Actual Infinity in Reality?

[polymath257]

The universe is expanding, but not into a true literal nothing. That word "into" implies something. Otherwise, a logical interpretation of the sentence "the universe is expanding into nothing" would that the universe isnt expanding at all.

Sometimes physicists have to be careful with how they word things. No wonder why we get theists misrepresenting what many of us believe about the universe or cosmos.

The *best* answer to the question 'what is the universe expanding into?' is THE FUTURE.

The universe is expanding into the future.

And, while that may sound like a trick of language, there is a strict sense in which it is completely correct in general relativity.

So, as discussed above, spacetime is a four dimensional construct: it consists of all of space and all of time as a single geometric entity. Spatial cross sections of this entity (i.e, space) are 'larger' for later times than they are for earlier times. That is what is means to say space is expanding. It just means that a later 3 dimensional cross section is larger (in some sense) that an earlier one.

A very basic analogy may make this easier to comprehend. Suppose that the surface of the earth represents spacetime. In this analogy, think of higher latitudes as being later in time. Different longitudes are different spatial locations.

So, a 'time slice' is a cross section of the surface of the earth that has the same latitude: a latitude line. This represents space at a specific time.
 

Now, in the southern hemisphere, as we move north (which is later in time), the latitude lines get larger: space is expanding. In the northern hemisphere, as we move north, the latitude lines get smaller: space is contracting.

Also, we have a 'beginning' at the south pole and an 'end' at the north pole. In this analogy, time is finite with 'singularities' at the poles.

Now, what are the latitude lines expanding into? Different latitude lines aren't even in the 'same space' because they are at different times. They expand or contract into the future!

Now, this is a *very* simplistic analogy, but the essences are there: spacetime is a single geometric construct, space corresponds to cross sections of that geometry. Different times correspond to different cross sections, and expansion/contraction are determined by comparing two different cross sections.