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M-theory and gravitational waves

  1. Jul 16, 2010 #1
    I've read that the discovery of gravitational waves will further support the big bang theory at the expense of M-theory. What's the specific difference between the two models that relates to gravitational waves? Is it that a brane collision would not produce the waves at the beginning of the universe?
  2. jcsd
  3. Jul 17, 2010 #2
    You must have a few things mixed up.

    1) First, M-theory is not a theory about the beginning of the universe, it's a theory about what the basic entities in nature are (namely, 2- and 5-dimensional branes, which are formed from the 0-branes of "Matrix theory", and which become 1-dimensional fundamental strings when some of M-theory's 11 dimensions are compactified). Ekpyrotic cosmology - a cyclic universe where two branes clash and rebound repeatedly, like cymbals - is inspired by the string-theory idea of braneworlds, but it's not the only or even the standard cosmological picture in string theory.

    2) Gravitational waves are just a standard phenomenon in any theory that contains Einstein's gravity (general relativity), including string theory. They have not been observed directly, but the 1993 physics Nobel was awarded for measurements of a binary pulsar system which is spiraling inwards because it is losing energy at a rate consistent with the production of gravitational waves.

    You must be talking about observations from the early universe, e.g. features of the cosmic microwave background. That data *is* used to distinguish between different theories of how it all began: big bang vs ekpyrotic, or one theory of inflation versus another. Presumably the brane collision in the ekpyrotic cosmology does produce distinctive effects, but I don't know what they are. If you search recent papers on arxiv.org you might find something.
  4. Jul 18, 2010 #3
    mitchell's description above matches my understanding: repeatedly colliding branes (not M theory specifically) producing a "small bang" is the ekpyrotic model versus a one time "big bang" model...the only thing I'd question is the comment about inflation...

    Inflation is an 'add on', a separate theory, to complement the big bang model and provide smoothing (uniformity) observed in our universe and the two I think are now routinely called the big bang model as if they were developed together as an integrated entity. In the one cyclic model I have read about, noted below, dark energy before the brane collison acts as a smoothing mechanism, not inflation. But there are many big bang models and perhaps more than one cyclic model as well.

    You can read about the specific milestone (experimental) tests that both models must pass in Chapter 9 of Paul Steinhardt and Neil Turok's THE ENDLESS UNIVERSE [which means endlessly collding branes]...As of 2006 it looked like both models pass five tests and a sixth is to measure the characteristics of gravitational waves. Apparently it was believed at the time of the book the necessary WMAP measurements can probably distinguish between the models..
    Be back later if I can figure out the gravitational wave differences....
  5. Jul 18, 2010 #4
    thanks for the responses. I now understand that it's the effect of gravitational waves on the cosmic radiation background rather than the waves themselves that is the difference.
    measurable effects would support the big bang model
  6. Jul 19, 2010 #5
    jraace: I am unsure what you last post means.... You can compare your thinking to these excerpted explanations from the same book. [Steinhardt and Turok] I posted above:

    page 201-208: Gravitational waves are the key to the sixth milestone test (distinguishing inflationary from cyclic model). For inflation, the tidal (my term, the authors call them warps and wiggles) effects ranging from those that have been stretched to those that are still microscopic in size have roughly the same height and depth and all contribute to the distortion of space (SCALE INVARIENCE) . Gravitational waves leave a distinct imprint on the cosmic background radiation pattern.

    The cyclic model gravitational waves are completely different: The energy density of the universe during the phase when long wavelngth gravitational waves are generated is miniscule compared to the inflationary model. The gravitational waves produced are not scale invarient: their amplitude increases sharply as their wavelength decreases. This is because accelerating branes cause quantum jitters to increase as the colliding branes approach each other.

    And finally, they discuss around page 209-212 that energy density fluctuations and gravitational waves are both sources of non uniformities causing polarizations. Gravitational waves create temperature variation and polarization by squeezing in one direction and stretching in a perpendicular direction (tidal effects). Gravitational waves produce both E mode (because of their resemblance to electric field patterns) and B mode polarizations, (ROTATIONAL) energy density variations.

    The presence of B mode would confirm the inflationary model. So far they have not been detected, and the E mode that has been detected supports both inflationary and cyclic models. Apparently a new satellite and instrumentation is required and is feasible. It was thought the European Space agency PLANCK satellite then scheduled for a 2008 launch (and results around 2009 or2010) could make the necessary measurements. Neither LISA nor LIGO were thought to have the needed sensitivity.
  7. Jul 21, 2010 #6
    Apparently no test results yet to distinguish between inflationary and cyclic models.

    From Wikipedia:


    The European Space Agency website is here:
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