In the first moments after the Big Bang, the universe underwent a rapid expansion — inflation. Space stretched faster than light, and quantum jitters of the vacuum turned into real particles. This is how dark matter, the invisible substance that holds galaxies together, could have been born.
The authors of the study considered a hypothetical particle: the dark photon. It is an analog of the ordinary photon, but with a tiny mass. In models where inflation alters the geometry of space, the properties of the dark photon depend on the course of the expansion itself. Calculations showed that if this dependence is ignored, one could be off by hundreds of times in the number of particles produced.
The scientists took the inflation model proposed by Alexei Starobinsky in 1980. In this model, the expansion is driven not by a field but by corrections to spacetime curvature. For this scenario, they honestly integrated the particle production equations from the beginning of inflation to the present day. The result is a mass prediction that can be tested in laboratory experiments searching for dark photons.
Thus, quantum jitters of the vacuum, frozen in the first moments, turn out to be the key to unraveling the composition of the modern universe. The narrow mass window is not just a number: it's an invitation to directly test the theory.
as written by the authors arXiv:2607.29149v1
We present a detailed investigation of scenarios in which dark-photon dark matter is produced from quantum fluctuations during inflation. In particular, we focus on inflationary models that necessarily involve a Weyl transformation, dependent on the inflaton amplitude, in order to move to the Einstein frame. In such models, the kinetic function of the longitudinal mode of the dark photon varies throughout, and even after, the inflationary period. We show that this variation of the kinetic function has a substantial impact on the resulting relic abundance of dark photons. As a representative and phenomenologically important example, we analyze the Starobinsky inflation model, for which we perform an accurate computation of the relic dark-photon abundance. By imposing the relevant observational constraints, we find that, in order to reproduce the observed dark-matter density in the present Universe, the dark-photon mass must lie in the range $$5.6 < m < 7.4\,\mu\mathrm{eV}$$ within the framework considered in this work.
Dark photons, if they exist, could mix very weakly with ordinary light. They are being sought by 'listening' to an empty metal cavity with ultra-sensitive radio receivers.