Early this year, astronomers announced the first ever observations of a neutron star collision using both gravitational waves and its light in all the wavelength. This discovery has already had a huge impact in several different fields and it is now being used to better understand the expansion of the cosmos.

Observations of near and distant galaxies show that the universe is expanding at an accelerated rate. To explain this, astronomers have postulated the existence of a mysterious substance known as dark energy or a cosmological constant, which pushes everything apart. The lack of any concrete proof of the existence of dark energy has led to the formulation of many alternative theories to explain what we see and don't see.

The simultaneous observations of light and gravity from this neutron merger, however, can help cull some of those theories. This detection showed that gravity moves almost exactly at the speed of light (with a precision of 5 parts over 10 million billions) but not all explanations for the expansion of the universe allow that. The results are published in Physical Review Letters.

“Our results make significant progress to elucidate the nature of dark energy,” co-author Miguel Zumalacárregui, from the Lawrence Berkeley National Laboratory and UC Berkeley, said in a statement. “It’s really about the timing. The simplest theories have survived, the favorite explanation is this cosmological constant. That’s as simple as it’s going to get.”

The theories that are most affected are the ones that tend to modify Einstein’s theory of general relativity by changing how gravity interacts at large and small scales. They require differences between the speed of light and the speed of gravity, and without that, they don’t work.

While this is a comforting news for dark energy, it doesn’t mean that alternative theories are dead. Modifications could still allow them to explain phenomena we see in the cosmos. And at the same time, there are issues that are currently not addressed by the standard model of cosmology, which has dark energy as one of its cornerstones.

Two different measurements of the rate of the universe's expansion, known as the Hubble rate, have found slightly different values for it, making it quite the head-scratcher for researchers, but gravitational waves could help solve this mystery.

“Maybe we have underestimated some events, or something is unaccounted for that we’ll need to revise the standard cosmology of the universe,” Zumalacárregui added. “I’m really excited for the coming years. At least some of these nonstandard dark energy models could explain this Hubble rate discrepancy."

The gravitational wave observatories, LIGO and Virgo, will be back online in late 2018.