Spacetime and the cosmos
1998
The expansion speeds up
Supernova Cosmology Project and High-Z Supernova Search Team
Is the universe’s expansion slowing down?
Two teams measured distant exploding stars to find how much the universe’s expansion was slowing down. It turned out to be speeding up. Whatever drives it is called dark energy, and nobody yet knows what it is.
- From any galaxy
- speed = Hubble rate × distance
- Today’s measured rate
- about 70 km/s per megaparsec
Space itself stretches, carrying galaxies apart. From any galaxy you pick, every other galaxy moves away, and the further it is, the faster: Hubble’s law. There is no centre. (Real galaxies also have their own small motions, and nearby ones held by gravity do not spread out.)
How they figured it out
Everyone expected gravity to be slowing the expansion. Two rival teams set out to measure by how much, using a particular kind of exploding star, type Ia supernovae, which all reach roughly the same peak brightness.
The distant supernovae were fainter than expected: further away than a slowing universe allows. The expansion is speeding up. Both teams spent months checking their work before believing it. The leaders shared the 2011 Nobel Prize.
The equation
The rate at which the universe grows is itself growing.
See it in real life
A ball that speeds up as it rises
Throw a ball upwards and it slows. Imagine instead it sped up as it climbed. That is what the universe is doing, and the thing pushing it, dark energy, is the biggest unknown in physics.
Where engineers use it
Standard candles
Judging distance by brightness is used in lidar and optical rangefinders too.
Where it came from
- A ruler for the universe 1912
Same technique
Both judge distance by how bright a star of known power appears.
Where the comparison stops: The 1998 teams used exploding stars (type Ia supernovae), visible billions of light-years away, far beyond Cepheids’ reach, and calibrated with Cepheids nearby.
- The expanding universe 1922–1929
Led to
Measuring how the expansion changes over time revealed it is speeding up.
- Galaxies spin too fast 1970
Led to
Dark matter and dark energy together make up most of the universe; we see only about 5%.