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String theory cosmology on inflation, CMB, gravitional waves?

  1. Mar 25, 2010 #1
    cosmology strings vs loops

    How well does string cosmology account for inflation, CMB, gravitional waves, evolution classically using FRW metric? does string cosmology offer either qualitative or quantitative predictions? This paper addresses these in the loop framework,

    Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves
    Jakub Mielczarek, Thomas Cailleteau, Julien Grain, Aurelien Barrau
    11 pages, 14 figures
    (Submitted on 24 Mar 2010)
    "Loop quantum cosmology provides an efficient framework to study the evolution of the Universe beyond the classical Big Bang paradigm. Due to holonomy corrections, the singularity is replaced by a "bounce". The dynamics of the background is investigated into the details, as a function of the parameters of the model. In particular, the conditions required for inflation to occur are carefully considered and are shown to be generically met. The propagation of gravitational waves is then investigated in this framework. By both numerical and analytical approaches, the primordial tensor power spectrum is computed for a wide range of parameters. Several interesting features could be observationally probed.
    Last edited: Mar 25, 2010
  2. jcsd
  3. Mar 25, 2010 #2


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    There's been lots of work on inflation within string theory. Here are some reviews:


    The overarching assessment is that it's rather difficult to do, because supergravity corrections tend to spoil the necessary flatness of the inflaton potential. Perhaps the most technically advanced models of inflation within string theory are based on brane inflation. McAllister, Baumann, Klebanov, and others have several recent papers detailing impressive progress in this area.

    The issue of primordial gravitational waves (those generated by inflation) is an important one in string inflation. Measurable amount of GW's require the inflaton field to travel at least a distance of order the Planck mass during inflation. This is very difficult to implement in string theory because this distance is precisely where we expect our effective field theories to break down. In the context of brane inflation, the field variation is limited by the size of extra dimensions, which, for a similar reason, are constrained to be smaller than Mpl. So, it appears very difficult for string theory to provide us with a measurable amplitude of primordial gravitational waves (however, there are some exceptions: N-flation and models based on axion monodromy). As far as astrophysically produced GW's (eg from pulsars), these are understood in terms of general relativity. String theory reduces to GR at low energies, and so these are compatible.

    As far as classical evolution with the FRW metric, this is irrelevant to the ultimate UV complete theory of gravity. In cosmology, we are ultimately in the realm of effective field theory, in which all the effects of high-energy gravity are integrated out -- we say that the theory decouples from the high-energy degrees of freedom. There are however additional fields predicted by string theory that occur at low energies (eg the dilaton field). The dynamics of these additional fields have been utilized to paint very different pictures of the early universe than those of the standard big bang model. For example, the early work on the Pre-Big Bang model was string theory inspired: http://arxiv.org/abs/hep-th/9211021, in which the universe undergoes a contraction prior to the big bang and ensuing expansion phase. String/brane gas cosmology is another alternative to inflation: http://arxiv.org/abs/hep-th/9109048 in which the early universe is thought to have evolved under the dynamics of a hot gas of strings. Lastly, the ekpyrotic (or colliding brane) model of the big bang: http://arxiv.org/abs/hep-th/0103239 and it's cyclic incarnation: http://arxiv.org/abs/hep-th/0111098 are examples of string theory-based alternatives to more standard scenarios.
    Last edited by a moderator: Apr 24, 2017
  4. Mar 25, 2010 #3
    I want to say thanks for the above. As a cosmologists, how promising do you think the above is in comparison to Loop cosmology, which reproduces the observationally supported FRW metric in the classical regime? Is cosmology more dominated by strings or loops?
    Last edited by a moderator: Apr 24, 2017
  5. Mar 25, 2010 #4


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    String theory also produces the FRW metric, so this isn't a point of contention between the two models. I think what's important here is how close LQG and string theory are to being correct theories of gravity. The cosmologies they lead to are certainly secondary to that. It makes little sense to me to work on Loop cosmology before you have even ironed out all the issues of LQG, of which there are many (and similarly for strings). I'm not an expert in either field, so I can't seriously weigh in on which is more developed, although I've heard most of the arguments on both sides. I would consider any work on cosmology based on these theories as sheer speculation at this point.
  6. Mar 25, 2010 #5
    Are you a cosmologist? How many of the issues of LQG are relevant for LQC? The loop cosmology is symmetry reduced, and has an effective Hamiltonian. Can strings reproduce 4D gravity with inflation, with 6 dimensions curled? Regarding the loop paper I linked the author in addition to showing inflation as a generic feature of LQC, offers some observational predictions, specifically on B-type cmb polarizations, is this impressive or not? is inflation a generic feature of string cosmologies?
  7. Mar 25, 2010 #6


