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How exactly does the universe expand?

  1. Apr 13, 2013 #1
    Good Evening everyone, I have 2 short questions about expansion.

    From what I think I have learned based on various posts, expansion really means the distances between remote galaxies that are increasing over time. Am I right? Thank you marcus for the pinned thread about the balloon analogy, that really helped!

    If my understanding of expansion is correct, I'd like to know: What exactly is it that causes the expansion? Is it dark energy?

    Secondly: As the universe expands and the distance between objects increases, is new space actually created and 'injected' in between them? Or does the space between objects actually 'stretch'? I think I've taken the balloon analogy too literally, to the point where I imagine space behaving like rubber or some other stretchy material. I don't have the faintest idea how it behaves, I'm hoping somebody else might be able to enlighten me.

    As always, thank you for your time and patience!

  2. jcsd
  3. Apr 13, 2013 #2
    As I am on my phone atm and I'm feeling lazy atm. I'll post an article myself and other PF members wrote. On expansion and redshift. Its lengthy though lol. So if I rewrite it It will be broken down into three sections.

  4. Apr 13, 2013 #3
    Thank you so much Mordred! That is extremely helpful, even if it's way beyond my level of understanding, I'm a patient guy... I'll decipher what I can from it. Cheers!
  5. Apr 13, 2013 #4


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    Thanks for the appreciative words about that pinned thread, quite a few people have contributed to it by now.
    Cosmologists have been unlucky in the choice of some of their terminology. "Big Bang" seems to give public a misleading mental image, it is just the start of expansion and they're still investigating and trying to fit testable models to that part of universe history. No consensus as yet.

    "Dark energy" is another unfortunate phrase. It's being used as an alias for a curvature constant traditionally denoted by a greek letter Lambda we've had around for around 95 years, as a naturally occurring term in the Einstein GR equation. There is no observational evidence that it corresponds to an actual ENERGY. On the contrary, the last 5 years or so of evidence points to it being a simple constant. So in the professional literature authors often do not say "dark energy"---nowadays they often simply say "cosmological constant" without reference to energy.

    This constant had a negligible effect on early history of universe. It began having a noticeable effect only after a few billion years. Until around year 7 billion the expansion of distances was slowing down, and now it is only just barely speeding up.

    Lambda is a very slight curvature and it was dwarfed, totally overwhelmed, by all the stuff going on earlier. So the answer to your question "Was it dark energy?" is NO, not if by "dark energy" you mean what people usually mean by it namely the cosmological constant.

    There may have been some actual energy field, long since decayed, that was at work in the early universe and which participated in kicking off the expansion! We can speculate! Maybe a big brother of the Higgs field! The question itself "What started expansion, right at the start?" is an exciting question. Various ideas have been proposed. Some people are working on models where there is a bounce. Quantum effects make gravity repellent at extremely high density, so a prior contraction rebounds, starting the expansion. There is no professional consensus so far.

    One thing they do agree on is that classical 1915 Einstein GR equation explains how expansion once started will continue without constant input of energy. The equation only allows for a gradual predictable change in the rate. There is one enormously difficult to accept, but apparently true, thing about cosmology and that is geometry has a mind of its own. It is not like what Euclid said, it is not fixed static with triangles always adding to 180 degrees. Geometry RESPONDS to the flow of matter and to its own past. If it gets started growing, it will continue at least for a while. If it gets started bending (say because of a flow of some matter) it will continue at least for a while. And bending makes triangles add up to something besides 180.

    We have to accept this because it is also our law of gravity, that turned out to be more successful than Newtons. Gravity=dynamic geometry. We don't have anything better than this at present. It has been tested a lot, in all sorts of ways, at many different scales (earth, solar system, other stars, distorted lensing effects of clusters of galaxies and unseen clouds, and (yes) expansion of distance. It is all part of the same thing, the same simple equation that WORKS. And it is the best law of gravity we have so far. So it puts our intuitions in a bind. Our intuitions say that geometry cannot be dynamic and influenced by flows of matter, it has to be fixed exactly the way Euclid said.

    That is the predicament we are in, our minds are in. Space is not a substance IN geometry. It IS geometry. It does not require "energy" to make geometry change. Geometry has an equation (a "mind of its own") describing how it behaves==an equation which is also the simplest most correct law of gravity we have so far. We don't have an explanation for why this equation (although Ted Jacobson, in a famous 1995 paper came close to explaining--made real progress) and so so we just have to accept it for the time being. With our 2000 year old Euclidean intuitions screaming in pain.

