This week the Nobel Prize in Physics will be awarded to the leaders of two teams that found a crazy result in their study of the expansion of the universe. The award is for “the discovery of the accelerating expansion of the Universe through observations of distant supernovae.”
For almost a century, the Universe has been known to be expanding. The expansion has been measured by looking at the brightness of stars or galaxies – and using that to estimate their distances from us. And separately the speed with which they are moving can be worked out by determining the red-shift of spectral lines. The general rule – found by Edwin Hubble in the 1920s is that the further away a galaxy is from us, the faster it is moving away from us. This is regarded as a consequence of the Big Bang about 14 billion years ago. This relationship was studied in greater depth by two teams working in the late 1980s and 90s.
They used the very bright explosions of a particular type of star as a standard candle. The type Ia supernova that they used are ideal – they are an old compact star, as heavy as the Sun, but as small as the Earth. When they explode, they emit as much light as the whole galaxy that they are in. In the entire universe about ten type Ia supernovae occur every minute. But the Universe is huge. In a typical galaxy only one or two supernova explosions occur in a thousand years. So the teams took images of the sky three weeks apart, and looked for variations in the appearance of distant galaxies. Once a supernova was identified, other telescopes were used to get the brightness and the redshift of the star. This enabled the teams to measure how far away the supernova is and how fast it is moving away from us. Only supernovas sufficiently far away were used – 6 billion light years was far enough, this ensured that local variations in motion were not included.
What they found was utterly unexpected – these supernovas were a lot fainter than expected – indicating that they were much further away than thought. This implies that the universe’s expansion had speeded up. The acceleration is thought to be driven by dark energy, but as to what that dark energy is – well, it’s just not known. What is known is that dark energy is about three quarters of the Universe. And the findings of the 2011 Nobel Laureates in Physics have shown us a Universe that is unknown to science.