An orchestra of millions of instruments plays a single note, but each enters at its own moment and with its own volume. That's how the stochastic gravitational wave background sounds — a hum from black hole mergers that are individually indistinguishable. For a long time, it was thought that black hole pairs circle in nearly perfect circles. But orbits can also be highly flattened ellipses. The authors of the new work showed that then the overall 'noise' changes, and this can be heard.
When two black holes of stellar mass approach each other, they lose energy through gravitational wave emission. On an elongated orbit, waves are emitted especially strongly at the moments of closest approach. This is like the creaking of a swing: if a child sits still, the creak is steady, but with vigorous rocking, the sound becomes sharper and louder at the peaks. So here: eccentricity (scientifically, eccentricity) makes the gravitational hum noticeably louder at high frequencies.
The scientists built a model of the background for two cases. In the first, all pairs have the same, very high eccentricity — above 0.9 at an orbital frequency of 0.0001 Hz. Such a background is confidently distinguishable from the circular orbit background. In the second case, the eccentricity varies among pairs, distributed according to a thermal law. Here, everything depends on how early the pairs began to approach: if at a frequency of 0.00001 Hz, the background is almost like that of circular orbits, but if at 0.0001 Hz, the differences are already noticeable. This is important for the future space detector LISA, which will capture waves precisely in this range.
Another surprise: orbital eccentricity allows distinguishing pure evolution of a pair in vacuum from evolution in a dense gaseous environment. If there is a lot of gas around the black holes (density above one ten-millionth of a gram per cubic centimeter), friction with the gas further circularizes the orbits, and this is reflected in the background. By analyzing the hum, one can understand in what environment the black holes lived. Finally, if LISA registers such a background, it will provide an upper estimate of the eccentricity of orbits for the same pairs when they reach the frequencies of ground-based detectors. This will improve templates for searching individual signals. A short phase of eccentric orbits at the beginning leaves a long hum in the Universe's memory.
Gravitational waves from a single pair of black holes can be emitted unevenly: on an elongated orbit, the burst of radiation at the moment of closest approach is thousands of times more powerful than on the rest of the orbit.
In Carl Sagan's novel 'Contact', aliens transmit to humans the blueprints for a machine that uses gravitational waves. Real detectors like LISA might one day pick up not only natural hums but also artificial signals.