    I think this is is one reason people fall into the verbal trap of calling what is really a naturally occurring (automatically occurring) constant in the Einstein equation by the name "dark energy".
    Our intuition is taking revenge on us for all the indignities it has suffered over the past 100 years. :biggrin:

    for an article debunking the "mysterious dark energy" buzz-think
    or if you don't have the link handy, google "rovelli prejudices". It will be the top hit with that two-word search.
    Last edited: Apr 13, 2013
  6. Apr 13, 2013 #5


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    the balloon analogy is a good tool but it has its problems, as explained here:

  7. Apr 13, 2013 #6
    Well deserved! I might have mentioned it in another thread, but I always look forward to reading your posts especially, as well as several others here - so thanks for all your help.

    I'm reading about the cosmological constant now as I'm extremely new to all this. The terminology confuses me more than the math (okay, that's a lie!) but as you say, unlucky! I recently read about the experiments they are conducting deep underground in mines and so on, trying to detect dark matter or WIMPS. The idea of it being a particle, or discovering something exotic like a table of dark elements was firmly entrenched in my mind.
  8. Apr 13, 2013 #7


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    There's no evidence that "dark energy" (the slight intrinsic spacetime curvature) is related to DARK MATTER.

    There are half a dozen good reasons supporting the dark matter hypothesis. It condenses into largescale structure at a different rate from ordinary matter, and would tend to dominate structure formation. Computer simulations with DM qualitatively match observed structure.

    We can SEE clouds of DM, where it is denser, by how it optically distorts the galaxies behind the clouds. So density maps (like contour maps) have been made. Very interesting (called weak gravitational lensing.) Clusters of galaxies (of ordinary matter) tend to be surrounded by these clouds.

    The mechanism by which DM condenses into largescale structure does not allow it to curdle into small scale structure the way OM does. Can't get hot and blow off surplus kinetic energy like OM can.
    Pretty clearly it is a good hypothesis, so it makes sense for people to make detectors (in space, or down in mines, etc.) to try to detect DM particles. Best wishes to them! Confirmed detection sometime in next 10 years? By 2023 maybe? Would be a major triumph, perhaps the most important physics/astrophysics discovery of the decade.

    But that is not "dark ENERGY". DM gathers in huge wisps and clouds by its own gravity. The small intrinsic curvature of space-time is a CONSTANT, the same everywhere at all times. Very different.
  9. Apr 13, 2013 #8
    That was one of the reasons of the sheer size of the article. I felt it was important to describe each term or constant as they showed up in the article. However to save space some of the
    descriptives were kept brief

    Also thank you for the appreciation of the article its nice to get positive feedback from the intended audience
    Last edited: Apr 13, 2013
  10. Apr 13, 2013 #9


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    Thanks! Your comments are heartening. Here's an interesting passage from page 2 of that paper by Bianchi and Rovelli you can get by googling "rovelli prejudices"

    In fact, it may not even be true that Einstein introduced the λ term because of cosmology. He probably knew about this term in the gravitational equations much earlier than his cosmological work. This can be deduced from a footnote of his 1916 main work on general relativity [9] (the footnote is on page 180 of the English version). Einstein derives the gravitational field equations from a list of physical requirements. In the footnote, he notices that the field equations he writes are not the most general possible ones, because there are other possible terms. The cosmological term is one of these (the notation “λ” already appears in this footnote).
    The most general low-energy second order action for the gravitational field, invariant under the relevant symmetry (diffeomorphisms) is

    S[g] = (1/16πG)∫ (R[g] − 2λ)√g

    which leads to (1). It depends on two constants, the Newton constant G and the cosmological constant λ, and there is no physical reason for discarding the second term.
    From the point of view of classical general relativity, the presence of the cosmological term is natural and a vanishing value for λ would be more puzzling than a finite value: the theory naturally depends on two constants; the fact that some old textbooks only stress one (G) is only due to the fact that the effects of the second (λ) had not been observed yet.

