Mass Discrepancy: Is Dark Matter Hiding in Plain Sight?

The mass discrepancy drives the search for dark matter. Find out how a new approach could solve this riddle.

We all know that our Universe consists of matter and energy. Yet, one of the most daunting challenges of modern science is that cosmologists can’t account for about 95% of the matter and energy in the cosmos.

Cosmologists refer to the bulk of the matter and energy in the Universe as “dark.” They can tell that it’s there, but they can’t explain what it is in any detail.

In terms of dark matter (DM), when scientists observe the way stars and galaxies interact, such as rotation curves, they’re able to estimate the mass of disk galaxies based on the law of gravity. The other way to assess a galaxy’s mass is to determine roughly how many stars it contains and calculate the mass on that basis.

The problem is that the Answers Don’t Match

The problem is that these two answers don’t match. They aren’t even close. The difference in the stellar mass to light ratio is what cosmologists call the mass discrepancy.

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The mass of a galaxy, based on the gravity cosmologists observe, is about 30 times more than the mass they get based on the number of stars within it. So, some sort of mass must exist out there that humans aren’t able to perceive.

Astronomers have noticed this mass discrepancy for over a century. Lord Kelvin had detected it in 1884 when he told an audience, “many of our stars, perhaps a great majority of them, may be dark bodies.”

Fritz Zwicky Showed 400 Times More Gravity at Work

Other astronomers remarked on this variance as well. Still, cosmologists credit Fritz Zwicky with truly drawing it to the attention of the scientific world. His observations of the Coma Cluster of galaxies showed that there was 400 times more gravity at work than he could account for visually.

He referred to the mass discrepancy as “missing matter” or “dark matter.” Zwicky revealed a scientific puzzle that cosmologists have yet to solve. 

In the 1970s, Vera Rubin and Kent Ford used more sophisticated spectrograph technology. Their results showed that the mass of galaxies must be about six times more than we can account for from their visible mass. 

Dark matter Seems to Be 85% of All the Matter

From 1980 on, a range of confirming evidence has flowed in to support the idea that dark matter must exist. Since dark matter seems to represent 85% of all the matter in the Universe, understanding its composition is a priority for cosmology.

What kind of hidden matter would account for this mass discrepancy? All the other matter that we know about consists of protons, neutrons and electrons. So, it makes sense that dark matter would also consist of some new kind of particle.

This particle would have to have two properties. Since it causes these observed mass discrepancies, it must have a mass all its own. 

Weakly Interacting Massive Particles or WIMPs

Since it is imperceptible, it must not interact very much with the photons that make up electromagnetic radiation, like visible light or x-rays. So, scientists have named these hypothetical constituents of dark matter “weakly interacting massive particles,” or WIMPs for short.

There’s just one problem with this idea. Nobody has ever observed or detected anything whatsoever that remotely resembles a WIMP. Physicists have tried all sorts of exotic experiments to discover them directly or indirectly, but they’ve all failed.

Last week, the journal Physical Review Letters published a study from the Berkeley Lab and UC Berkeley. It suggests that the data needed to prove that dark matter exists may be hiding in plain sight. 

Experiments Already Conducted Could Contain the Evidence

The researchers are suggesting that the data points from experiments their colleagues have already conducted could contain the evidence that WIMPs exist. They believe that they could mine the results of earlier attempts to find either WIMPs or the mysterious tiny particles called neutrinos for the information they need.

Their research focuses on what happens when a hypothetical WIMP strikes the nucleus of an atom, which then absorbs the WIMP’S energy. They believe that if this happens, its protons and neutrons could eject electrons or neutrinos.

This absorption process would produce a signal that previous experimenters would have recorded, even though they were looking for other things. Sorting through the records of completed particle collector studies might deliver the explanation for the mass discrepancy for which everyone is looking.

“The Data is Already Basically Sitting There”

Jeff Dror is a postdoctoral researcher in Berkeley Labs’ Theory Group and UC Berkeley’s Berkeley Center for Theoretical Physics. As he explained, “The data is already basically sitting there. It’s just a matter of looking at it.” 

Dr. Dror and his team are calling for a paradigm shift around the mass discrepancy. Given that experiments to date have found nothing, the researchers suggest that their colleagues should rethink the basic assumptions about the nature of WIMPs and what indirect signals to research.

Dr. Dror put it this way, “It’s easy, with small modifications to the WIMP paradigm, to accommodate a whole different type of signal.” 

“Small Modifications to the WIMP Paradigm”

The team’s new approach would be to look for particles with much smaller masses than particle physicists had assumed that WIMPs would have. This method opens up a wide range of overlooked energy signals.

In their paper, the team describes “orders of magnitude of unexplored parameter space.” Although the team hasn’t proven this, these passed over absorption signals may hold more leads than those on which their predecessors have focused.

The big advantage of the Berkeley team’s proposal is that it doesn’t involve setting up an elaborate new experiment. Relying on existing data reduces the time, effort and cost involved in explaining the mass discrepancy.

“Huge Amount of Progress With Very Little Cost”

“You can make a huge amount of progress with very little cost if you step back a little bit in the way we’ve been thinking about dark matter,” according to Dr. Dror. This proposed study certainly wouldn’t be the first time in the history of science that thinking outside of the box produced a long-awaited discovery.

We all feel a need to understand our place in the Universe. It’s hard to tell each other that story when we can’t comprehend the nature of 95% of our world.

Dr. Dror is enthusiastic. “One of the biggest questions in the field is the nature of dark matter. We don’t know what it is made out of, but answering these questions could be within our reach in the near future. For me, that’s a huge motivation to keep pushing – there is new physics out there.”

We always have more to learn if we dare to know.
Learn more:
Berkeley Lab
Directly Detecting Signals from Absorption of Fermionic Dark Matter
Mass discrepancies in galaxies: dark matter and alternatives
The 5 Big Questions We Need Cosmology to Answer
Ancient Galaxies and Even Older Dark Matter

Are Diamonds Cosmology’s New Best Friend?


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