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Isn't it true that the concept of length exists only within the universe? Are we simply measuring the observable universe, that is to say, the spatial limits of our observation?

Prish Chakraborty
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    The universe is by definition the totality of everything that exists. The visible universe is, also by definition, the part of the universe that we can "see" and do measurements on. From that it follows that we can't say anything about the part of the universe that is outside of the visible universe. The good news is that science will never ask us to do anything that we can't do. So, yes, spatial and temporal measurements are only part of our effort to understand the visible universe. – CuriousOne Jun 11 '15 at 06:54
  • possible duplicate of How do we know the size of the universe? – John Rennie Jun 11 '15 at 06:58

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It is difficult to say. When you say "The size of the Universe" some people think about the observable Universe, while others, like me, think about the entire Universe, beyond just what we can observe. When it comes to length in the entire Universe, it's useless. the Universe is infinite so no matter what measurement we create, it will be insufficient to measure the entire Universe. In a sense, when people say the size of the Universe, it is easier to assume they are talking about just the parts we can observe.

DragonSlayer3
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  • Under current "received truth" the universe is deemed to be finite in size. – Russell McMahon Jun 11 '15 at 09:18
  • I do not accept the thought that the universe is finite, it just doesn't make sense. Think of it like this. Take an egg and pretend that everything inside the shell is our universe. Then put the shell in a jar, and the jar in a box. Once we reach the shell we think we've reached the end of the universe. If we go through the shell we will be inside the jar. Keep going and you're in the box, go a little further and you hit the wall of the box. The point is, there is always more. If we keep going, it never stops. The only way to find an end is when we lose interest or will power and stop. – DragonSlayer3 Jun 12 '15 at 09:21
  • What any of us "accept" or what to any or all of us makes sense has no affect on reality. The egg/box/whatever analogy utterly fails in this case. Reality "just stops" is a good easy ludicrous but about as good as we can manage summary. If you want something related that exercises the mind try: The two alternatives re "reality" is that it has always existed or that it came into existence from absolutely (absolutely abs ...) nothing. Neither makes any sort of "sense" to any form of rational logic. (No problem for irrational logic :-)). So -please posit a viable 3rd option. – Russell McMahon Jun 13 '15 at 07:39
  • I guess when it come to if the universe is Infinite or not it's like Last Thrusdayism, there's no proof for it but there's no proof against it. – DragonSlayer3 Jun 15 '15 at 08:13
  • "Expanding" and "open and infinite" are not the same. Even if 'open' and expanding, the extent of space is held to be currently finite under current understandings. Just because current understandings will not be worth the hard drives they are written on 100 years from now (or 20, or maybe 10) does not mean we are intellectually 'free' to hold completely opposing views 'just because we want to'. To do so is random-religion just as surely as turtles-all-the-way-down. Interestingly. – Russell McMahon Jun 15 '15 at 14:20
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What do we mean by the size of the universe?

Exactly that. We mean what we say.

Isn't it true that the concept of length exists only within the universe?

Since the universe is everything, yes. But that doesn't stop us talking about the size of the universe.

Are we simply measuring the observable universe, that is to say, the spatial limits of our observation?

That's what we can measure, but when we're talking about the observable universe we say observable universe. When we're talking about the whole universe, we don't.

Note that some people say the universe is infinite, see Dragonslayer's answer above, and this NASA article on WMAP. However we have no evidence that it is, and IMHO an infinite universe is at odds with big bang cosmology, which I feel is broadly correct. Search on universe size of a grapefruit and you find articles saying the universe was the size of a grapefruit near the time of the big bang. More recently however, since WMAP found that the universe was "flat", some people have shifted to saying the observable universe was the size of a grapefruit, and that the universe was infinite even when the big bang occurred. I don't buy this myself, because IMHO an infinite universe cannot expand. I'm with Russell McMahon above. What's interesting about the finite flat universe is that it has some kind of edge. But of course, that's one for another day.

John Duffield
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