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Big bangs in the multiverse.

  1. Jul 19, 2012 #1

    MathematicalPhysicist

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    In the multiverse we have multiple universes, in this model does every universe start with a big bang? does this model explain how these big bangs start to occur?

    Any litertaure on this model that discusses these issues?

    Thanks in advance.
     
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  3. Jul 19, 2012 #2

    Bobbywhy

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    When I entered "multiverse" into the arxiv search box it comes up with 126 separate papers. Probably among those 126 papers the subject of big bangs is discussed. But that is only my imagined guess. You'd have to go there yourself to find it.

    http://arxiv.org/find/all/1/all:+multiverse/0/1/0/all/0/1
     
  4. Jul 19, 2012 #3

    Chronos

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    The basic 'multiverse' model asserts, 'big bangs' occurs continuously throughout an unlimited number of little patches of 'observable' universes. Many astrophysicists consider this an 'imaginative' idea. Like nearly all such models, it is invulnerable to falsification via observational evidence - which, IMO, places it on similar footing as refrigerator light fairies.
     
  5. Jul 19, 2012 #4
    I'm studying inflation, and I've encountered a field named "inflaton", postulated to explain inflation. This model is called "the slow-roll inflation". The Inflaton is a massless field which can assume arbitrarily random values, and when it assume a sufficiently large value, it "explodes" in a Universe. But this can happen more than one times, leading to parallel separated universes.
     
  6. Jul 19, 2012 #5
    No, that's a misrepresentation of eternal inflation. In the inflationary multiverse, slowly expanding regions of space are separated by rapidly expanding space still undergoing inflation. The idea is that the decay rate of the inflaton field is smaller than the speed at which the size of the universe doubles. So, as regions of inflating space settle down to their true vacuum states and stop inflating and begin expanding slowly, these regions are driven apart by the still inflating space. More bubbles continue to form (as regions of the inflation field reach their true vacuum through quantum processes), but the rapidly expanding inflating region will keep them separated because of how enormously fast it's expanding. This creates an ensemble of slow expanding pocket universes separated by space still undergoing inflation. Hence, a multiverse.
     
  7. Jul 19, 2012 #6

    I would read this:
    http://arxiv.org/abs/hep-th/0702178

    or watch this lecture:


    Guth argues that a strong case can be made that the universe underwent inflaiton and that inflation is generically eternal ie that the decay of the inflaton field that led to our universe didnt just happen once but happens eternally as the inflaton field expands fatser than it decays.
    Now one can certianly take issue with this, perhpas we dont need inflation at all as people like Turok, Penrose, Steindhart and Magueijo have proposed or perhpas one can construct models of inflaiton that are not generically eternal. Marcus who is one of the most knowledgable people here on comsology forums will hopefully point you in the right direction for such models.

    Nevertheless to accept thje multiverse hypothesis is to accept that inflation occured and its generically eternal, both of these assumptions may be false but to compare them to believing in faireis seems somewhat unfair to me. But read Guths paper and see what you think.
    Eternal inflation is not the only mdoel of the multiverse but i think its the one that attracts the most attention in the literature.
     
    Last edited by a moderator: Sep 25, 2014
  8. Jul 19, 2012 #7
    One such model:

    Spontaneous Inflation and the Origin
    of the Arrow of Time
    Sean M. Carroll and Jennifer Chen 2004

    http://arxiv.org/pdf/hep-th/0410270v1.pdf

    This is a paper with a wide ranging discussion of cosmologicallly related concepts and is quite readable. You should note that the paper does not used the world MULTIVERSE as I recall...but the discussion and concepts lead directly there....

    Big bangs are themselves currently speculative.....no one knows if they are big or not...because our models fail at those apparent 'singularities'. Another way to say the same thing: The known laws of physics are not much help because they describe how things evolve in time not how time can begin.

    In simple terms you can take a 'rare' event as the cause of our universe, statistically unlikely but guaranteed to happen eventually by quantum mechanics, or you can take the 'ho hum' view that events leading to new universes happen all the time but are not observable by us. An anthropic approach suggests that our finely tuned universe allowing our kind of life arises among a multiverse most of which are not tuned so as to allow our existence: so in a sense inflation gave the anthropic principle 'new life'.,,allowing our 'special universe'; But, alas, so far 'root causes' of new universes elude us!

    Also, you might try searching for 'eternal inflation' as a corollary to 'multiverse'....
     
  9. Jul 19, 2012 #8
    I met Sean Caroll last weekend; I wanted to clear up the difference between the Caroll/Chen model and eternal inflation. What the former suggestes is that space can tear into new pieces whcih then undergo inflation , this is different to eternal inflation where the space is still connected , he wrote a superb popular book on the subject :
    http://preposterousuniverse.com/eternitytohere/
     
    Last edited: Jul 19, 2012
  10. Jul 19, 2012 #9
    I see MarkM posted while I was composing...his description fits the Carroll -Chen paper I referenced...here are some brief notes I made..quotes or near quotes from the paper:

    The 'large curvatures" above means 'big bang conditions' and the authors say rather than starting from such entropically disfavored [rare] conditions, universes in their proposal pop out from 'high' entropy [but not very high] flat de Sitter space where thermal quantum fluctuations are sufficient to spawn new universes.
     
