## Definition **Dark energy** is the name given to the unknown component of the universe responsible for the observed accelerating expansion of space. Detected indirectly through observations of distant Type Ia supernovae in 1998, dark energy appears to act as an anti-gravitational influence that, since approximately 5 billion years ago, has overcome the collective gravitational pull of all matter and caused the rate of cosmic expansion to increase rather than slow. It constitutes an estimated 68% of the total energy content of the universe, making it the dominant component of the cosmos by energy, yet its physical nature remains entirely unknown. ## Discovery The detection came from two independent teams studying Type Ia supernovae — thermonuclear explosions of white dwarf stars that have accreted enough mass from a companion star to exceed a critical threshold. Because all such events follow similar physical processes, they reach comparable peak luminosities; astronomers can use them as *standard candles* to determine distances across billions of light-years by comparing observed brightness with intrinsic brightness. In 1998, both teams found that supernovae at cosmological distances were dimmer than expected — meaning they were farther away than a decelerating universe would place them. The expansion of the universe was not slowing under gravity; it was speeding up. The discovery was awarded the Nobel Prize in Physics in 2011. ## Energy Budget of the Universe Combining observations of the cosmic microwave background, galaxy clustering, and supernova distances, cosmologists estimate the composition of the universe as follows: - **Dark energy**: approximately 68% - **Dark matter**: approximately 27% - **Ordinary (baryonic) matter and radiation**: approximately 5% The matter and light that humans can directly detect — stars, planets, gas, dust, everything in the [[Standard Model of Particle Physics]] — constitutes less than 5% of the total energy content. Dark energy alone outweighs all visible and dark matter combined. ## Candidate Explanations No explanation has been confirmed. The leading candidates are: **Cosmological constant ($\Lambda$)** — Einstein introduced a constant term $\Lambda g_{ab}$ into his [[General Relativity]] field equations in 1917 to produce a static universe (before Hubble's discovery of expansion). He later called it his "greatest blunder." In the modern context, $\Lambda$ can be reinterpreted as a constant energy density inherent to empty space — a vacuum energy that exerts negative pressure and drives acceleration. Quantum field theory predicts that the vacuum should have energy (from quantum fluctuations), but the predicted value differs from the observed value by approximately 120 orders of magnitude — one of the most notorious discrepancies in all of physics. **Quintessence** — A dynamical scalar field that varies in space and time, driving acceleration in a way that could change over cosmic history. Unlike the cosmological constant, quintessence is not fixed; its equation of state $w = p/\rho$ (pressure divided by energy density) could evolve. **Inflaton field link** — Galfard speculates that the inflaton field responsible for cosmic inflation in the very early universe may have re-activated approximately 5 billion years ago, becoming the agent of the current acceleration. If so, dark energy and the early inflationary epoch could share a common origin. ## Implications for the Cosmic Future If dark energy remains constant (cosmological constant), the universe will expand forever, with galaxies receding from one another until the observable universe becomes increasingly cold, dark, and sparse — the so-called *heat death*. If dark energy grows stronger over time, it could ultimately overcome even atomic and nuclear forces in a *Big Rip* scenario. The fate of the universe thus hinges on a component of reality that physics cannot yet describe from first principles. ## Related - [[General Relativity]] - [[Architecture of the Cosmos]] - [[Standard Model of Particle Physics]] - [[The Problem of Quantum Gravity]] ## Sources - [[The Universe in Your Hand (Galfard 2015)]]