One of the leading candidates for dark matter may have been ruled out too soon. It was thought that dark photons should have noticeably heated the early Universe, but astronomers detected no such heating. Using computer simulations, physicists obtained a different result. Dark photons transferred almost no energy to the plasma, so the absence of observational traces no longer argues against their existence.
The Invisible Majority
Dark matter about outweighs ordinary (baryonic) matter — the material that makes up stars, planets, and our bodies — by a factor of five. It remains elusive because, as far as we know, it has no discernible electromagnetic interaction and does not emit, absorb, or scatter light. We judge its existence mainly by its gravitational influence on visible matter.
Electrons, protons, and neutrons, by contrast, interact with the electromagnetic field. This difference prompted physicists to search for particles beyond the Standard Model. Many hypothetical candidates have been proposed.
A Hidden-Sector Particle
One of them is the dark photon,a hypothetical analogue of the ordinary photon, which is a carrier of interaction in the so-called dark sector. This is the name of a hypothetical set of particles and interactions that almost do not interact with ordinary matter (only gravitationally, as far as we know) and can be connected to it only very weakly
The idea emerged back in 1986, when Bob Holdom described a mechanism for kinetic mixing between two independent electromagnetic fields. Such a mechanism allows for a very weak connection between the normal and dark sectors. Since then, the dark photon has remained one of the most popular hypotheses in theoretical physics.
A Trace in the Hot Plasma
The young Universe was filled with dense, extremely hot matter. Calculations from previous years predicted that dark photons within it would convert into ordinary photons. A side effect should have been additional heating of the plasma and noticeable signatures in observations.
Astronomers found no such traces. As Space.com notes, the allowed parameter range narrowed so much that many cosmological observations effectively ruled out the existence of dark photons altogether.
Nonlinearities Shut Down the Process
The first doubts arose when the authors of the study noticed suspiciously high energy values. The problem lay in the assumption of linearity, according to which energy was transferred into the plasma gradually and uniformly. “For the last fifteen years, people have been using the linear approach. If you apply this approximation, you get a very large energy transfer,” said Junwu Huang of the Perimeter Institute for Theoretical Physics.
Simulations showed that this picture was incomplete. As soon as the first portions of energy entered the Standard Model plasma, numerous nonlinear effects appeared in the system. They shut down the conversion after only a tiny fraction of the dark photons’ initial energy had been transferred.
The findings were peer-reviewed, and the paper was published on August 13 in Physical Review Letters. The study was authored by three scientists from two institutions.
More Room to Search
Previous calculations had drastically underestimated the allowed strength of dark-matter interactions. “These exclusions claimed that the dark matter interaction strength had to be one hundred million times weaker than it may actually be,” said Anson Hook of the University of Maryland, College Park.
The broader parameter range also affects other hypothetical particles beyond established theory. “If you calculate the early-Universe plasma correctly, experiments will be able to probe new regions of parameter space and perhaps actually see something,” added Mohamad Shalaby, a co-author of the study from the same institute.
