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This is somewhat triggered by this question: It's established that universal energy is not constant. But is the net change positive or negative?

When I say entire universe, is that equivalent to "the observable universe"? Since the universe beyond the observable universe cannot affect us in anyway, it might as well not exist. The fact that more of the universe becomes observable over time, just means that the universe is expanding.

Of course, when we consider our space time models, we do model the parts of the universe that are not observable. So in our models, the universe is more than the just the observable part.

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First, just a clarifying point: the fact that more of the universe becomes observable over time has to do with the finite speed of light, not the expansion of the universe (which is indeed happening at an accelerated rate).

And of course it is best to consider the universe as a whole. Even though we can't see the dark side of the moon, we still know indirectly it's there. Now that we live in a space age perhaps know directly, but you get the point. You only lose understanding by cutting off your imagination at the end of the observable universe, so why do it?

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  • $\begingroup$ I might be wrong, but doesn't the non-observable universe lie outside of our causal past? I don't think your dark side of the moon analogy is so great because it is causally connected to us. $\endgroup$ – Ryan Unger Feb 13 '15 at 1:03
  • $\begingroup$ Actually, we don't know its there since we can't observe it. We have models that say it's there, but it's not the same thing. Where as we can observe the dark side of the moon (at least in principle). Yes, I agree this is not the same thing as "inflation" $\endgroup$ – yalis Feb 13 '15 at 1:35
  • $\begingroup$ I mean the dark side of the moon analogy was meant to show that just because we can't observe some region, (either in principle or in practice) doesn't mean it's not there. And it's not simply an aesthetic or academic difference because these inaccessible regions can help us put together a predictive theory. $\endgroup$ – Surgical Commander Feb 13 '15 at 1:55
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"Universe" can have several meanings. Some describe the visible universe (small u), others describe the whole Universe (capital U), whatever that might be.

That we can described the "visible" universe, perhaps implies a visible and non-visible universe.

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  • $\begingroup$ We can imply it through the model. Model the entire universe and say that only this part is observable to the earth folk. But we cannot prove that the non-observable part exists. $\endgroup$ – yalis Feb 13 '15 at 1:39

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