Dark Matter Hurricane Gives Better Chance to Identify Axion Particles

Dark matter "hits the Solar system slap in the face"

Heads up, we’re in a hurricane we can’t see. Its superpower of invisibility comes from the fact that it’s made of dark matter.

And astroparticle physicists found an excellent use for this invisible space hurricane: solving the mystery of what makes up the mysterious substance. In a paper published November 7 in the journal Physics Review D, researchers from Universidad de Zaragoza, King’s College London, and the UK’s Institute of Astronomy predict higher chances of identifying dark matter by using detailed satellite data on the movements of the stars.

A Reminder That Humanity Is a Drop in the Bucket of the Universe

The grand total of normal mass observed by humanity — your cat, the sun, a Tide pod — makes up less than five percent of the universe. Roughly 68 percent is dark energy, and the final 27 percent is dark matter. Just dark space made of who-knows-what, potentially driving the accelerating expansion of the universe. Of the current theories, scientists favor Weakly Interacting Massive Particles (WIMPs) or axions as the mystery particle.

We’re basically swimming in the stuff, which scientists like Ciaran A. J. O’Hare, Ph.D., a theoretical physics postdoc at the University of Zaragoza and the study’s first author, call “dark matter wind.”

“The reason we use this phrase is that we’re embedded in a halo of dark matter and we’re rotating in the galactic disc (this rotating wheel of stars and gas), but the halo is very different,” O’Hare tells Inverse. “There is no disk of dark matter, no preferred rotation, just buzzing around in random directions.”

Basically, we may not know what the particle is, but since we know the direction we’re rotating, scientists like O’Hare can determine where dark matter should be coming from, which is where stellar streams — the remains of dwarf galaxies stretched across the sky — come in.

“Streams are really generic predictions of how we understand galaxies to form,” says O’Hare. Most streams are wispy and small, but a wealth of rich data collected by the European Space Agency’s Gaia satellite on the distance and velocity of over a billion stars gives researchers the details needed to identify streams that are hard to see with the human eye. Plus, scientists know these dwarf galaxy remains come with dark matter.

The S1 stream, identified thanks to Gaia, storms our way as a hurricane for two reasons.

“The really kind of remarkable thing we found about the S1 is that we’re not only sitting inside, but the direction we’re going is the opposite, we’re moving upstream.” explains O’Hare. “In testing for it, once we see dark matter, if the S1 stream is there, we can be extra sure the signal we’ve seen is dark matter because we can associate it with this object we can see out in space.”

How Do We Detect Dark Matter?

Despite the fact that dark matter slaps our galaxy in the face, detection proves a challenge. Experiments typically create a collision and measure the energy, light, or heat scatter. Previous experiments targeting WIMPs looked at nanometer-scale scatter, and the detectable energy window remained narrow. For clearer evidence of the stream, the group opted to look for different hypothetical particles, axions, under experimental conditions that allow them to look for millimeter-scale scatter (still tiny, but detectable) with a much higher chance of success.

Current experiments are trying to level-up in two directions, according to O’Hare. Some researchers are making the experiment as big as possible, to give particles more to interact with. On the other hand, exponentially more energy events for dark matter occur at lower energy levels, which require reading tinier and tinier signals.

Despite the challenges, the S1 stream exists at a promising crossroads. “The main thing I really like about the S1 stream is that it’s basically giving us another thing to look for,” says O’Hare. “It’s the interface of astronomy and particle physics. It’s what astronomy can tell us about particle physics, and what particle physics can tell astronomy.”

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