Gravitational wave 'memories' could help us find elusive cosmic strings

Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and "Space Radio," and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

Many theories of the early universe predict that the cosmos should be flooded with cracks in space-time, called cosmic strings, but no cosmic strings have been detected yet. Now, a new proposal suggests that we may need to look for them not through traditional astronomy, but through their gravitational-wave signature, which may persist in space-time long after the cosmic string has vanished.

When the universe wasn't even a second old, it underwent the most radical and transformative phase transitions of its existence — a feat not repeated even until the present day, billions of years later. These phase transitions completely reorganized the universe as the (hypothetical) unified force splintered off into the separate fundamental forces: gravity, the strong nuclear force, the weak nuclear force and electromagnetism.

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As the forces splintered off, the fundamental quantum vacuum energy of space-time realigned itself to support the new forces. But this realignment wasn't perfect. Think of water in an ice cube tray freezing into an ice cube. As the phase transition occurs, different regions of water can form ice crystals oriented in various ways. In some parts of the ice, the water molecules will line up in one direction to form a crystal; in other parts of the ice, the water molecules will line up in a different direction. No matter what, all of the water turns into ice when it's cold enough, but defects can appear between the domains of differing crystal arrangements.

You can see those defects as cracks and flaws in these ice cubes. But to see the cracks and flaws that developed in space-time, we have to look much harder. The defects that appeared during the early phase transitions of the universe can come in a variety of dimensions, but the most common is likely a one-dimensional fissure, known as a cosmic string (which is unrelated to the superstrings of string theory). 

Making waves 

Encountering a cosmic string would be a fearsome sight. It would be no wider than a proton, but it would pack an entire Earth's worth of mass in less than a mile. Because of the way a cosmic string would pull on the fabric of space-time, you could make a complete circle around one without having to travel a full 360 degrees. Cosmic strings also split the light coming from any background objects, making them appear double. Depending on how they interact with other particles and forces, cosmic strings may glow with intense radiation and high-energy particles. They would also likely stretch across the entire universe.

The best way to search for a cosmic string isn't by looking for the string itself, but for when they manage to tangle up with themselves or others. When that happens, loops of stringy stuff can form. These loops are incredibly unstable, vibrating like crazy until they emit enough energy that they disappear. 

Many theories of the early universe predict that cosmic strings should be ubiquitous. Indeed, cosmologists once thought the strings would be so common that they would form the skeletons of the largest structures in the cosmos. But survey after survey has turned up nothing — no double images of distant galaxies, no flashes of radiation when the loops vibrate themselves to death.

So a new paper recently posted to the preprint database arXiv.org suggests a new approach: Instead of looking for direct signs of cosmic strings, we should search for indirect signs, like the imprint they leave on space-time itself. 

Making a memory 

Loops of cosmic strings are incredibly massive and extremely unstable. That's a potent combo that can make a healthy quantity of gravitational waves, which are ripples in the fabric of space-time. But individual loops won't contribute much; current instruments don't have the sensitivity to detect a single vibrating loop the same way they can detect merging black holes.

But gravitational waves aren't exactly like water waves or sound waves. They don't just pass through space-time; they can permanently deform space-time, creating a background "memory" of their passing. Imagine every sound producing a tiny, permanent hum wherever it goes — that's what gravitational waves can do.

In the recent study, the astrophysicists explored the nature of the gravitational wave memory left by vibrating cosmic string loops. They found that the strongest waves were produced when cusps and kinks formed in the loops and that their memory would also be the strongest.

Because astronomers have failed to find evidence of any cosmic strings yet, theorists have long wondered if cosmic strings dissipated long ago, leaving no evidence behind. But if, instead, we search for their gravitational wave memory, we can detect the imprints of the cosmic strings even if they no longer exist.

But a key question remains: Can we detect these memories? Unfortunately, it's a little too early to tell. Our usual methods of understanding gravity tend to break down in the extremely strong environment near the cusp of a string loop, so we're not exactly sure what effects it might have on the surrounding space-time.

One possibility is that the gravitational energy released by the cusps immediately forms small black holes. In that case, there would be little gravitational wave memory left, since most of the energy would be redirected to the black hole formation. However, if all of the energy went toward forming gravitational waves, then their memories could be detectable with the next generation of detectors, like the space-based Laser Interferometer Space Antenna.

Gravitational wave memory is a key prediction of general relativity, and astronomers have been slowly developing the technology necessary to see these signatures. Finding the memories of cosmic strings would be a double bonus — a confirmation of Einstein's theory of general relativity and the first-ever detection of a remnant from the first second of the universe.

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