Theories Of Dark Energy, Gravity Pruned By First Neutron Star Merger Observation

Even as scientists try to solve the mystery of dark matter — by looking for hypothetical particles that could be the constituents of this mysterious substance which cannot be directly observed and yet accounts for over a quarter of all the mass-energy in the known universe — there is a bigger, related mystery always looming. Called dark energy, this phenomenon was theorized to account for the accelerating expansion of the universe and makes up over a third of all the universe’s mass-energy.

According to the standard cosmological model, dark energy acts as something like the opposite of gravity, which, if left to its devices, would have slowed down the expansion of the universe. But beyond that, there are only theories (that can’t be tested, given our current science and technology) to explain what exactly dark energy is, its properties and ways to measure them. However, the merger earlier this year between two neutron stars, the only event of its type ever observed, has ruled out some of those theories.

The collision and merger between the two neutron stars on Aug. 17 was observed using both the detection of gravitational waves as well as that of the intense burst of light that accompanied the event. Both the waves and the light arrived almost simultaneously at Earth, traversing the distance of about 130 million light-years, and that has implications for theories about dark energy and gravity.

For instance, there are some exotic and complicated theories about star mergers that posited a long time period gap between the arrival of gravitational waves and light from the event. A class of such theories, called scalar-tensor theories, would now either need to be done away with entirely or would have to be greatly modified.

Some other complex theories, on the other hand, such as the massive gravity theory — it assumes the existence of an elementary particle of gravity, called graviton — may also hold up to the measurements of the neutron star merger. That is, assuming the graviton exists, and also if it has a very slight mass.

But it looks like the favorite theory is one that has long been sidelined — that of the cosmological constant, introduced by Albert Einstein (who else?) over 100 years ago. This theory spoke of an inherent energy in space, more of which would be produced as more space came into being (through expansion), thus leading to the expansion accelerating.

“Our results make significant progress to elucidate the nature of dark energy. The simplest theories have survived. It’s really about the timing. The favorite explanation is this cosmological constant. That’s as simple as it’s going to get,” Miguel Zumalacárregui, a theoretical physicist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), said in a statement Monday.

Along with Jose María Ezquiaga, who was a visiting researcher at Berkeley Lab, Zumalacárregui published a paper on the impact of the neutron star merger (the event was named GW170817) on theories of dark energy and gravity. Titled “Dark Energy After GW170817: Dead Ends and the Road Ahead,” it appeared online Monday in the journal Physical Review Letters.