A team of astrophysicists have mapped the “Lyman-Alpha Forest”, and provided further supporting evidence that galaxy and galaxy cluster formations are better explained by dark matter than alternative theories.
While observing the universe, astronomers found that galaxies and galaxy clusters don’t behave in the way we would expect. In short, stars at the edge of galaxies move so fast (in most, but not all galaxies) that they should be yeeted off into intergalactic space given the amount of visible mass we can see in those galaxies.
There are alternative explanations, such as Modified Newtonian Dynamics (MOND) where it is proposed that gravity works differently at low accelerations. There are problems with MOND, and the currently favored hypothesis by most physicists is that space is filled with invisible “dark matter” which only interacts with other baryonic matter (the stuff which we can see) via gravity.
To explain the dynamics of galaxies and galaxy clusters, dark matter is expected to have about 10 times as much mass as ordinary baryonic matter, but so far we haven’t found any direct evidence of what it is. Candidates still range from weakly interactive massive particles (WIMPS) and axions to primordial black holes, though this latter candidate is also beginning to look unlikely. It has been suggested too that it could be caused by supermassive black holes behaving in ways we haven’t got our comparatively very puny heads around yet.
In the new study, astrophysicists from the University of California, Riverside, used the “Lyman-Alpha Forest” to attempt to indirectly map dark matter. Essentially, they looked at light from distant sources and mapped drops in light along the hydrogen wavelength. These drops in light correspond to matter the light has encountered along the way.
“It’s somewhat like shadow puppetry, where we guess the character placed between the light and the screen based on its silhouette,” Simeon Bird, associate professor of physics and astronomy and lead author of the study, said in a statement.
“Since each type of atom has a specific way of absorbing light, leaving a sort of signature in the spectrogram, it is possible to trace their presence, especially that of hydrogen, the most abundant element in the universe,” Bird explained.
The result of mapping this light is the “forest”, resembling many small trees. The team says that using hydrogen spectrograms can be used to trace dark matter indirectly, like pouring dye into a stream of water.
“The dye will follow where the water goes,” Bird said. “Dark matter gravitates so it has a gravitational potential. The hydrogen gas falls into it, and you use it as a tracer of the dark matter. Where it is denser there’s more dark matter. You can think of the hydrogen as the dye and the dark matter as the water.”
The team suggests that the structures seen in the resulting map are indicative of an unknown influence, or that dark matter is a particle. This is, of course, not “mystery solved” as we have never detected such a particle.
“It’s not completely convincing yet,” Bird added. “But if this holds up in later data sets, then it is much more likely to be a new particle or some new type of physics, rather than the black holes messing up our calculations.”
The study is published in the Journal of Cosmology and Astroparticle Physics.
