How We’ll Explore the Gravity Wave Spectrum
published during a waning gibbous moon.

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Gravity Wave Spectrum

The gravitational wave spectrum, sources, and detectors. Credit: NASA Goddard Space Flight Center.

Radio, microwave, infrared, optical, ultraviolet, X-ray, and gamma—astronomers have used the electromagnetic spectrum to reveal a wealth of information about the universe. But did you know that gravitational waves—those wrinkles in the fabric of space-time that we’re only just now beginning to observe—also come in a vast range of wavelengths?

So you—yep you—are making gravity waves every time you go for a jog.

When the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the detection of its first gravity waves, scientists gained an initial toehold into this unexplored sector of the cosmos. In the coming years, both LIGO and other observatories will peer into ever wider portions of the gravitational wave spectrum. While we have some initial ideas about what they’ll see, the universe they’ll reveal remains largely unknown.

Albert Einstein showed that massive objects curve the fabric of space-time. Should those masses move, they will produce ripples in this cosmic cloth. As this informative LIGO explanation puts it: “Any object with mass that accelerates (which in science means changes position at a variable rate, and includes spinning and orbiting objects) produces gravitational waves, including humans and cars and airplanes.” So you—yep you—are making gravity waves every time you go for a jog.

Gravity Wave Spectrum

This artist’s conception portrays two neutron stars at the moment of collision. New observations confirm that colliding neutron stars produce short gamma-ray bursts. Such collisions produce rare heavy elements, including gold. All Earth’s gold likely came from colliding neutron stars. Credit: Dana Berry, SkyWorks Digital, Inc.

Relatively small objects like people and container ships don’t create much distortion. But gigantic things like stars, black holes, and galaxies will cause such a large warping of space-time that an extremely clever and careful experiment can see it. LIGO and other gravitational wave detectors are all essentially looking for a slight but periodic change in either space or time (which are intimately connected in the hopes of spotting a passing ripple.

LIGO searches for some of the smallest gravitational waves we can detect, those whose wavelength is about 1,000 kilometers. These are produced by neutron stars—which contain the mass of the sun squashed down within a radius smaller than a city—or black holes orbiting one another. Should these exotic objects crash together, they will release a gravity wave burst that travels out at the speed of light from its point of origin. As the waves pass by LIGO’s 4-km-long arms, they distort space by a fraction of the size of a proton, an effect that the detector’s super precisely calibrated lasers can actually see. Any tiny vibration, like a passing truck, could be mistaken for a gravity wave so LIGO consists of twin observatories on opposite sides of the U.S.—reducing the interference from noise.

LIGO’s first detection pointed to two stellar-mass black holes—one with 29 and the other with 36 times the mass of the sun—merging together. As the observatories discover more and more such events, researchers will learn how often they occur in the universe. The instruments are currently undergoing upgrades to make them 10 times more sensitive and will eventually be joined by a third detector in India, as well as other gravity wave experiments in Europe and Japan. These efforts should one day allow scientists to pinpoint exactly where in the sky the gravity waves are coming from, letting them do follow-up observations with optical telescopes.

Because Earth is a noisy place, many researchers hope to one day launch gravitational wave observatories into space. That’s the idea behind the European Space Agency’s Laser Interferometer Space Antenna (LISA), a set of three satellites flying in tandem that would monitor the distance between them down to less than the diameter of a helium atom to try and detect gravity waves. These would be coming from supermassive black holes, those weighing more than a million times the mass of our sun, that are thought to exist in the center of all galaxies. When these beasts merge together, they can produce huge ripples in space-time, at least a billion kilometers in wavelength.

What else might gravity wave detectors find? For the time being, nobody knows.

LISA has an unfortunately rocky history, originally being funded jointly with NASA. Though the American space agency pulled out in 2011, ESA has forged on, launching a precursor mission, LISA Pathfinder, late last year to test the technology. If everything goes according to plan, the full-scale LISA mission could launch in 2034 and open up a large part of the gravitational wave spectrum.

To see bigger gravity waves researchers will need to focus on a much grander scale, creating a gravitational wave observatory out of the local universe. That’s the idea behind a pulsar timing array, which precisely measures the signal from pulsars—highly-magnetized neutron stars that rotate more than a thousand times per second, emitting a beam of radiation that sweeps past the Earth like a cosmic lighthouse. If a gravitational wave distorts space-time between a pulsar and our planet, the distance between them will grow or shrink and the pulsar’s beam will arrive correspondingly late or early. By correlating such deviations among many different pulsars, astronomers could see gravity waves whose wavelengths are measured in light years.

This animation shows the collision and merger of two neutron stars. Merging neutron stars can create an event known as a short gamma-ray burst. Credit: Dana Berry, SkyWorks Digital, Inc.

These waves would be emitted after galactic collisions, when the central supermassive black holes of each galaxy would slowly orbit each another for thousands of years. Scientists think this process occurred many times in cosmological history but the details are murky—did galaxies of equal size come together or were giant galaxies more likely to gobble up smaller ones? The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is one collaboration working on answering such questions, and teams in Australia and Europe are also hoping to spot gravity waves from these events within a decade or so.

Finally, there are the largest gravitational waves we might one day see, those from the beginning of time. Emitted during an era known as inflation, when our universe expanded exponentially, these waves should have left a faint but discernable imprint on the Cosmic Microwave Background, a remnant glow from shortly after the Big Bang. A couple years ago, a team using the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) experiment thought they’d spotted this signal though later analyses showed their results were contaminated with noise from interstellar dust. But even more careful experiments are still hunting for these primordial gravitational waves, which could reveal a great deal about how our universe began.

What else might gravity wave detectors find? For the time being, nobody knows. Astronomers didn’t suspect the existence of pulsars before radio telescopes were invented. Just as with the electromagnetic range, the gravitational spectrum no doubt contains a great deal of strange and mysterious stuff just waiting for us to find it.