accelerating expansion
Imagine raisins in a rising loaf of bread: as the dough expands, every raisin moves away from every other one. The universe expands like this, with galaxies as the raisins and space itself as the dough. The natural expectation was that gravity would slowly put the brakes on, easing the expansion over time. Accelerating expansion is the surprising discovery that the opposite is happening: the dough is rising faster and faster, and the galaxies are flying apart at an ever-increasing rate.
The discovery came in 1998 from two rival teams measuring Type Ia supernovae — exploding white dwarf stars that all reach nearly the same true brightness, making them excellent 'standard candles' for gauging cosmic distances. By comparing how bright these supernovae appear with how fast their host galaxies are receding (their redshift), the teams could trace the expansion's history. The distant supernovae looked fainter — and therefore farther away — than a decelerating universe would predict. The only way to fit the data was a universe whose expansion had switched from slowing down to speeding up a few billion years ago.
This result, which won the 2011 Nobel Prize in Physics, forced dark energy onto the cosmic stage and reshaped cosmology. The acceleration means a repulsive influence (dark energy) now dominates over the inward pull of all matter. It also implies a strange long-term future: if dark energy stays constant, galaxies beyond our local group will eventually recede so fast that their light can never reach us, and the distant universe will fade from view. A note of care: 'expansion' is space itself stretching, not galaxies hurtling through space, and there is no center from which everything flees.
In 1998 the Supernova Cosmology Project and the High-z Supernova Search Team independently found that distant Type Ia supernovae were about 25% fainter than expected in a decelerating universe — the telltale sign that cosmic expansion had begun accelerating around 5 to 6 billion years ago, as dark energy took over from gravity.
Faint distant supernovae revealed that the universe's expansion is speeding up, not slowing down.
Acceleration does not mean galaxies are being pushed faster through space; space itself is stretching at an increasing rate. And it does not contradict the big bang — the early universe did decelerate; dark energy only took over a few billion years ago.