In the first moments after the Big Bang, the universe expanded at an incredible rate — a phase known as inflation. Even the vacuum was restless back then: quantum jitters shook the fields like wind rustling through a wheat field. One such field, the dark Higgs field, could have randomly tipped away from its most stable state.
When inflation ended, this field began to oscillate and decay, spawning particles — dark photons. These became candidates for dark matter. Particles born this way get an initial kick, so they zip around faster than usual. That imposes constraints: overly fast particles would have disrupted galaxy formation. So the interaction between dark photons must be extremely weak — roughly like the gravitational pull between two feathers a kilometer apart.
If dark photons also mix slightly with ordinary photons, they could be caught in future experiments — either by a faint glow in space or by barely perceptible nudges in sensitive detectors. For now, cosmology remains the ultimate judge: the map of the cosmic microwave background and the spectrum of distant hydrogen clouds bear the imprint of those ancient events.
as written by the authors arXiv:2607.29682v1
We reconsider the production of dark photons $$A'$$ as dark matter, from the perturbative decay of a dark Higgs field $$h$$, that is stochastically misaligned from the minimum of its potential during inflation. This is a simple and predictive framework for generating the $$A'$$ relic abundance. It is constrained by structure formation, since the $$A'$$ are initially boosted, and inflationary isocurvature fluctuations, which require small quartic couplings $$\lambda h^4$$. We identify $$A'$$ masses between 100 eV and 1 GeV and gauge couplings $$g\sim 10^{-15}-10^{-10}$$ that are consistent in this scenario, and which become more tightly constrained if a generic level of kinetic mixing is present. The favored parameter region could be tested through future CMB or Lyman-$$\alpha$$ observations, and, in the presence of kinetic mixing, by direct detection experiments or diffuse soft gamma-ray searches.
Inflation lasted a tiny fraction of a second, about 10⁻³⁵ seconds, but during that time the universe expanded by a factor of 10²⁶. If a grain of sand grew by the same factor, it would become the size of the observable universe.