Confused about expansion

[Michael Ray Mooney]

Considering the growing consensus that the universe is infinite, being everything everywhere, how is it possible for infinity to expand? In all my research I find confusion among cosmologists about the important distinction between the part of the universe we cqn see and the whole infinite universe.

 

[Ibix]

being everything everywhere, how is it possible for infinity to expand?

Any two things that are a distance $x$ apart are a distance $x\times (1+\delta)$ apart at some later time, where $\delta$ is independent of the distance $x$ and the position of either thing but (in general) not the time. That's a uniform expansion, whether you can see the end of it or not, or even if there's not an end.

In all my research I find confusion among cosmologists about the important distinction between the part of the universe we cqn see and the whole infinite universe.

Then you've been reading popsci, which is often sloppy about that distinction. Professional discourse has no such confusion.

 

[PeroK]

Considering the growing consensus that the universe is infinite, being everything everywhere, how is it possible for infinity to expand? In all my research I find confusion among cosmologists about the important distinction between the part of the universe we cqn see and the whole infinite universe.

Here's an introduction to the metric expansion of space. In general, from a geometric point of view, the spatial distances between points in space can vary over time, whether the space is finite or infinite.

https://people.ast.cam.ac.uk/~pettini/Intro Cosmology/Lecture03.pdf

 

[phinds]

Google the Hilbert Hotel

 

[PeroK]

Google the Hilbert Hotel

Is that when Hilbert refurbished his infinite hotel and made all the rooms bigger? The infinite hotel expansion!

 

[Ibix]

Google the Hilbert Hotel

Wasn't that the Eagles' biggest hit?

Welcome to the Hotel David Hilbert
Not a finite place (not a finite place)
Not a finite space
Plenty of room at the Hotel David Hilbert
Any time of year (any time of year)
You can find it here

 

[Michael Ray Mooney]

Google the Hilbert Hotel

I';m familiar, but as a mathmatical thought experiment it doesn't address the concept of of an endless universe, (No end or boundary in any direction.)

 

[phinds]

I';m familiar, but as a mathmatical thought experiment it doesn't address the concept of of an endless universe, (No end or boundary in any direction.)

No? I think that it does. It explains how infinity is endless. It's the Hilbert Hotel in all directions.

 

[Ibix]

I';m familiar, but as a mathmatical thought experiment it doesn't address the concept of of an endless universe, (No end or boundary in any direction.)

What part of the concept of an infinite universe are you having trouble with? I already explained what's meant by expansion, and PeroK provided a link to further detail.

 

[Michael Ray Mooney]

No? I think that it does. It explains how infinity is endless. It's the Hilbert Hotel in all directions.

That doesn't address the issue of an "expanding universe." Infinity does not expand... get bigger. We can't have it both ways... infinite and yet expanding.

 

[DaveC426913]

That doesn't address the issue of an "expanding universe." Infinity does not expand... get bigger. We can't have it both ways... infinite and yet expanding.

It is not about the universe itself expanding or getting bigger - it is about the things in it getting further apart.

The Hilbert Hotel is almost a perfect analogy.

We assume the hotel itself is infinite in size (and, incidentally, homogenous). Yet, we can still expand the distance between every room in it.

 

[PeroK]

That doesn't address the issue of an "expanding universe." Infinity does not expand... get bigger. We can't have it both ways... infinite and yet expanding.

In your introduction, you said:

Hi everyone. I've had an avid interest in several fields of physics all my adult life, now 79. (Long retired as a psychologist.) My fields of interest include cosmology, astrophysics, relativity theory, quantum physics and philosophy of science applied to physics. I look forward to participation here.

It's hard to believe any of that is true, to be honest. How can you possibly have an "avid interest" in cosmology and astrophysics, yet be stumped by the fundamental concept of a spatially infinite, expanding universe?

You can't have it both ways!

 

[PeterDonis]

We can't have it both ways... infinite and yet expanding.

Yes, we can. The mathematical model of a spatially infinite, expanding universe is perfectly consistent.

In all my research I find confusion among cosmologists about the important distinction between the part of the universe we cqn see and the whole infinite universe.

What confusion are you talking about? You need to back this claim up with specific references.