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Was there a 'birth of gravity'?

  1. Aug 4, 2009 #1
    'Birth of gravity' is a lead in question to an area I fully admit ahead of time I know little.

    What I'm wanting to ask is:

    At what point in the big bang model is gravity REQUIRED in order to explain our known data?
    Obviously, it has been around since the time of the surface of last scattering -- but was it
    'needed' before hand?

    My simple review of the early stage models seems to make reference to densities using General Relativity like relations. So at another level the question becomes:

    Are these really needed? What would be different if instead it was assumed that there
    was no gravity at a given 'era'?

    I'm honestly asking for info on this. Trying to decipher the technical papers leaves one with questions about assumptions.
     
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  3. Aug 4, 2009 #2
    The hot big bang model of a flat universe with gravity and a cosmological constant explains observations well back to 1 second after the big bang and maybe even to the Planck time of 10^-43 sec. Before this, better quantuum gravity models are needed. Some suggest that all the fundamental forces (strong & weak nuclear, electomagnetic, and gravity) were unified for some time beteen t = 0 and 10^-43 sec).
     
  4. Aug 4, 2009 #3
    How about a flat universe without gravity? What role does gravity play from 10^-43 to 'time of last scattering'? I see its involved but all I get from it is a 'time scaling' -- an effective 'z' or some such.

    I don't see where its really needed for the processes.

    The process we call now gravity was likely in action -- but even then it was extremely weak compared to other effects. With a 'flat' model what does it actually do besides provide a convenient time scaling concept? (if it does that?)
     
  5. Aug 4, 2009 #4
    The first two terms in the Friedmann equation would be zero and the only term left would be with the cosmological constant. This is the De Sitter solution - exponential expansion that depends on the size of the cosmological constant.
     
  6. Aug 4, 2009 #5
    Only assuming General Relativity applies at the cosmological scale. Taking some terms in a relation to zero is not the same as 'no gravity' -- it still imposes the ASSUMPTION of the Friedman metric -- spherical symmetry AND spatial three dimensions. Both are questionable

    The original question still remains: For what is 'gravity' required -- specifically before the time of the surface of last scattering? What results of the various era processes would be different if there was no effect from gravitation?
     
  7. Aug 5, 2009 #6

    Chalnoth

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    Basically, the current cosmological model assumes gravity for all times. Presumably quantum gravity effects may become important at early times, but this is by no means clear. Unfortunately there just isn't any way we know today to really say how gravity might "turn on", and our one candidate theory of everything (string theory) predicts that gravity always exists.

    The real problem is that it is by no means clear what would happen if there was "no gravity". The issue is simple: gravity, as it is understood today, is about the relationship between matter and the curvature of space-time. If you don't have any space-time, then we don't even know how to talk about what's going on.

    I would say, for instance, that edgepflow's statements are a bit too simplistic, as they assume a particular way in which gravity might be different: matter still responds to space-time, but space-time doesn't respond to matter. It's still a situation with gravity, just gravity of a different character than what we have today. I just don't think there's any way of tackling what things would be like with no gravity at all. Postulating different gravity is much easier, though there a multitude of ways to do so.
     
  8. Aug 5, 2009 #7
    I'll give you a 'target' here.

    There is no real proof that the concept of 'space-time' as used in General Relativity 'exists'.

    However, my point is that as far as I can tell, there is no requirement for the presence of 'gravity' for the 'processes' of universe evolution as described in the big bang theory.

    In fact, the first point in the evolution of the universe that I can see for the need of 'gravity' is at/after the 'surface of last scattering'. Its at that point the matter of the universe starts its 'clumping' that develops into the universe of today.

    So I'm postulating that one could say the 'birth of gravity' could have been at that point. IF SO, then theories that use a particle-like exchange to explain gravity would make sense -- gravity not 'materializing' until the universe was 'transparent'.

    HOWEVER, if gravity is required to explain earlier evolutionary processes, then this is a major blow at such a 'particle-like' exchange theory of gravity -- simply put it would seem that such a 'gravity' would not be 'in-effect' as yet.


    So I believe the question is an important one and needs addressing.
     
  9. Aug 5, 2009 #8

    marcus

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    Chalnoth, Steven Weinberg does not agree with you on a couple of points here.
    You could watch the last 12 minutes of the talk he gave at Cern on 7 July.

