Dark matter is an aspect of astrophysics which continues to pose big mysteries. While we’re pretty sure it exists, we don’t yet have a clear picture of what exactly it is. On the one hand there is a great deal of evidence to support dark matter. I’ve written a series of posts a while back outlining some of this evidence, …
Part of the evidence we have for dark matter is through its gravitational effect on the motion of stars. The first evidence for dark matter came from motion of stars in our galaxy, which indicated there must be a large quantity of unseen mass in our galaxy. So why is it that when we look for the gravitational effect of dark matter on nearby stars, we don’t see anything? It turns out that tells us something very interesting about the nature dark matter.
You might wonder why anyone would announce that they had found nothing. Isn’t science about discovering things? Finding nothing is pointless because you can’t prove a negative, right? Not quite. It actually depends on the type of nothing you’ve found.
When we left our story we, had two proposed models: MOND, which posits that for very small forces, the acceleration of an object doesn’t quite go to zero, and dark matter, which introduces an invisible “something” that makes up most of a galaxy’s mass. Neither one of these seem particularly appealing, so why (as I stated yesterday) do I favor dark matter as a solution?
Both MOND and dark matter were introduced to address a problem with the way galaxies behave. At a basic level, the stars in a galaxy such as ours orbit the galactic center in roughly circular orbits. The speed of a star in its orbit should be governed by Newton’s law of gravity. So, using Newton’s gravitational theory, we can predict a star’s speed given its distance from the center and the distribution of matter in our galaxy. Newton’s beautiful theory doesn’t agree with the experimental data.
When we look at our galaxy, we find it has more mass than the matter we see. Much more. This is the first clue that there must be dark matter in the Universe.
The first evidence of dark matter came from observing the motion of stars in our own galaxy. It turns out stars were moving far more quickly than they should. Ever since then astronomers have tried to determine just what dark matter is.