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Vacuums and the Big Bang

  1. Jul 24, 2012 #1
    I apologize if this is a "stupid" question, but I have been trying to find information on this for weeks and it has been to no avail thus far.

    I know that there are two theories in regards to what came "before" the singularity; the cyclic universe and the "universe came from a vacuum fluctuation" theory. I understand the former relatively well, but I just do not understand the latter, and was wondering if someone could offer a hypothetical explanation.

    I understand that vacuums can be responsible for the temporary creation of particles. What I do not understand is how those particles could expand and become our current universe. Is the answer just "it has something to do with quantum gravity, which we do not understand," or is there a hypothetical situation in which the above could occur? If my conclusions are erroneous, please explain.

    Thank you so much.
     
  2. jcsd
  3. Jul 24, 2012 #2
    about cyclic universes


    http://arxiv.org/pdf/1204.4658v1.pdf
    Audrey Mithani, Alexander Vilenkin

    ...The first two of these scenarios are geodesically incomplete to the past, and thus
    cannot describe a universe without a beginning. The third, although it
    is stable with respect to classical perturbations, can collapse quantum
    mechanically, and therefore cannot have an eternal past....


    http://arxiv.org/pdf/1204.5385v1.pdf
    Leonard Susskind

    ...Combing the Mithani-Vilenkin's observations with the ones in this note, we may
    conclude that there is a beginning...
     
    Last edited: Jul 24, 2012
  4. Jul 24, 2012 #3

    Chronos

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    It is possible the universe emerged from some prior state that was fundamentally different from any state known to exist today, but, it is unclear if that hypothesis is testable. The classic cyclical universe model has thermodynamic issues.
     
  5. Jul 24, 2012 #4
    Well, I disagree a bit with your categorization of pre-big bang models. There are an enormous number of different theories, the 'from nothing' part is usually associated with different models. For example, chaotic inflation holds that inflation began out of a vacuum, producing a universe. Sean Carroll's model also involves universe production out of a vacuum, but in his model, an eternal, cold, de Sitter space perpetuates the production of the universes. There are also a plethora of cyclic models, not just one or two.

    So, the vacuum fluctuation is a part of many cosmologies. It's based off of a widely accepted theory called inflation - in inflation, the early universe is filled with a scalar field called the inflaton field. The field's potential energy curve would be such that it had to overcome a large potential energy 'hill' to reach the lowest energy state. This is called a false vacuum. One example is supercooled water, water that is cooled below it's freezing point. It wants to get to the energetically favorable frozen state, but doesn't have enough energy to undergo the freezing process. It is incredibly unstable, disturbing supercooled water will cause it to freeze.

    This desirable energy state, a solid in the case of supercooled water, is called a true vacuum. The inflaton field wants to reach the true vacuum, but it doesn't have sufficient energy to get out of it's false vacuum. So, it was stuck.

    It turns out that a scalar field in a false vacuum produces an enormous negative pressure. This functions as an extremely powerful expansion, expanding the universe by unthinkable amounts. After a short period of time, quantum tunneling allows the inflaton field to reach it's true vacuum, and inflation ends. Also, the inflaton field then decays away into a hot bath of radiation, reheating the universe to a sufficient temperature for big bang nucleosynthesis.

    So, there are a few models of inflation that allow it to begin from a vacuum. There is chaotic inflation, for one. In this model, the inflaton field is in a normal state, and the universe is cold and empty. Quantum tunneling allows the inflaton field to get into it's false vacuum - even though classically this is impossible, it can occur in quantum mechanics. From there, inflation takes over.

    The other idea that has gained popularity is the Fahri-Guth mechanism. The idea is that an inflationary false vacuum can form in empty space. In their paper (which is unfortunately not available on the internet), they showed that such a state was unstable, and should collapse - and actually could tunnel out of it's space-time to prevent a naked singularity from forming. This would become a new space-time, and inflate as according to normal inflation. This 'baby universe' would be connected to the original universe through a wormhole. Since these are unstable, it will collapse, and the mouths in each universe will become black holes in each universe. They will then be totally seperated.
     
  6. Jul 24, 2012 #5

    marcus

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    You are going to get conflicting information. I hope you don't mind this. In science the really interesting questions are often surrounded by people confidently asserting conflicting ideas.

    If you have a few moments to peek into the collective mind of the world's experts (chaotic as it is) you could glance at the list of topics at this big triennial conference that just happened in Stockholm. Over a thousand participants this time!
    http://www.icra.it/mg/mg13/
    The list of parallel sessions (don't bother only in case of curiosity):
    http://www.icra.it/mg/mg13/parallel_sessions.htm

    Nonsingular cosmology made a strong showing this time. Typically that means a big bounce cosmology where a contraction rebounds, starting the expansion that we see. No attempt to explain where the contracting universe "came from". That's a problem for another day. The problem of why existence exists is for philosophers and the general public to think about. these people just want to understand the mechanism underlying the start of the expanding universe we see.

