Gravity pulls everything toward everything else, so after the Big Bang the expansion should be slowing down. In the 1990s two teams set out to measure how fast it was slowing, using type Ia supernovae billions of light-years away. In 1998 both found the opposite: the expansion is speeding up.
Why it works
A type Ia supernova's brightness gives its distance, and its redshift tells how much the universe has expanded since its light left. Comparing the two maps the history of the expansion. The distant supernovae looked fainter than expected in a universe that was slowing down, typically by about 20 to 25%. Fainter means farther ($b \propto 1/d^2$): a supernova at $75\%$ of the expected brightness is $\frac{1}{\sqrt{0.75}} = 1.15$ times as far. So the universe expanded more than expected since those explosions: the expansion has been accelerating for roughly the last 5 billion years. Something works against gravity on the largest scales. We do not know what it is, so we call it dark energy. The leading idea is an energy of empty space itself (Einstein's cosmological constant). The discovery won the 2011 Nobel Prize in Physics.
Key idea
Distant type Ia supernovae show that the universe's expansion is speeding up. The unknown cause, called dark energy, makes up about $68\%$ of the universe's energy; dark matter about $27\%$ and ordinary matter about $5\%$.
Worked example
Worked example: Fainter than expected
A distant type Ia supernova looks $75\%$ as bright as predicted for a slowing universe. How much farther is it than predicted?
- Same luminosity: $\frac{d_{\text{actual}}}{d_{\text{predicted}}} = \sqrt{\frac{b_{\text{predicted}}}{b_{\text{actual}}}} = \sqrt{\frac{1}{0.75}}$.
- $= 1.15$: about $15\%$ farther, so space has stretched more than a slowing expansion allows.
Answer: About $1.15$ times as far.
| Ingredient | Share of the universe (about) |
|---|---|
| Dark energy | 68% |
| Dark matter | 27% |
| Ordinary matter (stars, gas, planets, us) | 5% |
Common mistake
It seems obvious that gravity must be slowing the expansion, and for the first several billion years it was. But measurements of distant supernovae, the microwave background and the clustering of galaxies all agree that the expansion is now speeding up. Check the evidence: the far supernovae are dimmer, so farther, than a slowing universe allows.