Some of the dark matter thought to have been ruled out by the thermal history of the Universe may actually still be possible.
Dark matter is invisible, but its gravity affects galaxies and large cosmic structures. Its nature remains unknown. One of the candidates under consideration is the dark photon, a hypothetical particle that would resemble the ordinary photon in certain properties. It could make up all or part of this invisible matter.
To understand this, we need to go back to the young Universe. Matter there formed a plasma, meaning a very hot gas containing charged particles. Under certain conditions, dark photons could have transferred their energy to this plasma. Models then predicted heating strong enough to leave traces observable today.
The absence of these traces was used to establish very strict limits. Some possible masses for dark photons were therefore considered incompatible with cosmological observations. This reasoning nevertheless relied on an assumption: energy transfer had to continue efficiently when the right conditions were met.
Anson Hook, Junwu Huang and Mohamad Shalaby reexamined this step using numerical simulations tracking the behavior of the plasma. Their result substantially changes the scenario. When the transferred energy becomes comparable to the thermal energy of the plasma’s electrons, the plasma reorganizes itself. Density variations then appear and disrupt the conditions required for the transfer.
In practice, the mechanism eventually slows itself down. Dark photons therefore cannot continue injecting as much energy as previous calculations predicted. According to the researchers, the energy actually deposited remains close to the energy already carried by the electrons at the time of conversion. It thus remains far below the levels capable of producing the sought-after cosmological signatures.
This difference substantially changes the limits derived from observations. The constraints in question become 3,000 to 10 million times weaker, depending on the mass considered. The effect spans ten orders of magnitude in mass. In other words, a very broad range previously considered ruled out could once again be compatible with the available data.
The result is not a detection of dark matter and does not prove that dark photons exist. Instead, it removes a series of arguments used to rule out certain properties of dark photons. Other search methods, including laboratory experiments and astrophysical observations based on different phenomena, therefore remain necessary to test this hypothesis.