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, …
Hot and Cold
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.
Push Me, Pull You
If the universe is expanding, how is it possible that galaxies can collide with each other? It turns out that not only are colliding galaxies possible in an expanding universe, the fact that they occur is due in part to dark matter and dark energy.
Galactic Pile Up
MACS J0717 is a large galactic cluster formed by the collision of four galaxy clusters. It could help us learn more about dark matter , as well as the structure of the universe.
Star Killer
Primordial black holes are hypothetical micro-black holes that some have thought could solve the dark matter problem. New research argues that if such black holes exist they could collide with neutron stars, destroying them. The fact that we haven’t seen any such events further supports the idea that primordial black holes cannot solve the dark matter problem.
The Shape of Things
The most widely accepted model for the universe, sometimes called the standard model of cosmology, is known as the ΛCDM model. The Λ or “lambda” refers to the dark energy parameter known as the cosmological constant (which often uses lambda to represent its value). The CDM stands for Cold Dark Matter, which is the type of dark matter currently best supported by observation. In the ΛCDM model, the matter of the universe consists of regular matter (about 5%), cold dark matter (about 27%). Dark energy is then caused by a cosmological constant, which is a property of space and time itself, giving rise to dark energy.
Much Ado About Nothing
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.
Ugly Theory, Beautiful Data
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?
Beautiful Theory, Ugly Data
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.