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Thread: Does Special Relativity affect one's perception of Expansion?

  1. #1 Does Special Relativity affect one's perception of Expansion? 
    Time Lord
    Join Date
    Mar 2007
    Wouldn't observers in different frames perceive the rate of Hubble Expansion to be different?

    I've been thinking this through. The speed of light is the same for all observers, regardless of their frame of motion. Space appears to contract in the direction of motion.

    Suppose there is an object very far away in space and two observers are trying to see it. Observer A is stationary relative to the object. From his perspective the object appears to be outside the Hubble Sphere, so he can't see it. Observer B is moving away from the object at relativistic speed. From his perspective, the object appears to be inside the Hubble Sphere, and he can see it.

    Imagine the paradox. The Observer moving away at relativistic speed is the only one who can see it!

    I'm thinking the only way this resolves is if the two observers also perceive the rate of expansion to be different. Then observer B doesn't see the object, because the rate of expansion is higher in that direction for him, and despite the object appearing to be a shorter distance away it is still outside the Hubble Sphere because of the greater rate of expansion.

    This makes universal expansion more interesting, I suppose, because the thing that is expanding (space) has properties that differ from our everyday experience. It also makes me curious about expansion appearing to be the same in all directions for us on Planet Earth. We must be more or less stationary relative to the majority of the celestial objects we are looking at when we measure the Red Shift. At least not moving at relativistic speed relative to them.

    Some clocks are only right twice a day, but they are still right when they are right.
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  3. #2  
    mvb is offline
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    Dec 2012
    Delaware, USA
    There is no problem. The Hubble "sphere" flattens by Lorentz contraction as well. It is, after all, defined by the objects that are on it, whose recession velocities are very close to the speed of light. Also note that since the definition involves the speed of light, it is invariant as well.

    Markus Hanke and Kerling like this.
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  4. #3  
    Time Lord
    Join Date
    Mar 2007
    The rate of expansion is invariant? Even though the instantaneous distance itself is not invariant? So the distance is not invariant, but the rate at which that distance increases is invariant?

    I guess that makes more sense than it may seem at first. The rate at which light traverses a distance is invariant even though the distance itself is not invariant. So I guess the rate at which the distance grows could be invariant also............

    It's just weird.
    Some clocks are only right twice a day, but they are still right when they are right.
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