What is the nature of dark energy?
Observations of distant supernovae in 1998 showed that cosmic expansion is accelerating. The unknown cause — about 68% of the universe's energy budget — is called dark energy, but we do not know whether it is a cosmological constant, a dynamical field, or a sign that general relativity fails on large scales.
Why it is hard
The cosmological constant expected from quantum vacuum energy overshoots observations by up to 120 orders of magnitude — the worst prediction in physics. Distinguishing a true constant from slowly evolving “quintessence” requires percent-level measurements of expansion history across billions of years.
Current evidence
Type Ia supernovae, the CMB acoustic peaks measured by Planck, and baryon acoustic oscillations all point to w ≈ −1. DESI's 2024–2026 results hint at mild evolution of w with time, but the tension with a pure cosmological constant is not yet decisive.
Possible approaches
Precision wide-field surveys (Euclid, Rubin/LSST, Roman, DESI) mapping expansion and growth of structure; improved supernova calibration; testing gravity on gigaparsec scales; and theoretical work on vacuum energy, modified gravity and holographic scenarios.
Related topics
Sources
- Dark Energy — NASA Science
- Planck and the cosmic microwave background — European Space Agency
- DESI DR2 Lyman-alpha Results and Cosmological Constraints — Dark Energy Spectroscopic Instrument
- Planck Science Highlights — European Space Agency