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Expansion of Space

  1. Oct 7, 2014 #1
    So what I don't get about the expansion of space, and what I'm assuming one of you can explain to me, is that it seems like if space were expanding, how would we have any way of noticing it? It seems like as the distances between particles expands, so must the size of the particles themselves and proportionally, so would
    the speed that they could travel through
    said space, so how would we have any
    way of measuring or noticing this
    occurrence? And I get the redshift and
    other evidence for it but there is a clear
    contradiction in my understanding of it
    and I would be ecstatic if someone could
    explain this fully for me.
  2. jcsd
  3. Oct 7, 2014 #2


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    No, particles (and in fact anything smaller than a galactic cluster, which is HUGE) do not expand as the universe expands. Google "metric expansion" and check the link in my signature.
  4. Oct 7, 2014 #3


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    Or, said another way, the expansion is a gravitational effect that only impacts the universe on large scales. Things smaller than a supercluster of galaxies generally don't expand (this drops out of the equations when considering what happens with a universe that has some locations that are more dense than others).
  5. Oct 7, 2014 #4
    Okay what I was able get so far, is that the forces affecting particles and thus the particles themselves do not expand spatially, but how could space expand without the forces that occupy and are defined by space also expanding? It seems like you all are saying that the four fundamental forces only listen to some sort of absolute space independent of the rest of space that we observe expanding, which is absurd.
  6. Oct 7, 2014 #5
    This contradiction has been unsolved in my understanding of physics for a long time, maybe if someone explained the whole thing from the bottom up?
  7. Oct 7, 2014 #6


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    The forces expanding doesn't make any sense. Again, the expansion is an effect within General Relativity. The forces stay the same. There is no contradiction, because metric expansion has precisely zero impact on the forces.
  8. Oct 7, 2014 #7
    Yes, perhaps I should explain my reasoning behind the forces expanding. Photons for example, are responsible for electromagnetic interactions. I am just an amateur but it is my understanding(which may be incorrect of course, that's why I'm here)that In order for particles to interact through electromagnetism, photons must be exchanged between the particles, photons occupy and travel through space, so as space expands, it would seem like the range, and effect of the photons would expand with it, along with the particles responsible for all the other forces. And I don't mean to say that there's a contraditiction in modern physics, simply that there is a contradiction in my obviously false understanding of it. I wish to identify and correct that, which is why I'm here.
  9. Oct 8, 2014 #8


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    An expanded photon is just a photon with less energy. It changes nothing about how the electromagnetic force behaves.

    And again, the expansion is just an average effect. When you take into account the fact that the universe isn't completely smooth, you get zero expansion for gravitationally-bound systems (i.e., anything smaller than a supercluster of galaxies, as I mentioned above).
  10. Oct 8, 2014 #9


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    First and foremost, the exchange of photons in an interaction is an exchange of virtual photons. Virtual photons are not real photons like you see from a light bulb. They don't travel through space and the don't even exist for any meaningful amount of time. So no, the range and strength of the EM force does not change as space expands.

    Also, you seem to be thinking that space "drags" objects along with it as it expands. This is not an accurate way of looking at it. The expansion of space is a change in geometry that makes the distance between objects increase over time unless they are bound together by some force. No new space is being created nor is space moving since space isn't a physical object.
  11. Oct 8, 2014 #10
    It is a good, recurring question. Here is my limited understanding, having studied neither general relativity nor quantum field physics but having a general interest in cosmology:

    Cosmology is currently understood by the use of semiclassical physics.

    - The classical stands for non-quantum physics, and that is how general relativity (GR) is understood.

    - The semi stands for quantum physics modifying it slightly.

    Here GR is kept weak (low gravitational energy densities) and so called quantum field theory is used to describe the other particle fields. And so they also describe forces, who are just bosonic particles such as the photons of quantum electrodynamic field theory.

    The particle fields constitute the particle physics vacuum. So here we have free-coupled it from GR. We don't have to consider the effects of GR on particles or forces, nor the effects of particles and forces as such on GR. (GR of course connects gravity and spacetime with particles and forces in the spacetime considered through mass-energy and pressure/strain considerations, it is what it does. But that is as far as it goes.)

    The result is that you can expand spacetime at will, without the vacuum and especially here forces changing character. :w An example is the semiclassical model of so called LCDM cosmology, our current cosmology.

    The scale where gravitational (and EM forces, i.e. mainly chemistry) will keep objects together is immense. And the effects are locally small. The current expansion of the universe is ~10-10 parts/year in every direction.

    You will have to go out to scales of 10 000 light years (ly) to see an expansion of 1 ly over 1 million years. The Milky Way is ~ 10 000 ly thick measured over the disk. As it happens galaxies have strong enough gravity to keep together indefinitely anyway.

    By looking at spectral lines, that the properties of the vacuum including force behavior stays constant, has been checked to billions of light years. That means most of the observable universe and most of the elapsed time since the current universe emerged.

