Bringing WIMP Theory into Question: Is There Another Dark Matter Explanation?

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Weakly Interacting Massive Particles (WIMPs) are thought to dominate dark matter and huge efforts are under way to detect them. By their definition, WIMPs are massive theoretical particles, and they are very weakly interacting with normal matter. WIMPs are therefore notoriously difficult to detect, if they exist that is.

However, some physicists aren’t so confident that WIMPs are key to the hunt for dark matter. In a new study, two US researchers have re-opened the debate about dark matter, suggesting the bulk of it could be composed of heavier, strongly interacting particles, or possibly smaller, even more weakly interacting particles than WIMP theory. The physicists also go as far as suggesting that the Universe would be an even more interesting place where WIMP-less dark matter dominates…

“We know little about dark matter, since we can’t measure it directly,” said Jonathan Feng, a physicist at the University of California, Irvine. “But there are theories and models. WIMPs are attractive because they happen to appear in many popular theories of new particles and interactions. But what if there are other well-motivated possibilities for dark matter besides WIMPs?”

Feng, with co-author Jason Kumar, published a paper in Physical Review Letters called “Dark-Matter Particles without Weak-Scale Masses or Weak Interactions,” and the results have called into question the validity of focusing on WIMPs as the main component of the dark matter thought to make up the majority of mass in our Universe. The problem with dark matter, as stated by Feng, is that we cannot measure (observe) it directly and we therefore have little clue what it is. We know it’s there, the motion of galaxies and galactic clusters indicate a gravitational influence of something other than what we can see (i.e. luminous matter), but dark matter does not interact via the electromagnetic force, making it a particularly difficult entity to study.

There are strong theoretical reasons to believe WIMPs are at the centre of dark matter studies, but Feng and Kumar have composed models that suggest other weakly, and strongly, interacting particles can explain some of the phenomena we are observing.

“WIMPs are a very specific example of dark matter, but there is a broader class of particles,” Feng added. “We found that some of the models also predicted the right amount of dark matter for the universe, but with dark matter that was much more strongly or weakly interacting than WIMPs. We are wondering if almost-exclusive attention for WIMPs is really warranted.”

Indeed, a lot of attention is focused on WIMP theory, what if dark matter researchers are being blinkered by one theory at the detriment of a more subtle explanation? One of the key points raised is that there is strong evidence supporting dark matter candidates with a mass of around 1 GeV. This finding comes from the DAMA (Dark Matter) project at the Gran Sasso National Laboratories, Italy, that investigates the signal from possible dark matter interactions in the galactic halo. WIMPS are far bigger, with a mass of 100 GeV. Could this signal be from a far lighter weakly interacting dark matter candidate?

There are also suggestions from other research that strongly interacting particles are annihilating all the time, generating high energy photons that can be observed pervading the entire Universe. Although this is theory, Feng is optimistic about energetic photon experiments.

However, re-analysing the WIMP dominance over dark matter creates some interesting scenarios for the Universe. By considering WIMP-less dark matter, some rather exotic explanations begin to form.

“There are theories that there is a shadow world behind ours. It is a mirror world that is like ours, but doesn’t interact with ours,” Feng said. “With WIMP dark matter, that possibility is remote.”

“WIMP-less dark matter requires new forces that we don’t really know much about. If you could have evidence of this type of dark matter, it might be a hint that this shadow world exists.”

A shadow world may sound a little eccentric, but it is also built of viable theories just as the generally accepted WIMP theory is. This new study is certainly a reminder that dark matter is an unknown quantity, and researchers should be open to other particles and not just WIMPS…

Publication: Physical Review Letters
Source: Physorg.com