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.
The Dark Matter Model Works
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.
Other Gravitational Models Don’t Work
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.
Dying of the Light
Part 6 in the equations series. Boltzmann opens our eyes to a world where the warmth of our morning coffee forces us to confront our own mortality.
Memory Hole
Part 5 of the equations series. Got something to hide? Toss your secrets into a black hole, and no one will ever know. Or will they?
Unity
Part 4 of the equations series. Flying kites in a thunderstorm leads us to a single elegant theory describing lightning, magnets and light. Don’t try this at home.
Time After Time
Part 3 in the equations series. How a beam of light overturned 300 years of physics, and changed our view of the universe.