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What is Hot Dark Matter?

Updated on March 18, 2013
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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 100,000th 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).

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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.

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    • ib radmasters profile image

      ib radmasters 4 years ago from Southern California

      So how does dark matter react with light traveling across the universe. For light to get here from across the universe, it would seem that it didn't react with anything in space.

    • writerbeth profile image
      Author

      writerbeth 4 years ago from England

      Ahah, you've hit on quite the question there. And you are right, the light is affected by the dark matter by something called gravitational lensing. If an object has great mass then because of the forces of gravity, light bends around it. We then see the galaxy, or whatever the source of the light is, distorted because of this. That is strong gravitational lensing but there is also weak gravitational lensing that

      can be measured because of the shearing and bending of light and in part that tells us where the dark matter is. That is ridiculously brief but I hope it sort of helps - hmmm, I can feel another hub coming on! Thanks for reading, Beth

    • ib radmasters profile image

      ib radmasters 4 years ago from Southern California

      Beth

      Then you can solve the question of light

      -- Particle

      or

      -- Wave

      I had another thought..

      When the early morning light shines through the window and into a room, we can see the dust particles. They seem to fall based on gravity, but they are also affected by the wind in the room.

      The real thought is that if you took a picture of the dust in the air, would it look like a picture of dark matter in space. Of course, if you could see it, or imagine how it would look based on theory.

      So when meteors, asteroids and comets race around the solar system, do they cause a wave that pushes dark matter around, like a boat going through water?

      In addition, can dark matter be gravity?

      Thanks

    • writerbeth profile image
      Author

      writerbeth 4 years ago from England

      The first answer is a simple – both! I’m afraid due to wave-particle duality all particles also have a wave nature and that includes light.

      As to the second point, who knows what dark matter looks like? It is called ‘dark’ matter not because it is dark but because no-one is sure what it is. It is just a mysterious thing that is giving extra mass to the universe, so it could look like nothing or it could be a WIMP as I mentioned in my general Dark Matter hub. If it is a WIMP, then it isn’t going to be a lump of stuff that can be seen, it is just all around and as such I don’t think there would be a ‘wave’ effect either.

      As for gravity being dark matter. Dark matter is something giving extra mass to the universe, but gravity is a force that acts on objects with mass so it’s a bit like comparing apples and shoes, they aren’t really the same thing. Hope this makes things clearer, Beth

    • ib radmasters profile image

      ib radmasters 4 years ago from Southern California

      Beth

      I don't think that you understood my point with the following.

      When the early morning light shines through the window and into a room, we can see the dust particles. They seem to fall based on gravity, but they are also affected by the wind in the room.

      The real thought is that if you took a picture of the dust in the air, would it look like a picture of dark matter in space. Of course, if you could see it, or imagine how it would look based on theory.

      So when meteors, asteroids and comets race around the solar system, do they cause a wave that pushes dark matter around, like a boat going through water?

      And as for gravity and dark matter we don't know how either of them works, or whether there is a relationship between them.

    • ib radmasters profile image

      ib radmasters 4 years ago from Southern California

      bye bye

    • writerbeth profile image
      Author

      writerbeth 4 years ago from England

      Oh dear, and sorry ib radmasters, I guess I didn't understand if I haven't answered your question correctly. Maybe one of the other readers here can help?

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