Fabian Schmidt in APS Physics:
But the theory has fallen short in one respect: explaining the finding that the Universe is expanding at an accelerating rate. According to GR, the sum of all known radiation, visible matter, and dark matter should exert an inward “tug” on the Universe, slowing down its rate of expansion over time. So to account for acceleration, physicists have been forced to consider three possibilities [1], all of which are often loosely referred to as “dark energy.” The first option—and also the simplest and most favored—is the existence of a cosmological constant, or vacuum energy, which counteracts gravity by exerting a constant negative effective pressure. The second imagines that the cosmological constant is actually dynamical. Finally, the third possibility is that gravity behaves differently on large distance scales, requiring a modification of GR. Using the recent detection of a gravitational wave and light from a distant binary neutron merger, four research groups have now placed some of the tightest constraints to date on this third scenario [2–5].
The extraordinary observation that made this work possible occurred on August 17, 2017, when the gravitational-wave detectors at the Advanced LIGO and Advanced Virgo experiments picked up a loud signal [6]. Within 2 s of the event, known as GW170817, an instrument onboard the Fermi gamma-ray satellite detected a short burst of gamma rays from a similar location in the sky [7]. Follow-up observations by telescopes across the globe confirmed that the gravitational wave and gamma rays came from the same source—a binary neutron star merger in the NGC 4993 galaxy, approximately 130 million light years away from Earth (see 16 October 2017 Viewpoint). The fact that the two signals traveled from such a great distance, yet arrived at Earth just a few seconds apart, implies that gravitational waves travel at the same speed as light to within 1 part in 10151015 [8]. Previous constraints on the relative speeds had only been at the level of 1 part in 5, so this single observation improved our knowledge of a fundamental property of nature by 14 orders of magnitude.
More here. [Thanks to Farrukh Azfar.]