    He proposed an alternative TOE to string that is a good deal simpler and more straightforward--and warned string theorists in the audience that string might not be needed, ordinary "good old" QFT might work after all, running couplings, asymptotic safety.
    Said "string might not turn out" to be how the world is---the fundamental theory (not merely an effective one, but fundamental and predictive out to arbitrarily high energy) might be an asym. safe development of the QFT we already have.

    And in those last 12 minutes he described his own current work, which describes a way in which gravity could "turn on." At the big bang, since the G constant runs with scale, it might in fact be zero in the UV limit. The cosmological constant on the other hand might be very large in the UV limit, and so it would decline as the universe expanded. So that inflation would end.

    Weinberg cited a paper by Martin Reuter and Alfio Bonanno which already explores these ideas, using the (scale/density dependent) running constants of asymsafe QG to explain early universe behavior, like turning on of gravity, like inflation (without needing a graviton!) and like graceful exit from inflation (without needing a slow-roll potential gimmick).

    OK, so you may think this is weird or speculative. But it was Weinberg talking and he said there will be a conference on this very thing at Perimeter (in November). So I checked the conference website and participants included Weinberg and Martin Reuter and Renate Loll and Vincent Rivasseau and a bunch of other impressive people.

    So bizarre as it seems, there is a precise mechanism being studied by which gravity can "turn on" and inflation can occur (without fantasizing a graviton field) and then automatically "turn off"---and this is being studied by good people---and the subject has become hot.

    Very little has been said about this in public, which I guess is normal with new lines of research.

    If anyone is curious, here is an early paper on it by Bonanno Reuter.
    http://arxiv.org/abs/0803.2546
    Primordial Entropy Production and Lambda-driven Inflation from Quantum Einstein Gravity
    Alfio Bonanno, Martin Reuter
    (Submitted on 17 Mar 2008)
    "We review recent work on renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values. In particular we argue that QEG effects can account for the entire entropy of the present Universe in the massless sector and give rise to a phase of inflationary expansion. This phase is a pure quantum effect and requires no classical inflaton field."

    For the last 12 minutes of Weinberg's talk, go here, and skip the first 58 minutes by dragging the button.
    Here is the video:
    http://cdsweb.cern.ch/record/1188567/
    And here is a more central link if you want additional context.
    http://indico.cern.ch/conferenceDisplay.py?confId=57283

    The Asymptotic Safety conference (in November at Perimeter) has a webpage here:
    http://www.perimeterinstitute.ca/en/Events/Asymptotic_Safety/Asymptotic_Safety_-_30_Years_Later/ [Broken]
    The list of invited speakers is here:
    http://www.perimeterinstitute.ca/en/Events/Asymptotic_Safety/Invited_Speakers/ [Broken]

    It won't say anything in the conference description about turning on gravity, they are not trying to communicate to a broad audience. The conference is for the few experts who know about it. But we know Weinberg's current work on Asymptotic Safety is about the turning on of gravity (and the turning off of inflation) because he said so, and Weinberg will be there because he's one of the invited speakers listed (as are both Bonanno and Reuter) so one can read between a few of the lines.

    ======================================
    EDIT TO REPLY TO NEXT POST

    Hi Chalnoth, the reason Weinberg treats it as a theory of everything is that it includes the Standard Model of particle physics, or some modified version thereof. What he is talking about is QFT as a candidate theory of everything.

    By enlarging what he calls "good old" QFT to include gravity (along with everything else) using asymsafe methods. (a UV fixed point). What he suggests, if you listen to his talk, is that "that is how the world is". It is all subsumed in a single mathematical framework--just that the framework is not stringy. But you can choose not to call it a TOE if you prefer.

    I agree that it is interesting stuff. Everybody should listen to Weinberg's 7 July talk, or at least the last 12 minutes. It is low key but offers a remarkable change of viewpoint, considering who the speaker is.
     
    Last edited by a moderator: May 4, 2017
  10. Aug 5, 2009 #9

    Chalnoth

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    Well, I suppose it depends upon what you mean, but I would call that a candidate theory of quantum gravity, not a candidate theory of everything. The thing about string theory is that everything stems from a single mathematical structure. This wouldn't be the case for this theory, and is also why I don't include loop quantum gravity in that statement.

    Very interesting stuff, though.
     
  11. Aug 5, 2009 #10
    I gather that what keeps this theory from being called 'crackpot' is the names of the people proposing it.
     
