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Is it possible that it is just our observable part of the universe that is expanding, in the time that we exist, and other parts are both expanding and contracting at different rates and times?

Would light from a shrinking or slower/faster-expanding part of the universe reach us?

Could that be an alternative to the big-bang theory, in that the red-shift is a temporary situation local to our time and part of the universe? A "throbbing" energy that leads to the non-uniformity that we see in galaxy clusters; something like foam awash on the ocean surface.

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  • $\begingroup$ I'm not as educated as some in this, so I'll leave a comment, but there's 3 primary observations. 1) The observable universe is expanding and 2) the expansion is uniform, not bigger in some directions than others and 3) the universe doesn't appear to have any built in curvature, so while it's possible that there might be some fluctuation beyond what we observe, as Adrianmcmenamin points out, there's no evidence of that kind of fluctuation, so if it does exist, it would need to be enormously large. $\endgroup$
    – userLTK
    Commented Aug 17, 2015 at 21:34
  • $\begingroup$ It could all turn into sewing machines just past the limits of observability, and we'd be none the wiser. $\endgroup$
    – Wayfaring Stranger
    Commented Aug 18, 2015 at 12:34

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There is no evidence to support the idea that some part of the universe (at a cosmological scale) is contracting. Obviously one could construct such a theory but there is currently no observational data to support it.

Until relatively recently there were three broad ideas about the Universe's expansion - that gravitational pull would slow the expansion but by never enough to halt it, that (a boundary case) there was exactly enough matter in the universe for expansion to halt at an infinite point in the future, or that eventually gravitational attraction would first slow and then reverse the expansion, leading to a big crunch.

In fact, though, the observational evidence suggests that the expansion of the Universe is accelerating - that there is some "dark energy" that is speeding up the expansion. This is now the generally accepted view, though arguments continue as to what this "dark energy" is.

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We do not know what the dark energy is , which is responsible for the expanding universe.

However, if it's a property of space-time, the following two points will result:

  1. The universe expansion is isotropic, meaning it expands the same way in all directions.

  2. The expansion is homogeneous, meaning it expands the same way in all its parts, not only what we have observed.

The homogeneity and isotropy of the universe are fundamental principles, from which the laws of conservation of impulse and angular momentum follow. All the observational results support them. If these laws would be wrong, all the physical theory should be revised.

So in case dark energy is a property of the universe, all the universe should expand the same way as the observable universe.

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  • $\begingroup$ If we don't know what dark energy is, how can we say it is "responsible" for the expanding universe? There is an energy in the expansion, but can we really say that the energy is responsible for the expansion as opposed to a corollary of it? I know this is more an argument about the philosophy of science than anything else: but surely it is better to say "the universe is expanding isotropically (NB: at a cosmological scale) for an unknown reason which astrophysicists call 'dark energy'". Or something like that. $\endgroup$
    – adrianmcmenamin
    Commented Sep 5, 2015 at 13:58
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Yes, we don't know what dark energy is. It's better to say that whatever is responsible for expanding, we call it dark energy.

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