Dark Energy
Dark energy is a hypothetical form of energy that permeates all of space and is postulated to drive the accelerated expansion of the universe, constituting approximately 68% of the total energy content of the cosmos. It was first inferred from Type Ia supernova observations in 1998, which showed that distant supernovae were fainter than expected, implying the expansion of the universe is speeding up rather than slowing down. The simplest model for dark energy is the cosmological constant (Λ), representing a constant energy density of free space, though alternative models such as quintessence propose a dynamic scalar field.
Key Formula
H² = (8πG/3)(ρ_matter + ρ_radiation + ρ_Λ)
LaTeX: H^2 = \frac{8\pi G}{3}\left(\rho_m + \rho_r + \rho_\Lambda\right)
| Symbol | Meaning | Unit |
|---|---|---|
| H | Hubble parameter | km/s/Mpc |
| G | Gravitational constant | m³/(kg·s²) |
| ρ_m | Matter density | kg/m³ |
| ρ_r | Radiation density | kg/m³ |
| ρ_Λ | Dark energy density (cosmological constant) | kg/m³ |
Composition of the Universe (Planck 2018 Results)
| Component | Fraction (%) | Nature | Effect on Expansion |
|---|---|---|---|
| Dark Energy (Λ) | 68.3 | Unknown; fills space | Accelerates expansion |
| Dark Matter | 26.8 | Non-baryonic; gravitational | Decelerates expansion |
| Baryonic Matter | 4.9 | Atoms; visible/invisible | Decelerates expansion |
| Radiation | < 0.01 | Photons, neutrinos | Decelerates expansion |
Interactive Tools
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Related Terms
Expanding Universe
The expanding universe is the observational discovery that the fabric of space itself is stretching over time, causing galaxies that are not gravitationally bound to each other to recede from one another at velocities proportional to their separating distances. First confirmed observationally by Edwin Hubble in 1929 through measurements of galaxy redshifts, this expansion is described by the Friedmann–Lemaître–Robertson–Walker (FLRW) metric in general relativity. The rate of expansion, parameterised by the Hubble constant H₀ ≈ 67–73 km/s/Mpc, has been measured to be accelerating due to dark energy.
Dark Matter
Dark matter is a hypothetical form of matter that does not interact with the electromagnetic force, making it invisible to the entire electromagnetic spectrum, yet its existence is inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. It is estimated to constitute approximately 27% of the total mass-energy content of the universe, compared to only 5% for ordinary baryonic matter. Leading candidates include Weakly Interacting Massive Particles (WIMPs), axions, and sterile neutrinos, though no direct detection has been confirmed as of 2025.
Hubble's Law
Hubble's Law is the empirical observation that the recession velocity of a galaxy is directly proportional to its distance from the observer, expressed as v = H₀d, where H₀ is the Hubble constant. First published by Edwin Hubble in 1929 based on measurements of galaxy redshifts, it provided the first direct observational evidence for the expanding universe predicted by general relativity. The Hubble constant H₀, currently estimated at approximately 67–73 km/s/Mpc from different methods, also allows astronomers to estimate the age of the universe as roughly 1/H₀.
"Dark" signifies its imperceptibility through any known observational means (Old English "deorc"). "Energy" derives from Greek "energeia" (activity, operation), introduced into physics by Thomas Young in 1807. The concept was introduced in 1998 following the Nobel Prize-winning work of Perlmutter, Schmidt, and Riess on Type Ia supernovae.