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Accelerating galaxies?

  1. Jun 3, 2009 #1
    Accelerating galaxies???

    If Gravity is constantly pulling galaxies toward each other, why are they accelerating away from each other???
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  3. Jun 3, 2009 #2

    DaveC426913

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    Re: Accelerating galaxies???

    That's kind of a broad question. Where did you read this? It would help to understand what your preconceptions are.

    Gravity pulls galaxies together to collect in local clusters. However, within our local cluster, some galaxies are moving toward us some are moving away. This is just part of the normal residual relative motion the galaxies started with. The Milky Way and Andromeda are part of our local cluster and Andromeda is speeding toward us. Te force pulling glaxies into a cluster is pretty weak.

    Independent of that, the universe is expanding. This manifests as a force that pulls galaxies (and, technically, everything else) apart. But this force is so exceedingly weak that the normal pull of galaxies close to each other overwhelms it, keeping them in their local clusters. However, on a bigger scale - between galactic clusters where gravity is extremely weak - the force is strong enough to pull the clusters apart.
     
  4. Jun 3, 2009 #3
    Re: Accelerating galaxies???

    i heard dark energy has preety good roles,is it true ?? someone help
     
    Last edited: Jun 3, 2009
  5. Jun 3, 2009 #4

    DaveC426913

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    Re: Accelerating galaxies???

    The expansion of the universe can be seen as the manifestation of some form of energy, even though we don't know what form it takes.
     
  6. Jun 3, 2009 #5

    Chronos

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    Re: Accelerating galaxies???

    The are two unconfirmed components in the current LCDM model of the universe, dark matter and dark energy. Dark matter is believed to be comprised of neutrino like particles that exert gravitational force but otherwise barely interact with other matter, not even one another. The evidence for the existence of dark matter is strong. Dark energy is a more recent hypothesis based on the Perlmutter supernova study. This study indicates expansion of the universe is accelerating. Dark energy was deduced to explain this. This puts a big kink in the calculations. Since, under GR, energy and matter are interchangeable, you can get some pretty big numbers for the dark energy content of the universe. Neither 'dark' particles have yet been detected. It is highly probable the dark matter particle will be detected. The jury is still out on dark energy.
     
  7. Jun 6, 2009 #6
    Re: Accelerating galaxies???

    i just wanted to a simple thing about a feature of the motion if there can still be increase in velocity in decrease in acceleration uniformly,(when i thought of it ,i felt it could be ) but i want help from experts around here
    I am asking this question beacause there is something i hqave seen to tell
     
    Last edited: Jun 6, 2009
  8. Jun 6, 2009 #7

    Borg

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    Re: Accelerating galaxies???

    I'll take a shot at this - please be nice if I'm not describing everything exactly right or with perfect detail.

    The spacetime fabric that the galaxies reside in is expanding. This expansion is commonly compared to raisins in a rising loaf of bread. As the bread rises, the raisins get farther apart. The farther that they start from each other, the faster that they appear to accelerate. The raisins aren't actually moving, the space that they reside in moves.

    In 'smaller' areas such as a galactic cluster, gravity is able to cause accelerations that are greater than the spacetime expansion in that area. So, galaxies that are near to each other to start with, can be gravitationally attracted.

    For large distances between galactic clusters, the gravitational force is much weaker than the spacetime 'acceleration'. So, while the galaxies in a cluster can accelerate toward each other, clusters that are far apart, do not.
     
    Last edited: Jun 7, 2009
  9. Jun 6, 2009 #8
    Re: Accelerating galaxies???

    i would be grateful if someone help with my abve question
     
  10. Jun 6, 2009 #9

    Wallace

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    Re: Accelerating galaxies???

    No No No! The expansion of the Universe is not, and does not require, a force. The Universe simply expands because something (namely the Big Bang, or more technically, the inflationary era in the early Universe) gave everything a kick that cause everything to begin to move away from everything else. This expansion has since continued due simply to inertia, if something is moving it just keeps moving, and that includes the mutual recession of any pair of galaxies. The effect of gravity slows this expansion rate down (as everything attracts everything else) and indeed the expansion rate slowed for the first 10ish Billion years of the life of the Universe (the number are very rough). In general in an expanding universe there is no force trying to pull galaxies apart, galaxies are not expanding, hence there is no momentum to overcome, hence they do not expand. It is not because of a scale dependant force that is 'weak' because galaxies are small. There is not such force (at least not in general, and one is not required for a universe to be accelerating).

    On the other hand, we have discovered in the last ten years or so that the expansion is accelerating. Now, acceleration is the change in velocity and hence this does require a force (or the general relativistic equivalent). In fact this force must be repulsive, so it is very strange, and the energy responsible for it is called 'dark energy', which we know very little about (in fact other possibilities could explain the data without needing to invent dark energy, but this is the most promising model at present). What this means is that there is a force that is trying to push everything (including galaxies) apart, however this force is not require to sustain expansion, only accelerated expansion.

