Quote:
Originally Posted by Peter Wilson
Look, don't mince words.
If there was nothing beyond 5 Mpc, but otherwise everyting looked the same up to 5 Mpc, we would say, "The universe is 5 Mpc across and contracting." (According to GR as currently formulated, the universe must do one or the other: expand or contract.) It is not until we look beyong 5 Mpc that we observe expansion. So we say, "The universe is expanding."
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If there was nothing beyond 5 Mpc, the universe would not be homogeneous, and we would need another model instead of the now-used Robertson-Walker metric.
Quote:
Originally Posted by Peter Wilson
But this is incorrect. It is not doing one or the other; it is doing both at the same time: contracting locally/expanding non-locally.
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Technically, you are correct. But cosmology deals only with the behaviour of the universe "as a whole", i. e. on very large scales. When one talks about the behaviour of the universe "as a whole", local effects are simply neglected.
"on the whole", the universe is expanding.
But that obviously does not mean that cosmology is
entirely ignoring the more local effects - as I already pointed out.