LISA will measure ripples in space-time.

A Space-time Symphony

If you could hear the sounds of space and time, the universe would be a noisy place. When those bizarre, light-bending, space-curving, and time-warping objects—black holes, neutron stars, and white dwarfs—meet, mingle, and merge, they disturb the fabric of space-time, sending ripples of gravitational waves across the cosmos. But it’s not just black holes and their brethren that create these waves. The Big Bang itself, and maybe even more exotic objects called cosmic superstrings, all make their own undulations of space-time.

Although first predicted by Einstein’s theory of general relativity in 1916, gravitational waves have yet to be detected. While scientists hope ground-based observatories like the Laser Interferometer Gravitational-Wave Observatory (LIGO), run by Caltech and MIT, will identify a signal soon, detection is virtually guaranteed by the much-anticipated Laser Interferometer Space Antenna (LISA). LISA will aim at much lower frequencies than LIGO, and will be capable of detecting more sources. When launched, it will be the only instrument of its kind in space, a mission that will observe the universe as never before, listening to the cosmic cacophony that so far has been silent to us.

Gravitational waves are vibrations of space-time itself, and they jiggle everything they pass through, such as a planet or spacecraft—similarly to how sound waves jiggle the tiny bones in your ear, allowing you to hear. Unlike most telescopes, which point in a certain direction to detect signals, gravitational-wave detectors such as LISA measure waves from all directions, as an ear does. In this way, detecting gravitational waves is like hearing, and with so many potential sources out there, the trick is to figure out which is the black hole and which is the white dwarf. “It’s like listening to an orchestra and trying to tell which is the cymbal and which is the flute, or which is the first violin and the second violin,” says E. Sterl Phinney (BS ’80), professor of theoretical astrophysics, chair of the LISA Science and Sources Working Group, and the leader of the team that developed NASA’s Beyond Einstein program. LISA will address a myriad of topics, from the astrophysics of black holes to particle physics, to fundamental mysteries about the birth of the universe and the nature of gravity. In September, the National Research Council, which provides science policy advice for the government, recommended that LISA be made the flagship mission of the Beyond Einstein program.

Among the more promising phenomena the spacecraft will study is the merging of supermassive black holes. These events are some of the most violent and powerful in the universe, and likewise produce some of the strongest gravitational waves. When two of these behemoths meet, they spiral in toward each other. According to astronomers, nearly every galaxy has a supermassive black hole at its center, and when galaxies collide, the central black holes often merge—which can happen somewhere between once and 300 times per year.

Astronomers are finding that the evolution and formation of galaxies are inextricably tied to their merger history and to their central supermassive black holes. But since their black holes are always shrouded in gas, dust, and stars, scientists can’t directly observe them. Gravitational waves, however, zip through everything at the speed of light, and with LISA, researchers would be able to make the first direct observations of merging black holes. “They will tell us something very fundamental about how galaxies evolved,” says Tom Prince, professor of physics, the U.S. mission scientist for LISA and cochair of the LISA International Science Team.

LISA should also be able to detect a supermassive black hole eating a relatively tiny one, a few times the mass of our sun. But because the stellar-mass black hole is millions to billions of times smaller than the supermassive one, it works as what physicists call a “point test mass.” As the smaller black hole circles its giant partner, it follows every curve of space-time. The gravitational waves betray its path, telling physicists how space-time bends around the supermassive black hole. For the first time, physicists would find out if black holes behave as they think they do, Phinney says.

At the heart of LISA are the free-floating test masses like this one. Tiny shifts in distances between the test masses would mean a gravitational wave is passing through. The cubes’ polished surfaces reflect lasers between the spacecraft to measure the shifts.

Merging supermassive black holes could also serve as the most accurate yardsticks yet of the universe. A black hole binary system, in which two black holes orbit each other, loses energy as it produces gravitational waves. The strength of the waves reflects how much energy is lost. As the system loses energy, the two black holes spiral closer together, spinning around each other faster and faster, increasing the system’s orbital frequency. How quickly the orbital frequency changes tells scientists how fast the system is losing energy, which then tells them how strong the gravitational waves are. Just as light looks dimmer with greater distance, the strength of detected gravitational waves drops if the source is farther away. By comparing the measured strength of gravitational waves with the theoretical value, researchers can figure out how far away the system is. If the two black holes are coupled with an electromagnetic source, such as when the black hole eats surrounding gas and dust, LISA will make the most accurate measurements yet of the universe’s expansion.

Measuring cosmological expansion means measuring dark energy, the mysterious stuff that makes up roughly 70 percent of the universe. “LISA could revolutionize dark-energy studies,” Prince says.

Furthermore, gravitational echoes of the Big Bang give astrophysicists a powerful way to study the universe during its first second of existence. Conventional observations, by way of electromagnetic waves—light—only allow researchers to look back to when the universe was 300,000 years old. Before then, the universe was a hot plasma soup, too thick for light to pass through. But because gravitational waves can pass through the primordial soup, LISA may be able to reveal the universe in its infancy.

But wait, that’s not all. One of the more exotic gravitational-wave sources could be vibrating cosmic superstrings, long, one-dimensional objects that stretch across the universe. Waves on those strings, which were produced during the Big Bang, would move at the speed of light. They would flop around like a loose garden hose, creating gravitational waves, Phinney explains. If these strings exist and are detected, they would be a great discovery, he says. “It’s something of a long shot, but it’s a really exciting opportunity.”

While the science promises to excite and amaze, the spacecraft is a remarkable feat of engineering in and of itself. LISA consists of three identical spacecraft in a triangular formation. In order to detect the frequencies researchers want, the triangle has to be gigantic—five million kilometers per side, or the same distance you’d cover if you drove to and from Pasadena and New York about 1,120 times. Each craft holds two identical instruments, and each instrument encases a shiny, free-floating, four-centimeter cube that acts as a test mass. Laser beams that bounce between a cube in one craft and a cube in another form the three sides of the triangle. When a gravitational wave zips by, it shifts the distance between the test masses by a tiny amount. The laser beams also shift, giving scientists a measurement of the gravitational wave. The shifts in distance are so small that the instrument needs to be accurate to 10 picometers—smaller than any atom. Meanwhile, all this is trailing Earth by 20 degrees of its orbit around the sun, a distance equivalent to 25 million kilometers.

One of the biggest challenges engineers had to overcome was that of designing a spacecraft that would protect the test mass and keep it in its smooth orbit. Given the extreme sensitivity of the instrument, normally negligible effects such as the force from sunlight and the gravitational field of the spacecraft itself must be accounted for. One solution was to install microthrusters to counteract every inadvertent bump.

In 2010, the LISA Pathfinder mission will test this delicate ensemble. The mission, led by the European Space Agency and with JPL supplying the thrusters, will test the technology in a true zero-gravity environment. There’s no environment on Earth that’s as quiet as the space environment that LISA will experience, Prince says. So to make sure that researchers understand how the instrument works, they have to send a prototype into space.

The real LISA, a collaboration between NASA and ESA, won’t fly until 2018 at the earliest. The greatest hurdle so far, Phinney says, is whether NASA will provide enough funding. “The two big questions are when it will happen and whether the U.S. will have a major role in it,” Phinney says, noting that the U.S.—and Caltech in particular—has been a scientific leader for LISA over the past couple decades. “It would be a shame if the U.S. were to just drop out of it.”

Funding and politics aside, the science of LISA sells itself, drawing enthusiastic supporters, Phinney says. Scientists are confident the mission will eventually launch. When it does, scientists can finally tune in to the universe and its space-time symphony. —MW