Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The Grid did not begin at BB?

  1. Nov 16, 2009 #1
    The Grid, dark energy not created at BB ?

    In his book “The lightness of being” page 105, Frank Wilczek says:
    “The concept of Grid density is essentially the same as Einstein’s cosmological term, which is essentially the same as “dark energy” “.

    Dark energy, contrary to the other energy substances of our observable universe, nowadays is considered to have constant density forward and backwards (at least in (and around) our observable universe). I suppose it gives a strong indication that the BB of our observable universe could not be a pinpoint beginning.

  2. jcsd
  3. Nov 16, 2009 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    One of the curious things about Wilczek's book is that he uses the word Grid in place of the familiar word "vacuum".

    I think he is trying to jolt us into thinking about the vacuum in a new way. The computation method called "Lattice QCD" has become a kind of paradigm or reigning idea, and in a sense for a pragmatist one can say

    "How you calculate is what it is."

    His using the word Grid has a kind of shock value. It is potentially confusing, which is bad. But it forces readers to wonder about the vacuum, and re-think. So maybe the net effect, on balance, is good.

    Another key idea that he wants to drive into the public's mind is "condensate". The idea that something can come into existence in the vacuum because (under the circumstances) it takes less energy for it to exist than for it not to exist.

    Quarks are fermions so they can't occupy the same point. They have to be apart. But when they are apart, a gluon field arises between them. Energy arises in the vacuum by necessity, by a kind of economy.

    When one wants to calculate the flickering blobs of gluon (you probably saw the computer animation that Wilczek showed at Nobel talk) one uses a QCD Lattice---in simple terms, a Grid.

    So at the cost of confusing us with strange metaphors and unconventional terminology he wants to force us to imagine the vacuum *his* way.

    And *his* way is incomplete! He doesn't know where the cosmological constant comes from or why the figure for dark energy is what it is. Nobody does know this. Indeed there is a kind of discomfort about it that one can feel in physicists. They are embarrassed and somewhat disturbed by their ignorance about the vacuum energy density.

    If that one thing were understood, so much else could fall into place----or stand like an arch with the center stone put in.

    You take Wilczek's Grid picture as an indication that our universe wouldn't have had a point beginning.

    Well yes! I certainly agree with that. A beginning with possibly some finite very high density. Just not a mathematical point.

    However you invoke the idea of "dark energy" being constant. Empirically it does seem to be constant at cosmological scale. But but but......
    How can we extrapolate that to tell the vacuum energy density at big bang conditions?

    Or even how can that tell us the vacuum energy density in the empty space inside a proton. Inside a proton there are 3 quarks, and they occupy almost no room. And they have very little mass. Only around 3 percent, as I recall, of the proton's mass. So inside the proton is mostly empty space. And some 97 percent of the mass of the proton arises from Grid energy---empty space energy.

    I find this uncomfortable, almost painful, to contemplate. It is so paradoxical it makes me a little sad---thinking about how ignorant I am compared to a physicist like Wilczek and yet how even more ignorant he is. What stupendous ignorance!

    I don't think the vacuum energy around the time of big bang could be the same as the cosmological dark energy that we measure, which however seems not to be changing.
    Asleep or awake, we all wear this hair shirt of paradox.

    But yet you are right about it not being a mathematical point. I don't think working cosmologists expect to find an actual singularity. More likely a bounce, or some other finite-density transition.
  4. Nov 17, 2009 #3
    As the Universe expand, more "grid" are coming into existence. This vacuum/grid has an energy density (in fact a large one, as I read) which is not decreasing in time nowhere in Universe, so a continuous increase of total energy of it exist. Where is that energy coming from?
  5. Nov 17, 2009 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    In classical General Relativity, as formulated in 1915, there is no global law of energy conservation.
    It is not mathematically provable that one should expect energy to be conserved.

    So on what basis can you ask "where does that energy come from?"

    If you think energy conservation is a universal law, then you would already have had a problem in 1950 when people begin thinking about the cosmic microwave background.

