[Sacha Quadrelli]

Sacha Quadrelli asked the Naked Scientists:
   Hello there,

You may have tackled this one before, but here goes anyway: Theoretically, as there are no "edges" to the universe, if you were to head off in the same direction at about the speed of light and cross the entire universe, you would eventually end up at the place you started from. And if there are no edges, where or what is it expanding into?

Thanks and great show. Keep them coming.  Cheers,  

Sacha Quadrelli
What do you think?

[imatfaal]

Hi Sacha

We don't actually know the form of the universe - and will even have long arguments about the meanings of the terms used to describe it.  Locally the universe is flat - no matter on what scale we look we cannot find evidence that the universe is a sphere or torus etc (of however many dimensions you wish to imagine).  There is a good argument that on large enough scales (ie more than we can presently see) we might find that the universe has a curvature.  The universe being curved does not mean that it has no edge and no boundaries.  The universe being flat does not mean that it has edges or boundaries.  It is not normal and defies viewing in the minds eye!

On the question of expanding into what?  This is a perspective problem - everything we know in our personal human view is made up of something - even if it is just made up of empty space.  The universe is not expanding into anything. It is everything and it is getting bigger.  Anything outside the universe for it to expand into is by definition part of the universe and thus it is expanding with the universe.  It is possible that the universe is spherical and there are other spherical universes - but this is so beyond the realms of any testability it is science fiction rather than science.

[yor_on]

Distance is defined through its metric, 'gravity'. But 'space' is classically (macroscopically) also a nothing, allowing you to move without friction. That 'nothing' is expanding, but tell me, how do you give a 'nothing' a shape?

Gravity.

[Soul Surfer]

The amount of universe that we can see and detect is in effect spherical. but the size of the sphere depends on how far back in time we are looking and also whether we consider the size of the sphere when we were detecting the radiation or as it is now.  Although we don't know exactly how big it is now because we cant see it as it is now and the expansion of the universe may not be totally uniform.

All this has absolutely nothing to do with exactly how big the whole universe is or what shape it is but many theories suggest it could be many orders of magnitude bigger than we will ever be able to see.  There is one possibility that keeps cropping up through history.  That is the possibility that the whole of our universe could be SMALLER than what we can see.  This means that what we see is a bit like looking at ourselves between a pair of mirrors where we can see images of ourselves going off into the distance (except that this is in a full three dimensional sense)  this might be detected in certain repeated patterns in things like the cosmic microwave background or even images of distant galaxies and their statistical distributions.