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    I am a cosmologist, but I'm not familiar with inflation in LQC, or LQC for that matter. I glanced quickly at the Ashtekar and Sloan paper claiming that inflation is generic in LQC, but I haven't had time to digest the argument. I think LQG is relevant to LQC because LQC is based on LQG! If we found out tomorrow that LQG had a fatal flaw and had to be discarded, would it make sense to continue talking about LQC? I understand that it exists as an effective theory -- but I would find it troubling to work on an effective theory whose UV completion was just found belly up in the water. Of course one could claim that the effective theory is the low energy limit of some UV completion, but if it's not LQG than that kind of belies the whole motivation for considering LQC in the first place.

    I mention above that string theory can produce inflation as a 4D effective theory with extra dimensions compactified. The most successful models are based on flux compactifications of type IIB string theory.

    The B-type CMB polarization is characteristic of tensor modes (primordial gravity waves) and will be present in most models of inflation of sufficient energy scale. LQC predicts, I believe, a blue spectrum for GWs, whereas all other theories must predict a red spectrum. Just how LQC does this without brutally violating some hallowed principle of physics, I'm not sure. I guess that could be impressive. But again, I rest the value of the cosmology on the underlying theory that motivates it.
  8. Mar 25, 2010 #7
    Earlier Marcus and I discussed a Steinhardt no-go theorem on inflation and curled dimensions. Is there any way B-type CMB polarization could be observed detected? I'd be interested in having Marcus comment on blue spectrum GW. As a cosmologist what do you think is most promising UV- complete approach if obviously not LQC.
  9. Mar 25, 2010 #8


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    Right...the no-go theorem. Ignored like anything else that runs counter to an ideology :redface:
    Sure. ESA's Planck Surveyor is up there right now looking for it and there are several ground based searches (BICEP is one, there are others that escape me now). Planck will be able to see B-modes generated by tensor spectra that have amplitudes at least 5% as large as the scalar (density) perturbation amplitude. However, many inflation models (including many motivated by strings) will have tensor amplitudes much below this threshold.
    That would be great. In standard inflation based on the FRW universe, the amplitude of tensor modes is proportional to the Hubble parameter. So, a blue spectrum (which means an amplitude that increases as one goes to smaller scales (forward in time)) requires a Hubble parameter that increases with time -- pretty hard to do in an expanding universe! I don't know how LQC gets around this...

    As for my favorite UV completion?? In all fairness, I can't say. I've not studied LQG and my dabblings in string theory are not sufficient for me to weigh in on that theory either. However, I have hung around with sufficiently many string theorists that have tried to shape my opinion on the matter :smile:
  10. Mar 25, 2010 #9
    Do you and string friends find Steinhardts no-go theorem convincing? Even if it's possible to construct a string theory that allows for inflation, is it natural or fine-tuned? Inflation appears to be generic in LQC.

    What do most cosmologists research? What are a list of issues you'd like a more complete QG to address in cosmology?
    Last edited: Mar 25, 2010
  11. Mar 25, 2010 #10


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    There are many no go theorems in the business, dealing with different regimes within the full moduli space or alternatively parameter space of the 'inspired' model. Steinhardt's argument is that various objects in extradimensional models must be deep in the NEC violating regime in order to evade his bounds. Which is very possible in stringy cosmology (orientifolds, certain types of flux compactifications and various branes can violate the NEC). Of course, what he really wants to do is to argue that these models are no better or worse than his cyclic model (which also violates various energy conditions), and were heavily criticized on that point. But anyway, these sorts of things are usually desirable for stringy models, b/c of the enormous freedom in model building and is actually one of the possible methods for finding an eventual vacuum selection method.

    However, generally speaking, energy conditions, and much of the rest of this business is based on semi classical analysis, and remains somewhat speculative when naively transfered over to the full quantum gravity theories. Its messy, counterintuitive and bizarre, and far from settled (most physicists are greatly skeptical about pretty much everything to do with so called quantum cosmology, simply b/c of how far from experiment and fundamental theory these things can get, even in so far as they often are not necessarily trivially related to the parent theory).
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