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Does expansion exist on the micro level as well as the galactic level?

  1. Sep 17, 2009 #1
    Does expansion exist on the micro level as well as the galactic level?

    I've never read about it, but it occurs to me that there can be no uniqueness about where the expansion of the universe exists.

    I have to envision an atom as a 'kind' of solar system, and a molecule therefore as a 'kind' of galaxy, and an ore sample of this molecule as a galactic cluster. With this simplistic view in mind, it seems to me that the space between ALL particles, not just large objects, must be increasing- and frankly, that the particles themselves (since they are not "solid" objects, but rather consist of "smaller" entities (smaller expanding "universes")) must be expanding also.

    Why have I read nothing of this? I'm not well-read admittedly.

    By the way, if this were the case, wouldn't we feel the expansion of the physical object beneath our feet as a force pushing up against the bottom of our feet? Come to think of it, our feet would also be expanding down toward the ground, thereby creating some sort of feeling.
  2. jcsd
  3. Sep 17, 2009 #2


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    Expansion does not extend down to the microscopic level. In fact, it doesn't affect things smaller than clusters of galaxies. The reason for this is that the forces holding these objects together (gravity, electromagnetic and nuclear forces) are stronger than universal expansion.
  4. Sep 17, 2009 #3
    Are we accepting that clusters of galaxies are expanding along with the space between them or not?

    You implicitly say that stronger forces stop the expansion on levels smaller than clusters. You mean that these forces stop THINGS from moving.

    When I stand on the surface of the earth, is it not said that I am accelerating at 32 ft/sec/sec, although I'm not MOVING toward or away from the center of the earth? Can there not be accleration or even expansion without movement?

    It does not make intuitive sense to me that at some arbitrary level, expansion would no longer exist.

    We may have to discover that at some relevant frame of reference, the forces of which you speak are not stronger than one another.
    Last edited: Sep 17, 2009
  5. Sep 18, 2009 #4


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    In the present epoch, as Janus noted, it safe to say expansion does no operate at levels below intergalactic space. The future is less certain. Google on 'the big rip' for discussions.
  6. Sep 18, 2009 #5
    I agree and have puzzled about this contradiction as well. I explain it thusly:

    Hubble parameter measures expansion of space and it is greater at increasing distances. Since these distances are HUGE and measured in light years, the information provided by these observations are VERY OLD. I conclude that the universe is no longer expanding since we do not see a hubble shift in our neighborhood, although we could detect it.

    here is what i wrote:
    http://forums.anandtech.com/messageview.aspx?catid=50&threadid=2320348&enterthread=y [Broken]
    Last edited by a moderator: May 4, 2017
  7. Sep 18, 2009 #6


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    The forces of which I speak all get stronger as the distance between the objects get closer together. The expansion of the universe does not. So yes, there is a boundary between expansion and nonexpansion. it is that point where galaxies are close enough for their mutual gravitational attraction is greater than their tendancy to move apart. Galaxy clusters do not expand because the individual galaxies are close enough to each other for gravity to hold them together. Galaxy clusters move apart because they are not close enough to each other.

    An analogy: You are standing on a tile floor in your stocking feet. The floor has 1ft square tiles. The floor starts to expand (the 1 ft tiles eventually become 2 ft tiles, etc.)

    You will see a person standing a few tiles away recede from you. (as the tiles expand, the distance between the center of the tiles increases. )

    Now imagine a third person standing next to you, and you are holding hands. The person few tiles away will still recede from you, but the person standing next to won't. This is because your grip is stronger than the friction between both of your stockings and the floor. You are like two galaxies in a local cluster gravitationally bound to each other. The tiles still expand under your feet, you just don't move apart with them.
  8. Sep 18, 2009 #7
    If gravity has this effect, then measurements of the hubble parameter in the vicinity of intense gravity wells (black hole, quasar,etc) should be 0. So, 2 galaxies that are very far away might have a certain redshift but a supermassive black hole next to a different galaxy in another part of the universe (but just as far away) should have much less redshift. Has this been seen?
  9. Sep 18, 2009 #8
    Whilst i appreciate that i am not at the high level of a PF mentor,

    Surely the above cannot be correct.
    If so how did inflation come about?
    Space must expand at every level.
    However, we have gravity too. this causes things to collapse.
    There is a battle between expansion and gravity/electrical forces
    On the microscopic, gravity and electromagnetic forces win and so whilst space itself expands the particles, planets etc don't.
    One has to get far away and into what is called the 'Hubble flow' before expansion effects are greater than 'local effects'.
    For instance the andromeda galaxy is close by and will eventually collide with our own. Gravity wins because the expansion effects are smaller that gravity effects.
    BUT.... all space exapnds no matter on what scale.
  10. Sep 18, 2009 #9
    Intense gravity wells etc have gravitational redshift. this will be added on to cosmological redshift.
    IE a photon of light escaping a black hole, gains lots of gravitational potential energy. Since E = hf, its energy reduces, frequency reduces wavelength increases.
    Gravitationally redshifted.
    Not zero.
  11. Sep 19, 2009 #10


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    Gravitational redshift is a non contributor to H0. The vast majority of photons generated by black holes, etc., originate too far from the gravity well to be noticeably redshifted.
  12. Sep 19, 2009 #11
    If Newton were alive today and knew what we know, I think he would have added another term to his gravity equation. This would be a negative term (repulsive) and have a very very small constant times distance squared and perhaps independent on their masses. This term applies all the way from short distances to extremely long distances but is so small that it is not detectable at distances shorter than inter galactic distances. Perhaps something like:

    F=G*(m1*m2)/r^2-R*r^2 and may have a m1*m2 relationship as well.

