Despite dozens of years devoted to the study of puzzling dark matter, the substance can still hardly be detected and measured by instruments. And now, researchers have been exploring an intriguing hypothesis: what if galactic rotation and galaxies’ extra-gravity are affected not by dark matter per se, which was found to be the case in the 1980s, but the mass of light instead?
While we don’t know for sure what dark matter is and can’t detect it directly, there is apparently something that accounts for extra gravity, and this might not be dark matter, a team of researchers, including plasma physicist Dmitri Ryutov, Dmitry Budker, and Victor Flambaum suggested. In a new paper, published in The Astrophysical Journal and cited by The Science Alert, they adduced an argument that light particles (photons) could be at least in part the source of the phenomenon, which causes something that looks like gravity to us.
The effect they described is some kind of "negative pressure" caused by electromagnetic stresses related to photon mass. When viewed in the context of a mathematical system called Maxwell-Proca electrodynamics, these electromagnetic stresses can generate additional centripetal forces, acting mostly on interstellar gas. The team calls this Proca stress, assuming that it operates pretty much like gravity.
There are a couple of nuances, however, which are yet to be studied, with regard to Proca stress applied to short-lived vs. long-lived stars. On the one hand, short-lived stars that are formed from gas and then turn into gas before completing one orbit would be strongly coupled with it; the Proca stresses acting on the gas would be indirectly operating on these stars as well.
However, with long-lived stars, such as the Sun, it must be different: it orbits the galactic centre once every 230 million years, so it's had a few turns on the roundabout. According to the team's estimates, it should have a highly elliptical orbit under Proca stresses, which is the assumption contradicted by modern observations.
"We don't currently consider photon mass to be the solution to the rotation-curve problem. But it could be part of the solution", Budker said, adding that researchers should keep an open mind as long as they are in the dark about what dark matter actually is.
Meanwhile, NASA scientists have managed to calculate the mass of the Milky Way, most of which comes from mysterious dark matter, while only a fraction of it is accounted for by the nearly 200 billion stars in it.
"The Milky Way weighs in at about 1.5 trillion solar masses (one solar mass is the mass of our Sun), according to the latest measurements. Only a tiny percentage of this is attributed to the approximately 200 billion stars in the Milky Way and includes a 4-million-solar-mass supermassive black hole at the centre", NASA said in a 7 March post.