    Also GR has been checked similarly, showing further that the underlying semiclassical approximation is viable and predicting what is seen.
    Last edited: Oct 8, 2014
  12. Apr 7, 2015 #11
    Consider that space isn't expanding but is being created. If one takes two hydrogen atoms and combines them we get a single helium atom. With fusion reactions we reduce the amount of space taken up by two atoms and place the matter in the space of 1 atom thereby creating (freeing) space. Thus stars get smaller (occupy less volume) so the distance between their outer regions get further apart. Every day the sum of atoms in the universe gets smaller but the gravitational affects remain constant.
  13. Apr 7, 2015 #12


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    This is not necessarily true even as a description of what happens locally in a fusion reaction; but even if it were true, it would have nothing to do with the expansion of the universe.

    To the extent this even happens (see above), the effect is miniscule; but in any case, as above, it has nothing to do with the expansion of the universe.
  14. Jun 15, 2016 #13
    Space itself doesn't exist. Space is a measurement the same as time. Space doesn't mean void. The ancients didn't understand air was composed of atoms and molecules because they couldn't see them - only felt the effect of air moving and saw the results. We don't see space but can see the results. For instance, why does gravity alone fail to predict star velocities at the outer regions of the galaxy? Galaxies and the stars within them move at tremendous speeds. What are the propulsion mechanisms and limiting factors associated with their movements? If we consider energy produced by stars, galaxies, etc. causing "space" to expand then we might assume that the expansion can reduce the limitations that exists. If so then the galaxies at the edge of the universe may well be traveling beyond the speed of light essentially being pushed faster than the propulsion from energy it generates by the expansion behind it.
  15. Jun 15, 2016 #14


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    Were you to paint an ink drop on a rubber sheet then stretch it, the dot would expand. If you looked at the dot under a microscope, you would see tiny unexpanded dots of ink with spatial separation. That is like the universe, the tiny unexpanded dots are galaxies and galactic clusters, the spatial separation represents the expansion of the universe.
  16. Jun 15, 2016 #15


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    A couple of things here. First, far away galaxies are already receding from us (and we are receding from them) at a velocity greater than c. That's nothing special.

    Second, your description of expansion and energy is incorrect. No energy is generated from expansion. If anything expansion reduces energy in the universe through redshift, but that topic is actually very complicated, non-intuitive, and nuanced, and I couldn't hope to begin to explain it well. In addition, there is no "behind" in this expansion. From the point of view of a far away galaxy, everything else in the universe other than a few nearby galaxy clusters is moving away from it while it itself remains stationary. Which is exactly what we see from our own point of view. All observers, regardless of their location in the universe, would see this exact effect as long as they remain at relatively low velocities with respect to the CMB frame.
  17. Jun 15, 2016 #16
    If I mislead you I apologize. I'm implying that energy is driving the expansion and has been for ? billions of years. One must consider that tremendous acceleration began just after the time of the "big bang" and continued henceforth. Even small increments over the life of the universe will result in very large velocities. I am concerned about the measurement of distances which I may comment on in another topic since it is more related to gravitational lensing.

    If one considers to be located at the edge of the universe and continuing in an outward direction above c, then a universe wouldn't exist at that reference frame. One would be invisible since light couldn't catch up if one holds c as the speed of light in a vacuum. Thus to see the universe one must be a part of its past

    Consider that "behind" implies two motions are occurring co-linearly in a specific sequence of which behind refers to the lagging occurrence.
  18. Jun 15, 2016 #17


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    @D2Bwrong, your statements are not being posed as questions, they seem to me to be statements of personal theory based on misconceptions of cosmology. You would do well to read up on some basic cosmology before posting such unsupportable claims further, as personal theory do not go over well here.

    Asking whether or not your beliefs are true is one thing, but posting them as fact when they are simply misconceptions is not a good idea.

    First, there IS no "edge of the universe". Second, as Drakkith has already pointed out, there are galaxies withing the Observable Universe that are receding from us (with no proper motion involved) at much greater than c but their light still reaches us and they most certainly do exist.
  19. Jun 15, 2016 #18

    Jonathan Scott

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    I like the simple analogy of the universe being like a cone, where the distance from the point is time and space is around it. If you move away from the point, space as a whole gets bigger. However, it doesn't get bigger locally. If you draw parallel lines on it, they remain parallel (as a cone can be made of flat paper). However, if you follow the paths of points which are spread out evenly around a circle, then as they move away from the point they get further apart.
  20. Jun 15, 2016 #19


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    We already have ideas about how expansion started and why it has accelerated over time. There's no need to propose your own ideas.

    There is no known edge and there is unlikely to be one, especially the kind of edge you are imagining. Far away galaxies are not speeding off into empty space.
  21. Jun 16, 2016 #20
    Yet Einstein believed the U. was statc, until Hubble contadicted that. Was it General Relativity? I thought the expansion was explained by the Big Bang Theory (BBT).
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