    R would be the expansion or repulsive constant.

    If this were the case at some distance the R term would balance the G term and gravity force F would be zero, beyond that distance expansion would overwhelm attraction.

    Does this make any sense?

  13. Sep 19, 2009 #12
    But surely one needs a 'standard candle' to determine Ho?
    Cepheid variables work as do supernovae 1a.
    Is there a standard candle black hole? ie a black hole that is equally bright (or dark????) no matter where it is in the universe?
    Otherwise, surely it is not a matter of the photons 'not being noticeably redshifted' but that they just cannot be used?
  14. Sep 19, 2009 #13


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    To a first approximation (and I must thank https://www.physicsforums.com/member.php?u=51971" as a major help for my understanding this) expansion simply means that things are moving apart from each other. It isn't a force pushing things apart. The universe expands because things are moving apart from each other ... not the other way around.

    That's the first approximation. The second approximation is that there are actually forces that act to slow down or to increase the rate at which things are moving.

    Gravity works to pull things together, and hence to slow down expansion.

    Dark energy works to push things apart, and hence to speed up expansion. As I understand it, "dark energy" is just a name we give for the apparent existence of something we don't really understand but which is the cause of expansion to be speeding up.

    But on sufficiently small scales (a cluster of galaxies or less) you can just consider the nice simple model you are all used to, of a bunch of stuff moving around in space, and subject to gravity.

    A galaxy isn't expanding, because its constituent parts are not moving apart from each other. No matter what motions you see elsewhere, the galaxy is all hanging together very nicely, and there is no particular "force" to counteract that, or to pull it apart. (Dark energy might in principle; but in the present epoch dark energy is far too weak to pull apart a galaxy or a galactic cluster.)

    That's the bottom line. The only thing that might push things apart is a "dark energy", and what we infer about dark energy now is that it is way WAY too weak to matter at small scales. Apart from that, there's nothing at all about the expansion of space that would lead one to expect anything to be pulled apart, on any scale, unless it is already moving apart right now.

    Cheers -- sylas
    Last edited by a moderator: Apr 24, 2017
  15. Sep 20, 2009 #14
    Don't think so, Space expands carrying all with it. The expansion we see now is just the dying embers of inflation and its scalar fields.
    On top of that we have the peculiar motion of the heavenly bodies within it.
    On the small scale, gravitational effects are far bigger than expansion of space effects so they dominate.
    But on the large scale expansion effects dominate and things move away from each other.
    Its a good thing that objects themselves don't get any bigger as otherwise we would not see the expansion.
    If distances got bigger and our 'ruler' got bigger at the same rate then we would measure the distances as constant. Its like a thermometer. The only reason the mercury rises is that mercury expands but the glass doesn't (hardly). If everything expanded then the mercury level would stay the same.
    Congratulations on the edit Sylas
    Last edited by a moderator: Apr 24, 2017
  16. Feb 25, 2010 #15
    I was under the impression that the expansion of galaxies is NOT measured by rulers, but by the red shift. In which cases it would be allowable for the rulers to be getting bigger at the same rate as distances.
  17. Feb 25, 2010 #16
    I should ask, how do we know that the universe is "expanding"? Is it not the red shift? And of course we make the assumption that red shift means that things are getting more meters apart. But the red shift would still be observable if all things, including space, were expanding, would it not?

    And are we certain that what you call the strength of "universal expansion" is not greater in small distances (close quarters), than it is at the galactic cluster level?
  18. Feb 26, 2010 #17


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    Redshift tells us the universe is expanding. Relativity tells us it is not a classical doppler effect. Spectral line shifts do not lie. We do not see discordant redshift in spectral lines. That tells us it is a global [spacetime] effect.
  19. Feb 27, 2010 #18
    Another way to look at the expansion effect in small scale is to start with earth and sun orbit. Th expansion and the gravitation will happen at the same time, though gravitation is much larger. Here, discrete modeling may help, that is say the sun-earth distance increases a little instantly due to expansion, then the earth orbit will change slightly and as a result the sun-earth distance increase will be diminished considerably after a while (actually the earth orbit will be slightly more elliptical.). and this diminishing effect will be greater in a mutibody case like in a galaxy.
    I think mathematically we can make equation and even can find a solution for a simple case, and the result will be in line with above rough thinking.
    In much smaller scale like our body cells, the dominant force is electric or inter-atomic force which will adjust each atomic motion continuously, diminishing expansion effect considerably.
    This is a more detail thinking of the expansion effect to small scale.
  20. Feb 27, 2010 #19

    I'm not disagreeing with you and let me ask you:

    Does what you're saying agree or disagree with the proposition that while according to the red shift the universe is expanding, it is also possible that at the same time all objects and the space in between them are expanding also?
  21. Feb 27, 2010 #20
    The explanation by advisor and mentor helped me a lot to understand. On this Axuality's question, I 'd say the answer is yes in infinitely short moment.. i. e. the expansion applies even in subatomic scale, but this expansion does not accumulate as time goes on in short distance.
    In long distance beyond galaxy cluster, the expansion accumulates as time goes on. Thanks.
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