But for decades scientists have been searching for low-frequency gravitational waves, thought to be constantly rolling through space like background noise. "We now know that the universe is awash with gravitational waves," Michael Keith of the European Pulsar Timing Array told AFP. - Using dead stars as clocks -As gravitational waves travel through space, they very subtly squeeze and stretch everything they pass through. Some spin hundreds of times a second, flashing beams of radio waves at extremely regular intervals, like cosmic lighthouses. Keith said the "background hum of all these black holes" was "like sitting in a noisy restaurant and hearing all these people talking".