A DISTANT supernova that appears to run in slow motion has produced the best
evidence yet that the Universe really is expanding. A study of light coming from
the galaxy confirms that the cosmological red shift is almost certainly due to
expansion, not some exotic 鈥渢ired light鈥� effect.
Doubts about the cause of the red shift have surfaced occasionally since its
discovery by Edwin Hubble and his colleagues at the end of the 1920s. Hubble
found that the spectrum of light emitted by distant galaxies was shifted towards
lower (redder) frequencies. The effect is analogous to the Doppler effect in
which the whistle from a receding train drops in pitch. The more distant the
galaxy, Hubble found, the larger this red shift. These observations have
generally been accepted as evidence that the Universe is expanding, and that
distant galaxies are receding fastest.
But a few doubters have argued that the redshift is a result of photons
dissipating their energy on their journey through space. If they are right, then
the red shift would still be an indicator of distance, but it would not imply
expansion and there would be no need for the Universe to have been born in a big
bang.
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The new proof of the expanding Universe draws on Einstein鈥檚 special theory of
relativity, which has been confirmed many times by experiment. If the Universe
is indeed expanding, then to an observer on Earth, time should appear to run
more slowly in distant galaxies. So to test the expansion idea, astronomers
simply need to watch a 鈥渃lock鈥� in some distant galaxy.
A supernova explosion provides an ideal clock. A supernova increases its
brightness to a peak, then declines again, following a 鈥渓ight curve鈥� with a
characteristic shape and duration. If the Universe really is expanding, a light
curve from a distant galaxy should have a similar pattern but appear slowed
down.
The principle is simple, but it has taken until now to make the extremely
precise observations of the light curve of a very distant, and therefore very
faint, supernova. Researchers from nine different centres around the world,
headed by Bruno Leibundgut at the European Southern Observatory, have pooled
their talents to measure and analyse the light curve from supernova 1995K, which
occurred in a galaxy at a red shift of 0.479. The shape of the light curve
exactly matches that of equivalent supernovae in our Galaxy, but it is stretched
in time by exactly the amount expected if the galaxy was receding from us at
nearly half the speed of light, as one would predict from the red shift (
Astrophysical Journal, vol 466, p L21).
Martin Hendry of the University of Sussex says the new study has 鈥渃losed an
important loophole in cosmology. Until now there has been no independent
verification that red shift is caused by Universal expansion. The paper is very
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When more supernovae have been analysed in this way, astronomers will get a
better idea of how fast the Universe is expanding. From the rate at which this
expansion is slowing down, they should also learn the density of the Universe.
However, this will take several more years of work, says team member Robin
Ciardullo of Pennsylvania State University.
