Physicists Resurrect an Old, ‘Strange’ Dark Matter Theory

Dark matter might not be nearly as exotic as most theories about the stuff suggest. Instead, it could be macroscopic clumps of material formed from common particles already found within the Standard Model of particle physics. This argument comes courtesy of physicists at Case Western University, as presented in a new paper posted to the arXiv pre-print server.

Dark matter is usually thought of in terms of exotic, so-far undiscovered particles. The leading candidates are known as weakly interacting massive particles, or WIMPs. This is where most of our dark detection efforts are focused, but a small handful of projects are also hunting for "hidden light" particles called WISPs, or weakly interacting slim particles.

Both varieties of particle are characterized by a disinterest in the fundamental forces of nature. WIMPs feel only gravity and the weak force (which drives nuclear decay), while WISPs feel gravity and, just the tiniest bit, electromagnetism (light, thermal energy, etc.). In the absence of these interactions, both sorts of particles behave as sorts of ghosts, existing but not existing.

These particles, while refusing to interact with photons (particles of light, e.g. the carriers of the electromagnetic force), add up to enormous masses. Together, dark matter makes up about 85 percent of all matter in the universe. This mass, acting as a sort of gravitational scaffolding, is what allows for the formation and persistence of galaxies. We live because of dark matter.

The catch is that we've never really detected dark matter, at least directly. We know it's out there because of its gravitational effects, but despite an impressive array of deep-underground detection experiments, we've yet to see an actual dark matter particle.

For those in the business of describing reality, this absence is alarming. For one thing, it provides fertile ground for alternative theories to grow. One example is known as MOND, for modified Newtonian dynamics. Basically, it says that there is no dark matter, and the gravitational effects we observe are merely the result of an ecstatic force of gravity. That is, Newton's equation for gravitational attraction changes dynamically with distance.

At first glance, the Case Western theory is almost as extreme. For one thing, it too suggests that there are no dark matter particles, at least none that exist outside of current knowledge. Instead, there are macroscopic (baseball-sized, say) clumps of "regular" matter formed from unexpected combinations of Standard Model particles. The physicists behind the current paper, led by CWU physicist Glenn Starkman, call this dark matter simply "macros."

The defining component of macros would be the strange quark, a highly unstable, extremely light variety of particle observed in high-energy collision experiments. (Quarks as a particle class are one of the fundamental constituents of matter.) Starkman and his team suggest that in the very early universe it may have been possible for these strange quarks to get together with more reasonable particles into stable nuclei of matter. They would have to do this with 90 percent efficiency to account for the dark matter we see in space, leaving the non-dark world with enough (but not too many) particle leftovers to form neutrons and protons.

"As pointed out, there is no experimental evidence for any particle candidate for the [dark matter] yet," Geoffrey Taylor, a physics researcher at the University of Melbourne, noted in an email.

"That some heavy dense objects with properties consistent with [dark matter] constraints, might be speculated is reasonable," he said. "There is no theoretical motivation for such objects. but a cursory look at the paper suggests this simple approach using only necessary constraints from experiment and theory give a range of possibly interesting candidate DM objects."

In order for macros to fit our view of reality, a few things would have to be true. The clumps would have to be more massive than 55 grams, or else they would have been observed in Skylab's strongly-interacting dark matter detectors. Macros would then have to be less than 1024 million billion billion grams, or else they would be massive enough to bend starlight.

This bending of starlight hasn't been observed. Possible masses for macros are further constrained by the indirect astrophysical history provided by sheets of mica buried several kilometers below Earth's surface.

"If the Macros have a low enough mass, their number density would be high enough to have plausibly left a historical record on earth," the current paper notes. "If they have a low enough [density] so that they would have penetrated deep (about a few kilometers) into the earth's crust, a record would have been left in ancient muscovite mica." No record has been found.

The role of density here is worth unpacking a bit. The density of a given dark matter candidate is given by a ratio of σX/Mx, where σX is a region of space (in which interaction might take place), and Mx is a mass. The standard dark matter models assume a very small space compared to the material's mass, with the result being very low densities and less interaction (weakly interacting). It's possible, however, to have strongly interacting dark matter if, instead of making the region of space very small, we make the mass very big.

This is intuitive: Adding a droplet of red dye to a glass of water and dumping a bucket of that same dye to a swimming pool might come up with a similar dilution, or dye density.

The dark matter macro theory isn't as out there as it might seem. In 1984, the astrophysicist Edward Witten proposed something similar: "dense, invisible quark nuggets."

"Many models that could be defined as Macros have been written down before, including Witten's nuggets of quark matter 30 years ago," Manoj Kaplinghat, a physics and astronomy professor at the UC Irvine not affiliated with the current paper, told me. "Glenn Starkman was part of a seminal effort in this direction back in 1990. The present article attempts to systematize the description of 'Macros' in terms of their mass and how strongly they interact with normal matter, so that viable models can be clearly identified.

"How interesting these viable models are depends on individual taste," Kaplinghat said, "but you can't argue with the motivation for looking into this, which is that we know very little about the nature of dark matter."