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Dark energy related to 10-dimensional universe

  1. Jul 11, 2012 #1
    Hi, I am an amateur fascinated by quantum physics and... trying to understand it. Sorry if my question is too radiculous.

    I read some time ago (http://www.dailygalaxy.com/my_weblo...-universe-had-10-dimernsions-at-big-bang.html) that there might have been 9-dimensions plus time at the Big Bang. The article says some of them expanded more than the others. Could it be the other way around? Could some of them 'have collapsed'? Could Big Bang then be a 'collapse' of some dimensions and the space/matter/energy would have been pushed out so the other dimensions would expand? Could then this have been the case with inflation phase and contemporary accelaration of the expansion of the universe? We would be in 4D (plus time) universe changing to 3D? Could that be the 'dark energy' responsible for the expansion of the universe?

    Again I'm sorry if this is an absurd question but I'm kind of entry-level.
  2. jcsd
  3. Jul 11, 2012 #2
    You're referring to string theory, not quantum mechanics. String theory is a theory of quantum gravity - it unifies Einstein' theory of general relativity with quantum mechanics. These two theories are the two pillars of modern physics. However, general relativity is what we call a classical theory, like Newton's laws of motion and Maxwell's equations of electrodynamics. In order to turn a classical theory into one that can describe interactions of the quantum scale, you have to 'quantize' it. That is, you need to impose certain rules from quantum mechanics on it (more precisely, you promote physical observable properties to mathematical objects called 'operators', which act on the state of a particle or group of particles.)

    This works great for every other classical theory, but not general relativity. When you try to introduce a particle that corresponds to the gravitational field (which you may recall is the geometric curvature of space-time in general relativity.), called the graviton, everything breaks down. Without going into detail, it's because particles can participate in virtual processes before they can interact. For example, a photon may become an electron and a positron, but then annihilate back into a photon. So, you must account for all of these processes by integrating over a large amount of different momenta. Since the Uncertainty Principle says that your uncertainty in position multiplied by your uncertainty in momentum must always be less than a specific value, and since particles interact at exactly zero distance, you must integrate to infinity. Once again, this still works fine for everything else - but introducing gravitons causes this integral to blow up to infinity, a nonsensical answer. However, since, in string theory, every particle is really an extremely small string, interactions occur at non-zero distance. This turns out to resolve the issue, and let's you retrieve sensible calculations.

    On top of this, string theory is a 'theory of everything'. The different vibrational modes of the string produce the properties of different particles. This gives rise to all of the particles of nature, and all of the forces.

    However, it include some other states, too. Namely, tachyons, which have negate mass squared, and particles with negative energy states. In order to rid string theory of the latter, you need to introduce 26 dimensions of spacetime into the theory. To deal with the first, you need to incorporate a hypothetical symmetry called 'supersymmetry'. This removes the tachyons, and reduces the number of required dimensions to 10. However, it was discovered that there are 5 general ways of formulating a string theory - 5 different theories. It was realized, by Edward Witten and others, that these strong theories fit together through 'dualities'.

    This leads to a unified string theory called 'M-theory', with 11 space-time dimensions.

    So, obviously, the questions is 'where are these extra dimensions?'. In string theory, they're compactified - they're made extremely small, so that they're unobservable.

    However, as you're question mentions, our three dimensions would have been equal in size to the other seven space dimensions - so, why did our 3 grow large? We don't know exactly. That field of string theory is called 'string gas cosmology', and deals with how the geometry if the extra dimensions would affected the early universe, and how they were compactfied.

    Also, it is very important to keep in mind that string theory is as hypothetical as it gets - it has no empirical evidence. We have no idea if these extra dimensions actually exist.
  4. Jul 11, 2012 #3
    could be. there is no theory for what caused the 'big bang' nor even if there was a 'big bang'. nobody knows what happened.

    the big bang model of cosmology is the FLRW model
    [see wikipedia:http://en.wikipedia.org/wiki/Big_Bang_model] [Broken]
    for what happens AFTER the initial 'bang'.

    All our theories assume an initial 'bang' and we have models and mathematics for what happens just after.
    Last edited by a moderator: May 6, 2017
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