    There was not much about "quantum fluctuations" or about the ideas of people like Hawking, Vilenkin that go back to the 1980s and 1990s. Fashions change. I really think there is no dominant model at present. People will talk to you in tones of absolute certainty but a lot of it is still just hypothetical and speculative. The picture of the human mind grappling with the start of expansion is neither pretty or simple.

    I have faith that beauty and clarity will emerge out of the current chaos. I trust us to manage this.

    Here is my personal favorite cosmology session of the Marcel Grossmann meeting:
    http://ntsrvg9-5.icra.it/mg13/FMPro...tField=order2&-SortOrder=ascend&-Max=50&-Find

    I'm particularly excited by the work that Francesca Vidotto and Andrea Dapor presented at that session!
    And Francesca is a PF member who has recently been contributing to discussion at the "Beyond Standard" forum! She is too busy to be around here regularly but comes in once and a while. Here is one of francesca's posts https://www.physicsforums.com/showthread.php?p=3998710#post3998710
    She has 8 or 10 more in that thread as I recall.
    Andrea and two other people just released a preprint of some research where they showed how you could have a bounce cosmology in which the inflation right after the rebound was caused by the Higgs field. Nice result!
    I can't speak with any kind of assurance but I really like what all the people in that particular conference session are doing. They are a young bunch. Mostly postdocs.

    Can't resist pasting in the abstract of one of the two presentations that Andrea gave during that session:
    http://ntsrvg9-5.icra.it/mg13/FMPro...s&talk_accept=yes&-max=50&-recid=42162&-find=

    Speaker
    Dapor, Andrea
    Co-authors
    Michal Artymowski, Tomasz Pawlowski
    Talk Title
    Loop Quantum Cosmology for nonminimally coupled Scalar Field
    Abstract
    We conduct a LQC-quantization of the FRW cosmological model with nonminimally coupled scalar field. (This model is interesting from the classical point of view because it allows heavy fields (such as the Higgs) to produce inflation.) Making use of a canonical transformation (between Jordan variables and Einstein variables), we recast the theory in a minimally coupled one, for which standard LQC techniques can be applied to find the physical Hilbert space and the dynamics. Though the analysis of the genuine quantum system can be performed, we focus on the semiclassical sector - obtaining a "classical" effective Hamiltonian. At this level, we can transform back to the Jordan frame, and study the dynamics. It turns out that the initial singularity is replaced by a "mexican hat"-shaped bounce, joining the contracting and expanding branches.
    ==============

    If you click on any of the talk titles in the program you get an abstract summary of the talk so you can check to see, for example, what Francesca was talking about. She also gave a talk at another sessions like QG1 A.
    http://ntsrvg9-5.icra.it/mg13/FMPro...tField=order2&-SortOrder=ascend&-Max=50&-Find
     
    Last edited: Jul 24, 2012
  7. Jul 24, 2012 #6

    marcus

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    Here is another session of the Stockholm conference that dealt with nonsingular cosmologies. Mostly bounce I gather.
    http://ntsrvg9-5.icra.it/mg13/FMPro...tField=order2&-SortOrder=ascend&-Max=50&-Find

    I had forgotten, I see Andrea gave a talk at this session too---same stuff Loop bounce with Higgs-driven inflation.

    Not much of the older ideas of "quantum fluctuation" and "eternal inflation" were in evidence anywhere as far as I can tell.

    There is another triennnial series of big international conferences that deal with cosmology, the big bang, the geometry of the universe and such like topics. As it happens that one will be held NEXT YEAR in Warsaw starting July 8. This is the socalled "GR" conference.
    Next year will be the 20th in the series so it will be called "GR 20".

    These are the two main big international conferences where you can see what the current state of the field is. Who chairs the sessions, what the lineup, what are the hot topics etc.
    So we will get a chance next year to check and see if our impressions hold up of how things are going.
     
  8. Jul 24, 2012 #7

    phinds

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    How EXTREMELY well said, Marcus.
     
  9. Jul 24, 2012 #8
    Also confidentally asserting conflicting ideas is how science proceeds. A lot of science involves "boxing matches" between rival ideas, and when you are in a "boxing match" you are going to push your idea as hard as you can, knowing that other people are going to push their ideas as hard as they can.

