The universe: infinite or finite

[Lucas_Jackson]

@Jackknifed_Juggernaut

I hope this isn’t too tangential of a question, but I think it’s related. In 150 billion years, the CMBR will be undetectable, and all other galaxies outside of our own will be invisible due to the expansion of the universe. If a new civilization, similar to ours, were to form in another solar system, would they ever know that other galaxies ever existed?

This actually came up in another thread. IIRC, the answer is no. That is a lonely thought.

[EnolaStraight]

@Lucas_Jackson

@EnolaStraight

So infinities can be generated from finite initial conditions.

Can you expand on this a bit? Does this depend on frame of reference? I’m confused.

Umm…no, I can’t.
This was the only thing I could come up with.

[Chronos]

@Francis_Vaughan

But I really don’t believe that many physicists think that matter is actually squeezed out of existence in an infinity dense point. That raises more questions than it answers.

I don’t think I would quite say that. I don’t think that any physicist would be surprised if quantum gravity (once we figure it out) says that the mass of a black hole is actually confined to a very small (but finite) region… but we don’t have quantum gravity yet, and so we don’t know. The best model we have says that it is in fact an infinitely-dense, zero-size point, so I think that, if we’re honest with ourselves, we sort of have to accept that (at least for now).

@Francis_Vaughan

Sometimes things work much better with a different coordinate system. The favourite next in line is polar coordinates. A huge amount of grief vanishes for some problems just doing that.

And, of course, for other problems polar coordinates introduce new griefs. A lot of the skill in data analysis comes in knowing just how to represent your problem, for any given problem.

@Pardel-Lux

It seems the trouble is not as bad as click-bait would suggest, and those “little red dot galaxies” are not fully formed after all:

Webb telescope just solved the 'universe-breaking problem' | Mashable

I mean, clickbait is never an accurate representation of science. At any given moment, any given science has some observations that just don’t make any sense. And then someone figures out how to make sense of them. Most often, it’s just experimental error. Sometimes, it’s a discovery of a new but fairly unsurprising phenomenon. Occasionally, of course, it does actually upend everything we thought we knew… but that’s really, really rare.

@DrCube

I think the disconnect here is that sometimes we use the term “universe” to mean “everything that exists,” and other times to mean “everything we can observe”. When they say “the universe was once a single dense point” they’re talking about the latter.

No, we’re talking about the whole shebang.

@FlikTheBlue

At least that’s my understanding of what the Planck time means, that is that considering smaller amounts of time makes no sense in the real world.

A very common misconception. The only Planck units that are known to be absolute extrema are the Planck speed (AKA the speed of light) and the Planck angular momentum (AKA Planck’s constant). Some of them aren’t even of particularly remarkable size: The Planck momentum, for instance, is about how much momentum a running housecat has.

Now, the Planck distance and time might be fundamental indivisible quanta of spacetime. I don’t think anyone would be shocked if it turned out that that was so. But they might not be, too.

[Max_S]

@Roderick_Femm

Would it make sense to think of this as new space being created (rather than existing space being newly occupied) by the expansion of existing matter?

I guess you could think of it that way, but it’s counter-intuitive. Intuitively, if you can define a hypothetical object’s position with spatial coordinates, it seems that space exists in the universe even if the object itself doesn’t.

Imagine a hypothetical Russel’s teapot 100 billion light years away from us, and the coordinates it might have. If you think of space in terms of cartesian coordinates, then intuitively that space already “exists” even if you make the philosophical assumption that all matter in the universe is <50 bn ly away from us.

But if you think of space in terms of e.g. homogeneous coordinates, then conceptually space has a hard boundary. Using homogeneous coordinates is a little counter-intuitive, though.

Imagine you are on a rowboat at sea, with no landmarks or stars or other navigational aids, except that there are other boats. You can’t feel wind or current, and it’s always noon. If you had some kind of navigational aid, you could use that to base your coordinate space and derive relative north, south, east, and west. So as an alternative to describe the position of other ships in terms of 1) the angle from the bow of your ship and 2) the ratio of the distance between your own ship and the other ship, compared to your own ship and the horizon, measured by holding a ruler vertically in front of your eyes. This is a homogeneous (polar) coordinate system where a point is defined as (\theta, |r| < 1), \theta being the angle and r being the ratio. From your point of view, with this concept of space, it makes no sense to speak of crossing the horizon - you don’t see other boats flying in the sky. Ships that approach the horizon just get smaller and smaller until you can’t see them any more, but they’re still in the water and you can infer their position approaches but never reaches the limit (\theta, 1). By analogy, the water is space and you can see no new space (water) is ever created no matter how far out objects (boats) recede.

~Max