  11. Jul 19, 2012 #10

    marcus

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    Skydive, thanks for the kind words!

    Guth's argument fails for the reason you suggest. One can indeed construct such models--they are simpler and more open to observational test.

    Guth came along before there was much development of quantum GR. He formed his ideas using classical GR and was forced to make elaborate assumptions. By contrast in quantum GR (for instance in the context of Loop cosmology) inflation arises naturally without the extra baggage of assumptions that, in Guth's scenario, make it generically eternal.

    http://arxiv.org/abs/1103.2475
    Probability of Inflation in Loop Quantum Cosmology
    Abhay Ashtekar, David Sloan
    (Submitted on 12 Mar 2011)
    Inflationary models of the early universe provide a natural mechanism for the formation of large scale structure. This success brings to forefront the question of naturalness: Does a sufficiently long slow roll inflation occur generically or does it require a careful fine tuning of initial parameters? In recent years there has been considerable controversy...
    ...dramatically different predictions can arise because the required measure on the space of solutions is intrinsically ambiguous in general relativity. We then show that this ambiguity can be naturally resolved in loop quantum cosmology (LQC) because the big bang is replaced by a big bounce and the bounce surface can be used to introduce the structure necessary to specify a satisfactory measure.
    The second goal of the paper is to present a detailed analysis of the inflationary dynamics of LQC using analytical and numerical methods. By combining this information with the measure on the space of solutions, we address a sharper question than those investigated in the literature: What is the probability of a sufficiently long slow roll inflation WHICH IS COMPATIBLE WITH THE SEVEN YEAR WMAP DATA? We show that the probability is very close to 1. ...
    34 pages, 3 figures
     
  12. Jul 19, 2012 #11
    Hi Marcus , two questions that seem relevant to me are
    (1)how does inflation start?
    (2) how does inflation end ?
    As I understand it LQC provides us with a fundamentally new answer to (1) inflation is the result of the big bounce. This gets rid of Turok's fine tuning objection to inflation, right?

    However does LQC differ from Guths narrative in the way that inflation ends? Guths paper seems to me to argue that inflation is eternal due to the way it ends ie the inflaton field expands at a faster rate than it decays, so there is always some new space undegoing inflation -I belive this is what one needs to assume to get a mutlvierse. Am I wrong?
    So I'm not clear how whether inflation stars as some random fluctuation or whether its through a quanutm bounce, how does this challenge Guth's argument? Can you elabaorate? Does LQC not have an inflaton field? Does it give us a reson to change the assumption of the relative decay rates? or is there some other affect that negates Guth's argument?
     
  13. Jul 19, 2012 #12
    skydive:
    Glad you posted that. Are your sure that is correct? From the Carroll-Chen paper I inferred their old and new spaces WERE causally connected. But now that I reread their summary I am not so sure, Maybe you or someone can infer from this:

    Otherwise, I'll try and reread the appropriate section of their paper if I can find it!

    Can a model be 'time symmetric' AND 'tear' ???
     
  14. Jul 19, 2012 #13
    skydive:
    I see the Carroll Chen paper, section 3.2, discusses STARTING INFLATION if
    you are interested. I saved this key piece via notes

    Seemed a bit 'hand wavy' to me, but so are many other such descriptions trying to describe a 'start'....
     
  15. Jul 19, 2012 #14
    Hhm not sure where this is in the original paper but in Caroll's book "From eternity to here"its the text that surrounds Fig 85, page 356 in the Uk edition. He distinguishes between baby universes that come from space time splitting off and pocket universes from eternal inflation. Just to check that was the key difference I asked him last weekend and he confirmed thats correct.
     
  16. Jul 19, 2012 #15
    well, thta must then be the case.....thanks.

    Either way, won't upset me and my Yorkies!!!
     
  17. Jul 20, 2012 #16

    marcus

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    An interesting paper on LQC inflation came out today!
    http://arxiv.org/abs/1207.4353

    As I recall from earlier discussions, LQC gets around several problems associated with inflation because the bounce has a natural brief period of faster than exponential growth. I Hubble rate H increasing to Planck scale (!), they call it "superinflation", rather than steady or declining H (as in slow roll inflation with quadratic potential). The bounce provides a brief kickoff like that generically, without extra assumptions.

    Then you can add on a common type of quadratic potential inflaton and it doesn't require fine tuning to ensure it delivers 60 efolds, because the kick of superinflation is guaranteed to get it all energized, high up on its potential hill. You just let it peter out by itself.

    No multiverse is involved, basically because the scenario does not rely on any low probability events. You don't need a quantum fluctuation to get started, or a random decay to get stopped. You don't need fine tuning to ensure that the inflaton field will give you an adequate 60 efolds.

    I'm remembering back to some Ashtekar et al papers, which you can look up. Essentially the message was that the bounce makes adequate inflation a fairly straightforward proposition if a rather commonly assumed type of inflaton field is put in the picture, and no bother about multiverses.

