One of the leading ideas to explain dark energy is called the “cosmological constant.” This idea originated with Albert Einstein, who stated that this constant keeps the universe static in size rather than letting it contract due to gravity. Once scientists discovered in the 1920s that the universe is expanding, Einstein retracted the idea and called it his greatest blunder. With the finding that the expansion is accelerating, however, scientists reintroduced this type of modification. An implication of this model, that energy density is constant while the universe expands, is hard to accept because it implies something that repels instead of attracts like gravity, but the model does agree with all the data we currently have.
Another idea is that Einstein’s general theory of relativity, the basis for gravity, needs a modification. Gravity agrees with all the measurements made up until now; any change to the theory has to maintain that agreement. One way to test for modifications to gravity is to study the growth of clusters of galaxies since the Big Bang. Both the expansion of the universe as well as gravity influence the number and size of galaxy clusters; the former is pulling them apart while the latter is pulling them together into the clusters. The precision measurements required to study this are now just barely within our reach.
We have other ideas, too. We don’t even know if dark energy is a thing; it could be a property of space that we don’t yet understand how to think about. In any case, it is one of the great mysteries in science. Over the next decade, new projects such as the Dark Energy Survey, which will start collecting images in September 2013, will take the next step in unraveling the nature of dark energy.
Fermi National Accelerator Laboratory,