What is Hot Dark Matter?

Before we discuss hot 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 by electromagnetic radiation and is postulated to exist because of the detection of its gravitational effect. Its possible presence is used to explain how galaxies formed after the Big Bang and Hot Dark Matter is one of the three hypotheses that try to explain what happened. The others are Warm Dark Matter (WDM) and Cold Dark Matter (CDM).

The difference between these three is the distance that particles could move just after the Big Bang before they were slowed down because of the expansion of the universe. The distance the particles moved is known as the free streaming length and 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 about to form into a galaxy and if the free streaming length of particles is thought to be larger than this protogalaxy i.e. moving quite fast, then we are talking about hot dark matter.

Theoretically, Hot Dark Matter consists of particles that travel at ultra relativistic velocities. That means that they travel very close to the speed of light. It is speculated, that the best possible candidate for these particles is the neutrino. There are two main reasons for this. Firstly, that they have a very small mass (less than 100,000^th of the mass of an electron) and that they only interact using two of the four fundamental forces i.e. gravity and weak interaction. Not only that, but these two forces are so weak that they are very difficult to detect. They are classed as being Weakly Interacting Light Particles or WILPs (as opposed to those particles postulated in Cold Dark Matter theory, WIMPs).

If Hot Dark Matter theories were to work, then they would describe the formation of galaxies after the Big Bang to be what is called a top-down formation scenario. Basically, that large pancake-like structures were created which then broke down into smaller pieces to become clusters of galaxies and break down further to create individual galaxies.

Unfortunately, recent microwave background radiation measurements from the COBE satellite have shown that it is not possible for the individual galaxies to be created this way and in fact from deep field observations, it looks like the small galaxies formed first, with the clusters of galaxies forming later.

Therefore, it is suggested that maybe, this is not the explanation for dark matter and the creation of the universe but instead may be a small part of the overall answer with Cold Dark Matter being favoured for the definitive solution.