When two dead stars collide in a distant galaxy, two messengers bring us the news: a shiver in space, a gravitational wave, and a flash of light. Both messengers race toward us across the entire Universe, but their paths can differ slightly. Light sometimes gets delayed by cosmic dust, while gravitational waves pass through everything almost without noticing obstacles.
The authors of the study found a way to cross-check the accounts of these two messengers. They derived simple mathematical rules that connect distances measured by light and by gravitational waves. The rules act like a lie detector: if both methods give consistent results, our picture of the world is correct. If a discrepancy appears, it means we are missing something about how the cosmos works.
What's especially interesting is that the rules do not depend on whether our Universe is flat or slightly curved, like the surface of a giant soap bubble. Astronomers usually assume the cosmos is flat for simplicity, but this assumption can introduce errors in calculations. The new formulas allow us to bypass this assumption and test whether the world is actually curved. It's as if a cartographer learned to draw an accurate map of the terrain without knowing in advance whether they are standing on a plain or a hillside.
The gravitational wave from a neutron star merger detected in 2017 traveled 130 million light-years, and by the time it arrived, its amplitude had diminished so much that LIGO's detectors picked up mirror displacements a thousand times smaller than a proton's size.
In Carl Sagan's novel "Contact," an alien signal contains instructions for building a machine that uses gravitational waves for travel — here, we use them as another way to "hear" the cosmos.