ÌÇÐÄÊÓƵ

Researchers, in a recent , introduce a new mechanism that may finally allow ultralight dark photons to be considered serious candidates for dark matter, with promising implications for detection efforts.

Around 85% of all matter is believed to be dark matter, yet this elusive substance continues to puzzle scientists because it cannot be observed directly.

One of the candidates for dark matter particles is dark photons. These hypothetical particles are similar to regular photons but have mass and interact only weakly with normal matter.

However, theoretical progress in dark photon dark matter models has been hindered by the "kinetic mixing constraint."

Dark photons in the early universe frequently formed cosmic string networks due to kinetic mixing with ordinary photons, preventing them from surviving as individual dark matter particles.

These string-like structures cannot clump gravitationally to form galaxy halos or reproduce the dark matter signatures we observe, essentially disqualifying them as viable dark matter candidates.

Researchers, David Cyncynates of the University of Washington and Zachary Weiner of the Perimeter Institute for Theoretical ÌÇÐÄÊÓƵics, have now proposed a novel solution that could overcome this fundamental limitation.

"I was struck by a suggesting that many proposed dark photon models might not work as dark matter," Cyncynates told ÌÇÐÄÊÓƵ.

"That raised the question: are all such models really ruled out, or can we find scenarios that still work—especially ones that could be tested by the next generation of experiments?"

Breaking free from cosmic strings

The cosmic string problem arises because dark photons typically acquire their mass through a mechanism similar to how ordinary particles get mass—via interaction with a field called the dark Higgs.

When this process occurs early in the universe at high densities, dark photons become trapped in long, string-like configurations that stretch across cosmic distances.

"Dark photons want to assemble into cosmic strings when they're quite dense in space," said Weiner to ÌÇÐÄÊÓƵ. "High densities are hard to avoid, since any dark matter candidate must have been first produced early in cosmic history when the universe was much denser."

The key insight of the new research is the "timing." By delaying dark photon production until much later in cosmic history, the researchers found they could avoid the density conditions that lead to cosmic string formation.

"Our model sought to minimize this effect by delaying the epoch at which dark photons are produced as late as possible—just in time for them to play their role as cold dark matter during the formation of the cosmic microwave background anisotropies," explained Weiner.

The researchers' model introduces a scalar field that evolves over cosmic time, effectively changing the theory's parameters as the universe ages. This field suppresses the dark photon mass in the early universe, then allows it to grow through a process called tachyonic instability.

This delayed production mechanism works through what the researchers call a "runaway potential." As the scalar field evolves, it creates conditions where transverse dark photon modes become unstable and grow exponentially, generating the dark matter abundance we observe today.

"The trick is that this new field makes the dark photon much lighter in the early universe than it ends up being today, which makes them easier to produce than in other scenarios," said Cyncynates.

Opening doors to detection

"The simplest scenario, where dark photons are created during inflation, only works if they're almost completely invisible to regular matter, which is bad news for detection," noted Cyncynates.

"The dark photon can then get away with having stronger interactions, opening the door to detection in lab experiments."

The researchers identified several upcoming experiments that could detect their predicted dark photons. These include cavity-based searches that use ultra-sensitive detectors in shielded environments to pick up weak signals that dark photons would create.

"Experiments like DM-Radio, ALPHA, Dark E-field, and MADMAX could all detect the kind of dark photons our model predicts," said Cyncynates.

Some experiments use radio-frequency techniques to search for dark photon conversions, while others rely on how light behaves inside plasma to enable resonant conversion of dark photons into regular photons.

Beyond laboratory detection, the late production mechanism creates distinctive signatures in cosmic structure formation. The delayed production leads to enhanced small-scale structures, including minihalos with characteristic masses and sizes that could be observable with future telescopes.

"Future telescopes could see hints of the enhanced small-scale structure that's characteristic to our proposal, for example via jitter in the motion or brightness of stars," explained Weiner.

"But direct detection in the laboratory would be essential evidence to suggest that an observed astrophysical signal is in fact due to dark photon dark matter."

The enhanced structure formation arises because dark photons produced late in cosmic history retain memory of the production process, creating density fluctuations that are absent in conventional dark matter models.

Implications for the future

The study provides concrete experimental targets and observable predictions that could guide future dark matter searches.

The model proposed focuses on dark photons acquiring mass through a dark version of the Higgs mechanism, similar to how ordinary particles like the W and Z bosons get their mass. However, the researchers note that alternative mass-generation mechanisms might face different constraints.

"An alternative possibility, a so-called Stückelberg mass, may not be as strongly constrained, but it is not currently known whether cosmic strings form or how they behave in that case," said Weiner.

The researchers believe their model opens up new parameter space that was previously thought to be excluded, offering fresh hope for detecting one of the dark matter's most interesting candidates.

© 2025 Science X Network