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in physics we were learning about the universe and the big bang. we were being told that everything came from one point and 'exploded' outwards, and is continuing to expand. I felt that this explanation was unclear, so I said that it was more like everything being one the surface of an expanding 'balloon', but with an extra dimension, a space which is the hyper surface of the expanding universe. my teacher disagreed and said it was just like a big explosion. I think this is wrong-it implies that there is an edge of some description, or that the universe is just an expanding sphere of stuff inside infinite space. what is the best way to visualise the universe?

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You were correct, you teacher was incorrect.

It is the space that expands - much as a surface of a balloon does.

An explosion is a poor analogy in contrast because - as you suggest - it implies something to expand into.

For another way, not as accurate as the balloon analogy but maybe helpful - imagine being trapped inside an expanding loaf of bread in an oven. You have no knowledge of the world outside the loaf - and you may assume the loaf is infinite in extent. But as the dough rises the gaps inside the loaf get larger without the mass of the loaf itself increasing. You could compare our position in the universe to being inside such an infinite loaf.

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  • $\begingroup$ +1, but the loaf analogy is no less accurate in general; it's just different. For the case of an infinite flat FRW universe, an infinite loaf is better than the balloon. However, the balloon analogy is more accurate finite positive-curvature FRW universe. ... So it depends on what cosmological model you're trying to describe. $\endgroup$
    – Stan Liou
    Mar 13, 2015 at 15:44
  • $\begingroup$ Bread dough analogy is more helpful if you include poppyseed galaxies in your recipe. Distance between seeds over time will follow a Hubble expansion like law, where the rate of recession between any two seeds increases linearly with the distance between the seeds. $\endgroup$
    – Wayfaring Stranger
    Mar 13, 2015 at 23:34

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