    The basic reason is that in physics standard practice is to include all terms allowed by the symmetry of the theory. The underlying symmetry of GR is the diffeomorphism group, that is, "general covariance" as Einstein called it. This more than anything else dictates the nature of space-time. The differ group (general covariance) only allows TWO gravitational constants, G and Lambda. So the natural thing is to include both in the equation and expect both to play a role.
    A priori there was no reason to assume that one of the two naturally occurring constants would vanish, be exactly equal to zero. However no effect of Lambda was observed until 1998.
    So this constant of nature was a "sleeper" for over 80 years :biggrin:
    Check the article out! It is written for wide audience and there are plenty more goodies like the above passage.
  11. Apr 13, 2013 #10
    That dark energy debunk article was an enjoyable reading. Thanks for posting it
  12. Apr 14, 2013 #11


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    Well, the interaction between gravity and the contents of the universe determine how the expansion changes with time. What caused the expansion to start in the first place is whatever physics set up our universe in the first place. And we don't yet know the answer to that.

    Yes, new space is created by the expansion. Stretching would imply that there is some sense in which space resists the expansion, or that it becomes somehow thinner after expanding. Neither is the case.
  13. Apr 14, 2013 #12
    That is a great correlation, never thought of it before but makes
    sense. Thanks for that

    Not to imply I thought of space stretching. Your straight forward explanation cuts to the chase so to speak.
    Last edited: Apr 14, 2013
  14. Apr 14, 2013 #13


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    AAARGH! more verbal trouble. :biggrin:

    a stretch limo is not any thinner, it is just lengthened
    a Boeing 747 stretch airliner is not any narrower, it is just elongated

    whether or not the thinning down idea is implied by the word, or just elongation, depends on the context, I think.

    I like the word stretch for the elongation of wavelength and of distance because it is one syllable and familiar-sounding. So if it is all right with you all I will continue using it with the understanding that no thinning is implied when I apply it to distances and wavelengths.
    Last edited: Apr 14, 2013
  15. Apr 15, 2013 #14


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    I would like to ask whether one should think of this statement "new space is created" as a helpful notion or perhaps as an interpretation of the cosmological redshift, rather than of a physical fact, which is measurable.

    Because remembering the article
    Expanding Space: the Root of all Evil? which quotes Weinberg
    I feel tempted to add, how can nothing increase?

    Nevertheless, I have some hope, that it is somehow compelling to talk about the creation of space, though knowing that it isn't a substance. How should a layman understand the true physical fact of increasing distances otherwise in terms of geometry merely? Without taking reference to increasing space?
    Last edited: Apr 15, 2013
  16. Apr 15, 2013 #15
    I don't know, but I don't see why it shouldn't. I don't see why there couldn't be an infinite "supply" of nothing. Nothing could be produced at no cost. I don't see what the problem is, if that is what the universe feels like doing.
  17. Apr 15, 2013 #16


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    Well, I can measure that something increases, but nothing? Or how would you describe the difference of state before and after nothing has increased?
  18. Apr 15, 2013 #17
    Hi Thomas,

    is an interesting question without firm answers. That's because nobody really knows "what is space? What's the difference between 'space' and 'distance'. Is space continuous [as in GR] or discrete [as in QM]? Where does it come from? Where does it go?" We've discussed these kind of questions...and yours...so if you search you'll find many good ideas in these forums.

    [For me it's hard to figure space is 'nothing' because changing geometry over time [like cosmological expansion] results in particle creation...[also discussed in these forums.]

    In the article linked to above [Prejudices....] they write:

    So we think the universe is expanding and that the expansion is currently accelerating based on generally agreed upon observations.
    I don't think GR does 'explain' it...but it at least provides a model that predicts it. 'dark energy'
    vacuum energy provides a potentially finer grain of understanding, but that may not be the correct understanding.

    Also, keep in mind neither GR nor QM work perfectly....they don't handle BH and Big Bang'singularities'...so we have more to learn.

    and while we are at it, use caution regarding 'cosmological distances'...that's also not so easy
    to understand as you might think. We all need to develop approaches to interpreting what THAT means.

    If you are familiar with Newtonian kinetic and potential energy, no GR, no tensors,etc, you might find Leonard Susskind's third Cosmology tutorial very interesting...on Youtube. It provides lots of basic insights to your questions by showing how some simplified math underlies our understanding of expansion. Susskind even does a simple calculation from scale factors [expansion rates] to show that around year 7 billion the expansion of distances which was slowing down began to speed up.....
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