Looking ahead to the LISA gravitational-wave detector

That concept has already been proven by the LISA Pathfinder mission, which employed just one spacecraft and two mirrors to test the technology behind the larger LISA endeavor, Larson says. The mission’s goal was to determine whether changes between the two mirrors could be measured to the precision needed to ensure LISA detects gravitational waves. “It was enormously successful,” he says. “We exceeded what LISA needed to do. So we’re very excited. That was a huge boost for us. And so that’s really a lot of the impetus, together with the LIGO detections, it’s really got everyone pressing full steam ahead right now.”

LISA will detect gravitational waves in the range of 0.1 mHz and 1 Hz; by comparison, LIGO operates in the frequency range spanning 10 to 1,000 Hz. Just as light has several different wavelengths or frequencies, and each type of light (such as optical, infrared, and X-rays) tells astronomers something different, gravitational waves span a range of frequencies as well. LISA will be able to detect binaries shedding gravitational waves in wider orbits and with heavier masses than LIGO, opening up a new window on the universe to study objects such as white dwarf binaries and supermassive black holes. As soon as it’s turned on, LISA will detect a “hum” of sources in all directions, giving researchers a treasure trove of data that will illuminate many new and different aspects of the universe in which we live.

LISA is currently in Phase A, Larson says, which means contractors have taken a look at what the scientists want to do and will come back within about a year and a half with designs on how it can be accomplished. Once those designs are submitted, he says, the project will select one and continue moving forward. LISA is spearheaded by the European Space Agency (ESA), with NASA serving as a junior partner in the mission.

Picking apart a cosmic tune

If LISA will detect so many sources at once, how will astronomers ever separate them? It’s a bit, Larson says in his talk, like how one picks out individual voices in a room during a party. Amidst the background noise, you can easily focus in on the conversation you’re having because it’s happening nearby; astronomers will use several techniques to do the same, isolating signals against the background to hone in on a given source of gravitational waves.

One of those techniques relies on the Doppler effect. Consider a pair of white dwarfs or a white dwarf-neutron star pair circling each other, emitting gravitational waves. As Earth — and LISA — orbits the Sun, it will get nearer and farther from the pair. That will cause the pitch to fluctuate, higher as we get closer, and lower as we get farther. That pitch variation will allow astronomers to determine where on the sky that binary is located, helping them to map out the many, many white dwarf binaries expected to dot the Milky Way. (White dwarfs are the stellar remnants of Sun-like and smaller stars, which are common.)

Larson’s team also expects to hear supermassive black holes in merging galaxies collide. Those mergers will cause a sudden, loud uptick in gravitational waves — a characteristic “chirp” that should be fairly obvious against the white dwarf background. In that case, he says, it’s like picking out a particularly loud person in the room, whom you can always hear no matter where you are.