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.
The Old Model Doesn’t Work
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.
Five Facts
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.
Nerds Seeking WIMPs
The Soudan Iron Mine in Northern Minnesota is home to several experiments in particle physics and cosmology. I’ve written about one of the projects there, known as the Cryogenic Dark Matter Search (CDMS). Another experiment is Main Injector Neutrino Oscillation Search (MINOS), which detects muon neutrinos produced at Fermilab in Northern Illinois. MINOS is about 48 feet long, and contains 6000 tons of steel layered between scintillators. The entire detector had to be lowered down a narrow mine shaft piece by piece and then assembled on site.
Line of Perseus
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.
Electric Boogaloo
As I’ve noted before, the idea of dark matter isn’t invoked just to make the standard theories work. Despite our incomplete understanding of dark matter, there is significant evidence to support it. But how do we know that regular matter isn’t enough to account for the unseen mass in the universe? What if it were something like dark mode plasma?
Galaxy Rangers
In an earlier post I wrote about one of the mysteries of dark matter. While dark matter matches most observations very well, it doesn’t do well in the area of dwarf galaxies. In particular, computer simulations predict that there should be many more dwarf galaxies than we observe. This has been taken to mean that either the simulations are somehow flawed, or dark matter isn’t the complete solution we’ve thought. But now new research has found that dark matter simulations might be right after all.
Still in the Dark
Dark matter has an interesting history. It was first proposed to account for the fact that stars in our galaxy move much faster than they should around the galactic core. Evidence of dark matter has been seen in galactic collisions like the Bullet Cluster, as well as through gravitational lensing by galaxies. On the other hand, 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. Then there is the puzzling aspect of dwarf galaxies.
Have You Heard the One About…
This week I’ve gotten a number of questions about various proposed models in astrophysics, such as the one about how anti-gravity could explain dark matter and dark energy. Or the one where dark matter is a result of quantum interference on a cosmic scale. Or the one where the cosmic microwave background is actually due to thermal turbulence rather than the big bang. There are lots of ideas that show up in the literature and in the press, so how do you judge the quality of a particular idea?