We Might Soon Be Able to See Missing Ripples in Spacetime

• Ripples in spacetime known as gravitational waves contain huge amounts of information, but they are incredibly hard to detect.
• Even with next-gen tech, it’s going to be very hard to separate a singular wave from the noise in the data.
• Experts say that a constellation of gravitational wave detectors might be the answer, and there are already two proposals on the table.

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Even in the vastness of space, sometimes things crash into each other. When two rocks do it, you can get a plethora of asteroids, or meteors. When a planet and an asteroid do it, you might form a new moon. But with something as big as a black hole merger, you get something remarkable: gravitational waves.

Gravitational waves are often referred to as “ripples in spacetime,” and they emanate from sources of immense energy like black hole mergers. First discovered in 1974—almost 60 years after Einstein first predicted them—they travel through the fabric of the universe at the speed of light, bringing with them information about where they came from and how exactly gravity works.

But even though we’ve been able to detect gravitational waves for a while now, it’s no easy feat. Since 2015, researchers have only been able to detect about 100 black hole mergers by spotting the waves they give off, even though it is estimated that there are hundreds of thousands of mergers every year.

Why can we see so few? Well, the rest of the waves have a habit of sort of blending into the background. They come together and meld into an almost completely flat, “blurry” backdrop that scientists call the stochastic gravitational-wave background (SGWB). In the SGWB, one wave is almost impossible to distinguish from any of the others.

So, researchers are looking to up their remarkably low hit rate. And they think they’ve found a way.

According to a new paper out of the International School for Advanced Studies in Italy (SISSA), creating a constellation of interferometers—instruments that gather information from ways waves interfere with each other—might do the trick.

“A constellation of space interferometers orbiting the Sun could enable us to see subtle fluctuations in the gravitational background signal, thus allowing us to extract valuable information about the distribution of black holes, neutron stars and all other sources of gravitational waves in the universe,” Giulia Capurri, a PhD student from SISSA and first author on the study, said in a press release.

Being able to spot those individual fluctuations, which researchers refer to as anisotropies, would allow us to truly map the SGWB and identify the massive sources of energy that create gravitational waves. But it’s going to be difficult, even for next-gen tech like the Laser Interferometer Space Antenna (LISA) and the Einstein Telescope. “Identifying [anisotropies] requires a very high level of angular resolution not possessed by current and next generation survey instruments,” Capurri stated.

However, the team argues in the paper that a constellation of several LISA-like detectors would be able to combine information from the cosmic microwave background and the SGWB to effectively isolate anisotropies. A group of devices like this would cover about the distance between the Earth and the Sun, and allow for significantly better resolution in the hunt for very subtle signals. Resolution—the ability to visually distinguish one object from another—is the whole ball of wax when it comes to separating a single gravitational wave from the noise of the SGWB.

Luckily, even though launching a gravity wave-detecting constellation is fairly far off, we might have options. Currently, there are two active proposals for a project like this—the European Space Agency’ Big Bang Observer (BBO) and the Japanese multi-institutional Deci-hertz Interferometer Gravitational-wave Observatory (DECIGO).

“Together with further similar projects whose details will be published in due course, [BBO and DECIGO] will be crucial for developing an optimal design for future observational instruments that we hope will be built and commissioned in the coming decades,” Carlo Baccigalupi, a professor of theoretical cosmology at SISSA, said in a news release.

Hopefully, a constellation of detectors will finally give us the key to unlocking the secrets of gravitational waves.