• Question: What is black matter? What is dark energy? Where do ultra-energy particles come from?

    Asked by xfactorforthewin to Joel, Kristian, Tim, Venus, Zachary on 15 Nov 2013.
    • Photo: Joel Goldstein

      Joel Goldstein answered on 15 Nov 2013:

      I think you mean “dark matter” – that’s material that we think must exist in the universe as we can see its gravity acting on stars and planets. We don’t know what it is, and we’ve never directly seen any here on earth.
      “Dark energy” is similar. It’s energy that we have only observed indirectly through its effect on distant galaxies. Something seems to be pushing the universe apart, but we have no idea what it is or where it comes from.
      Finding out what dark matter and dark energy really are is one of the highest priorities for modern particle and astrophysics.
      Ultra-high energy particles are produced by astrophysical phenomena, like pulsars, supernovas and black holes. Particles with energies far higher than those in the LHC are constantly bombarding the earth from space.

    • Photo: Tim Hollowood

      Tim Hollowood answered on 15 Nov 2013:

      ok so I think you probably mean dark matter. It’s a different kind of matter that we don’t know much about because it doesn’t interact with us except via gravity. Dark energy is harder to understand, it’s a sort of soup of energy that fills space and it’s having a big effect on the universe making it expand forever. Ultra-energy particles are floating around the universe, they can come from very energetic processes like those going on as matter falls into a black hole.

    • Photo: Kristian Harder

      Kristian Harder answered on 15 Nov 2013:

      Ok, you’ve got your explanations there, but I’d like to mention one more thing that I find quite important. You’ve heard that we have never seen dark matter directly. We only think that it’s there because we can see galaxies move and we see light distorted under the influence of gravity in a way that cannot explained by the gravity of the objects nearby. You could explain this by additional matter (and thus additional gravity) that we cannot see.
      So, what are the options:
      Is it normal matter that we just can’t see because it’s dark (such as black
      holes, or dust)? probably not, because dust would either obstruct the view to objects behind it, or it would heat up eventually and start to glow, and
      also the distribution of the unknown matter tells us that this is probably not the case. So we conclude it must be a different kind of particles than the ones we know about.
      But what if we are wrong? What if the reason that stuff doesn’t move according to the gravity of the objects we see not because there are additional invisible objects, but because we misunderstood how gravity works? Maybe the effect of gravity on such a huge scale is simply not what we expected it to be from measuring it in our solar system. And hey, you may have heard that gravity is indeed the least well understood force we know about. There is no quantum theory of it. We are unable to experiment with it in particle physics because it is soooooo weak that it has pretty much no effect whatsoever on individual elementary particles in our experiments. (Gravity may not feel weak if you fall down a ladder and break your leg, but that’s only happening because the entire planet is pulling at you!)
      So, there are indeed attempts to explain motion of galaxies and the movements of light by modified laws of gravity. So far, none of these attempts was as good as the assumption of dark matter in explaining what we see, but I wouldn’t give up on that option yet. 🙂
      Of course if we do find a weird and previously unknown particle at the LHC that has all the properties we expect from dark matter, then that would settle it. So, for us experimentalists, that’s one of the things we’ll focus on for the near future. 🙂

    • Photo: Zachary Williamson

      Zachary Williamson answered on 15 Nov 2013:

      black matter? I suspect you mean dark matter, yes?

      Nobody knows where dark matter comes from, it’s one of the great unsolved mysteries. There’s about 10 times more of it than ordinary matter but we can’t see any of it!

      There are 2 theories of dark matter I like to speculate about. The first, is that it’s something called a Weakly-Interacting-Massive-Particle (WIMP). Basically some sort of really heavy particle (it would have to be heavy because there’s so much of it!) that only interacts with other forms of matter via a very weak mechanism, and basically drifts through the universe doing nothing.

      The second theory is that dark matter is something called a sterile neutrino. Neutrinos are a lot like the hypothetical WIMPs, except they’re very small and light (and so can’t account for all the dark matter out there). A sterile neutrino would be extremely similar to a WIMP, except it wouldn’t interact with anything *at all*, and would drift through the universe doing absolutely nothing.

      We have a handful of experiments looking for both types (my experiment is one of them, although its not our main focus), but so far we haven’t found a thing.

      Dark Energy is really interesting, but we know almost nothing about it. What we do know, is that distant objects billions of light years away from us, are A: moving away from us at great speed, and B: the speed at which they’re moving away is increasing.

      There’s a problem with this though! The gravitational attraction of all the objects within the universe *should* be slowly shrinking the universe. After all, gravity pulls everything together. But we see the opposite happening! This means there needs to be some sort of force that not only counteracts gravity, but exceeds its strength. We have *no idea* what that force could be, so we call it ‘dark energy’ because it’s, well, hidden to our eyes.

      Ultra-energy particles come from 2 sources normally. Supernova explosions (where a giant star runs out of fuel and collapses into a black hole), or from when neutron stars get eaten up by a black hole.

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