In Brief
A team of theoretical physicists are suggesting a theory that combines black holes, gravitational waves, and dark matter to discover a hypothetical particle called axions. These, the researchers said, might be discovered using the LIGO.

An Axiom for Axions

As much as our understanding of the universe has grown over the past decade, we are still a few missing of the threads that make up the interstellar tapestry. Among other things, we’ve yet to understand what accounts for more than 25 to 27 percent of the mass and energy and 80 percent of the gravity in our observable universe. This dark matter, scientists believe, could be made up of particles that remain undetectable and largely unknowable for us.

However, scientists have recently put forward a new model to help us understand these particles — and possibly to “hear” them as well.

The dark matter particles of interest here are called axions, which are hypothetical subatomic particles that are very light, electrically neutral, and have eluded scientists for more than 40 years now. A team of researchers from the Perimeter Institute for Theoretical Physics in Canada proposed a theory that can help explain what these are and locate them.

To do this, they combined several of the craziest concepts in physics — such as black holes, gravitational waves, and dark matter. “The basic idea is that we’re trying to use black holes…the densest, most compact objects in the universe, to search for new kinds of particles,” researcher Masha Baryakhtar told Gizmodo.

Tuning In and Listening

Of course, it helps that gravitational waves — a fundamental part of Einstein’s general theory of relativity — have been discovered and confirmed in 2016, providing a handle to work with. The theory Baryakhtara and colleagues developed, which was published in the journal Physical Review D, puts forward a way to understand black holes and axions by following a “gravity atom” model.

In this model, a black hole works like an atom with axions as electrons. Atoms and electrons interact via electromagnetism, whereas axions interact with a black hole via gravity. Axions jump around a black hole, gaining and losing energy and releasing gravitational waves. As the black hole rotates, it supercharges the space around it, producing more axions through what’s known as a superradiance effect. This could produce 10^80 axions — as many as the total number of atoms in the entire universe.

Now, with the estimated abundance of axions, the researchers believe it’s possible to actually hear the hum of the gravitational waves produced by this atom-like behavior of the black hole-axion system. “You’d see this at a particular frequency which would be roughly twice the axion mass,” said Baryakhtar said in the Gizmodo interview. To detect these, scientists can rely on the very instrument that detected gravitational waves in the first place, giant detectors called Laser Interferometer Gravitational Wave Observatory (LIGO).

These can be adjusted to tune in on axions. “With the current sensitivity we’re on the edge” of detecting axions, Baryakhtar explained. “But LIGO will continue improving their instruments, and, at design sensitivity, we might be able to see as many as 1000s of these axion signals coming in.”

Of course, this is still just theory, and theoretical physics is a tough field to work in. But instruments like LIGO are helping us understand the vast and largely unknown space outside of our world. Just as it’s helped discover gravitational waves, which previously existed only in theory, the LIGO may soon discover particles like axions. We may be entering a new era in physics.