The Earth may be swimming through an ocean of… dark matter – and waves in that invisible ocean lapping against our planet’s upper atmosphere could generate detectable radio waves that could finally help us find this elusive part of the universe, according to new theoretical research.
A wealth of astrophysical and cosmological evidence points to the existence of dark matter, from the unexplained rotation curves of certain galaxies to the growth of the largest structures in the universe. Attempts to explain this wide variety of observations with alternative formulations of gravity have failed, so the vast majority of astronomers think that dark matter is an unknown form of matter that only rarely interacts with light or with normal matter.
But that’s a very broad idea that covers many possibilities. Dark matter can consist of solid particles, but searches for those types of particles have been largely unsuccessful. So an intriguing alternative is that dark matter is exceptionally light, either in the form of theoretical particles known as ‘axions’ or as an exotic form of photon that carries a bit of mass.
With that incredible lightness – millions of times lighter than the lightest known particles – dark matter could act in very strange ways. In particular, the dark matter would do that, instead of appearing as individual point-like bullets behave more like large waves that slosh through the cosmos.
In a recent research published on the preprint server arXiv, physicists examined models of ultra-light dark matter that was not completely dark, allowing it to interact extremely rarely with normal matter. Most of the time, these interactions barely registered and produced nothing noticeable. But on rare occasions, the dark matter and normal matter interacted enough to produce a significant amount Radio waves.
Related: A ‘dark mirror’ universe may exist in ours where atoms couldn’t form, new research suggests
This would happen when the dark matter has a plasma and when the frequency of dark matter waves aligned with the frequency of plasma waves. When this happened, a resonance would occur, amplifying the interaction and producing radiation in the form of radio waves, the team’s models suggested.
The universe is no stranger to plasmas – all stars spew plasma into space in the form of stellar wind – so theorists had already explored the production of radio waves due to the interaction of dark matter with environments such as the solar corona or the interstellar medium. But in this new study, the scientists discovered an interaction point much closer to home: our planet’s ionosphere.
Earth’s ionosphere is the thin, hot layer of the upper atmosphere and consists of a loose collection of ionized (charged) particles: a plasma. Of course, there are waves lapping through it, and the researchers found that those waves can interact with waves of hypothetical dark matter that might be washing over Earth.
The radio waves produced by this interaction would be barely noticeable. But the researchers found that by using a carefully tuned radio antenna to look for a specific frequency of radio waves over the course of a year, they might be able to detect these waves.
This idea is especially promising because Earth’s ionosphere offers several advantages over other sources of dark matter-produced radio waves. First, the ionosphere naturally reflects many radio waves from deeper space, making it relatively devoid of contaminating signals. Second, the ionosphere is right above us, easily accessible and already continuously monitored and studied.
It’s a long shot. This form of dark matter is highly theoretical and it would take years, if not decades, to perfect the observational technique for looking for these radio waves. But if it works, it would be a goldmine, allowing us to study one of the most mysterious elements in the universe right on our cosmic doorstep.