Space Lasers Will Seek a New Kind of Gravitational Waves

Orbital observatories such as the Hubble Space Telescope and James Webb Space Telescope (JWST) can see far back into the universe—so far, in fact, that they have revealed some of the earliest galaxies to form in the first 500 million years of our universe’s 13.8-billion-year-history. Much further back, we can see the remnant heat left over from the big bang, the cosmic microwave background (CMB) radiation, which was emitted about 400,000 years into cosmic history. But what about the period after the CMB appeared and before the first stars and galaxies formed, deep in the so-called cosmic dark ages? Is it possible we could witness the birth of some of the first objects in the universe? The task is an almost impossibly tall order with Hubble and JWST: such objects are simply too small and faint. But with a new cadre of gravitational-wave observatories, detecting the stirrings of some early cosmic arrivals—specifically, black holes—should be a cinch.

The best news: those observatories are already being designed. “The era of gravitational waves has arrived,” says Chiara Mingarelli, a gravitational-wave astronomer at Yale University.

Last month the European Space Agency (ESA) approved the latest milestone in this era, a €1.5 billion ($1.6 billion) space observatory called the Laser Interferometer Space Antenna (LISA). Comprising three free-flying spacecraft each separated by 2.5 million kilometers in their orbits around the sun, LISA will search for a specific frequency of gravitational waves unseen by the current crop of ground-based detectors. These are the Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo collaboration in the U.S. and Italy—joined by the Kamioka Gravitational-Wave Detector (KAGRA) in Japan, although that observatory’s operation has had setbacks—as well as various pulsar timing arrays such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) in the U.S. and Canada, which made its first major definitive detection last year. LIGO and its ilk chiefly detect gravitational waves from black holes with masses near that of our sun at the point that they merge together. Pulsar timing arrays witness waves from the slow, deathly inspiral of much larger supermassive black holes, which lurk at the centers of galaxies like our own. But “we are missing the range in between,” says Nora Lützgendorf, lead project scientist for LISA and an astrophysicist at ESA.

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That’s where LISA comes in. Planned to launch as early as 2035, its three spacecraft will each contain two small, baseball-sized floating cubes made of gold and platinum. By firing lasers between these cubes—and between its three spacecraft—LISA will measure the tiny shifts in the distance between them that are produced by gravitational waves stretching and compressing space as they wash over our solar system. To make such delicate measurements requires something more than flashy lasers, though: each cube must freely float in space to achieve as close to perfect stillness as possible while whirling around the sun, protected against any conceivable perturbation from external forces such as the solar wind. “The spacecraft will shield from the adverse effects of deep space,” says Oliver Jennrich, also of ESA and a LISA project scientist. That will let LISA detect gravitational waves with a wavelength—the peak to peak of the incoming ripple—of seconds to hours, compared with milliseconds for LIGO/Virgo and years for pulsar timing arrays. “All three detectors are complimentary,” says Mingarelli, part of the NANOGrav consortium.

Such a wavelength is a window on merging supermassive black holes at the centers of galaxies with lower masses than those pulsar timing arrays are sensitive to; LISA should detect such behemoths from 10,000 to 10 million solar masses, compared with one “billion to ten billion solar masses” for pulsar timing arrays, which detect black holes harbored inside much larger galaxies than the ones LISA will observe, says Stephen Taylor of Vanderbilt University, NANOGrav’s chair. These arrays are unlikely to ever see the actual moment when two of these objects collide because inspiraling supermassive black holes can take millions of years to come together across their vast orbital separation. But LISA could observe the mergers of its target population of smaller black holes and witness their final moments weeks or even just hours before the mergers take place. Using LISA to monitor galaxies across the entire sky, “we expect a couple of mergers a year,” Lützgendorf says. “This is still uncertain because we do not know how often supermassive black holes merge in our universe. We have never probed this.”

LISA, with its compatriots, will allow us to understand the nature and number of black holes in the universe in a way no other form of astronomy can match. Because its measurements are at a smaller scale than those of the existing detectors, LISA will not witness the final mergers of its stellar-mass black holes, which formed from collapsing stars and orbit like stars inside a galaxy, but it will be able to pass the baton for this observation to LIGO and Virgo. It should be possible for such ground-based observatories to spot the telltale “chirp” as LISA’s targeted black holes spiral closer together and shift their emitted gravitational waves to shorter wavelengths where LISA’s view fades. “We can combine those two measurements and learn a lot,” Lützgendorf says—including better estimates of the masses of the merging black holes.

Illustration shows gravitational waves from the collision of two black holes reaching LISA. Accompanying diagram shows how LIGO uses laser beams to detect gravitational waves and explains how LISA will work similarly but with even longer arms to pick up smaller signals.
Credit: Moonrunner Design Ltd. (illustration); Jen Christiansen (laser interferometer diagram)

LISA may also witness the unions of intermediate-mass black holes, a mysterious class of these gravitational monsters that are larger than stellar-mass and smaller than supermassive black holes. So far scientists have struggled to detect and study them. Mergers of these mid-sized black holes may have been the seeds of their supermassive kin in the early universe, which could help explain how those giants grew so quickly. If intermediate-mass black holes existed in the early universe, Mingarelli says, “they’re going to be merging with each other, and LISA would see them.” LISA will be sensitive to other, never-before-seen events, too, such as the moments when smaller neutron stars or black holes fall into larger supermassive black holes and the delicate, deathly dancing of stellar corpses called white dwarfs as they co-orbit each other. “There should be many tens of millions of white dwarf binaries in the Milky Way,” says Shane Larson at Northwestern University, part of a NASA-led U.S. LISA team.