  12. Aug 5, 2009 #11

    marcus

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    Not in my case, I'll tell you. I heard Martin Reuter explain this program in October 2005, via internet audio, and I was enormously impressed. I have been waiting for exactly this kind of break to happen----a big name like Weinberg affirming that it has promise (after some years in the minor leagues)----and people crossing over in droves.

    But in the meantime I paid close attention to Reuter's talk at Zakopane in spring 2007 and his talk in Morelia in summer 2007 and in Nottingham 2008. And even more attention to papers Roberto Percacci wrote and his chapter in Oriti's book. I've reported on AsymSafe for over 4 years now. And what we now see, and Weinberg describes, was always in the cards.

    So don't imply that I need a Nobel prize name in order to make me realize that something is not crackpot. All you need is to have the time to pay attention, and not have your mind full of cliché fallacies like ("string is the only game in town because blah blah etc")

    A lot of people are in exactly the same situation as I am, and know something (usually more than I do) about what is going on.

    Now about the people who needed Weinberg to tell them, please don't disrespect them because Weinberg has called an awful lot of shots right over 40 plus years. Nobel has nothing to do with it. He has built up enormous respect and credibility by being right over and over and over. So it makes sense to look at the name on the label.

    In this case, I and a bunch other people didn't need to look at the name. But if I didn't know anything of my own accord about something, then I know enough to take Weinberg's word seriously. The way I wouldn't take just any old Nobel laureate.

    Look, the guy just made the move from particle physics to cosmology! His book Cosmology is now the book on cosmology. That is an enormously smart move, because it is going to be the sky (rather than the big machines) that requires the most new physics from us and tells us the most surprises, and gives us the biggest challenges. First he writes the book on Quantum Field Theory, and then he makes the ultrasmart move from QFT to Cosmo, and then he writes the book on Cosmo. Go figure.

    It is efficient to pay attention to people with that much credibility, even if you disagree, and I certainly have disagreed with Weinberg back in the days when he was supporting string theory as "our one best hope." No one is right all the time, but certain people you listen carefully.

    BTW if you have some idea of "birth of gravity" of course it could very well be crackpot because there is a right way and a wrong way to go about things, but if you do congratulations for creativity. And I would advise just off the top that you try to read the bonanno reuter paper and some of the reuter papers that lead up to it.

    His Morelia talk 2007 is the easiest introduction, it is audio-visual and pitched for non-specialists. I don't know if it is still online though. Other than that I don't know a good introduction. The point is there is a regular way that constants like G and Lambda can change with scale or energy, called running, that can be calculated. Keywords like "renormalization group flow" or RG flow. This has worked in other contexts like QED and QCD. This is what you have to understand the basics of if you want to be talking on the same page as these guys.

    The Morelia talk was at Loops 2007. You might google Loops 2007 Morelia and see if any of the talks are still on line.
     
  13. Aug 5, 2009 #12

    Chalnoth

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    Not really. Weinberg's proposing a reasonable direction of quantum gravity research. It has its issues, to be sure, but it isn't completely unreasonable. The primary issue I can see, if I'm understanding it correctly, is that, just like with normal quantum field theory, we expect that the theory must be replaced by some other at very small length scales. That doesn't mean the theory isn't worth pursuing, just that if it turns out to be accurate, it won't be complete (a feature that isn't a problem as long as it's got a range of useful applicability).

    If you want to know what one called a "crackpot" should do to shed that categorization, here's a good checklist:
    http://blogs.discovermagazine.com/c...alternative-science-respectability-checklist/

    It's pretty short, just three items. But overcoming each is essential for any new scientific theory to even get a decent glance. To be accepted it must overcome further hurdles of experimental testing and verification.

    P.S. If you're wondering why I was extraordinarily skeptical in my first post here, the explanation is simple: there wasn't any detail offered. It was far too vague.
     
  14. Aug 5, 2009 #13
    I would prefer that the basic level science is really looked at before trotting out a new whizbang theory.

    I honestly do not see any need for dark energy. Dark matter at the cosmological scale
    cannot be justified because the mass value and density cannot be justified. Dark matter related to galaxies cannot be justified -- its based on the assumption that gravity is a property solely of the individual matter pieces -- independent of symmetry -- again incorrect. There is the continuing assumption that photons can 'produce' a gravitational field -- again wrong -- they can appear to be effected by one but not produce one of their own.

