LISA, a $1.6 billion gravitational wave observatory scheduled to launch in the next decade, will revolutionize the way we see gravitational waves – infinitesimal perturbations of space-time first predicted more than a century ago and discovered only eight years ago.
LISA: simplicity and accuracy
LISA stands for Laser Interferometer Space Antenna and consists of three spacecraft orbiting the Sun in a fixed triangular formation. LISA is an interferometer, which means that the mission will sniff out gravitational waves using laser interferometry – measuring the distance between masses using incredibly precise laser beams about 5 million miles (8 million kilometers) long, with each side of the LISA triangle about 1.6 million miles (2.5 million km) long.
The lasers are crucial, but they are only part of the LISA design – they are merely measuring sticks for the distances between three metal cubes, one in each of the three LISA spacecraft. The cubes are made of a gold-platinum alloy to minimize the magnetism that can act on them. Again, the goal of LISA is to travel in space without touching anything but space-time and the gravitational waves that affect them.
“The basic idea of the project is that we are launching these cubes,” said Saavik Ford, an astrophysicist at the American Museum of Natural History. “We just want them to sit there and experience the joy of space-time without any other forces acting on them, and it’s that last part that’s the most difficult.”
“You have to maneuver the spacecraft when the masses [gold-platinum cubes] are falling to make sure that the spacecraft itself doesn’t drift into the masses and hit them, which would be terrible,” Ford added.
As for understanding the complexity of LISA, Ford’s then-graduate student Jake Postiglione draws an analogy: The technical challenge is like pointing a laser from New York to Los Angeles (if the Earth were flat) and trying to hit the eyeball of a fruit fly with it. Both the laser and the fruit fly are moving as this operation unfolds.
The scale of the engineering challenge is “frankly mind-boggling,” Ford said, “and I’m very glad it’s not my department.”
NASA is providing several elements of LISA’s instrumentation, including a laser system, telescopic systems, and devices that will control the electric charge levels on the test cubes.
The frequency of orbiting objects is determined by how often they complete a full orbit around each other. Our gravitational wave detectors are good at detecting certain frequencies for a variety of reasons, but all existing detectors have one major limitation: They are stuck on Earth.
A cosmic oracle for ancient black holes
Gravitational wave detectors differ in the types of orbital frequencies they detect. Ground-based detectors – namely the LIGO-Virgo-KAGRA collaboration – are excellent at detecting the high frequencies that correspond to smaller masses, such as black holes the size of stars. But when these masses get a little bigger-say, more than two hundred times the mass of our Sun-their orbital frequencies become similar to the noise our planet makes.
“There’s a frequency at which the Earth itself is so noisy that it’s already your problem,” Ford says. “You literally can’t do it. One way or another, you have to go into space.”
In space, pulsar synchronization arrays are a useful measurement tool for the largest black holes, although the Earth is still part of the equation. In this system, observatories on Earth track reliable flashes of light from rapidly rotating objects (pulsars); when the time it takes for the light to reach Earth is slightly delayed or accelerated, it indicates that space-time has been stretched or compressed by gravitational waves. In 2023, a team of researchers working with pulsar chronometers found convincing evidence of a gravitational wave background in pulsar data.
The black holes observed with pulsar chronometry are typically billions of times the mass of the Sun and are at the center of monster galaxies – they even outnumber the dwarf Sagittarius A*, the black hole at the center of the Milky Way, which has a mass of approximately four million solar masses. If black holes were porridge, LISA would be Goldilocks. The mission will sniff out low-frequency gravitational waves, which are almost indistinguishable from noise by ground-based detectors. Nevertheless, the space observatory can detect massive black hole mergers, when star-sized black holes turn into supermassive ones, as well as intimate double mergers of compact objects and other astrophysical flares and background phenomena.
“Pulsar synchronization arrays give us information about the stochastic background for massive double black holes at very low frequencies, and LIGO has basically set the limits for the velocities of different families of mergers of compact stellar-mass objects,” said Emanuele Berti, a theoretical physicist at Johns Hopkins University. “Thinking has changed in different ways, but I would say that the most interesting science we can do with LISA is centered around massive binary black hole mergers, because that’s something we just can’t study on the ground.”
Dodging noise in space
Although LISA will have much less interference in space than on Earth – ideally zero – the observatory will have to sift through cosmic noise. There are objects in the universe that make black holes much more difficult to observe because they also emit gravitational waves. The most annoying form of this compact interference is double white dwarfs: compact shells of former stars that orbit each other and eventually merge, mixing space-time like whisks in a mixer. The exception to this noise is when the binaries are so pronounced that they can be isolated and recognized for what they are. A cosmic double-edged sword, these “check binaries” will help astronomers confirm LISA’s capabilities once the mission is in place.
LISA will simultaneously detect noise from millions of sources, many of which are within our galaxy, according to NASA. Scientists will separate the wheat from the chaff with a huge amount of data processing and compare it with existing theories and models of known objects in the universe. With more than a decade to go until LISA’s launch, scientists are working on simulating data processing to prepare for the real thing.
Tracking space evolution
“There are really only two questions in astrophysics: “How did we get here?” and “Are we alone?” Ford said. “Every single thing we do is aimed at answering some little piece of one or the other, and sometimes both of those questions.”
“We don’t play the black hole game to answer the question ‘are we alone? “But the question ‘how did we get here?’ is quite important to understanding these black holes.”
Understanding the birth, life, and death of stars – and the role of these nuclear fusion furnaces in producing elements – is inextricably linked to the presence of black holes. Moreover, the types of stars formed by galaxies and the number in which they form can be related to the mass and behavior of the black holes in the cores of these galaxies. Black holes can be disorderly eaters – often regurgitating stellar material and spewing it into space – making them active participants in the evolution of the universe around them.
“There are several papers on so-called Little Red Dots that indicate that there are faint AGNs [active galactic nuclei – luminous galaxy nuclei powered by supermassive black holes] that probably originate from accretion of massive black holes,” Berti said. “All of this evidence once again indicates that massive black holes must have existed quite early in the history of the Universe. It’s always been a mystery, but now it’s becoming an even bigger mystery.”
The Webb Space Telescope’s observations of the Little Red Dots show the spots of light as they were when the universe was between 600 million years old and 1.5 billion years old. While recent research suggests that these dots are signs of the growth of previously hidden black holes in the early universe – and cosmological models are not “broken” as headlines suggest – LISA observations will help reveal the exact nature of the mysterious light sources.
LISA will observe the expansion of black holes and better characterize the array of compact objects in our Universe. This information can also be applied to existing cosmological models and dominant theories, such as Einstein’s general relativity. The data from the “ground truth” (so to speak) will be a convincing stress test for those ideas about the universe, one of which was confirmed when LIGO first detected gravitational waves in 2015. There are many known unknowns in the pitch-black vastness of space-time, but LISA scientists are determined to lift the veil – at least a little – on some of the most fundamental mysteries of the universe.