    One thing that's interesting is that physicists often communicate in a peculiar way that confuses people that aren't used to it. A lot of communication is to not only communicate facts but also emotional state. You not only are trying to communicate an idea but what you think about that idea. The thing about "boxing matches" is that you try to strip out the "emotional" parts of communications. So when a physicist talks about it idea, it often sounds like they are 100% sure of it, even if they aren't, because having two sides communicate "as if" they are 100% sure of something is how you end up with intellectual boxing.
     
  10. Jul 24, 2012 #9
    One other cool thing is that it will ultimately get settled with data. We have a lot of detailed maps of the cosmic microwave background, and we have enough data to figure out what happened when the universe inflated, and there is a good chance that there is something in the data that will tell us about the pre-inflationary state of the universe.

    To put it poetically, the big bang was loud and produces a lot of sound waves which we can observe because the sound was frozen in various places in the universe. We can look at the sound of the big bang and then figure out what happened.

    The hard part of coming up cosmological ideas is not to come up with ideas. That's easy. The hard part is go show that they are wrong. If you can't show that an idea is wrong, then it's sort of pointless to spend lots of time coming up with the idea. The cool thing is that we are getting flooded with data now.
     
  11. Jul 25, 2012 #10
    as for penrose cyclic model

    http://arxiv.org/pdf/1012.1268v1.pdf

    ...Thus, we conclude that there is no evidence for the CCC model in the current WMAP data...

    http://arxiv.org/pdf/1012.1305v3.pdf

    ...However, we find that this variation is entirely expected in a sky which contains the usual CMB anisotropies. In other words, properly simulated Gaussian CMB data contain just the sorts of variations claimed. Gurzadyan & Penrose have not found evidence for pre-Big Bang phenomena, but have simply re-discovered that the CMB contains structure...


    http://arxiv.org/pdf/1012.1656v1.pdf

    ...By comparing with Monte Carlo simulations of the CMB sky, we find that the low variance circles of Gurzadyan & Penrose (2010) are not anomalous...
     
    Last edited: Jul 25, 2012
  12. Jul 25, 2012 #11
    as for cyclic models at whole

    http://arxiv.org/pdf/gr-qc/0701070v1.pdf

    ...We show that, contrary to expectation, it is unlikely that such models can offer a physically viable solution to the flatness problem...

    ...This behavior is well known, and is often used to model non-singular oscillating cosmologies. However, we find that the continual transfer of energy required to create cycles of indefinite size and duration results in subsequent increases in the non-zero minimum of expansion. Thus, by sourcing energy from this negative energy scalar field it becomes more negative, and increases in the maximum of expansion are accompanied by increases in the minimum of expansion. This behavior does not appear to be consistent with a physically viable cosmology...


    as for loop models
    http://arxiv.org/pdf/1204.5604.pdf


    ...However this modified Friedmann equation could have been obtained in an inconsistent way, what means that the obtained results from this equation, in particular singularity avoidance, would be incorrect. In fact, we will show that instead of a nonsingular bounce, the big rip singularity would be replaced, in loop quantum cosmology, by other kind of singularity.

    ...And thus, the current statement that, in loop quantum cosmology, the big rip singularity is replaced by a non-singular bounce would be incorrect....
    .
     
    Last edited: Jul 25, 2012
  13. Jul 25, 2012 #12

    marcus

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    Both papers deal with restricted scenarios based on very special assumptions. They are not relevant to nonsingular cosmology (where the "big bang" is replaced by a bounce) in general.

    Not sure what you mean by "cyclic models at whole". The word "cyclic" is not used to describe Loop models in general because often the models studied have only one bounce---they thus have no periodic cycle. Some versions do cycle, but it is not typical. Moreover the word "cyclic" is used by other cosmologists to describe a lot of different sorts of models. Steinhardt and Turok have called their "braneworld" scenarios cyclic.

    In any case the paper you cite (by Barrows and Clifton) does NOT apply categorically to everything called "cyclic". It is based on restrictive specialized assumptions referring to some earlier research. So it is somewhat misleading to suggest that it applies to some ill-defined category "as a whole".

    Your "as for loop" paper by de Haro has nothing to do with ordinary Loop cosmology models. It is about "phantom energy/big rip" scenarios where you do not have the usual cosmological constant.

    However thanks for your interest and for looking up the research papers.
     
    Last edited: Jul 25, 2012
  14. Jul 25, 2012 #13
    If you understand the first part, 'temporary particles', please explain to me!! [LOL]

    For the record, nobody really knows that! We believe Heisenberg uncertainty and quantum jitters at Planck scales [quantum foam] sets some limits at small scales, but why is that?? And why do those models break down at the Big Bang and at the singularities of black holes?? Something is wrong but a solution, a more fundamental explanation, awaits a theory of quantum gravity.

