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ETpro's avatar

If the universe is infinite, how big is what it is expanding into? (Strange Universe 2011)

Asked by ETpro (34498points) February 25th, 2011

We can observe data that tell us the Universe appears to be at least 46 to 70 billion light years in radius. Even though that is much further than light could travel in the 13.7 billion years since the Big Bang, the accelerating expansion rate of the Universe means that the light we see from the most distant objects left the location they were in 13.5 billion years ago or so, and just got here, but they have since moved much, much further away from us. Some theorize that the Universe is actually infinite. If it is, how can it expand when it is already everywhere there is and fills everything there could ever be?

One might think that the Big Bang was like a stick of dynamite exploding inside a bolder, blowing everything outwards at great speed. This is a flawed concept. Such human-bound physics could not account for pieces of the boulder accelerating. Nor is it necessarily true that the Big Bang occurred at a single point. So an infinite Universe is possible.

Do you think the Universe is infinite? If it is, what is it expanding into? Do you think this conundrum proves the Universe is finite, or that it shows we cannot grasp the morphology of all its dimensionality?
Here are the previous questions in the Strange Universe Series for 2011.
1: —How do you envision space in more than 3 dimensions, then rotate it to see what happens?

The 2010 list of 20 questions in the series is shown here.

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6 Answers

ragingloli's avatar

There is no answer to that question, since there is no space beyond the universe.

The_Idler's avatar

The Universe is not expanding, space is expanding.

The Universe has no external dimensions, because dimensions are spatial properties, and space is inside the Universe.

Besides, the Universe could be finite, but boundless in the 3 spatial dimensions.
A bit like how the Earth is finite, but boundless in the 2 surface dimensions.
The Earth is not infinite, but you could walk forever and ever in any direction, and never reach the edge…

Nullo's avatar

I wouldn’t be surprised to find out, thirty years from now, that it’s all really just a forced-perspective trick.

I figure that the Universe is effectively infinite, whether it really is or not. Certainly, I doubt that we’ll ever be able to find out.
Wasn’t it popular some years back to say that the Universe was, to misuse the term, round? Such that if you start off in one direction you’d eventually arrive back at your starting point?

ETpro's avatar

@ragingloli There definitely is an answer to the question. It isn’t the sort of logical trick question which cannot be answered: such as, “If god can do anything, can he build a mountain so high he can’t climb it?” A legitimate answer may be we currently do not know.

@The_Idler That’s what the OP said. It asked what it is expanding into if it is infinite in scope already. Per the definition of infinity, wouldn’t that form a logical contradiction? Infinity has to fill everything that is and ever could be, such that it is impossible to divide it.

@Nullo In human understanding of today, you are probably right we can’t answer this question definitively. But there may be ways of discerning things we have not yet tappped into. If you imagine (whatever your belief) that there is a Creator, do you not thing the Creator would be able to answer this question? If so, then what if the Creator chose to open your eyes to the answer? Could you not then see it?

gasman's avatar

Finite vs Infinite
One must distinguish between (1) the observable universe, which has a measurable age & size; (2) a proposed multiverse, of which the observable universe is but one insignificant part of a complex, fractal structure. In some models the multiverse is a “foam” of universes, each created by its own big bang causing a new universe to bud off from the rest. Another use of the term “multiverse” is the many-worlds interpretation of quantum mechanics, whereby all possible quantum states co-exist (in some sense) in parallel universes.

The observable universe is popularly modeled as the 2-dimensional surface of a vast 3-dimensional sphere. In this scenario, ants crawling around the surface would perceive it to be flat, yet as the balloon grew in size each ant would see his neighbors receding from him with some measurable redshift, proportional to how far away the other ant is (Hubble’s Law).

Meanwhile the ants never find any kind of barrier or wall to mark the end of their universe, and erroneously conclude that it is infinite. Now jump to 4 spatial dimensions instead of 3, plus time, and you’ve got a model of big-bang cosmology (ants = galaxies), whose predictions agree well with observations in varied ways.

Just as the ants see 2 spatial dimensions when there are actually 3, we infer a 4-dimensional structure to our apparently 3-dimensional universe. In the simplest model it is a hypersphere. The universe—to use a very old term—is “finite but unbounded.”

Big Bang versus ordinary explosion
Dynamite explodes into its surrounding space. Newton regarded space as a sort of universal framework in which events play out, a highly intuitive idea that works for human-scale phenomena. In big bang cosmology, however, space itself is what is expanding from the singularity. In this sense the universe is not expanding into anything.

Size of observable universe
Although relativity requires that nothing may move through space at faster than the speed of light, no laws are violated when space itself (there’s that idea again) expands at faster than the speed of light. Thus the observable universe has a diameter of about 46 billion light years (ref) even though the big bang occurred less than 14 billion years ago.

ETpro's avatar

@gasman Thanks for a great answer. Your analogies really capture the concept well.

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