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Space between galaxies

  1. Oct 13, 2011 #1
    As I understand it, it isnt just the sapce between galaxies is stretching but new space is being created between galaxies, is that correct?
    If so are there any models that attemtp to explain what is generating the new space ? wheres does it come from?
  2. jcsd
  3. Oct 13, 2011 #2


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    I'm not sure it's useful to endow space with these properties: the ability to stretch implies that there is some inherent tension in empty space. This is not correct. Classically, empty space is empty space -- it makes little difference whether you imagine it stretching or whether you imagine 'more space' being created. What's physically important is the metric tensor, which represents the gravitational potential, and it describes the expansion without distinguishing between these two possibilities.

    However, in GR, we can't forget about the other main player -- energy. And this becomes especially important when we start thinking about quantum mechanics. Now we have to worry about the nonzero energy of the vacuum, and, hence, one might say the energy of empty space. But I still prefer to think about the energy existing in the space, not of the space. Vacuum energy, as you probably know, is a gravitational oddity -- it causes the universe to accelerate and maintains a constant energy density as the universe expands. This means that, locally, we have energy constantly being generated to fill in the void. Strange, yes, but this is just the nature of the quantum vacuum -- it has energy! As space expands (whether we imagine it stretching, or being created, or whatever), there's more space there, and hence, room for more quantum vacuum, and yes, more energy.
  4. Oct 13, 2011 #3


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    Part of a response here is attitudinal, Phil. My attitude about it agrees and overlaps with Brian in some respects, not in others.

    I don't think of space as a material with objective physical existence. Not a "thing". (there are some famous Einstein quotes from 1916 to that effect)

    So it cannot stretch or expand or be created. What expands are distances.
    Geometry expands.

    We tend to think of geometry as "God given" eternally the same. But Gen Rel says geometry arises out of nature and the angles of a triangle defined by 3 rays of light do not always add up to the same thing.

    Gen Rel says if you have some uniformly distributed matter or radiation (as we do) defining a universal restframe for you (as it does for us) then distances between stationary points can change.
    Nature decides geometry and how it changes. And there is a reason that around here the three angles add up to 180 degrees. It is contingent on prevailing conditions. And geometry is explainable to a certain extent. Not just laid down for ever by God or Euclid. Kind of nice, that.

    So I don't need any "dark energy" in my picture. There is just a constant of Nature, a certain curvature, which occurs along with Newton's constant G, in the 1915 equation which has become our Law of Gravity. The constant is Lambda. It is just a constant like G is.

    I don't need "space stretching" or "new space created" ---and any involvement of the particle theorists' "vacuum energy" in the geometry of the universe is negligible in my picture.

    And the big fuss that particle theorists make about the fact that their calculation of vacuum energy is ridiculously big is their problem. They evidently calculate it wrong. It does not have anything to do with cosmology or "dark energy". One should not confuse people by talking about "dark energy". There is just a curvature constant Lambda which becomes a fake energy if you drag it to the wrong side of the equation where it does not belong.

    So my attitude is pretty much in line with the one laid out by one of the world's top quantum gravity people (Rovelli) in this 2010 paper.
    To get it, just google the title:
    "Why all these prejudices against a constant?"

    You don't even need the quotes. Just google the title and you get the free online PDF
    of the article. I see it is fairly popular---has been cited 25 times already.
    It helps explain the attitude I'm expressing here.
    Last edited: Oct 13, 2011
  5. Oct 13, 2011 #4


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    Thanks for the reference Marcus. I'll check it out as well.
  6. Oct 14, 2011 #5
    THanks for your replies guys. Very informartive. Btw Marcus, I got Rovelli's new book. will read on my vacation. Thinking of loops , in LQG though, space time is made up of a space time atoms right? so does the number of sppce time atoms increase as the universe expands or..?
  7. Oct 14, 2011 #6


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    Phil, I'll be interested to hear any reactions to the book. It has changed the way I see history---I see the sixth century BC much more clearly now, and the roots of the sci/tech tradition.

    With a math model, different people will have different verbal descriptions. There might not be one correct translation of the math picture into English words. You might not like the words I use at all and prefer e.g. Abhay Ashtekar's or you might prefer e.g. Lee Smolin's.

