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Brane cosmology and extra dimensions

  1. Jan 9, 2013 #1
    I have a few questions so I hope it is okay if I ask them all here in this thread. They are all related somewhat.
    According to brane cosmology, our 4-D universe is embedded inside a higher dimensional space, a hyperspace if you will, called the bulk. The reason we cannot perceive or interact with the bulk is because we are "stuck" in our brane.

    Lets say that we somehow became unstuck and were able to navigate into the higher dimensional space, what would we perceive? Would we be able to see into our 3d brane universe from the outside? Would we be able to interact with things IN the brane?
    All this is just like asking, what would we perceive if we took a ride on a photon or went into a black hole. I dont mean to suggest that it is actually possible to leave our brane.

    While we are on the subject, in brane cosmology, how many spacial dimensions does the higher dimensional space have?
    In brane cosmology, are there still compactified dimensions in our own brane universe? Would there effectively be extra spacial dimensions inside AND outside our universe?

    One more thing if its alright. The universe's expansion is described as a metric expansion of space, where the the expansion is intrinsic and therefore does not need or refer to anything out side the universe to expand. But if we live in a higher dimensional bulk as described in brane cosmology, would it be appropriate to say that the universe is actually expanding into this higher dimensional bulk?
     
  2. jcsd
  3. Jan 9, 2013 #2
    If you haven't already , you should get a general perspective here:

    http://en.wikipedia.org/wiki/Brane_cosmology

    [It answers some of your questions]

    My comments below will relate to the ekpyrotic cyclic model of Neil Turok and Paul Steinhardt where the current universe came about from the finite collision of two branes:

    The known laws of physics are not much help because they describe how things evolve in time not how time can begin. The big bang and cyclic model agree after the one second mark, but differ dramatically before that time.

    Cyclic Model: the Ekpyrotic Universe [From the author's book]

    “according to the new cosmological (cyclic) model, (cyclic) bangs are triggered by the decay of dark energy (from the previous cycle). Dark energy decay {at the end of one universe} is a remarkable process that can smooth and flatten the universe, and create small density variations just as effectively as the process of inflation does{at the beginning of a universe} in the inflationary model….the universe emerging from the bang is naturally flat and smooth.”

    My comments in parenthesis {}.

    By combining Heterotic M theory (with it's two parallel branes as envisioned by Witten and Horava), a cyclic universe, endlessly repeating, can be theorized in ten dimensional space between the branes. Dark energy plays three fascinating roles at different cycle stages: it speeds up the expansion rate, as currently observed, it acts as a stabilizer by acting as a shock absorber (analogous to an automatic pneumatic door close cylinder), and it decays over long periods, shutting itself off. Dark energy is dynamic! It means the universe does NOT have to expand forever.

    Heteroic M theory was the stimulus for this model: ten space dimensions bounded by two parallel braneworlds, with tube like membranes between them….
    Based on Horava and Witten model: branes are boundaries for the tenth dimension which reachs between them…space exists only between the branes,
    A few key elements: branes and extra dimensions, dark energy and the slow decay of dark energy.
    Repeated collisons between branes form new cycles and universes,
    Collisions are unavoidable and fill the branes with hot but finite and nearly uniform radiation. ..Brane kinetic energy is converted to matter and radiation.
    Dark energy dominates before the bang, stretching the branes making them flat, smooth and parallel…

    A cycling between high pressure and low pressure dark energy, gravity, PE and KE
    enables the process the continue forever via conservation of energy…
    Quantum brane jitters (ripples) cause the branes to collide at slightly different times at different places..early locations would have slightly lower temperature and density today.


    Usually, string theories seem to have such tiny additional dimensions....

    In the Turok Steinhardt model, the branes encompassing the bulk are being pushed further apart by dark energy. That's different, I think, from your statement.

    A bit more more on their model here:
    http://en.wikipedia.org/wiki/Ekpyrotic
     
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