    But rather than go back to those 2011 Ashtekar papers, I would say have a look at the paper by those three young researchers. Tom Pawlowski is a Warsaw postdoc. They have some new ideas. I was just looking at the paper and was excited by it. It is is less speculative and more concrete about inflation. They do some interesting numerical simulations. You might like it.
     
    Last edited: Jul 20, 2012
  18. Jul 20, 2012 #17

    marcus

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    Skydive, this is the paper I mentioned in post#15, when I was replying to your earlier question.http://arxiv.org/abs/1207.4353
    Inflation from non-minimally coupled scalar field in loop quantum cosmology
    Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
    (Submitted on 18 Jul 2012)
    The FRW model with non-minimally coupled massive scalar field has been investigated in LQC framework. Considered form of the potential and coupling allows applications to Higgs driven inflation. The resulting dynamics qualitatively modifies the standard bounce paradigm in LQC in two ways:
    (i) the bounce point is no longer marked by critical matter energy density,
    (ii) the Planck scale physics features the "mexican hat" trajectory with two consecutive bounces and rapid expansion and recollapse between them. Furthermore, for physically viable coupling strength and initial data the subsequent inflation exceeds 60 e-foldings.
    14 pages, 5 figures
     
  19. Jul 20, 2012 #18
    If I understand things correctly a multiverse is argued to be generated in slow roll inflation beucase whilst some of the false vacuum/inflaton field decays into a true vacuum state , the remaining section which has not decayed yet, is expanding exponentially, so the total volume of inflating space never declines and hence inflation is eternal. So in order to prevent a multiverse something presumbaly has to happen to get the whole inflaton field to decay everywhere at the same rate and at the same time. Is that the case in LQC or have I misunderstood?
     
  20. Jul 20, 2012 #19

    marcus

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    I think you are imagining eternal inflation scenario rightly. I'm not expert but I'll suggest an alternative picture:

    A universe with finite spatial volume collapses and rebounds. There is no "horizon problem" since as it collapses it is all in causal contact.
    The entire universe enters superinflation together at a single moment. The scalar field (Higgs or whatever is serving as inflaton) is everywhere primed to the same energy. "Pushed up to the same place on the potential hill" by superinflation.

    The entire (still quite small) universe stops superinflation at the same moment when the Hubble rate reaches Planck max, and enters inflation mode. The scalar field rolls down uniformly and the entire universe exits from inflation at the same moment.

    I found Tom Pawlowski's article quite interesting at first glance yesterday. I'll take another look now. You might wish to also, as a way of getting a more definite idea of inflation in Loop cosmology.
     
  21. Jul 20, 2012 #20

    marcus

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    I'll quote some sample excerpts from Pawlowski http://arxiv.org/abs/1207.4353
    ==quote 1207.4353 page 1==
    The inflation paradigm is one of the most successful ideas allowing to explain recent precise cosmological observations. However one of its main problems is the construction of the physically viable scenario featuring sufficiently long inflation epoch (> 60 e-foldings) with sufficiently large probability. In this context a lot of hope is attached to the models featuring a non-minimal coupling of gravity and a scalar field (e.g. the Higgs field [1–3]), ... Such mechanism of driving the inflation is also efficient in generating the correct primordial curvature perturbations.
    ...
    At this point it is worth noting that the type of the potential considered, while usually associated with the models of Higgs inflation, is not restricted just to this particular field...
    ...Thus, recent results from CMS [8] and ATLAS [8] (mH ≃ 125 GeV) are both consistent with SM Higgs inflation.

    While the considered model is very successful on the classical level it still suffers the standard problems related with the presence of initial singularity, which are expected to be solved by quantum gravity. One of the leading approaches to provide quantum description of spacetime itself is Loop Quantum Gravity (LQG) [9–11]. The cosmological application of its symmetry reduced version, known as Loop Quantum Cosmology (LQC) [12], has indeed provided a qualitatively new picture of early Universe dynamics. The prediction of the so-called big bounce phenomenon [13] offered a new mechanism of resolving long standing cosmological problems. For example, the existence of a pre-bounce epoch of the Universe evolution provides an easy solution to the horizon problem, while preliminary studies indicate that the dynamics in the near-bounce superinflation epoch prevents the catastrophic entropy increase [14, 15] usually considered a danger to bouncing cosmological models (following the consideration of [16]). What’s even more important, the spacetime discreteness effects amount to a dramatic increase of the probability of inflation in the models with standard m2φ2 potential scalar fields [17] (see also [18]). Indeed for such models the probability of inflation with enough e-foldings to ensure consistence with 7 years WMAP data happens with probability greater than 0.999997. These results make the loop approach very attractive in inflationary cosmology.
    ==endquote==
    In looking over 1207.4353, I followed reference [15] and found another interesting Pawlowski one:
    http://arxiv.org/abs/1106.3793
    "Effective dynamics of the hybrid quantization of the Gowdy T3 universe"
    Basically they study a compact highly inhomogeneous bounce. The challenge was to incorporate a lot of inhomogeneity (gravity waves) in the geometry. Parts of the conclusions section on page 18 were thought-provoking. But this doesn't directly relate to the topic of inflation, which our discussion was about.
     
    Last edited: Jul 20, 2012
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