How are the boundaries of the universe defined?

They aren’t.

The boundary of the_visible_ Universe is at the distance from whence light would have taken the age of the Universe to reach us, but that’s a different thing.

I should say that it is possible in principle to imagine that we live in a finite universe with a boundary which would then be defined by the condition that an observer was unable to keep travelling in a particular direction. Such a proposal is not empirically determinable from where we are in the Universe, is aesthetically displeasing and as I understand it would be something of a challenge for our best theoretical modelling.

If the Universe is finite, it seems far more likely that it is finite but unbounded (like the surface of a sphere: finite in area but in no direction (confined to the surface!) does an observer ever ‘stop’.

CrazyNeutrino said:

How are the boundaries of the universe defined?

See answer to Question 2.

Just kidding.

It depends what model of the multiverse you are talking about, and where you are looking from.

Just reading Mr Greene this morning who says that if you have an Inflationary Multiverse, with varying rates of inflation resulting in an infinite number of Bubble Universes, then from the outside each of the bubbles would look finite, but from the inside of any bubble, it would look infinite, and the inside of each bubble would contain an infinite number of sub-universes, and it’s one of these sub-universes that we see part of as our observable universe.

On this hypothesis you could define a boundary (somewhat arbitrarily I think) as a location in space-time with a particular density, but you could never get there from the inside, since the bubbles are infinite.

MartinB said:

Such a proposal is not empirically determinable from where we are in the Universe, is aesthetically displeasing and as I understand it would be something of a challenge for our best theoretical modelling.

I really don’t get that, not that my failure to get it makes it wrong, but it would be good if the pop-sci writers gave a bit more attention to explaining what is wrong with it, rather than just taking it as a given that it’s not the way it is.

Geez, I would have thought the first point ticks off everything that you wanted to know about it :-)

MartinB said:

Geez, I would have thought the first point ticks off everything that you wanted to know about it :-)

You mean the “Such a proposal is not empirically determinable from where we are in the Universe,”?

That’s OK so long as all the other proposals are treated in the same way.

The Rev Dodgson said:

MartinB said:

Geez, I would have thought the first point ticks off everything that you wanted to know about it :-)

You mean the “Such a proposal is not empirically determinable from where we are in the Universe,”?

That’s OK so long as all the other proposals are treated in the same way.

movable viewing platforms?

>>inside of each bubble would contain an infinite number of sub-universes, and it’s one of these sub-universes that we see part of as our observable universe

You are seriously suggesting that we live in sub-universe which is itself a sub-bubble?

Hmm, I can dig that but the terminology needs some work.

What is the universe again?

Ian said:

>>inside of each bubble would contain an infinite number of sub-universes, and it’s one of these sub-universes that we see part of as our observable universe

You are seriously suggesting that we live in sub-universe which is itself a sub-bubble?

Ian said:

Well Brian Greene is.

Ian said:

Hmm, I can dig that but the terminology needs some work.

What is the universe again?

Depends what definition you use, but multiverse seems to be the favoured name for everything that exists.

>>multiverse seems to be the favoured name for everything that exists.

Umm yeah. But that is the definition of universe.

Ian said:

>>multiverse seems to be the favoured name for everything that exists.

Umm yeah. But that is the definition of universe.

No, the word “universe” does not have a single correct definition.

The Rev Dodgson said:

No, the word “universe” does not have a single correct definition.

Yeah, that’s my point. Needs work.

I’d rather to continue to say that we inhabit “the universe” rather than say we live in a sub-sub-bubbleverse (even if you sing it).

CrazyNeutrino said:

How are the boundaries of the universe defined?

If there are boundaries to the universe I would suggest they are involved with BH’s. An EH may be measured from a BH, but all particles outside are involved in balance. The FoR of any particle in relation to the nearest EH may be it’s proximity to a universal boundary.

Ian said:

The Rev Dodgson said:

No, the word “universe” does not have a single correct definition.

Yeah, that’s my point. Needs work.

I’d rather to continue to say that we inhabit “the universe” rather than say we live in a sub-sub-bubbleverse (even if you sing it).

Go right ahead.