    My point is there is so much that is basically wrong -- or at least unproved -- that jumping off in new directions is a total waste. All these resources will end up diverted into pointless activities.


    I hadn't tried it till a little while ago. Below is Ned Wright's binned data, shifted by Reiss' latest values on the Hubble constant. I did an expansion velocity fit to the data just for 'fun'. The match was so good it surprised me -- see below. The theory I personally favor derives a value of 0.8660254...c for the expansion velocity of the universe (a cylindrical symmetry value whose model also predicts cylindrical symmetric galaxies having a constant rotation velocity. It also has 6 matter states with the first two very stable and a mass ratio of 0.5 -- the third is marginal. Just basic physics.). This fit gave 0.864c Its very linear -- so of course it will be ignored. It works and I have been honestly looking for ways to 'break it' -- can't find any. Its NOT 'the solution' because is raises very many more questions. I admit the approach was not the classic 'scientific method' -- instead I used the 'standard engineering method' -- i.e. if you don't have data or info on it you can't use it so don't try. Start with the simplest approach and see where it breaks -- before trying to make it complicated. Don't try to solve everything with one single grand scheme -- bound to fail (Murrphy always wins).

    My background is Engineering Physics and years of data analysis. I did do graduate work (MSC Engr Physics) that included a lot of cosmology -- but that was 35 years ago. Had to find a real job -- no openings back then for anyone wanting 'to build a universe'. Seems nowadays there might be.

    BTW -- I am a strong supporter of the general big bang model. I'm not a supporter of the way General Relativity has been used and abused. GR just can't be applied -- at least as it has been -- to cosmology -- its likely not much needed for that anyway.

    I've been purposefully trying to 'hold-back' -- I'm not really proposing a model so much as asking what do we really know for sure -- I've been away from cosmology for so long I'm not sure. My gravity concepts are based on what was going to be my PHD thesis -- 35 years ago the model predicted that intense gravity fields would be surrounded by a cloud of pair production. The January 2008 news report caught my eye and peaked my interest. The idea was that the gravity field was actually a loss from the electromagnetic field (QED like) -- this loss due to the QED exchange photons interacting to produce a positron/electron pair. This of course would mean that gravity effect would not really occur until the photons could 'escape'. As long as matter and energy were fully 'engaged' -- i.e. all photons scattered back there would be no loss possible. But when the universe became transparent -- then these photons (the ones from the recombination of gamma rays of pair production) would be 'lost' -- net result gravity. The distance reference used in my calculations is derived from a very simple particle in a box relation. In fact the orange lines in the data plots are entirely theory -- no datafitting for even an Ho value.

    That is what makes it 'crackpot' -- you should NOT be able to derive the age of the universe from constants like the Planck constant, the speed of light, the gravitational constant, mass of electron and average nucleon mass. But it does. Yes total crackpot.

    That is why I'm asking for flaws -- wanting them -- this can't be right.
     

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  15. Aug 6, 2009 #14

    Chalnoth

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    Overly-superficial analyses tend to produce the wrong result. Here's a good blog post for one of the main reasons why we are so confident today that dark matter is actually out there:
    http://blogs.discovermagazine.com/cosmicvariance/2006/08/21/dark-matter-exists/

    There are other really strong reasons to believe that dark matter exists, but the above is perhaps the most understandable.

    As for dark energy, the truth is we don't know exactly what's going on there just yet. Many possibilities have been eliminated, however, and now we're basically down to just two: we don't understand the action of gravity on very large distance scales, or there's some sort of matter out there that experiences negative pressure. The second option tends to be the more preferable as the few even somewhat well-motivated models for changing gravity at large scales just don't fit with observation.
     
  16. Aug 6, 2009 #15
    Doesn't do it for me. I've read the Bullet Cluster papers several times. I don't agree with the way they stated their conclusions. Looks more to me that the core material of both clusters was little effected by the collision (i.e. they missed each other) and continued on. So the high gravity regions identified simply contain the cores -- just highly compacted normal baryonic matter -- with little surrounding matter.

    Yes, no doubt there is dark matter -- obviously the normal baryonic kind. The problem is the non-baryonic kind. One popular possibility are WIMPS. There might be something 'WIMP-like' -- but there is a problem. The view of gravity I'm looking at has gravity as a weak part of the electromagnetic field (QED). So in this view for gravity to exist for a particle the particle must carry electromagnetic properties. My understanding of WIMPs -- which is very limited admittedly -- says they don't. A problem.

