Dark Energy

The Nobel Prize for Physics has just been offered to the three scientists, Saul Perlmutter, Brian Schmidt and Adam Riess, who found evidence in 1998 that the expansion of the universe was accelerating.

It was Edwin Hubble who first showed that the universe was expanding, and it was the Big Bang theory that provided an explanation. A consequence of the theory, however, was that the expansion would slow down due to the gravitational force between galaxies acting as a brake on their headlong flight.

Now that we have learnt that the rate of expansion is accelerating, we need to understand why. One answer suggests that the ballooning of space brings with it a mysterious dark energy which is an intrinsic property of that space. We call it dark because it only acts through the gravitational field and cannot manifest itself as light. It’s effect however is to inflate space further at an exponential rate which will eventually cause a doubling of the universe every 10 billion years.

The Nobel laureates started out by working within the orthodox model of a decelerating expansion and were seeking to establish an accurate measure of the rate. To do this they sought out type 1a supernova explosions in distant galaxies. These are formed from binary stars where one of the pair has collapsed into a degenerate dwarf which then sucks stellar material from its neighbouring star until it reaches 1.4 solar masses and goes supernova. The explosions are massive, spectacular and always release the same amount of light energy, which allows us to calculate their distance. The rate at which they are moving can then be determined from the redshift, a stretching of their light waves caused by their movement away from us.

By inspecting supernovae in galaxies seven billion light years away, the scientists discovered that their redshift was less than might be expected from their distance which could only be explained by their moving more slowly in the past. Searches have now been extended to galaxies ten billion light years away.

It’s Einstein who has the last laugh. In the cosmological equations that he derived from his theory of relativity, he included a cosmological constant which described the energy of space. This was abandoned in future work and Einstein famously regarded it as his biggest blunder. Now it appears that the cosmological constant describes Dark Energy and Einstein was right all along.

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