New Mechanism Suggests Dark Photons as Viable Dark Matter Candidates

Recent research published in *Physical Review Letters* has proposed a novel mechanism that positions ultralight dark photons as serious candidates for dark matter, a substance that constitutes approximately 85% of the universe's mass yet remains elusive and undetectable by conventional means. The study, conducted by David Cyncynates of the University of Washington and Zachary Weiner of the Perimeter Institute for Theoretical Physics, addresses the longstanding challenge posed by cosmic strings formed by dark photons in the early universe, which have hindered their viability as dark matter candidates.

Historically, dark photons, which are akin to regular photons but possess mass and interact weakly with ordinary matter, have struggled under the 'kinetic mixing constraint,' limiting their potential as dark matter. Dark photons were often trapped in long, string-like configurations known as cosmic strings due to kinetic mixing with ordinary photons during the universe's early dense state. These configurations lacked the ability to clump together gravitationally, disqualifying them from forming the halos of galaxies that are characteristic of dark matter.

The researchers propose that by delaying the production of dark photons until later in cosmic history, the conditions necessary for cosmic string formation can be avoided. This insight hinges on a mechanism that introduces a scalar field evolving over cosmic time, effectively changing the parameters governing dark photon behavior as the universe ages. This approach minimizes high-density conditions, allowing dark photons to behave as cold dark matter during the formation of cosmic microwave background anisotropies.

"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," explained Weiner in an interview with *Phys.org*. The implications of this model extend to laboratory detection, as the reduced mass of dark photons in the early universe makes their interactions with regular matter more pronounced. Cyncynates noted that upcoming experiments such as DM-Radio, ALPHA, Dark E-field, and MADMAX are poised to detect the predicted dark photons, which would represent a significant advancement in dark matter research.

The study also highlights that the delayed production mechanism creates unique signatures in cosmic structure formation by generating enhanced small-scale structures, including minihalos that future telescopes might observe. This distinctive characteristic arises from density fluctuations that are absent in conventional dark matter models, which could indicate the presence of dark photons.

The research opens new avenues for exploring dark matter candidates, challenging previous assumptions about the limitations of dark photon models. However, the authors caution that alternative mass-generation mechanisms, such as the Stückelberg mass, may not face the same constraints and require further investigation.

The findings not only reinforce the need for innovative approaches to dark matter research but also provide concrete experimental targets and observable predictions, potentially reshaping our understanding of one of the universe's most intriguing mysteries. As Cyncynates succinctly put it, "This model opens up new parameter space that was previously thought to be excluded, offering fresh hope for detecting dark matter's most interesting candidates."