Dark energy is a mysterious component of the Universe causing its accelerated expansion. Although it constitutes the majority of the Universe's total energy density, its exact nature remains one of the greatest unsolved questions in modern physics and cosmology. Since its discovery in the late 1990s, through observations of distant supernovae, dark energy has changed our understanding of cosmic evolution and spurred intensive research both theoretically and observationally.
In this article, we will examine:
- Historical context and the cosmological constant
- Evidence from Type Ia supernovae
- Additional methods: CMB and large-scale structure
- The nature of dark energy: ΛCDM and alternatives
- Observational discrepancies and current debates
- Future prospects and experiments
- Concluding thoughts
1. Historical context and the cosmological constant
1.1 Einstein's "biggest mistake"
In 1917, shortly after the creation of the General Theory of Relativity, Albert Einstein introduced the so-called cosmological constant (Λ) in his field equations [1]. At that time, the prevailing belief was a static, eternal Universe. Einstein added Λ to balance the gravitational force on a cosmic scale and thus ensure a static solution. However, in 1929, Edwin Hubble showed that galaxies are moving away from us, which meant an expanding Universe. Later, Einstein, believing that Λ was no longer needed for an expanding Universe, called it his "biggest mistake."
1.2 Early hints of a nonzero Λ
Despite Einstein’s regret, the idea of a nonzero cosmological constant was not forgotten. In later decades, physicists considered it in the context of quantum field theory, where vacuum energy can contribute to the energy density of space itself. However, until the end of the 20th century, there was no strong observational basis to believe that the Universe’s expansion was accelerating. Therefore, Λ remained more of an intriguing possibility than a firmly established phenomenon.
2. Evidence from Type Ia supernovae
2.1 Accelerating Universe (1990s)
At the end of the 1990s, two independent groups — the High-Z Supernova Search Team and the Supernova Cosmology Project — measured distances to distant Type Ia supernovae. These supernovae are considered “standard candles” (more precisely, standardized candles) because their intrinsic brightness can be determined from their light curves.
Scientists expected that the expansion of the Universe was slowing down due to gravity. However, it turned out that distant supernovae are fainter than expected — meaning they are farther away than predicted by the deceleration model. A stunning conclusion: The expansion of the Universe is accelerating [2, 3].
Main conclusion: There must exist a repulsive “antigravity” force that overcomes cosmic deceleration — today widely called dark energy.
2.2 Nobel Prize recognition
These discoveries, which changed our understanding of the Universe, led to the 2011 Nobel Prize in Physics being awarded to Saul Perlmutter, Brian Schmidt, and Adam Riess for the discovery of the accelerating Universe. Thus, dark energy quickly went from a theoretical hypothesis to an essential component of the cosmological model.
3. Additional methods: CMB and large-scale structure
3.1 Cosmic Microwave Background (CMB)
Shortly after the discovery of supernovae, balloon experiments such as BOOMERanG and MAXIMA, and later satellite missions WMAP and Planck, provided very precise measurements of the cosmic microwave background (CMB). The data from these observations indicate that the Universe is almost spatially flat, i.e., the total energy density parameter Ω ≈ 1. However, both baryonic and dark matter make up only about Ωm ≈ 0.3.
Implication: When Ωtotal = 1, there must still be a component filling the remaining part — dark energy, constituting about ΩΛ ≈ 0.7 [4, 5].
3.2 Baryon Acoustic Oscillations (BAO)
Baryon Acoustic Oscillations (BAO) in galaxy distributions provide another independent method to study the Universe's expansion. By comparing the observed scale of these “sound waves” in large-scale structure at different redshifts, astronomers can reconstruct how expansion changed over time. Large sky surveys like SDSS (Sloan Digital Sky Survey) and eBOSS confirm supernova and CMB findings: the Universe is dominated by dark energy, driving late-time accelerated expansion [6].
4. The nature of dark energy: ΛCDM and alternatives
4.1 Cosmological constant
The simplest dark energy model is the cosmological constant Λ. In this model, dark energy is a constant energy density filling all space. This leads to an equation of state parameter w = p/ρ = −1, where p is pressure and ρ is energy density. Such a component naturally causes accelerated expansion. The ΛCDM model (Lambda Cold Dark Matter) is the prevailing cosmological model combining both dark matter (CDM) and dark energy (Λ).
4.2 Dynamic dark energy
Despite its success, Λ also poses many theoretical challenges, especially the cosmological constant problem, where quantum field theory predicts a vacuum energy density much larger than observed. This has prompted consideration of alternative theories:
- Quintessence: a slowly rolling scalar field whose energy density changes over time.
- Phantom Energy: a field with w < −1.
- k-essence: a generalization of quintessence with non-canonical kinetic terms.
