The universe's expansion is slowing down
The old claim turned out to be wrong and was rejected by evidence.
TaughtThe universe has been expanding since the Big Bang, and gravity is gradually slowing that expansion. The open question is whether it slows enough to collapse again.
NowTwo teams studying distant supernovae found in 1998 that the expansion is accelerating. The cause, called dark energy, appears to make up roughly 68% of the universe.
What actually happened
The textbook question used to be framed as a tug of war. Expansion pushes outward, gravity pulls inward, and the fate of the universe depended on which won: a Big Crunch, or an endless coast. Nobody seriously proposed a third option where expansion speeds up.
In 1998 the Supernova Cosmology Project and the High-Z Supernova Search Team independently measured distant Type Ia supernovae and found them dimmer, and therefore further away, than a decelerating universe allowed. The expansion was accelerating. Saul Perlmutter, Brian Schmidt, and Adam Riess shared the 2011 Nobel Prize in Physics for the result.
We still do not know what dark energy is. That is worth saying plainly: this is not a case where an old answer was replaced by a confident new one. It is a case where a confident old answer was replaced by a much better measurement and a much bigger question.
Newsletter
Get new weekly revisions
Sources
- The Nobel Prize in Physics 2011nobelprize.org
- NASA: Dark Energyscience.nasa.gov
Who was taught this
Still standard through 2000, so anyone who finished school between 1950 and 1998 learned the earlier version.