Why does the Universe expand the way it does? Why does it accelerate? Einstein’s equations offer an explanation of gravity that works on the scale we know, but do they work on the grandest scales of space and time? Humanity now has a way to find out.
The General Theory of Relativity predicts the behaviour of gravity, and includes a term known as the cosmological constant. Einstein added this term to make the universe static and unchanging, as he believed it was. But when the expansion of the universe was discovered by Edwin Hubble, Einstein regarded it as ‘the greatest blunder of his career.’
Decades later, the discovery that the expansion of the universe was accelerating led to the concept of ‘Dark Energy,’ and it showed that Einstein’s great blunder was actually required for the equations to make sense. But this begs the question “Do the equations of general relativity actually apply to the large scale universe?”
Now, humanity has a way to test this. Scientists from the University of Edinburgh have shown that testing the speed of gravitational waves will show if Einstein’s relativity and dark energy are correct. Their calculations reveal that if gravitational waves propogate at the speed of light, relativity holds true and the cosmological constant represents dark energy. If they move slower, then Einstein’s theory must be revised, opening the door for new alternative theories of gravity that explain the large-scale acceleration of the universe.
It was only a year ago that LIGO confirmed the first detection of gravitational waves, and subsequent experiments could provide an answer to the dark energy question. After a century, we may soon have an answer to the last riddle of Einstein’s relativity.