    The CMB photons have, by now, lost 99.9% of their original energy. Expansion drains energy from photons. Where does that energy go? :biggrin:

    So far the various answers people have dreamed up do not seem all that satisfactory, at least to me. Maybe some day we will have a better theory to replace General Relativity and these questions will be definitively answered.
  6. Nov 17, 2009 #5
    I must admit that I do not really understand the Grid nor the almost equivalent Dark energy (Wilczek's words), but my idea is contrary to what you are saying. I suppose, maybe wrong, that dark energy with it's constant density already exist in the domain far outside our
    observable universe wherein the expanding spacetime, related to the other energies like
    E-M, dark matter and normal matter, is penetrating. So in my opinion Grid and Grid density both are constant, i.e. Grid's space is not expanding there and thus no energy is created.
  7. Nov 18, 2009 #6
    As I understood today's cosmology, outside the Universe is ... nothing. I mean that the Universe is not expanding into something that pre-exist, just between Universe objects more space are present from one moment to another.

    I really don't understand what this imply: we are unable to say if a global law of energy conservation can exist, or we can surely say that a global law of energy conservation can't exist?
  8. Nov 18, 2009 #7


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    GenRel is probably not the final theory. For example it has singularities, usually a sign that a theory is imperfect or incomplete and needs to be replaced.

    What I am saying is simply one small observation about GenRel. It does not have a global energy conservation law. If you believe GenRel, then you cannot mathematically prove such a law on the GenRel basis and you have to get along without such a law.

    As long as GenRel is our dominant framework for largescale spacetime geometry, we cannot be sure that energy conservation is true, because we cannot prove it in that logical framework.

    Indeed it seems to be either meaningless or else violated in certain cases---and people argue about this.

    This does not mean that this will always be the situation. Maybe GenRel will be replaced by another theory, a more fundamental theory of space-time-matter. Maybe the new theory will contain all the good results of GenRel where it is correct, and also all the good results of quantum mechanics, and yet in some sense the new theory might be deeper. It might tell where GenRel and QuantMech "come from". How they derive from some more basic model of existence.

    Maybe. And maybe this new theory, if we ever arrive at it, will allow us to prove a global energy conservation law! If that happens, then we will have a global energy conservation law. It might happen, and it might not. We don't know the future.

    But for now we only have limited approximate conservation laws. They do not apply over all of curved spacetime. They are very very very accurate if you just isolate a system inside an approximately flat volume. Or where there are some approximate symmetries.

    My attitude about it is agnostic. We don't know everything yet :biggrin: and there are unresolved issues. Other people may be uncomfortable with that and want to draw definite conclusions, which is OK too.
  9. Nov 18, 2009 #8
    Thank you marcus. It is much clear for me now.

    A lot of things are strange in GenRel (for common sense) but the idea that energy can come from nowhere is most odd.
  10. Nov 18, 2009 #9


    User Avatar
    Science Advisor

    I agree with everything marcus said, but I want to add something:
    There is a law of local conservation of energy in GR. You find it http://en.wikipedia.org/wiki/Friedmann_equations#The_equations".
    Using this law, you can see where the energy comes from:
    A normal gas has positive pressure. As it expands, this pressure does work, and the energy density decreases by that amount. That's how photons lose energy.
    Dark Energy has negative pressure, so, as it expands, it does negative work, and its density increases by that amount.
    If the pressure happens to equal the energy density (negatively, of course, in appropriate units, and as it is thought to be true for DE), the negative work exactly replaces the energy that got diluted by the expansion. The energy density therefore stays constant.
    If the negative pressure were stronger, it would overcompensate, and increase the energy density without bound. That's called "phantom energy" and would lead to a "Big Rip".

