Cosmic Expansion & Dark Energy

In 1998, two independent teams — the High-Z Supernova Search Team led by Brian Schmidt and Adam Riess, and the Supernova Cosmology Project led by Saul Perlmutter — published a result they had not been looking for. They had set out to measure how fast cosmic expansion was slowing down, taking it for granted that gravity would brake it over time. The data showed the opposite: the expansion is speeding up. Type Ia supernovae — exploding stars that all flare to nearly the same true brightness, making them reliable cosmic distance markers — appeared fainter, and so more distant, than a decelerating universe allowed. Some unknown component, soon labeled dark energy, was pushing the expansion to accelerate. The discovery won the 2011 Nobel Prize in Physics.

The universe is expanding: Hubble's law (1929) found that distant galaxies recede at a speed proportional to their distance. The expansion is intrinsic — galaxies are not flying through space so much as space itself stretching between them. General relativity ties the rate of that stretching, and whether it speeds up or slows down, to what the universe is made of: ordinary matter and radiation pull the expansion to a brake, while a cosmological constant (Einstein's Λ) — or any dark energy, an unknown something with a kind of negative pressure — pushes it to accelerate. The 1998 result implies that most of the universe's energy is in this accelerating form: roughly 70% dark energy, 25% dark matter (unseen mass felt only by its gravity), and 5% ordinary matter. The supernova evidence works because a Type Ia is a standard candle — an object of known true brightness, so its apparent faintness reveals its distance. Pair that distance with redshift — the reddening and stretching of a receding object's light, here caused by space expanding while the light is in flight — and you recover the whole expansion history. The acceleration has since been confirmed by several independent lines: the cosmic microwave background (the faint afterglow of the hot early universe), the regular spacing imprinted on galaxy clustering (baryon acoustic oscillations), the gravitational bending of light by intervening matter, and the growth of cosmic structure — all converging on the same ΛCDM model (dark energy plus cold dark matter plus ordinary matter). What dark energy is, nobody knows. The simplest guess is a constant energy of empty space — but quantum theory's naive estimate of that energy overshoots the observed value by a staggering factor of about 10¹²⁰, one of the widest gulfs between theory and observation in all of physics. The alternatives — a slowly evolving field (quintessence), or a change to gravity itself — remain unsettled.