Cold Dark Matter (CDM)

Before we discuss cold dark matter, if you don’t know what dark matter is, it might be an idea to see my hub ‘What is Dark Matter?’ which explains this.

Dark matter cannot be detected at all by electromagnetic radiation and it is thought to exist because of the detection of its gravitational effect. The idea that it is present can be used to explain how galaxies were formed after the Big Bang. Cold Dark Matter is the more likely scenario of the three hypotheses that attempt to explain this process. The others are Hot Dark Matter (HDM) and Warm Dark Matter (WDM).

The difference between all these hypotheses is the distance that the particles traveled just after the Big Bang. Until the particles were slowed down by the expansion of the universe, the distance they travelled was known as the free streaming length. This length determines whether we are talking about the dark matter being hot, warm or cold.

A protogalaxy is a cloud of gas that is prior to it forming into a galaxy, possibly of the size of a dwarf galaxy. If the free streaming length of particles is thought to be about smaller than this protogalaxy, then we are talking about Cold Dark Matter.

Cold Dark Matter is the simplest and most likely of the explanations for what happened after the Big Bang. Most particles are found to become non relativistic very early on, this means that they have a velocity that is very small in comparison to the speed of light, and therefore can be classed as particles of Cold Dark Matter.

The particles that are thought of as being part of this Cold Dark Matter theory are called WIMPs (Weakly Interacting Massive Particle). No-one knows what these particles might be and nothing has been found that fits the criteria yet. However, if they were to exist, this would make the Cold Dark Matter theory the most attractive.

Cold dark matter leads to a bottom-up formation of the universe with small objects being created first, and larger galaxies and clusters of galaxies being created subsequently. There are a few discrepancies between this theory and some observations of galaxies for example the fact that there should be more dwarf galaxies than is observable. However, computer simulations of dark matter particles operating as Cold Dark Matter particles, confirm that the universe would be created as it is today with these rules in place.