edge of universe?

Simon Bridge

Well ... people have started "playing around" and OP does not seem to be interested in the replies so...

Dead Boss

I don't think anybody mentioned this, but Euclidian geometry does not imply edge and I don't get why you'd think it does.

petm1

Looking out into space is the same as looking back in time. Looking out and seeing signals from one event in every direction such as the surface of last scattering appears to be seeing from the inside of the event sort of like seeing the inside walls of my den. Looking at the keyboard in front of my face I see the outside of the keyboard. The only edge I can see of the universe is the edge formed by matter I see these edges via photons which thanks to their spectral lines we can even think of them as having an edge.

I would think the GPS clocks prove that simultaneity of matter is real, it is the view we observers have of the present via photons that is the illusion.

Sorry I am so confusing.

meteor

hi, layman here. It's my understanding that actually in the framework of General Relativity, the 2 main accepted possibilities are that the Universe is either infinite in volume (this being the preferred possibility), or finite in volume but without an edge or boundary (they sometimes give the example of the 2D surface of the Earth).

My question for knowledgeable people here is: Is it possible within the framework of General Relativity to create a serious and viable model in which the Universe has an edge?
Or to put it differently: Does General Relativity preclude the existence of a finite Universe with an edge?

If such a non-crackpot, finite with-an-edge theory has ever been published for example in a place like arxiv, can you provide a link?

Chronos

The universe has a temporal edge called the big bang. You can't see beyond the big bang, or, technically speaking, the surface of last scattering [a few hundred thousand years after the big event]. This is not a point in space, but, a point in time that every observer in the universe has in common. Due to the finite speed of light, all observers can only see the past. So, discussing the spatial edge of the universe is like discussing the color of a musical note.

Simon Bridge

tldr: the question needs to be refined.

Long version:
Chronos' heroic attempt at describing how the Universe might be described as having an edge not withstanding, the question, as asked, still has the same problem as the first one in this thread: what do you mean by "edge"?

This is very important since there are very many ways of interpreting the word so that you'd have one in GR. In fact - we have edges in our universe in the trivial sense that we can construct geometric objects that have them or we can draw a line on the ground and that would be an edge.

A common conception of a non-trivial edge to the universe is the idea that if you keep travelling in one direction you will eventually run out of Universe - that what you mean?

Another approach is to look at the fastest travellers that are arriving here (light) and ask: where did they come from? where is that place now? and what is the furthest place they could have come from? ... that last would also be an "edge".

Then there is the ideas about what GR allows ... GR allows all kinds of things that we don't expect to find in nature - like matter with an imaginary mass. Merely satisfying GR does not seem a high bar. It's only math.

con.foundas

Interesting reply. Although, cause and effect along with other anomalies and factors associated with, to a large extent, classical reasoning becomes somewhat meaningless when certain assumptions are made re the big bang, we can't help thinking in this way. So assuming there is a beginning somewhere there with the respect to the big bang, we are inclined to assume there is an ending somewhere there; hence the edge of the universe.

If there isn't an edge to the universe, because we make wrong euclidean judgements or whatever, can we safely assume that the big bang occurred in the ways physicists assume, at the very point it 'began'? Or to put it another way, why allow for classical reasoning at least to slip in on one end (the beginning) and not the other 'end'; the edge of or on the expansion of the universe (which can [but probably shouldn't] allow for an ending)?

Once again, 'we cannot observe our future' is an excellent point to the query of an edge to the universe. But observations aside, we can conceptualize it and at least make assumption regarding, well, does it (this universe) end or not.

We may even have classical limitations with respect to the structure of our language (syntax etc) as this may not be a reliable vehicle to generating understanding and adequate meaning when attempting to describe such non classical or complex 'events'.

petm1

One temporal edge of the universe is the little twist in time we call the big bang but I think of the spatial edge of the universe as that represented by the stress-energy tensor of our present.

Drakkith

Forgive me, but I can't make heads or tails of this, it doesn't seem to make any sense.