What Is Dark Energy? A Name for Why Expansion Is Speeding Up

The measurement depended on standard candles, objects whose intrinsic brightness is known so that their observed brightness gives their distance. Type one-a supernovae serve because they explode at a characteristic mass and therefore with consistent energy, after a correction relating peak brightness to how fast the light fades. Two independent teams, one led by Saul Perlmutter and the other by Brian Schmidt and Adam Riess, measured distances to supernovae far enough away that the light had travelled for billions of years, and compared those distances with the redshift, which indicates how much the universe has expanded since the light left. The distant supernovae turned out to be fainter, and therefore further away, than an expansion slowing under gravity would put them. Both teams reached the same conclusion independently, which is why the result was accepted quickly despite being unexpected, and it won the 2011 Nobel prize in physics. Subsequent evidence from the cosmic microwave background and from the distribution of galaxies has confirmed it by entirely different routes.

Whatever drives acceleration must have an unusual property: negative pressure, meaning it pushes outward rather than resisting expansion, and it must not dilute as the universe grows, or the effect would fade. The leading candidates are few:

• •A cosmological constant, an intrinsic energy of empty space itself, which Einstein introduced in 1917 for a different reason and abandoned, and which fits the data extremely well with one free number
• •Vacuum energy from quantum field theory, which is the natural physical interpretation of that constant and which produces the worst quantitative prediction in the history of physics, overshooting the measured value by something like a hundred and twenty orders of magnitude
• •Quintessence, a dynamic field whose energy density changes over time, which would show up as an equation of state that varies with cosmic epoch
• •Modifications to general relativity at very large scales, which would remove the need for a new substance by changing the law of gravity instead
• •An error in the assumption that the universe is uniform on large scales, since inhomogeneity could in principle mimic acceleration, a possibility that has been examined and is generally considered insufficient
• •Measurements so far are consistent with a plain cosmological constant, and the current observational programme is essentially an attempt to detect any deviation from it

The accounting is unsettling. Combining supernova distances, the pattern of temperature fluctuations in the cosmic microwave background and the clustering of galaxies gives a consistent budget in which ordinary matter, meaning everything made of atoms including all stars, planets, gas and dust, accounts for around five percent of the total energy content. Dark matter, which is detected only through its gravitational effects and is likewise unidentified, accounts for roughly twenty-seven percent. Dark energy accounts for the remaining sixty-eight percent. The two dark components are unrelated despite the shared adjective: dark matter clumps and holds galaxies together, while dark energy is smooth and pushes space apart. Because dark energy does not dilute while matter does, its relative importance grows as the universe expands, which means it was negligible in the early universe, took over a few billion years ago, and will increasingly dominate.

Several large observational projects exist specifically to measure the expansion history precisely enough to test whether dark energy is constant. Surveys mapping millions of galaxies use baryon acoustic oscillations, a characteristic scale imprinted in the distribution of matter by sound waves in the early universe, as a standard ruler at different epochs. Weak gravitational lensing measures how mass along the line of sight distorts the shapes of background galaxies, tracking how structure has grown. Space telescopes are being flown with this as a primary objective. Early results from some of these programmes have hinted at a possible variation over time rather than a constant, which if it holds up would be the most consequential result in cosmology in decades, and which is not yet established. Separately, a persistent discrepancy between measurements of the current expansion rate from nearby and from early-universe methods, known as the Hubble tension, remains unresolved and may indicate that something in the standard model of cosmology is incomplete.

Two teams measuring distant type one-a supernovae found in 1998 that the expansion of the universe is accelerating rather than slowing, and dark energy names whatever causes it. It must exert negative pressure and must not dilute as space expands. A cosmological constant fits the data with one number, while the quantum vacuum interpretation of that constant overshoots by around a hundred and twenty orders of magnitude. It accounts for about sixty-eight percent of the universe's energy content.