How do we know the universe is infinite?

[bcrowell]

the distance between two points moving apart, where one is at infinity, cannot be defined; therefore you cannot define the velocity at which the points are moving apart; nor the time it takes. Everything becomes undefined. Unless you wave your hands in the air and say its "infinte", but that's not a logical argument.

Nobody said anything about a point at infinity.

 

[William White]

Nobody said anything about a point at infinity.

no you are playing philosopical games

you said an infinite "thing" is expanding (which is wooly in the extreme)

if this thing has no points at infinity, it is not a thing nor is it infinite. You said somebody was being illogical, yet use words like "thing" to describe something with a size (infinite) but no points.

you can of course, use other nouns instead of thing, and instead of points, but the principle is the same

 

[bcrowell]

Are you excluding the possibility of wormholes? Those seem 'non trivial'...but maybe not technically??

The general term for this sort of thing is topology change. There are both kinematic and dynamical reasons why we don't expect topology change to be possible. Two examples of topology change are: (1) a closed universe becoming open (or vice versa), and (2) creating a wormhole that didn't previously exist.

Standard cosmological models are homogeneous, so they don't have wormholes in them -- if wormholes do exist, they're just at a smaller scale, a level of granularity that we aren't considering.

Thanks. Ebony was 'senior doggie' among our three until a few weeks ago. Smartest for sure. She was almost 15 ...we really miss her. What's almost worse, my wife now talks to me instead of the dog! So I am trying to hang out in these forums and appear busy, hoping not to be noticed.

I'm sorry to hear about her death. So the icon is sort of a memorial? :-( My wife and I have been debating the extent of the resemblance between our dog and yours. We've always figured our dog, Lucy, is half schnauzer, but we don't know. Her face looks almost identical to Ebony's to me. Lucy has a white chest and paws, and a tail that curves over like a question mark. She's only 2, so I'm hoping we have many more years with her.

 

[bcrowell]

no you are playing philosopical games

You can like or dislike philosophy, but a hallmark of philosophy is that philosophers try very hard to define their terms carefully. I've defined my terms carefully, but I don't see that you have.

if this thing has no points at infinity, it is not a thing nor is it infinite.

I defined in #28 what I meant by an infinite universe. Your quoted statement is false according to the definition I gave. A simpler example is that by the definition I gave, the real number line is infinite, but it has no points at infinity.

 

[PeterDonis]

Thread closed for moderation.