Dark Matter

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Dark Matter Hasn't Killed Anybody Yet — and That Tells Us Something
Mike Wall, Science & Astronomy | 26 July 2019

The lack of dark-matter wounds can help researchers home in on the mysterious stuff's nature.

A ghostly ring of dark matter floating in the galaxy cluster ZwCl0024+1652, one of the strongest pieces of evidence to date for the existence of dark matter. Astronomers think the dark-matter ring was produced from a collision between two gigantic clusters.(Image: © ESA/Hubble)

Nobody has stumbled into an emergency room with an inexplicable lightsaber wound, as far as we know — and that tells us something about dark matter, a new study suggests.

Dark matter makes up about 85% of the material universe, meaning it's about six times more abundant than the "normal" stuff that makes up stars, planets, people and everything else we're familiar with.

But nobody knows what dark matter actually is; the mysterious substance appears to emit no light, so it's incredibly hard to study. (Researchers infer dark matter's presence based on its gravitational influence on stars and galaxies.)

Physicists have come up with a number of dark-matter candidates, including hypothetical elementary particles such as axions, weakly interacting massive particles (WIMPs) and sterile neutrinos. But other theories hold that dark matter consists mostly of larger objects, with masses of a gram or more.

These theorized macroscopic dark-matter objects, or "macros," may be incredibly compact, with densities akin to those of atomic nuclei. If that's the case, there won't be all that many of them zooming through space — meaning macros may be very difficult to find.

"We'd need a very large or a very old detector to find these things," said Jagjit Singh Sidhu, a physics doctoral student at Case Western Reserve University in Cleveland, Ohio.

But scientists don't necessarily have to build such detectors; they're already out there, if you know where to look, Singh Sidhu reasoned recently. He and Case Western Reserve professors Glenn Starkman and Ralph Harvey determined that scientists could potentially spot evidence of macros in ordinary granite countertops.

Macros, if they exist, likely rocket through space at about 560,000 mph (900,000 km/h) relative to our solar system. One that happened to hit a countertop would leave a path of vaporization in its wake, which would be obvious even after the rock resolidified.

The three researchers wrote up their idea and posted the unpublished study on the online preprint site arXiv.org this May. The paper caught the attention of Robert Scherrer, who chairs the physics and astronomy department at Vanderbilt University in Tennessee.

"Within a few days," Singh Sidhu told Space.com, "Bob emailed me and Glenn, saying, 'Why don't we just use people [as detectors]? Because these things would really hurt if they hit a person.'"

Which brings us to the new study. Singh Sidhu, Scherrer and Starkman did some calculations, taking into account the speed and possible sizes and masses of macros. They determined that getting hit by one would be very unpleasant indeed.

In the study, which the researchers posted to arXiv earlier this month, they compared a macro injury to a gunshot wound. But that analogy isn't quite right, Starkman said.

Macro victims "would look like a Jedi knight has stabbed them with a lightsaber," Starkman told Space.com.

There have been no reports anywhere in the world of stabbings by invisible Jedi knights, as far as we know. And this tells us some interesting things, Singh Sidhu, Scherrer and Starkman said.

"We know the total mass density of dark matter in the universe — that's a very well-fixed thing," Scherrer told Space.com. Macro masses are unknown but correlate with their total numbers.

"If they have lower masses, they're more abundant," Scherrer said. "So, if we put an upper limit on their abundance — if we say, the number density can't be above this number or we'd be seeing dead people — that actually is a constraint on the mass. It says the mass has to be high."

Using such reasoning, the trio was able to characterize macros somewhat. Specifically, they ruled out the "large and light" parameter space for these hypothetical objects; macros that are wider than 1 micron or so and also lighter than 110 lbs. (50 kilograms) should not exist, the researchers calculated.

Singh Sidhu said he hopes the new paper, which has been submitted to the journal Physical Review Letters, raises the profile of macros as viable dark-matter candidates.

"I just want people to consider all of the options," he said, explaining that WIMPs and other hypothetical elementary particles currently draw the attention of most dark-matter researchers.

The novel detection strategies laid out in the new paper — and in the countertop study, which has been submitted to the journal Physical Review D — could also end up inspiring other scientists to think outside the box, team members said.

"It helps to expand our horizons in terms of what kinds of things we should be thinking about in constraining dark matter," Scherrer said. "There may be other unusual ideas out there that we haven't thought of that could put limits on either this model, or other models for dark matter. It shows that you should cast a wide net."