• Question: What's your favourite theory about dark matter?

    Asked by anon-353931 to Kathryn, Lucien on 9 Mar 2023. This question was also asked by anon-353921.
    • Photo: Kathryn Boast

      Kathryn Boast answered on 9 Mar 2023:

      It depends if I’m feeling lucky!

      We don’t know what dark matter is, and we only have some evidence to tell us what it does. There are four fundamental forces at the heart of physics: gravity, the “electromagnetic force” (which is a combination of things like light, magnetism, electrostatics etc), the “weak nuclear force” and the “strong nuclear force” (which holds the centre of atoms together). Dark matter seems to use gravity – it can pull other things towards it, in the way that the sun pulls the earth towards it. But it’s not really clear if it uses any of the other forces. It doesn’t seem to use the electromagnetic force – if it did, then light would interact with it. We don’t seem to see it (which is why we call it dark) – so, no electromagnetism. We’re pretty sure it also doesn’t use the strong nuclear force, as we think we’d have noticed the effects on normal matter by now. So the question is, does it use the weak nuclear force?

      This is the big mystery, and no-one knows the answer. So far, we haven’t seen any evidence that it does, but most of the experiments that are searching for it assume that it will. So if I’m feeling like we’re going to get lucky, and one day we will detect dark matter in our experiments, then I’d bet that it uses the weak nuclear force. But on a bad day…….. Maybe it only uses gravity? In that case, we might never know what it actually is.

    • Photo: Lucien Heurtier

      Lucien Heurtier answered on 20 Mar 2023:

      Hey knee480emu, that’s actually the first time a student asks me this, that’s a great question!

      To give you some context, for years people have believed strongly in models of dark matter called ‘WIMP’ (standing for weakly interacting massive dark matter). This theory is quite neat, as it just assumes that dark matter is a new particle which only interacts feebly with ordinary matter, such that at some point, when the universe expands, its interaction with us is ‘beaten’ by the expansion of the Universe. Therefore, it is trying to interact from time to time, but each time it tries, it is pulled apart from its target by expansion. What’s fascinating is that assuming the strength of the interaction the same as the ‘weak’ interactions (the one responsible for radioactivity), the prediction is that there would be the right amount of dark matter today compared to observation. Also what’s great in this theory is that the only ingredients are the mass of the Dark Matter particle, and its interaction strength, there is no other extra assumption that needs to be made to explain physics.

      However, despite the number of detectors that have been constructed, the WIMP has not been found yet. That does not mean it doesn’t exist though, it has just not been found yet. There are many versions of WIMPs that exist (it could interact with protons, with electrons, with neutron, there are plenty of possibilities).

      Another exciting possibility (which so far is my favorite actually) is the idea that dark matter is made of black holes! This is very constrained by many observations, but it is possible. These black holes would need to be ‘primordial’, meaning that they have been produced in the very early Universe, a long time before stars existed, and thus do not originate from the collapse of a star. They would need to have a mass between 10^16 g and 10^20 g, that means less than a thousand billion times less than the mass of the sun! (so very tiny black holes)

      There are many models of dark matter though. You can search online and you will find plenty. What I’m really convinced is that dark matter exist, but I would not bet my house on one model or another, we need more observations!

      There have been very nice simulations done by the cosmology group in Durham, you can watched there: https://icc.dur.ac.uk/Eagle/highreader.php?page=evo

      If you go down to the video, you can choose what you want to see (gas, stars, or dark matter). And you will see that they are located at the same place. Actually, that’s why I believe that dark matter exists: without dark matter in those simulations, nobody understands how galaxies could have formed as there seems to not be enough matter in the universe to do that…

      Have fun searching online about this, this is a very passionating topic!

      Best
      Lucien

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