    Ok, I'm not sure that concepts like "negative pressure" and empty space doing work are less disturbing than a simple "there's no energy conservation". But you should have heard of it, and see that this basic law still has something to say in cosmology.
    Last edited by a moderator: Apr 24, 2017
  11. Nov 19, 2009 #10
    Dear Marcus, Skolon & Ich,

    many thanks for your responses to the Grid issue,

    A good friend of mine once said to me, if things are paradoxial then, as he has heard from a philosopher, "enlarge the frame, wherin it is placed, sufficiently to solve the paradox.
    I find and see the models/frames of physics are still (or always will) too narrow or sometimes "(not) even wrong" so that we can't avoid paradoxes or that we just say "the thing is a mystery".
    At least I think that the problem of "energy conservation" and notions as "nothing", "begin" etc are belonging to the realm of the unknown or even the false and are sometimes placed in too narrow frames.
    I feel happy with this situation but I continiously try to put things in frames which are less paradoxial from a reality viewpoint. Reality which is autonomous and never completely understandable by whatever models we develop. The impossible task of understanding reality is my first priority.
    To my feeling a frame made with buildingblocks like "there only exist being and transformation" and "creation and anihilation don't exist" helps me in feeling better during my interaction with this reality. Skolon I think you are right that many or most cosmologists accept a mysterious nothing, I simply can't. Sorry for using the word penetration, expansion should have been the better word.
    At this moment have no time yet to really give a better reaction to both your's, but I already whished to give you this one so that my starting statement could be better understood for follow up discussions which I hope I can follow and contribute to.
    Kind regards
  12. Dec 8, 2009 #11
    This is something I "as a non-physicist" don't understand about these vacuum ideas. Is the vacuum similar to a euclidean four dimensional reality? It's my understanding that the euclidean models, like Hawking's very famous one, have time as a spatial dimension. Isn't a time component necessary for causality? In other words, for some kind of quantum tunneling to happen, doesn't there have to be a before and after where the jump occurs over the hypothetical boundary? Am I totally mixing up the idea of a vacuum and hawking's idea?
  13. Dec 10, 2009 #12


    User Avatar
    Science Advisor
    Gold Member

    I think it is reasonable to assume net energy is conserved in the universe. Expansion dilutes energy directly proportionate to the volume of spacetime.
  14. Dec 10, 2009 #13
    Dear Chronos,
    Happy with your assumption! I still have additional questions here.
    1) Is it possible to see the couple of spacetime and energy density also as a local entity?
    2) Can you then imagine such entities seperately connected to the different kind of existing types of energy? (In such a case e.g. dark energy could possibly have its own spacetime?)
    3) Expanding or dillution, seen as local, don't necesary imply enlargement of a domain where this takes place, compare a longitudinal vibration on a string or in an organ-pipe.
    4) Is not the cosmological principle too restrictive while considering fluctuations in the universe?

  15. Dec 11, 2009 #14


    User Avatar
    Science Advisor
    Gold Member

    Dear hurk4:
    1. What 'coupling' of spacetime and energy density are you referring to?
    2. I have no idea what you are talking about.
    3. You are comparing apples to oranges.
    4. What fluctuations? What cosmological principle? How are they related?
  16. Dec 11, 2009 #15
    Dear Chronos,
    Thank you for your reaction which I think is clear to me.
    I should better stop asking you these questions.

    Kind regards
  17. Dec 16, 2009 #16
    Because the vacuum have a energy density (about 10-9 joules per cubic meter) and expansion imply more vacuum, can't we say that instead of a dilution of average energy an increase of total energy exist in Universe?
  18. Dec 17, 2009 #17


    User Avatar
    Science Advisor
    Gold Member

    Does an increase in 'space' introduce new energy into the universe? That is a possibility. I don't like it because it upends the laws of thermodynamice. My dislike, however, is my problem. There is nothing to suggest the laws of the universe are immutable over time, just the ones we have measured.
  19. Dec 29, 2009 #18
    I always wondered why the assumption is everything comes out of big bang. Is there something that excludes superimposing the BB on everything that 1) Is not predicted by it (dark energy, dark matter), or 2) Could self-exist outside of BB (physical laws, time, space, quantum mechanics). No doubt the inflationary epoch opened up a bit of room for our universe to exist in, so we would not run into matter that may have been sitting around before the BB.
    Last edited: Dec 29, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook