Does the univeres have size or shape?

[phildoe]

I have heared people say that the universe has no size or shape. My problem with this is how can the universe be expanding or how did it come from an infinitaly small spot(Big Bang). If it is expanding because an infinitly large object can not expand from an infinitly small point.

I think that the universe is like a 4 dimensional sphere. So if you go in any direction in space and keep going eventualy you will be back at your original spot. becaue even though you are going straight in 3 dimensions but the 4th dimension is slowly curving.

Kind of like the Earth even though you are going straight in 2dimensions you are slowly curving in the 3rd dimension.

 

[Ambitwistor]

Originally posted by phildoe
I have heared people say that the universe has no size or shape.

We have an idea of how big the observable universe is, but we don't know how big the universe as a whole is. As for shape, we know that it's very flat.

My problem with this is how can the universe be expanding or how did it come from an infinitaly small spot(Big Bang). If it is expanding because an infinitly large object can not expand from an infinitly small point.

We don't know whether the universe is infinitely large, but in models in which the universe is infinite, it didn't come from an infinitely small point. In infinite Big Bang models, the universe started out already infinite at the Big Bang; it was just infinitely dense.

I think that the universe is like a 4 dimensional sphere. So if you go in any direction in space and keep going eventualy you will be back at your original spot.

There are models in which space is a 3-dimensional hyperspherical surface. If those models are to be correct, the universe must be very much larger than the observable universe in order to be as flat as we know it is.

 

[phildoe]

How do we know that the universe is so flat?

 

[Ambitwistor]

Originally posted by phildoe
How do we know that the universe is so flat?

Through measurement of luminosity-redshift relations, the spectrum of the cosmic background radiation, and other cosmological parameters; the geometry of the universe determines those relationships.

 

[Nereid]

What observations?

Saying the same thing another way, only cosmological models in which the universe is flat provide acceptable matches to the best, current observations of the universe.

These observations have been done using a wide variety of astronomical instruments, across just about the entire electro-magnetic spectrum, and include some initial data from astronomical observations of neutrinos.

Behind these observations is a vast body of experimental results, mostly in physics. Key areas which provide the foundation for the astronomical observations (and their interpretation) include nuclear physics, atomic physics, and good ol' classical dynamics.

On a personal note, what's awesome about all this is that it hangs together so well; the observations and theory are tightly interlocked, and the conclusions - including the one about the universe being flat - are well-constrained.