Dark matter is a curious thing. We have no direct proof of what it is but there is plenty of indirect evidence that a substance that only interacts with gravity and not light permeates the entire universe. It's also a lot more abundant, outweighing regular matter by five to one.
Given that we don’t know its properties, however, there is plenty of room for speculation. Are dark matter particles heavy or light? Soon we might be able to detect one, but until then astrophysicists have to use another approach. Using computer simulations, they build galaxies and see how different they would look under the influence of dark matter with different properties. A new study has found that the very first galaxies would have looked very different depending on the nature of dark matter.
Reporting in Physical Review Letters, an international group of researchers conducted simulations of how galaxies may have formed under three different scenarios. The first was the most widely accepted model known as cold dark matter, where particles of dark matter are heavy (10,000 times the mass of an electron) and slow. The second model was for warm dark matter, slightly lighter and slightly faster. For the third scenario they tested the fuzzy dark matter scenario, which consists of ultralight particles a billion billion billion times lighter than electrons. This was not easy, as this hypothetical dark matter would be so light that it would be governed by quantum mechanics, more like a collection of waves than individual particles, which is why it is described as “fuzzy.”
These different scenarios have to create over time the galaxies that we know, but the way they get there are strikingly different.
“Even though in the late universe these different dark matter scenarios may predict similar shapes for galaxies, the first galaxies would be strikingly different, which will give us a clue about what dark matter is,” lead author Dr Philip Mocz of Princeton University, explained in a statement.
The team created three protogalaxies to test the scenarios out. In the cold dark matter scenario, galaxies formed in a bright near-spherical halo. The warm dark matter scenario showed that galaxies would have formed first in tail-like filaments. In the final scenario, the fuzzy dark matter also showed galaxies forming in filaments but these looked more like strings.
We may not currently know what the environment around these protogalaxies was like, but upcoming telescopes might soon be able to solve this mystery. The team is building up more detailed simulations of these fictional galaxies so that once observations come in, they can be compared and contrasted against the simulations.
“The first galaxies in the early universe may illuminate what type of dark matter we have today,” said co-author Professor Mark Vogelsberger, from MIT’s Kavli Institute for Astrophysics and Space Research. “Either we see this filament pattern, and fuzzy dark matter is plausible, or we don’t, and we can rule that model out. We now have a blueprint for how to do this.”