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Multiverse theory and Dark Matter

  1. Feb 17, 2016 #1
    I've been looking over the idea of the multiverse recently. I am trying to grasp exactly why so many physicists believe in the idea when it seems more philosophical than scientific. Are there any good indicators pointing towards the theory from QM or GR?

    The gripe that I also see with it (echoed by an equal amount of physicists) is that you can never verify it experimentally (partially because everything goes with an infinite amount of universes to choose from). However I have seen some suggestions flying around that a nearby parallel universe's gravity will leak into our own, and hence a measurement in effect would be possible. Curious as to whether or not this could explain the extra gravitational kick from dark matter as oppose to the ad hoc non-baryonic suggestions?

    Regards, Billy.

    Edit: I forgot to mention the many worlds interpretation of QM. Is this a source for the multiverse belief?
     
  2. jcsd
  3. Feb 17, 2016 #2

    Chalnoth

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    Multiverse ideas naturally extend from many models within quantum mechanics (General Relativity is pretty silent on the whole thing).

    For example, one of the fundamental features of the standard model of particle physics is what is known as spontaneous symmetry breaking (https://en.wikipedia.org/wiki/Spontaneous_symmetry_breaking). This is a component of the Higgs mechanism that gives particles their masses.

    One way that this can be visualized is to imagine a flat surface, on which there are fixed a grid of pens, each with their points fastened to the table. The pens can move around in any direction, but they're connected to one another with springs. High temperatures would, in this analogy, be represented by the pens having a lot of kinetic energy, so they're all bouncing around. If the pens have enough energy, they'll bounce around so much that there's no single direction that stands out.

    But as the temperature drops, the pens will tend to fall. The specific direction they fall in is random, but because of the springs attaching neighboring pens, they'll all tend to fall in one direction together. Now, there is a specific direction that is special: the direction the pens point. But that direction is random, and if the table is big enough and the energy is lost quickly enough, then some pens will point in one direction in one part of the table, while other pens will point in another direction in another part of the table.

    In the standard model, the direction that the pens fall has consequences for how the weak nuclear force interacts. In other proposed high-energy physics models, there are many other spontaneous symmetry breaking events that have other consequences for things like the relative strengths of the forces as well as other constants that are required to describe how particles interact. If these models are correct, then a multiverse where different regions of the universe have different physical laws is essentially inevitable.

    Plus, this kind of model has a very interesting prediction: defects. In the pen analogy, imagine what would happen if you had one pen in the center of the table that was standing vertically, while all of the pens around it were laying flat pointed away from it. In this situation, the pen in the center would be stuck in a high-energy state. But this is a 2D system: our world is 3D. In a 3D world, a defect like this would be a long string, known as a cosmic string:
    https://en.wikipedia.org/wiki/Cosmic_string

    So far, no cosmic strings have been detected. And unfortunately, the way cosmology works even if they do exist there might not be any within our visible universe.
     
  4. Feb 17, 2016 #3

    Chronos

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    A major problem with most multiverse models is they are inherently unfalsifiable. This places them beyond the purview of scientific inquiry; a condition more commonly referred to as mysticism.
     
  5. Feb 17, 2016 #4

    Chalnoth

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    By this definition, a majority of modern physics could be termed "mysticism". WIMPs are technically unfalsifiable, for example: they could have properties such that they could never be detected directly. Same with primordial B-mode polarization on the CMB.

    Falsification is far too simplistic a metric. Most ideas in theoretical physics end up not being entirely falsifiable because they come along with unknown parameters, and some subset of those parameters lies outside the range of detection. But if we were to go by your overly-simplistic analysis, we should just throw all of those ideas out. There would be no reason to search for supersymmetry, because it has a large parameter range where we could never detect it in our particle accelerators. We would have had no reason to hunt for exoplanets for some time, because we currently don't have the ability to detect planets like those that exist in our own solar system. There would have been no reason to bother running LIGO because no known gravity wave events would have produced a detectable signal.

    No, sorry, fixating on falsification just betrays an unwillingness to think critically about what actually separates science from non-science.
     
  6. Feb 19, 2016 #5
    I would say that if the set of assumptions or worldview/models that best allow us to explain and understand the universe we see around us also suggest the existence of some other part of the universe, then we should take the idea seriously, even if we can't observe it at the moment. I think consistency demands that much.
     
  7. Feb 20, 2016 #6

    Chronos

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    I agree we should not ignore observation, nor models that might account for anomolous data. But, I fail to see the risk reward benefit in dedicating resources on speculative models without good cause..
     
  8. Feb 20, 2016 #7

    Chalnoth

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    The resources involved are minuscule. People working in pure theory generally don't get much of anything in the way of grant money, and are a minority in the physics profession. And those who are considering multiverse models usually do so only as a small part of their broader research.

    Perhaps more to the point, multiverse models usually arise from other theories of physics which are being studied for other reasons.
     
  9. Feb 21, 2016 #8
    Wrong

    You don't understand how science works.
     
  10. Feb 21, 2016 #9

    Chronos

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    Just as 'proof' is unattainable in science, falsification can also be elusive. In models where the potential parameter space is unconstrained, supporting evidence is often as elusive as the proverbial needle in a haystack. This is especially discouraging when the haystack is asserted to be inaccessible. Does that justify dismissal of dark matter or the standard model, that enjoy significant empirical support? I would argue that is placing an unreasonable burden of proof upon just about any scientific model. I do, however, feel models, like multiverse, that are devoid of empirical support are deserving of a 'where's the beef?' award. I cannot take them seriously without credible empirical evidence. On the other hand, I'm perfectly OK with deducing a cow has crossed your path when you find pie on your shoe
     
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