Intergalactic collision brings new restrictions to dark electromagnetic interactions
Luís Oliveira e Silva, professor at IST and researcher at IPFN/GoLP, is co-author of a recent article that establishes a significant limit for the long-range interactions of dark matter. The paper was selected as an Editor's Suggestion in Physical Review D and was also published in APS Physics Magazine.
The nature of dark matter, which comprises more than 80% of the mass of the Universe, remains a mystery. Having been observed only through its gravitational interactions, we know very little about the underlying particle physics of dark matter.
The research focuses on a simple but powerful idea: if dark matter particles interact via a long-range force similar to electromagnetism, even weakly, those interactions could affect how dark matter behaves during collisions between galaxy clusters. The Bullet Cluster, a well-known astrophysical case of two colliding clusters, offers a rare opportunity to examine this. Observations show that the visible matter slows down as expected due to electromagnetic forces, while the dark matter appears to pass through unaffected.
An unbroken U(1) gauge force, similar to electromagnetism but acting only between dark matter particles, would mediate long-range dark-matter self-interactions. The authors refer to such a force as dark electromagnetism and to investigate this behaviour, they carried out large-scale simulations based on plasma physics.
The researchers modelled the collision of two counterstreaming plasma clouds, treating dark matter as a neutral plasma composed of particles with opposite charges under a hypothetical dark electromagnetic force. The equations that govern dark matter self-interactions are identical to those of a collisionless electron-positron plasma. The simulations show that if such a force existed with any significant strength, it would trigger instabilities that slow down the dark matter flows — a process not observed in the data.
By comparing these results with astrophysical observations, the team was able to place a very strong upper limit on the possible strength of this interaction. This establishes a strong upper bound on the strength of a dark electromagnetic self-interaction. The findings rule out a wide range of dark matter models involving unbroken gauge forces and demonstrate how tools from plasma physics can provide critical insight into cosmological questions.
Full article:https://journals.aps.org/prd/abstract/10.1103/PhysRevD.111.L071701
