Dark energy is a component of the cosmos that causes the expansion of the universe to accelerate, accounting for about two-thirds of the total cosmic energy density in the concordance Lambda-CDM model and effectively characterized by negative pressure with an equation-of-state parameter near −1. The concept is often identified with the Cosmological constant Λ in general relativity, though alternatives exist. According to NASA Science, dark energy comprises approximately 68–70% of the universe, while precision analyses of the cosmic microwave background and other probes find values consistent with Ω_Λ ≈ 0.68. NASA Science; Planck Collaboration, A&A 2020. (science.nasa.gov)

Historical development and discovery

Evidence for accelerated expansion emerged in 1998–1999 from measurements of Type Ia supernovae used as standardizable candles. Two independent teams—the High-z Supernova Search Team and the Supernova Cosmology Project—found distant SNe Ia to be dimmer than expected in a decelerating universe, implying acceleration and a dominant dark energy component. Key papers include Riess et al. (1998) and Perlmutter et al. (1999), findings later recognized by the 2011 Nobel Prize in Physics. Riess et al., AJ 1998; Perlmutter et al., ApJ 1999; Nobel Prize press release, 2011. (dash.harvard.edu)

Earlier frameworks anticipated a repulsive term in gravity: Einstein introduced Λ in 1917, and later developments recast it as vacuum energy. Reviews outline the observational case for acceleration and the theoretical landscape that includes dark energy or modifications to gravity. Peebles & Ratra, Rev. Mod. Phys. 2003; Frieman, Turner & Huterer, ARA&A 2008. (journals.aps.org)

Physical interpretation and models

In general relativity, accelerated expansion occurs when the total equation-of-state satisfies w < −1/3. The simplest model sets dark energy equal to the Cosmological constant with constant w = −1, identified with vacuum energy density. This raises the “cosmological constant problem,” the large discrepancy between observed Λ and naive quantum-field-theory expectations by many orders of magnitude. Dynamical dark energy models (e.g., quintessence) posit a slowly evolving scalar field with w ≠ −1 and possibly time variation; other approaches modify gravity on cosmic scales while preserving general relativity tests locally. Weinberg, Rev. Mod. Phys. 1989; Peebles & Ratra, Rev. Mod. Phys. 2003; Frieman, Turner & Huterer, ARA&A 2008. (journals.aps.org)

Observational probes

• –Supernovae: The Hubble diagram of SNe Ia at varying redshifts traces luminosity distance versus redshift and directly reveals acceleration. Riess et al., AJ 1998; Perlmutter et al., ApJ 1999. (dash.harvard.edu)
• –Baryon acoustic oscillations (BAO): A standard ruler imprinted in the large-scale distribution of galaxies provides angular-diameter and Hubble distances as functions of redshift. The first clear BAO detection in galaxy clustering came from SDSS luminous red galaxies in 2005. Eisenstein et al., ApJ 2005; WiggleZ BAO at z≈0.6, MNRAS 2011. (deisenstein.scholars.harvard.edu)
• –Cosmic microwave background (CMB): Temperature–polarization anisotropies and lensing constrain geometry and matter content, anchoring dark energy parameters when combined with SNe and BAO. Planck’s final results favor a flat ΛCDM model with Ω_m ≈ 0.315 and Ω_Λ ≈ 0.685. Planck Collaboration, A&A 2020. (aanda.org)
• –Weak gravitational lensing and growth of structure: Shear maps and redshift-space distortions probe how structures grow under gravity, enabling tests that distinguish dark energy from modified gravity. Frieman, Turner & Huterer, ARA&A 2008; Planck Collaboration, modified gravity constraints. (annualreviews.org)

Current empirical constraints

Joint analyses of SNe Ia, BAO, and the CMB find an equation-of-state consistent with a cosmological constant. The Pantheon+ compilation (1,550 SNe Ia) combined with Planck CMB and BAO yields w₀ = −0.978 (+0.024/−0.031) for flat wCDM, consistent with w = −1; Planck’s combined analyses report w₀ ≈ −1.03 ± 0.03 with SNe and BAO. Brout et al., ApJ 2022; Planck Collaboration, A&A 2020. (arxiv.org)

New large-scale structure measurements continue to refine the expansion history. The Dark Energy Spectroscopic Instrument (DESI) produced year‑one BAO results in April 2024, achieving sub‑percent precision on distances across multiple redshifts; the collaboration reported broad consistency with ΛCDM while noting hints that dark energy might evolve with time, a possibility to be tested with more data. DESI/Berkeley Lab news, Apr 4, 2024. (desi.lbl.gov)

Further DESI analyses of structure growth over ≈11 billion years have been reported as consistent with general relativity on cosmological scales while allowing a preference in some fits for time‑varying dark energy; these results are based on early DESI datasets and are subject to ongoing scrutiny and peer review. Reuters report, Nov 20, 2024. (reuters.com)

Major facilities and surveys

• –[Euclid (space telescope)]: ESA’s Euclid (launched 2023) is mapping billions of galaxies over a third of the sky to chart geometry and growth via weak lensing and galaxy clustering; early imaging and data previews were released in 2024–2025, with first cosmology data expected in October 2026. NASA/Euclid mission overview; NASA release, 2025; NASA update, 2025. (nasa.gov)
• –Nancy Grace Roman Space Telescope: NASA’s Roman mission (planned launch by May 2027) will conduct wide-field infrared surveys using SNe Ia, weak lensing, and clustering to probe dark energy, complementing Euclid in depth and methodology. NASA Roman dark energy page. (science.nasa.gov)
• –DESI: A ground-based multi-object spectroscopic survey measuring BAO and redshift-space distortions with high precision; first-year cosmology results appeared in 2024, with expanded analyses continuing. DESI/Berkeley Lab news, 2024. (desi.lbl.gov)
• –Vera C. Rubin Observatory (LSST): A 10‑year optical survey designed in part to constrain dark energy through weak lensing, supernovae, and large-scale structure; construction milestones in 2025 included installing the 3.2‑gigapixel LSST Camera, with first images in mid‑2025 and survey start targeted months thereafter. NSF news, Mar 12, 2025; Rubin monthly status, 2025. (nsf.gov)

Relation to other cosmic components

Dark energy is distinct from Dark matter, which clusters gravitationally and does not produce acceleration. Observational programs exploit their complementary effects—dark matter’s role in structure formation and dark energy’s impact on geometry and growth—to infer the composition and dynamics of the universe. NASA Science. (science.nasa.gov)

Notation and standard model context

In ΛCDM, parameters include Ω_m (matter), Ω_Λ (dark energy), H₀ (Hubble constant), and the spectral parameters of primordial fluctuations. The base model assumes spatial flatness and w = −1; combined CMB, BAO, and SNe data remain consistent with this framework while enabling precise tests for deviations such as w ≠ −1 or evolving w(z). Planck Collaboration, A&A 2020; Brout et al., ApJ 2022. (aanda.org)