    For me a quantum state describes as best we can how something is going to respond to measurement. The outcomes of measurement are not preordained and they depend on the order you do things. Measurement is a kind of discussion we have with nature and we don't know ahead of time how that will go. Fortunately there is a mathematical suitcase called HILBERTSPACE which can accommodate the contingencies of information we have about a system where the outcomes of measuring are not preordained and depend on the order we do things. What we know and aren't sure about (different potentials for discussion in other words) can be PACKED INTO that suitcase very conveniently without doing them too much violence.

    I don't focus on the substance of "spacetime" as much as on the interactive process of making geometric measurements (distances between things, angles, areas.)

    So a quantum state of geometry is about nature's possible responses to geometrical measurements.

    What you want a quantum theory of geometry to be able to calculate are probabilities---or somethings very much like probabilities called amplitudes.

    Loop is about defining a Hilbert suitcase of states of geometry and defining measurement operators on those states and calculating amplitudes of how the various states will evolve.

    I don't hear Loop saying that there is something called spacetime, or that this fabled substance of spacetime is made of fixed-size grains!

    I hear Loop saying that measuring the area of something has discrete levels, just like measuring the energy of a hydrogen atom you get discrete energy levels.

    For me, geometry and geometric measurement are real---and they may have a certain kind of discreteness----levels in other words that you encounter when you measure stuff.

    But "space" and "spacetime" don't have an objective physical reality for me. they aren't material and they aren't made of grains.

    I don't say "space expands". I say distances between stuff increase. Geometry expands.

    There are no grains of space. So when distances increase nothing needs to be created. there are no new grains that need to be created. Geometry (the potential for geometric measurment) simply has a life of its own---and distances simply increase---some of them, according to a certain pattern of distance change called Gen. Rel. or according to a quantum version of that which might be LQG.

    I fear my attitude is not very helpful or responsive to your question.

    The picture of expanding geometry I like best is what you get by googling "wright balloon model"
    It is an animated 2D toy analog of expanding 3D geometry. Whirling galaxies stay in the same (lat.+long.) place and distance between them increase. Wriggler photons actually move. always at the same speed. And often you notice that the distance between two galaxies is increasing faster than the speed the wrigglers are moving.

    As distances expand so do the visible wavelengths of the wrigglers. But the galaxies do not get larger because they are grav'ally bound.

    That's all classical---no Hilbert suitcase of quantum states, no amplitudes. But someday there may be a visual presentation of the dynamically evolving quantum geometry of the universe that is as clear as this classical balloon icon.
  8. Oct 24, 2011 #7
    I saw somewhere a fact that more the space between galaxies, the
    faster they fly apart. But is this true? Are they flying apart from
    each other because the space is growing between galaxies? There are a lot of things written on web, I'm unable to draw a clear picture out of this.
  9. Oct 24, 2011 #8


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    Yes, this is correct. It is the space itself that is expanding, carrying the galaxies along with it. See the balloon analogy write-up either in the Cosmology FAQ or the Sticky.
  10. Oct 25, 2011 #9
    Hi Marcus,

    You have one of the most enlightened attitudes that I have seen on this site to date.

    The ancient Greeks had a lucidity that leaves most others in their wake.

    Thats what I can't understand, if doppler x ray images of galaxies show half of the rotating sources moving towards the observer and the other half moving away from the observer then surely thats much of what you would expect observing rotating sources in a Euclidian space where the galactic center is also relatively stationary with respect to the observer. If we are actually observing multiple galactic year rotations and we fail to account for them properly we should probably be aware of the consequences of any misconceptions.

    On the observational mechanical level the difference between the physical structure of grazing/glancing incident x-ray telescope lensing and optical lenses reveal an anomally where x-ray observations of galaxy fields show many fewer actual spiral galaxies than are seen when compared with the number counted from an optical image of the same width of field. Only galaxies whose diameter of rotation could fit between the grazing incident concentric circle lens structure would appear on the x ray images while all galaxies would appear through the optical lens as long as their diameter of rotation was within the width of field.

    In simple Euclidian terms the remaining observed x ray spiral galaxies contain the light/wave from at least one galactic/cosmic year of their source(s) galactic rotation so the mass calculated from them will be out by the number of galactic rotations captured of each discrete source rotating around its respective galactic center.
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