As I’ve written about before, the existence of dark matter is well supported by observational evidence. There isn’t much debate in the astronomical community on the existence of dark matter and the fact that it makes up a large part of the mass of galaxies. We’d still like to have a direct observation of dark matter to be certain sure, but there is general consensus on dark matter.
Dark matter is an aspect of the universe we still don’t fully understand. We have lots of evidence pointing to its existence (as I outlined in a series of posts a while back), and the best evidence we have points toward a specific type of matter known as cold dark matter (CDM). One big downside is that we have yet to find any direct detection of dark matter particles. In fact, many of the likely candidates for dark matter have been all but eliminated. Another is that cold dark matter doesn’t agree with our observations of dwarf galaxies. Now a new paper presents a solution to the second problem that might even help with the first.
In this series we started with the basic observations of stellar motion in a galaxy, and how that led to the idea of some kind of dark matter. We saw that modified gravity models couldn’t explain things like the Bullet Cluster, while dark matter could. We also saw that dark matter must be something other than the normal matter that makes up you and me. It must be non-baryonic, weakly interacting with light, and make up a goodly portion of galaxies. Then again, our best candidate for dark matter is a hypothetical particle based on a particle physics model that is quite probably wrong.
Last time we saw that while alternative gravity models don’t agree with galactic stellar motion and gravitational lensing, the dark matter model does. While some of that dark matter is likely regular (baryonic) matter such as MACHOs (brown dwarfs, neutron stars, etc.), such objects cannot make up the majority of dark matter in the universe.
Yesterday we looked at alternative gravitational models (specifically Modified Newtonian Dynamics) as a solution to the problem of gravitational lensing and stellar motion not matching the observed masses of galaxies. We saw that observations of colliding galaxies such as the Bullet Cluster show that the mass distributions observed directly don’t agree with the distributions calculated by gravitational lensing. This pretty much kills the alternative gravity models, because you wouldn’t expect the two results to be radically different if they are both due to the same mass.
Last time we talked about where the problem lies. When we map the distribution of visible mass in a galaxy and calculate what the motion of stars should be in that galaxy, it doesn’t match the motion we see. The stars in a galaxy move as if there is ten times the mass we observe. Observations of the cosmic microwave background indicate that our measurement of galactic mass should be correct, but observations of gravitational lensing by galaxies agree with stellar motion. Something just doesn’t add up.
Astrophysics is mainly about gravity. Yes, it also depends upon chemistry, nuclear physics, optics and the like, but gravity is the overarching force. Gravity drives stars to shine, it drives black holes to form, and it drives the motion of stars and planets. So a good understanding of the universe depends upon a good understanding of gravity.
In the past I have done a week-long series on certain topics. I don’t do them all the time because they take more work than one-off posts, but they tend to be rather popular. So far the series have been fairly broad in scope, covering the quantum revolution, or science fiction vs. science fact. But this time I’m trying something a bit different: cover one topic in detail. Background, proposed models, observational evidence, and why we support one theory over the alternatives. We’ll start with dark matter. It was first proposed in the mid 1900s, and since then its existence has become both more confirmed and more bizarre.
The Perseus cluster is a massive galactic cluster consisting of thousands of galaxies. It is often a focus of study because it is both massive and reasonably close (about 240 million light years away). Recently we’ve discovered some interesting x-rays coming from the region. The results have been published the Astrophysical Journal, showing there is an unexplained emission line in the x-ray spectrum.