Cepheids

The top-ranked scientific justification for building Hubble was to determine the size and age of the Universe through observations of Cepheid variables in distant galaxies. This scientific goal was so important that it put constraints on the lower limit of the size of Hubble's primary mirror.

Cepheids are a special type of variable star with very stable and predictable brightness variations. The period of these variations depends on physical properties of the stars such as their mass and true brightness. This means that astronomers, just by looking at the variability of their light, can find out about the Cepheids' physical nature, which then can be used very effectively to determine their distance. For this reason cosmologists call Cepheids 'standard candles'.

Several groups of astronomers have used Hubble to observe Cepheids with extraordinary results. The Cepheids have then been used as stepping-stones to make distance measurements for supernovae, which have, in turn, given a measure for the scale of the Universe. Today we know the age of the Universe to a much higher precision than before Hubble: around 15 billion years.

Supernovae

Hubble's sharp vision means that it can see exploding stars, supernovf that are billions of light years away and difficult for other telescopes to study.

Most scientists today believe that the expansion of the Universe is accelerating. This result came from combined measurements of remote supernovf with most of the world's top-class telescopes, including Hubble, and it was a very surprising one. For many years cosmologists have discussed whether the expansion of the Universe would stop in some distant future or continue ever more slowly. From the new results it seems clear that the expansion is nowhere near slowing down. In fact, due to some mysterious property of space itself (called vacuum energy), the expansion is accelerating and will continue forever.

Hubble has given these supernovae measurements an added precision, mostly due to its high resolution. From the ground an image of the supernova usually blends in with the image of its host galaxy. Hubble can distinguish the light from the two sources and thus measure the supernova directly.

Gustav A. Tammann
Astronomer, University of Basle

"We certainly live in exciting times. Hubble has made enormous progress possible within cosmology. Today we have a much more unified cosmological picture than was possible even five years ago when people were talking of 'the cosmology in crisis'. We have seen a dramatic change from misery to glory!"

Bruno Leibundgut
Astronomer, European Southern Observatory (ESO)

"Hubble gave us the distance measurements of the first four supernovae that made us realise something was wrong with our present understanding of the Universe. Even though the definite proof that the Universe is accelerating came later, we could not reconcile our Hubble observations with a Universe where the expansion is slowing down."

The Composition of the Universe