Objects that dance together, be they black holes or dead stars, throw out gravitational waves as they revolve in the fabric of spacetime. It is these ripples that LISA and others observe, with the mass of the objects dictating the wavelength of gravitational waves produced and thus defining which observatory will be able to spot them. We now know such binaries are common across the universe and thankfully so. Without them, gravitational waves would not be driving an ongoing astronomical revolution; our detectors are far from sufficient to witness the faint gravitational drum of singular black holes or other massive cosmic objects.

The nature of how LISA works means that it will see all the signals from all of these events together as they take place. “We expect constant waves” from thousands of sources, Lützgendorf says. The true challenge of the observatory aside from the engineering hurdle—which scientists already know they can surmount thanks to a precursor mission in 2015, LISA Pathfinder, which tested the free-floating-cube technique in space—is teasing out the waves carrying this multitude of signals from the sea of cosmic noise. With traditional telescopes that rely on mirrors to reflect incoming light into a camera, you “know which way a telescope is pointing,” Jennrich says. Gravitational waves, however, are all-encompassing—a steady flow of ripples emanating out in all directions from multiple sources. “So you don’t know where things are coming from,” Jennrich says. Careful triangulation via LISA’s triangular shape in space and its orbit around the sun will attempt to identify and localize sources.

LISA will glimpse events back to the first 200 million years of cosmic history. This may include not just mergers but perhaps evidence for long-theorized primordial gravitational waves left over after phase transitions in the universe following the big bang, essentially ripples in spacetime generated from the expansion of the cosmos itself. “If there really is a background of primordial gravitational waves, those signatures might lie in pulsar timing arrays and LISA,” Taylor says. “We should be able to do a multiband search. I’m very excited about that.” LISA could delve into new physics, too, and provide fresh tests for Albert Einstein’s general theory of relativity. That includes testing the “no-hair” theorem, the idea that black holes can only be defined by their mass, spin and charge and are otherwise impossible to discern from one another. “We can probe if this is actually holding up or if they have properties that would go against the theorem,” Lützgendorf says.

Scientists are understandably thrilled by LISA, but already plans are afoot for additional gravitational-wave observatories. In Europe discussions are underway to begin construction of the ground-based Einstein Telescope, a so-called third generation gravitational-wave detector that would surpass the size and capabilities of LIGO and Virgo. Pending the project’s approval, a selection of a site for the telescope is expected by 2025 or 2026, with construction targeted for completion circa 2036, says Michele Punturo of the Italian National Institute for Nuclear Physics (INFN), coordinator of the Einstein Telescope. Aside from detecting more mergers and binaries, the Einstein Telescope would probe much further back into the universe than any gravitational-wave observatory ever before—perhaps into the first 20 million years of the universe’s existence. “We could detect gravitational waves coming from an event that occurred before the formation of the first star,” Punturo says. The project’s quarry would include primordial black holes—thought to have formed from pockets of hot matter in the seconds after the big bang—as well as intermediate-mass black holes similar to those sought by LISA.

LISA may not stand—or float—alone in its space-based search for gravitational waves. China has plans for a LISA-esque mission of its own, called Taiji, to launch in the 2030s. How the missions might complement each other is something “lots of us have started thinking about,” Larson says. Having two space interferometers could lead to boosts in each facility’s sensitivity, particularly with regards to detecting primordial gravitational waves, and would also ease the considerable efforts required to triangulate and localize sources of other incoming gravitational waves. Another idea under consideration is to use the moon as a gravitational-wave detector. By placing a sensitive seismometer on the lunar surface—perhaps via one of NASA’s upcoming Artemis moon landings—scientists could monitor the moon’s reverberations from the incessant wash of gravitational waves. “The waves ring up the moon like a bell,” says Jan Harms of the INFN, a proponent of the idea. That detector would be sensitive to wavelengths between LIGO/Virgo and LISA, potentially revealing otherwise unseen events such as “merging white dwarf binaries,” Harms says.

Ultimately, LISA and other similar missions could pave the way for even more ambitious space-based detectors—such as a proposal called the Big Bang Observer (BBO), which is a sort of “super-LISA.” This would involve not one triangle of three spacecraft but three or more triangles flying in formation around the sun. The overlapping system would allow scientists to “find every single neutron star in the universe and every single binary black hole system” emitting gravitational waves to which BBO would be sensitive, says Neil Cornish, an astrophysicist at Montana State University. But the purpose of making that map would be to effectively wipe it from the sky to reveal a terra incognita arising from just one remaining source of gravitational waves: signals spawned from the big bang itself. “With LISA, we can’t remove all the signals well enough,” Cornish says. With the BBO, primordial gravitational waves would flow forth. The success of LIGO/Virgo, pulsar timing arrays and now the approval of LISA could put discussions of such a tantalizing mission on the table. “LISA provides a good starting point,” Cornish says. “I’m hoping that will start to happen now.”

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