sometimes called the worst theoretical prediction in the history of physics) remains one of the most significant unsolved problems at the intersection of cosmology and fundamental particle physics, motivating alternative theoretical proposals beyond the simple cosmological constant, including dynamical "quintessence" models in which dark energy's density can evolve over cosmic time rather than remaining strictly constant.
Current and upcoming observational efforts — including large-scale galaxy surveys, further refined supernova studies, and precision CMB measurements — continue working to determine whether dark energy is truly constant (consistent with Einstein's original cosmological constant) or evolves with time (favouring alternative quintessence-like models), a distinction with major implications for the universe's ultimate long-term fate (see the Codex's forthcoming Fate of the Universe entry).
Concept Relationship Map
Requires: supernovae (Type Ia, standard candles), curved spacetime
Enables: cosmological constant, fate of the universe
Commonly confused with: dark matter (a gravitationally attractive phenomenon, distinct from dark energy's role in cosmic acceleration)
Sources
- Riess, A.G., et al. (1998). The Astronomical Journal, 116(3), 1009–1038.
- Perlmutter, S., et al. (1999). "Measurements of Ω and Λ from 42 High-Redshift Supernovae." The Astrophysical Journal, 517(2), 565–586.