“Cold, dark matter” has a certain ring to it, but new simulations of our corner of the cosmos suggest that dark matter – the stuff that is thought to underlie the universe – might be warm, with relatively fast-moving and lightweight particles. In cosmology’s standard model, dark matter is cold, made up of relatively heavy low-energy particles, and will happily settle into structures as small as planets. Hot dark matter has already been ruled out because its particles would move too fast for galaxies to form. But warm dark matter has smaller, faster particles that still allow for our familiar starry sky.
Most computer models produce a generic universe that doesn’t resemble ours in detail, but Gustavo Yepes at the Autonomous University of Madrid, Spain, and his collaborators on the Constrained Local Universe Simulations (CLUES) project have tuned theirs to resemble the galaxies and clusters nearest Earth. In their simulated universe, the CLUES team zoomed in on a small box of space – just 6.5 million light years to a side – which contains our local group of galaxies. Yepes and colleagues ran the simulation at high resolution, with about 60 million chunks of dark matter forming digital versions of the Milky Way, Andromeda and Triangulus galaxies. The team simulated the development of our corner of the universe with both warm and cold dark matter, and compared the results with real data on satellite galaxies from the Arecibo radio telescope in Puerto Rico.
“The cold dark matter simulations predict that there should be 10 times more satellites than have been detected in our Milky Way and its neighbour, the Andromeda galaxy,” says Yepes. The warm dark matter models may do a better job of matching the observed number of these low-mass galaxies, says Mike Boylan-Kolchin of the Max Planck Institute in Garching, Germany, who did not take part in the research. But he also points out that there could be other explanations for the discrepancies between what the cold dark matter scenario implies and what we can actually see: it may be that we don’t understand well enough how galaxies form, or we may not have detected some of these galaxies.
The formation of these satellite galaxies could perhaps be better modelled through “gastrophysics” – the astrophysics of space gases (see video above). In this version of the high-resolution simulation, another 60 million chunks are added to the mix. Their masses are just a fifth of those of their dark matter counterparts and they behave as gases.
Source: New Scientist