    Note that the explanation offered by Borg makes a small confusion. When models of the universe invoking things such as rising bread are used, one must be careful to realise that this mental picture is used to help you imagine how a uniform expansion (i.e. bread expanding everywhere) makes the raisins all move apart following a Hubble law (i.e. faster velocity with greater distance). However, it is a mistake to take from this that their exists some analogue of the bread, in this case 'the fabric of space-time' that causes the expansion to occur. This idea does not correspond to the physical theories behind cosmology. The only reason expansion occurs is because is did so in the past, and momentum is conserved. A change in the rate of expansion requires a force, but continued expansion at the same rate does not.
     
  11. Jun 6, 2009 #10
    Re: Accelerating galaxies???

    sorry to quote ,its simple confusion
    is inertia for uniform motion or acceleration?
     
  12. Jun 7, 2009 #11

    Borg

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    Re: Accelerating galaxies???

    I agree that the forces involved in expanding the bread have no correlation to the space-time fabric. The bread analogy was just used to help visualize the OP's original question.
     
  13. Jun 7, 2009 #12

    Wallace

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    Re: Accelerating galaxies???

    The point is that there are no forces at all required to keep the universe expanding, which is why the expanding bread thing can be confusing (since it that case there is a continual force from the bread driving the raising apart). In addition statements such as "the raisins aren't actually moving, the space that they reside in moves" aren't, in my view, particularly helpful. Space does not 'move', and galaxies, by any ordinary use of the word, certainly are 'moving' as the Universe expands.

    The notion of galaxies sitting still in an expanding space is a description of a particular co-ordinate system used to describe an expanding universe, but it shouldn't be elevated to physics.
     
  14. Jun 25, 2009 #13
    Re: Accelerating galaxies???

    observed facts: the further away a galaxy is from us the more red-shifted i.e. faster in its movement away from us. This is an astonishing observation. The universe is ever expanding. The furthest galaxies may exceed the speed of light in their disappearing act, and thus bypass the event horizon forever lost to us. In a billion years, if no catastrophe has already shattered our local galaxy, our skyline will be much dimmer with much fewer visible stars. Very problematic facts for proponents of the great-crunch. It seems that a greater part of our universe is formed by dark matter and an even more abundantly of dark energy. To complicate things even further dark matter is both hot and cold. It exerts generally attractive forces and to it is attributed why it was possible for galaxies to form in the first place and why a galaxy is not ripped apart by the high velocities at which it spins. Dark energy even more abundant than dark matter is repulsive in nature ever expanding the universe. A good read are the works of Paul J. Steinhardt and Neil Turok.
     
  15. Jun 25, 2009 #14
    Re: Accelerating galaxies???

    but if galaxies at the edge of or beyond our event horizon are approaching or exceeding the speed of light relative to us, does that not violate the law of constant speed of light?? Actually no since it is our universe (space) expanding. Many models by astrophysicists for a futuristic warp hyperspace machine exceeding the speed of light is based on emulating this phenomenon.
     
  16. Jul 10, 2009 #15
    Re: Accelerating galaxies???

    ive heard the speed of light is changing !!!
     
  17. Jul 10, 2009 #16
    Re: Accelerating galaxies???

    does the whole expanding universe thing go with the increase in entropy
     
  18. Jul 10, 2009 #17
    Re: Accelerating galaxies???

    Were did you hear that?
     
  19. Jul 10, 2009 #18

    DaveC426913

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    Re: Accelerating galaxies???

    It is a fairly well-known hypothesis.
     
  20. Jul 11, 2009 #19
    Re: Accelerating galaxies???

    I had a thought concerning the apparent acceleration of the expansion rate of the universe. However, I do not have the depth of understanding of astrophysics nor relativity to spot flaws in the idea. So, I submit it here for comment.
    If the expansion rate of the universe were actually decreasing, wouldn’t the Lorentz Contraction explain the fact that the rate appears to increase?
     
  21. Jul 11, 2009 #20
    Re: Accelerating galaxies???

    I don't know if you have read all of Wallace's posts on this thread. For the most part I'm in complete agreement with him. The expansion we currently see is due to the initial 'big bang'. We have been 'coasting along' since then -- for the most part with little change. There have been some theories that required the introduction of a 'dark matter' to help them fit the data (of the day). More recently, others are now arguing for a 'dark energy' to account for another perceived departure from the widely accepted theory.

    Neither of these ideas -- dark matter or dark energy -- have been 'proved'. In fact, the data used to come to these conclusions is based on models that have not been 'proved'.

    The current state of cosmology is such that a lot more 'proving' needs to be done. But there is hope for new data -- something that cosmology has always been very short on.

    Now all that said, when you mention Lorentz Contraction and its possible involvement with the recently perceived 'acceleration' the only way I can see this occurring (directly as you mention) is though a calculation similar to one done for a relativistic Doppler redshift. For reasons as you imply might best be reconsidered -- the Doppler redshift has not been accepted as a possible 'source' of redshift due to the stated belief that this perceived recession velocity is not 'real' in some way.

    As I understand it, the view is that this is considered a 'different effect' -- due to the General
    Relativity scale factors. Simply put, the current mainstream model uses General Relativity to address this. While a direct Lorentz Contraction (or Doppler effect) is Special Relativity.

    With today's limited data -- and its limited accuracy -- it is largely one of preference (in my opinion).
     
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