The universe is expanding with acceleration — but why? Quantum vacuum energy predicts a value 10¹²⁰ times larger than what we observe. The worst prediction in the history of physics.
🌌 The Discovery That Shocked Even Its Discoverers
In 1998, two independent teams of astrophysicists tried to measure how fast the expansion of the universe was slowing down. Instead, they found the opposite. The Supernova Cosmology Project (led by Saul Perlmutter, from 1988) and the High-z Supernova Search Team (led by Brian Schmidt, with a crucial role by Adam Riess) observed over 50 distant Type Ia supernovae — standard “brightness candles.” The supernovae were 10–15% farther away than a universe without a cosmological constant would predict. The universe isn't slowing down — it's accelerating.
"The discovery came as a complete surprise even to the Laureates themselves. The acceleration is thought to be driven by dark energy, but what that dark energy is remains an enigma — perhaps the greatest in physics today."
— Nobel Committee for Physics, 2011In 2011, the Nobel Prize in Physics was awarded to Perlmutter, Schmidt, and Riess. The term “dark energy” was coined in 1998 by cosmologist Michael Turner. It constitutes 68.3% of the universe (Planck 2013 data), while 26.8% is dark matter and just 4.9% ordinary matter. The acceleration began roughly 5 billion years ago (z ≈ 0.4).
❓ The Worst Prediction in the History of Physics
The most obvious explanation is Einstein's cosmological constant Λ — the intrinsic energy of empty space. Einstein introduced Λ in 1917 to maintain a static universe, and reportedly called it "the biggest blunder of his life" after Hubble's 1929 discovery of expansion. After 1998, Λ returned triumphantly.
But here lies the problem. Quantum field theory calculates that the vacuum has energy due to quantum fluctuations (zero-point energy). But the predicted value is 50 to 120 orders of magnitude larger than what is observed. With a Planck cutoff, the discrepancy reaches $10^{120}$. This is known as the "vacuum catastrophe" — “the largest discrepancy between theory and experiment in all of science.”
📊 The Universe in Numbers
Observed vacuum density: $\rho_{\rm vac} \approx 5.96 \times 10^{-27}$ kg/m³ (Planck 2015). Theoretical prediction: ~$10^{8}$ GeV⁴ vs. observed $2.5 \times 10^{-47}$ GeV⁴ — 55 orders of magnitude difference (with consistent regularization, Jérôme Martin 2012).
🔭 DESI 2024 — Is Dark Energy Changing?
In April 2024, the Dark Energy Spectroscopic Instrument (DESI) published its first results from over 6 million extragalactic objects across 7 redshift bins (0.1 < z < 4.2), using baryon acoustic oscillations (BAO). With constant w, they found $w = -0.99^{+0.15}_{-0.13}$ — consistent with a cosmological constant.
However, in a time-varying model ($w_0, w_a$), the data prefer $w_0 > -1$ and $w_a < 0$ at 2.6σ significance (DESI + CMB). Adding supernova data pushes this to 3.9σ. In March 2025, updated analysis reached up to 4.2σ in combination. If confirmed, this would mean dark energy is weakening over time — roughly 10% less than 4.5 billion years ago.
🧩 Competing Theories
If dark energy isn't constant, what is it? Quintessence proposes a dynamic scalar field that varies in time and space. Phantom energy (w < −1) would lead to a Big Rip — tearing apart galaxies, atoms, and spacetime itself. Modified gravity theories alter General Relativity at cosmological scales, but the gravitational wave GW170817 (2017) ruled out many of them. In string theory, the landscape offers ~$10^{500}$ possible vacua with different cosmological constants — an anthropic solution that Steven Weinberg (~1987) had estimated.
🌍 The End of the Universe?
The Nobel committee in 2011 commented poetically: "Some say the world will end in fire, some say in ice. Most probably it will end in ice." If dark energy remains constant, galaxies will recede into total darkness. If it strengthens, Big Rip. If it weakens — as DESI hints — perhaps the universe will return to deceleration or even collapse (Big Crunch).
Dark energy is the point where quantum mechanics meets General Relativity — and the two fail to agree by 120 orders of magnitude. Whoever solves this problem will fundamentally change our understanding of the cosmos.