Infinite space

[jeheron]

I recently attended a popular science lecture at the University of Sydney. The speaker said that if, given enough time, you travelled in a straight line out into space, you would eventually arrive back where you started from as if space was circular and you had circumnavigated it.

 

Is this remotely correct?

[ajb]

This is true if the universe is compact and without a boundary. Much like the sphere.

 

Such notions about the topology of the universe are still open.

[Martin]

I recently attended a popular science lecture at the University of Sydney. The speaker said that if, given enough time, you travelled in a straight line out into space, you would eventually arrive back where you started from as if space was circular and you had circumnavigated it.

 

Is this remotely correct?

 

What ajb says is correct. The universe could be spatially infinite, so that you might travel in a straight line forever and never get back to starting point.

 

or it could have a finite spatial volume which means (since one normally assumes no boundary) that it has to be roughly analogous to the 2D world of a sphere, more or less.

 

interestingly enough, for several years evidence has been accumulating on the side of the latter alternative----what your speaker was talking about

 

the key number is called Omega and it is a measurable density ratio.

 

if Omega is greater than 1, like say 1.01

then the universe is spatial finite, analogous to the surface of a ball (but nothing inside or outside, just the surface itself)

 

if Omega is exactly 0, then it is spatial infinite. like a flat 2D surface extending out indefinitely (but nothing above or below it, just the surface)

 

these are 2D analogies for a 3D space

 

Recently when Omega has been measured with the latest most complete data, there is still some uncertainty but they are getting numbers like 1.011

 

A paper came out in January 2007 with 1.011 as the "best fit".

However it is still not sure enough to say one way or the other. might be infinite volume, might be finite volume

 

in either case boundaryless----no "edge"

sometimes people get the notion that because a volume is finite it has to have a boundary but this is not true

[ydoaPs]

if Omega is greater than 1, like say 1.01

then the universe is spatial finite, analogous to the surface of a ball (but nothing inside or outside, just the surface itself)

 

if Omega is exactly 0, then it is spatial infinite. like a flat 2D surface extending out indefinitely (but nothing above or below it, just the surface)

 

What if [math]0<{\Omega}<1[/math]?

[insane_alien]

i think martin meant omega = to 1 rather than 0

 

if omega is less than one it corresponds to negative curvature(whatever that means)

[Martin]

i think martin meant omega = to 1 rather than 0

 

if omega is less than one it corresponds to negative curvature(whatever that means)

 

I sure did mean that! thanks alien.

 

Typo. Should have typed

if Omega is exactly 1, then it is spatial infinite. like a flat 2D surface extending out indefinitely (but nothing above or below it, just the surface)