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Galaxie recession vs. universe expansion?

  1. Aug 21, 2008 #1
    Do galaxies recede at the same rate that the universe is expanding? We know the universe is expanding by measuring the redshift of galaxies. But since a galaxy has mass and inertia, would it have a resistance to the accelerated expansion of the universe and therefore recede slower than the universe is expanding?
  2. jcsd
  3. Aug 21, 2008 #2


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    Yes. How else would you measure it? Galaxies, like the stars within them, have very modest proper velocities relative to the CMB.
  4. Aug 22, 2008 #3
    does the CMB have a redshift? is that how we know the universe is expanding? or do we know it is expanding because of the redshift of galaxies? Would our calculation of the expansion rate be too low because the galaxies have a resistance to the expansion which would make them recede slower than they would if they had no mass? Help clear me up on this confusion.
  5. Aug 22, 2008 #4


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    Yes, current measurement is about z=1090
    The wavelengths of CMB light have been expanded somewhat over 1000-fold
    CMB is one way. Galaxies are another. there are several ways to measure and record the history of space expansion, and they fit together consistently. A recent article on how the record of expansion has been determined is by Eric Linder
    It is called Mapping the Cosmological Expansion

    Recession speed is not ordinary motion that you are familiar with, and is not affected by inertia. The recession speed of a galaxy can accelerate without any force being exerted.
    If it were ordinary motion, then to make the galaxy accelerate you would need a force---as in Newton's law.

    Recession speed is just a rate that a distance betweem stationary observers is increasing. Gen Rel teaches us to expect that distances between stationary observers can change. That is what is counterintuitive about it. Largescale geometry is not Greek.

    the galaxies are hardly moving at all relative to CMB (i.e. relative to universe rest)
    their speeds (of real motion) are neglible---on order of a few hundred km/s.

    there is no inertia effect making them resist expansion. expansion just means the distances between galaxies (considered as stationary) are increasing.

    expansion is not like ordinary motion because it doesn't get you closer to anything. a galaxy which is receding from us at several times the speed of light can never catch up with and pass a photon. and it can never get to any destination, by means of its recession speed.

    the recession speeds of galaxies, from our standpoint, or from any other galaxy standpoint, are indeed accelerating----but inertia does not resist this because it is not like ordinary motion. it is merely the increase of distance between stationary things----exactly what the expansion of space is, no more, no less.

    So the answer to your question is No. Our calculation of expansion rate does not have to be adjusted to take account of the galaxies' inertia.

    There is no VERSUS in your title "galaxy recession vs. universe expansion"
    There is no versus or discrepancy because they are in essence the same thing.
    Last edited: Aug 22, 2008
  6. Aug 22, 2008 #5
    Thanks for your response, I'll check out those links in your sig. So my error is in trying to think of space/time as sepperate from the things in it? So we define the expansion of the universe as the recession rate of distant galaxies and the CMB? Would it be meaningless then to ask if there was no CMB and no galaxies or any other matter or energy in the universe. if it were just a pure space time vacum, even then it would still consist of a quantum vacum energy? Would we still be able to say it is expanding? I am trying to abandon the notion that space time is some kind of fabric that is stretching and carrying things along on it's surface, but being a 3 dimensional critter, I find it difficult.
    Just trying to get a grasp on the concept, excuse my ignorance.
  7. Aug 22, 2008 #6


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    that is roughly right, Talon. At least as far as what you said about galaxies.

    A galaxy is a more or less fixed collection of matter, typically not moving very fast wrt CMB. So you can equate it approximately to an observer stationary wrt CMB. Our distance to those approximately stationary (or exactly stationary) increases---the rate of increase is recession speed, that's what the expansion of space means----it is the expansion of distances between stationary things.

    But the CMB source is not a fixed collection of matter. The CMB originates from a unique event which all the matter in the universe experienced simulateously. . A one-time flash. The matter that we are getting the flash from is currently all the matter at some certain distance from us far enough so we are just now receiving the light (stretched to microwave noise). Tomorrow or a million years from now it will be DIFFERENT matter FARTHER from us. The matter that we are currently seeing the flash from is what is called the surface of last scattering.

    You know that is kind of difficult for me, philosophically. there is a very common wellknown empty universe model called the de Sitter universe. All it has is dark energy. No matter of any sort. It expands. But how would the people in it know?

    Well if there were people it wouldn't be perfectly empty. It wouldn't be purely a de Sitter universe.

    Cosmologists like the de Sitter universe because it closely resembles what they expect our universe will be like far in the future, and how it will behave when it is very thinned out. Expanded so much it is almost empty.

    What does expansion mean, practically speaking, if there is no CMB to mark a stationary observer, and no landmark galaxies, and no landmark of any kind? then only the old man in the sky, the omniscient imaginary observer, can see that it is expanding? I'm puzzled by this, cant get any traction on it.

    Our universe contents are 75 percent dark energy. So you could say we are almost in a de Sitter condition. In a pure de Sitter it would be 100 percent. Nothing else but. Mathematically it would be expanding all right, but how could you tell, practically speaking. I like things to be operationally defined, by some definite measurment that a person can make.

    have to give up. go back to the imperfect nittygritty reality with its approximate but workable definitions.
  8. Aug 25, 2008 #7
    so if the dimensions of space time are expanding and every point is the center of expansion and gravity and the nuclear forces are holding me together and hold me onto the earth. if gravity and the nuclear forces suddendy ceased to exist. would the atomic particles that I am made of suddenly go flying off in all directions, carried along with the expanding dimensions of the universe? and at what speed would these particles go flying off? Would it be a fairly slow speed because the expansion of the univers is slow, locally. or would it be a very fast speed since we can see distant galaxies receeding faster the speed of light?
    I suppose that due to the uncertainty of position and momentum, all these particles would fly off in random directions like the photons from a lightbulb?
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