4.3 Modified gravity
Some scientists, instead of acknowledging a new energy component, propose modifying gravity on large scales, for example, by applying f(R) theories, DGP brane models, or other General Relativity extensions. Although such models can sometimes mimic the effect of dark energy, they must also comply with stringent local gravity tests and data on structure formation, gravitational lensing, and other observations.
5. Observational discrepancies and current discussions
5.1 Hubble constant tension
With the improvement of Hubble constant (H0) measurement methods, a discrepancy has emerged. Based on Planck satellite data (extrapolating from CMB according to ΛCDM), H0 ≈ 67.4 ± 0.5 km s−1 Mpc−1, while local (distance ladder) measurement methods (e.g., the SH0ES project) find H0 ≈ 73. This roughly 5σ discrepancy may indicate new physics in the dark energy sector or other nuances not included in the standard model [7].
5.2 Cosmic shear and structure growth
Weak gravitational lensing studies, aimed at probing the large-scale structure of the Universe, sometimes show slight deviations from ΛCDM predictions derived from CMB parameters. Although these deviations are not as pronounced as the Hubble constant tension, they nevertheless encourage considerations of possible corrections to dark energy or neutrino physics and about systematic effects in data analysis.
6. Future prospects and experiments
6.1 Future space projects
Euclid (ESA): designed to perform large-scale measurements of galaxy shapes and spectra to better constrain the dark energy equation of state and large-scale structure formation.
Nancy Grace Roman Space Telescope (NASA): will conduct wide-field imaging and spectroscopy, studying BAO and weak gravitational lensing with unprecedented precision.
6.2 Ground-based studies
Vera C. Rubin Observatory (Legacy Survey of Space and Time, LSST): will create a map of billions of galaxies, measure weak lensing signals, and supernova indicators to unprecedented depth.
DESI (Dark Energy Spectroscopic Instrument): will record extremely precise redshift measurements of millions of galaxies and quasars.
6.3 Theoretical breakthroughs
Physicists continue to deepen dark energy models — especially quintessence-type theories that allow a varying w(z). Attempts to unify gravity and quantum mechanics (string theory, loop quantum gravity, etc.) may help better understand vacuum energy. Any definitive deviation from w = −1 would be a huge discovery, indicating truly new fundamental laws of physics.
7. Concluding thoughts
More than 70% of the Universe's energy appears to be dark energy, yet we still lack a definitive answer as to what it is. From Einstein's cosmological constant to the stunning 1998 supernova results and ongoing precise measurements of cosmic structure — dark energy has become a central part of 21st-century cosmology and a potential gateway to revolutionary physics discoveries.
Efforts to understand dark energy perfectly illustrate how the precision of the latest observations and theoretical insight intertwine. As soon as the new telescopes and experiments begin to provide even more detailed data — from ever more distant supernovae to detailed galaxy maps and especially precise CMB measurements — science will be on the threshold of new, significant discoveries. Whether the answer is a simple cosmological constant, a dynamic scalar field, or modified gravity, solving the dark energy mystery will irreversibly change our understanding of the Universe and the fundamental nature of spacetime.
Links and further reading
Einstein, A. (1917). “Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie.” Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften, 142–152.
Riess, A. G., et al. (1998). “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant.” The Astronomical Journal, 116, 1009–1038.
Perlmutter, S., et al. (1999). “Measurements of Ω and Λ from 42 High-Redshift Supernovae.” The Astrophysical Journal, 517, 565–586.
de Bernardis, P., et al. (2000). “A Flat Universe from High-Resolution Maps of the Cosmic Microwave Background Radiation.” Nature, 404, 955–959.
Spergel, D. N., et al. (2003). “First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters.” The Astrophysical Journal Supplement Series, 148, 175–194.
Eisenstein, D. J., et al. (2005). “Detection of the Baryon Acoustic Peak in the Large-Scale Correlation Function of SDSS Luminous Red Galaxies.” The Astrophysical Journal, 633, 560–574.
Riess, A. G., et al. (2019). “Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics beyond ΛCDM.” The Astrophysical Journal, 876, 85.
Additional sources
Frieman, J. A., Turner, M. S., & Huterer, D. (2008). “Dark Energy and the Accelerating Universe.” Annual Review of Astronomy and Astrophysics, 46, 385–432.
Weinberg, S. (1989). “The Cosmological Constant Problem.” Reviews of Modern Physics, 61, 1–23.
Carroll, S. M. (2001). “The Cosmological Constant.” Living Reviews in Relativity, 4, 1.
From cosmic microwave background measurements to Type Ia supernova observations and galaxy redshift catalogs, there is abundant evidence that dark energy exists. However, fundamental questions — such as its origin, whether it is truly constant, and how it fits with quantum gravity theory — remain unanswered. Solving these mysteries could open new paths in theoretical physics and provide a deeper understanding of the Universe.