A decade ago, physicists using twin detectors in Washington State and Louisiana detected gravitational waves—subtle ripples in space-time—for the first time. The landmark discovery, confirming Albert Einstein’s century-old prediction of such waves, was recognized with the 2017 Nobel Prize in Physics.
Gravitational waves are created whenever massive objects undergo any kind of acceleration. In the case of those first detections, the signal was produced by pairs of black holes within our galaxy that had been orbiting one another and eventually merged. Although gravity holds us down on Earth, the force is actually very weak, and so gravitational waves only distort space by a very small amount. In the case of the black hole mergers recorded in 2015 by the gravitational-wave detectors, known together as LIGO (Laser Interferometer Gravitational-wave Observatory), the passing waves caused the arms of the L-shaped detectors, each 2.5 miles long, to stretch and contract by an amount smaller than the width of an atom. Physicists describe these as high-frequency, short-wavelength gravitational waves: They jiggled the LIGO detectors only very briefly, corresponding to the few seconds that it took for the black holes to merge in a final death spiral.
Now, astronomers have recorded the faint background hum from a different kind of gravitational wave. These are lower-frequency, longer-wavelength gravitational waves that appear to be coming from every direction in the sky. While the LIGO detections involved collisions between black holes only a bit more massive than the sun, the gravitational waves responsible for this background hum are thought to result from orbiting pairs of supermassive black holes millions of times more massive than our sun, scattered about the cosmos. The data also suggests the hum is slightly more intense in the Southern Hemisphere sky—one of several puzzles raised by the latest findings.
Read more on Smithsonian Magazine