    Similar problems occur with other non-baryonic proposals.

    And that is on the "what is it" side.

    Then there is the question of whether the amount of dark matter requires something exotic. This basically comes about due to current baryonic dark matter estimates not being enough to account for the calculated mass of galaxies and galaxy clusters, etc.
    This has led to attempts like MOND to account for the problem.

    The gravity approach I'm looking at has two 'components' -- required by the model itself.
    One is the normal distribution of the matter that is being modeled -- for motion, energy and gravity effects -- and another for the symmetry in which the 'action' is happening. The symmetry is determined by the rest of the universe -- its symmetry in the LOCAL area surrounding the 'action'. In short the mass and mass distribution of the matter directly involved determine the 'strength' of the gravity fields involved -- but the environment can be different and determine how this is manifested. So in order to get a proper solution both a matter distribution and a local surrounding environment must be described. You can see how this might rather dramatically alter results in a given case if not recognized.

    Specifically, I have found that applying the idea using other than spherical symmetry can yield very interesting results. Cylindrical symmetry in particular in many cases. In this case, it is found that perfect cylindrical symmetry results in rotations with velocities independent of the radial distance -- constant and determined by the mass of the matter distribution and the height the cylindrical symmetry. Obvious galaxy rotation curve explanation. This can then produce results for mass determinations both less and more than the Kepler assumptions. Many times different. Our prolate and oblate galaxies might be prolate and oblate cylinders instead of spheroids as far as symmetry is concerned.

    Note two massive objects with cylindrical symmetry and non-aligned spin axes would produce an intersecting disk volume of high gravitaitonal force in the shape approximating a 'bar'. Interesting is it not.

    It does appear possible that such a situation could drastically lower the need exotic dark matter.

    Note, there is a real problem with this model -- just evoking it allows one to pick a symmetry and a matter distribution to fit a wide range of different circumstances. Its in someways too wide open. Could even turn a 'missing mass' problem to an 'extra mass' problem if misused.

    My point with this is that it may be our assumptions and techniques that are creating these issues -- exotic dark matter might not be needed or something even more odd might be. It has not been properly 'tied-down' as yet. Won't be until such questions as raised by this model are addressed.
     
    Last edited by a moderator: May 4, 2017
  17. Aug 6, 2009 #16

    wolram

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    Pardon me for being stupid, but is it not gravity that drives the hypothetical big crunch, does not the question become, how did the primordial plasma over come gravity.
     
  18. Aug 6, 2009 #17

    Chalnoth

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    Er, no, the primary mass component is not highly compact. It is broadly-distributed. Though it is certainly non-baryonic.

    Oh, well, it's just false that something must have electromagnetic charge to gravitate. Gravity doesn't couple to electromagnetic charge.

    Anyway, it seems like you are proposing a model that requires both some form of hypothetical matter and modified gravity. Occam's Razor clearly says that's much, much less likely than the dark matter observations just being produced by WIMP's.

    Except if you actually look at the rotation curves of real galaxies, they don't follow any simple model. They tend to be extremely messy and complex. The idea of constant rotation curves only applies to a small subset of highly-idealized situations.

    And no, prolate and oblate galaxies are certainly not well-modeled as cylinders.

    It might be if you could show how this came about. Your description so far is vastly too vague for me to make any judgment.
     
  19. Aug 6, 2009 #18

    marcus

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    That is an excellent question. One way to address it would be for you to start a fresh thread so that I don't have to get in the way of Chalnoth's discussion with Rymer, if I want to talk to you about that.

    Or we could just talk here. It's up to you.
     
    Last edited: Aug 6, 2009
  20. Aug 6, 2009 #19
    Well, I'm not sure the 'epoch' or 'era' you are thinking about as 'primordial plasma' relative to normal big bang concepts. So I can't answer that in those terms.

    In the terms that I have been working on, it didn't have to -- there was no gravity holding it back. And there still is no gravity today doing so. That is the reason the expansion velocity found by this non-mainstream model was constant. No forces so no change in velocity. An also no possibility of a 'big crunch'.
     
  21. Aug 6, 2009 #20
    What do you define as the core material? The gas, the galaxies or both?
     
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