Astronomers analyzed the gravitationally lensed image of a distant galaxy to test the nature of dark matter.

Gravitationally lensed quasars
Each of these snapshots shows four distorted images of a background quasar surrounding the core of a massive foreground galaxy. The gravity of the foreground galaxy magnifies the quasar, an effect called gravitational lensing. HS 0810+2554, the system studied to test the nature of dark matter, is the Mickey Mouse-shaped one at upper right.
NASA / ESA / A. Nierenberg / T. Treu

An international team of astrophysicists has provided the most direct evidence yet that dark matter does not consist of ultramassive particles. The work, based on supercomputer simulations, suggests that instead dark matter is made up of axions — hypothetical particles so light that they travel through space like waves. If correct, it would not only reveal what 85% of matter in the universe is made of but could also lead to new physics beyond the Standard Model. The study is published in Nature Astronomy.

Dark matter is tricky to study because it doesn't emit, absorb, or reflect light. So instead, a team led by graduate student Alfred Amruth (University of Hong Kong) looked for the material’s influence on gravitational lensing.

In this effect, a galaxy and its massive dark matter halo curves the fabric of spacetime around it. Light from a more distant source follows this curvature and bends around the galaxy as if passing through a lens.

When the foreground lens and the distant light source are closely aligned, astronomers see multiple images of the same background object. The positions and brightnesses of these images depend on the distribution of dark matter in the lens, providing a powerful probe of the mysterious substance.

For the past two decades, astrophysicists have struggled to correctly reproduce the positions and brightness of these multiple images, because they’ve assumed that dark matter is made of weakly interacting massive particles (WIMPs). If WIMPS are dark matter, then galaxies’ density should decrease smoothly as you move out from the center. Except that's not what astronomers actually infer from the lensed images.  

Amruth's team turned to an alternative dark matter candidate instead: axions. These ultralight particles were first proposed in the 1970s to solve a problem in particle physics regarding the strong force. Quantum theory says axions travel through space as waves rather than particles, leading to random density fluctuations as the waves interfere with each other. These random fluctuations would make the distribution of dark matter around a galaxy bumpy. (It’s worth noting that neither WIMPs nor axions have been detected directly.)

After assuming that dark matter is made of axions, Amruth and colleagues were able to recreate the observed positions and brightnesses of the quadruply lensed system HS 0810+2554: a foreground elliptical galaxy that splits the light from a background galaxy into four images.

‘We have reached a point where the existing paradigm of dark matter needs to be reconsidered,” says Amruth. “Waving goodbye to ultramassive particles, which have long been heralded as the favored candidate for dark matter, may not come easily, but the evidence accumulates in favor of dark matter having wave-like properties as possessed by ultralight particles.”

However, Edward Hardy (University of Liverpool, UK), who was not involved in the research, urges caution. “This paper is only a first step and extensive further analysis is needed before the nature of dark matter is settled,” he says. According to team member George Smoot, the James Webb Space Telescope should discover many more gravitationally-lensed systems, offering a more stringent test of the idea.

If that future work cements the conclusions of Amruth's research, Hardy says it would be a spectacular discovery. “There would be major implications,” he says, “not just for astrophysics but also for high-energy fundamental particle physics more generally.”


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May 2, 2023 at 2:41 pm

HS 0810+2554, I dug into this a bit more. The NASA ADS Abstract has a 2002 report on this QSO lensing. Discovery of a new quadruply lensed QSO: HS 0810+2554 - A brighter twin to PG 1115+080,, January 2002. The cosmology calculators show comoving radial distance for z=1.5, space must expand faster than c velocity, 1.0209580E+00 or 1.02 x c velocity using H0 = 69 km/s/Mpc. The BB model does not explain the origin of DM (whether WIMPS, AXIONS).

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May 2, 2023 at 2:43 pm

This link should work.

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May 4, 2023 at 6:34 pm

> Quantum theory says axions travel through space as waves rather than particles,...

Maybe I missed the big change, but I thought one of the foundational principles of modern quantum theory was wave-particle duality. It's not either/or (or "rather than"), but both. So supposed distinction doesn't make any sense.

Also, if axions are supposedly such "ultralight particles" -- now they're particles and not waves -- there must be lots of them.

It's the mass-energy density that causes the spacetime curvature underlying lensing, not what form the mass or energy is in. If the missing mass was bricks or baseballs it could still be clumpy/lumpy.

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Monica Young

May 5, 2023 at 9:36 am

You're absolutely right, there is still wave-particle duality! My understanding is that axions are so light, they behave more like a wave than a particle, whereas WIMPs are heavy enough to behave more like particles than waves. The difference in their behavior is what effects the gravitational lensing. And yes, if dark matter is indeed made of axions (which this study does not prove), then they would have to be present in abundance. There's also, of course, the possibility that dark matter consists of multiple components, which would complicate the picture further!

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May 5, 2023 at 9:11 pm

I think that the question of the true nature of "Dark Matter" is not going to be resolved as long as Astronomers, Physicists, etc... are going to insist on the ancient view that matter, particles, etc. exist... They and their obsolete philosophies have to be updated... There is NO SUCH thing as "Particles," there is NO SUCH thing as "Matter" in the sense that they hang on to... "Dark MATTER" is an excellent example of Draconian thinking... "Particles" are really bundles of energy, involved in a dance of electric, magnetic... possibly gravitic fields... perhaps others... As far as "Dark Matter" goes, it will soon be found that it is actually an unimaginably numerous cloud of increasingly infinitesimal "bundles" of energy fields, common throughout the entire Universe... After all, what we call "normal matter" makes up only a very small part of the Universe, while these more mysterious "micro-particles" or tiny "bundles" of energy exist everywhere, and are so common and numerous that they, whether they be called "muons" or whatever, are ubiquitous and unbelievably numerous, that they comprise most of the observed... and unobservable multiverse... and seem to have no trouble presenting themselves as the most common items which make up all of the "Cosmos," so... even if they blink in and out of existence commonly, they could be said to make up most, if not all of what can be observed, or implied...

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