    Consider this a corollary to MarkM's post above:

    An observer can detect new particles three ways: by simply accelerating herself, by observing an accelerating/expanding spacetime, by observing an accelerating causal horizon. Are those real of just mathematical constructs?/ I am not sure.
    But I do know different spacetimes [Rindler, Schwarszschild, de Sitter,etc,etc] provide different perspectives.

    let's take the first: an inertial observer and an accelerating observer passing by in free space will measure different temperatures...and make different particle counts...because their horizons are different. Similarly, accelerating outside a black hole horizon enables you to observe different temperatures and particles than if you are inertial [free falling]; in the former you get fried by radiation, in the latter you pass the theoretical horizon and are completely unaware of it! Supposedly all that is 'real' .

    Next consider this:

    that's from Carlo Rovelli, this discussion and his 2004 research paper:

    https://www.physicsforums.com/showthread.php?t=386051

    http://arxiv.org/abs/gr-qc/0409054

    So just post a question, 'What is a particle?", or better yet, see past discussions here, and youll see we have a lot of good ideas, but exactly why we have the particles we do, why they have the characteristics they do, is not really known from fundamental first principles. We know all that stuff because we measure it...not because we have a perfect underlying theory.

    Finally, and briefly, the horizons associated with acceleration: The existence, properties, and significance of a cosmological/causal horizon depend on the particular cosmological model being discussed...analogous to my comment above about different spacetimes offering different perspectives.

    Here is the way I first thought about particles popping out [becoming real] as a result of the presence of horizons: If you think of virtual particles as real and imaginary numbers, like some solutions to quadratic equations, Consider that the imaginary ones pass through a horizon...so they have no further causal effect..they no longer offset the 'real' number and that becomes a detectable particle. This explanation will draw lots of criticism, I'm sure, but it got me started...so maybe it will benefit you,too. I think I got the idea from the also simplistic description of Hawking radiation where a black hole horizon separates virtual particles with one disappearing behind the horizon and the other emerging as a real [detectable] particle. [This also is an intutive description according to Hawking himself when he tried to figure out what his own math might be suggesting.]
     
  15. Jul 25, 2012 #14

    marcus

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    Phinds, thanks for the supportive comment!
    Naty, I am told that it is not the "Higgs particle" that gives things mass, it is the Higgs FIELD, that extends throughout space.
    The particle is a manifestation of the field that occurs at certain energies in certain reactions the products of which appear in certain detectors. It is a quantum of the field. I think maybe we should think more in terms of the field sometimes.
    So I very much like this part of your post:
    ==quote Naty==
    ...
    ...Next consider this:
    ... uniquely-defined particle states do not exist in general, in QFT on a curved spacetime. More in general, particle states are difficult to define in a background-independent quantum theory of gravity....we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier…”​
    that's from Carlo Rovelli, this discussion and his 2004 research paper:

    https://www.physicsforums.com/showthread.php?t=386051

    http://arxiv.org/abs/gr-qc/0409054
    ==endquote==
     
  16. Jul 25, 2012 #15
    who say LOOP in the first paper ?
     
    Last edited: Jul 25, 2012
  17. Jul 25, 2012 #16
    What? Didn't you read the title of that paper? It states that a big rip singularity cannot be avoided in LQG. Which is perfectly fine, because very few believe the big rip is even a viable scenario.

    Marcus is speaking of the big bang singularity at t=0. If you read the abstract, they note that LQG resolves the big bang singularity.

    Why would you post a paper for a subject that it doesn't involve?
     
  18. Jul 25, 2012 #17
    sure ?


    http://arxiv.org/pdf/1207.3621v1.pdf

    ...that this singularity is replaced by a non-singular bounce...
     
  19. Jul 25, 2012 #18
    I corrected this above, it refers to the big rip singularity, NOT the big bang singularity. Why did you repost it?
     
  20. Jul 25, 2012 #19
    ...It is therefore a common hope that quantum effects, for instance new repulsive contributions to the gravitational force, can help to avoid singularities......
     
  21. Jul 26, 2012 #20

    As far as I can see LQC only tells us the big bang is replaced by a big bounce. But it doesnt have anything to say about the future of our universe or what might have been the origin (if any) of the universe that we bounced from. Hence it has sometHing in common with cyclic models in that the big bang is not the ultimate beginning but it doesn't give a complete cyclic description in the same way as , for example, Ekpyrotic or CCC models do. However I did see Param Singh on a BBC documentary describe it as cyclical, so Im wondering is there something I have missed or is there more diversity within the field or...?
     
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