The expansion of the universe is slowing down under the gravitational pull of matter.

Not a fact anymore

The expansion of the universe is slowing down under the gravitational pull of matter.

What we know now

Observations of distant Type Ia supernovae show that the expansion of the universe is accelerating. The cause is described as dark energy, although its physical nature remains unknown.

Why it changed

Two research teams studying distant supernovae expected to measure how quickly cosmic expansion was decelerating. Their observations instead showed that distant supernovae were dimmer than expected, indicating accelerated expansion.

Status: Overturned
Category: Astronomy
Accepted approximately: Mid-20th century to 1998
Changed approximately: 1998

The expansion of the universe itself had already been established. What changed in 1998 was the expected rate of change: gravity-dominated models generally predicted deceleration, while the supernova observations revealed acceleration.

Dark energy is the name given to whatever produces this observed effect; it is not yet a complete explanation of the underlying physics.

Evidence

Sources and what they establish

Historical context

The Nobel Prize in Physics 2011: Press release The Nobel Prize
Explains that the teams expected expansion to slow but discovered that it was accelerating.

Current evidence

The Nobel Prize in Physics 2011: Popular information The Nobel Prize
Summarizes the supernova evidence and the modern dark-energy interpretation.
What is Dark Energy? NASA
NASA overview of the unknown component associated with the universe's accelerating expansion.