In case you’re not up to speed on this, one of the stranger discoveries made by the Hubble Space Telescope is that the expansion of the universe is accelerating. According to everything we know about physics, this is impossible because the only force which operates over large (astronomically speaking) distances is gravity, and gravity only ever attracts. Gravity should be pulling the universe back towards itself, so the expansion should be slowing down. Nevertheless, the data show acceleration which means that there’s a gap in our theories. This missing ingredient, the mystery force that’s pushing the universe apart, is what we call Dark Energy.
The two most popular ideas for what dark energy actually is, are that there is a fifth force (in addition to the four we currently know: Gravity, Electromagnetism, Strong nuclear force and Weak nuclear force), or that gravity itself behaves differently to how we thought. This second theory is interesting, because it suggests that our current understanding of gravity (as explained by Einstein in his General Theory of Relativity) is incomplete. It suggests that we need to modify Relativity, in a similar way to how Einstein added to Newton’s Laws. This modified theory states that Gravity becomes weaker over greater distances (as currently accepted), but that if the distance becomes great enough then it eventually stops pulling and starts pushing – it becomes anti gravity.
In contrast, the more common understanding of Dark Energy is that it behaves like the Cosmological Constant which Einstein originally included in his General Relativity equations as a way to prop up the universe. Back in 1916 when Einstein first published the theory, nobody knew that the universe was expanding. The big bang theory was still decades in the future. The big mystery was what was keeping the stars and galaxies in place without them all simply falling into each other, and Einstein proposed the a constant, universal force (represented by his cosmological constant) as a possible solution. He later referred to it as the biggest blunder of his career, but Dark Energy has led many to consider that maybe he got it right after all, even if only by accident.
So how do we find out which theory is right? NASA’s Galaxy Evolution Explorer (GALEX) is an orbiting telescope designed to study distant galaxies in ultraviolet wavelengths. It has been studying the formation of galaxies so distant that we’re seeing them as they were seven billion years ago. A team of scientists using data from both GALEX and the Anglo-Australian Telescope have spent the last five years studying over 200,000 distant galaxies in the hope of determining which, if either, of these competing theories is correct. By carefully searching each of those galaxies for various types of object which shine with characteristic brightness (known collectively as Standard Candles), they have managed to map these galaxies out quite accurately. By also noting their redshift (using similar techniques to those which first discovered the expansion of the universe), they’ve calculated the velocity of each galaxy as it recedes from us. The end result has been a new map of the universe which shows us with unprecedented accuracy just how the universe is expanding.
And what was the point of all this hard work? Well each of the theories we discussed above is able to make specific predictions about the universe’s expansion. To disprove a theory, all that’s needed is to show that the predictions do not match up to reality. And the data show that the modified gravity theory simply doesn’t work. So for now, it looks as if Einstein’s careless blunder has ended up solving the problem that he never knew existed. Dark Energy appears to be a vanishingly small universally constant force after all. The cosmological constant can return to its original place in our textbooks.