New measurements of exploding stars are challenging an upstart theory that dark energy is just an illusion caused by our location within a giant void.
In 1998, astronomers reported that the universe's expansion seems to be faster now than it was in the past, based on measurements of supernova explosions in both nearby and distant galaxies. The latter provide a record of the past because of the time it takes their light to reach us.
That the universe's expansion could be accelerating was a surprise, since gravity should act as a brake on the expansion, slowing it with time. The most popular explanation is that energy of unknown origin – called dark energy – permeates space and acts as a repulsive force to speed up the expansion.
But some researchers have proposed an alternative: that the acceleration is an illusion that results from an uneven distribution of matter in the universe.
They accept that the expansion rate in the local universe is higher than in more distant regions. But instead of assuming the expansion rate has increased with time, they suggest our patch of the universe happens to contain less matter than average. Within this "void", the expansion rate is higher than outside because there is less gravity to slow it down.
But new, more precise measurements of supernovae, taken by the Hubble Space Telescope, clash with the simplest version of the void model. That model could be made to fit previous supernova measurements and other cosmological data, but only if the local expansion rate is about 60 kilometres per second per megaparsec or less. (One megaparsec is 3.26 million light years.)
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