# Edge of universe?

I'm not a physicist!
Just wondering what it is like on the edge of the universe? how many dimensions will be there, what force will be acted upon a close object, and what relativity will become?
I'm assuming the universe is euclidian (eucklidean? euclidean? my English sucks.) not hyperbolic or whatever geometry so there would be an reachable 'edge'. And under the assumpsion that the universe is constantly expanding.
Thank You
From a high school student
Victor Lu

Simon Bridge
Homework Helper
Just wondering what it is like on the edge of the universe?
What do you mean by "the edge of the Universe".
I'm assuming the universe is euclidian.
That would be a bad assumption.

Lets say for the sake of an example we were to postulate a Universe with a "reachable edge" - then that edge would have whatever properties we wanted it to have. It could be a brick wall.

What I am saying is that the question is too wide open.
Of more interest is the properties of the existing Universe that we live in.
This one is not Euclidean and has no reachable edge.

there is no universe such as you imagine...if you make an assumption that isn't accurate, you'll not get much in the way of insightful answers....why do you want to assume Eucledian geometry...a practical issue is that in our real universe you are at the 'center' of it, and so is every other observer!! [That is a very cryptic comment!]

try starting here:

http://en.wikipedia.org/wiki/Edge_of_the_Universe

insofar as is known, 'edges' are probably not what you envision.

You can read about an actual 'edge of universe' of sorts by checking Wikipedia subjects like
multiverse or eternal inflation....those posit multiple universes....

You might also read about black holes as they are causally disconnected from our
universe in some senses.....and horizons and cosmological horizons also have a sort of 'edge' if you want to call it that...

When you accelerate [change velocity] you create one type of causal horizon....that is a form of an 'edge'....

None of these is likely what you have imagined so far.

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Drakkith
Staff Emeritus
I'm not a physicist!
Just wondering what it is like on the edge of the universe? how many dimensions will be there, what force will be acted upon a close object, and what relativity will become?
I'm assuming the universe is euclidian (eucklidean? euclidean? my English sucks.) not hyperbolic or whatever geometry so there would be an reachable 'edge'. And under the assumpsion that the universe is constantly expanding.
Thank You
From a high school student
Victor Lu
We can see back in time about 13.7 billion years and out to about 90 billion light years in diameter and we cannot see an edge to the universe. This doesn't mean that there isn't one, but we simply have no reason to believe that there is an edge.

Chronos
Gold Member
We already reside at the temporal edge of the observable universe. No matter where you look, the universe is younger in every direction. Do you notice anything peculiar? Any other observer in the universe would experience this same illusion, so, the concept of an edge is meaningless. We perceive a temporal edge that ends in the past [at the big bang], but, we cannot observe our future.

We only "see" the present. We use artifacts, photons, from a past event to form an image within our eyes but we do not see back in time nor do we see out in space. The edge of our visible universe that we do see is the outside edge of matter where photons interact, the inside edge I would think of the oldest artifacts, cosmic background radiation.

Just wondering what it is like on the edge of the universe? how many dimensions will be there, what force will be acted upon a close object, and what relativity will become?
You are a part of the edge of the universe, we think in terms of four dimensions, Gravity is the force acting upon close objects and relativity is how we describe time.

Drakkith
Staff Emeritus
We only "see" the present. We use artifacts, photons, from a past event to form an image within our eyes but we do not see back in time nor do we see out in space. The edge of our visible universe that we do see is the outside edge of matter where photons interact, the inside edge I would think of the oldest artifacts, cosmic background radiation.
The term "see" can be a little ambiguous. I would take it to mean the comprehension of what the image formed on your retina represents and all the rules involved in how that image was formed. After all, I could claim you only ever see the past because by the time you are aware of it, the event has already passed. Also, your use of "edge" is odd, as the universe is not known to have an "edge" at all, only a horizon beyond which we cannot see thanks to the finite speed of light. This has nothing to do with where matter and light interact and is purely the result of the time of emission of the light.

You are a part of the edge of the universe,
I don't know what you are getting at here, as it doesn't seem to make much sense. I recommend not trying to take an unusual stance on what the "edge of the universe" means, otherwise we are likely to get bogged down into a pointless discussion on terminology.

Simon Bridge
Homework Helper
Sounds like reading too much Soren Kierkegaard to me :)
Note: n_kelthuzad (OP: Victor Lu) has yet to chip in to clarify the question - the answer very much depends on what is meant by "edge".

Garth
Gold Member
Hi Victor,

Just wondering what it is like on the edge of the universe?
On the other hand it could explain:

where all my odd socks have gone!

As a High School student keep asking questions and learn, soon you will be a physicist!

Because the universe looks 'isotropic', that is on the largest scales it looks the same in all directions right back to the Cosmic Microwave Background it is usual to assume that on the largest scales the universe is isotropic, and because we cannot claim to be at any special place in the universe (the 'Copernican Principle) it is therefore also logical to assume that the universe is 'homogeneous', that is on the largest scales it is the same everywhere. So the galaxies and clusters of galaxies on these scales can be treated like the molecules in an ocean. The ocean being the universe itself in this analogy.

With these assumptions there is no edge, as the medieval woodcut above imagined, the 3D space around us could be spherical (closed) like the surface of the Earth, so if you went off in a straight line you would end up coming back to your starting place, (though the expansion of the universe renders this trip impossible), it could be flat (Euclidean) or it could be hyperbolic (saddle shaped).

We don't know which it is; it is too close to call, although the weight of observations leans slightly to the closed, spherical case.

I hope this helps,
Garth

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Chronos
Gold Member
Because we are observationally limited by the hubble parameter, I doubt we will ever know with any certainty if the universe is closed or open.

This has nothing to do with where matter and light interact and is purely the result of the time of emission of the light.
I would think that the point where matter and light interact would always be an edge. A temporal edge to our universe using a photon as a ruler would be the difference between the durations of photons, oldest and youngest. The oldest from CMB and the youngest from matter, emission til reception or the travel time between the edges of our visible universe. The question was what is would be like at the edge of our universe, I did not think talking about something we can not see was needed to answer the question, shape of universe.

Drakkith
Staff Emeritus
I would think that the point where matter and light interact would always be an edge.
I don't understand this statement. What do you mean by "point" and "edge"? Are we talking about a horizon somewhere, or something else? Where is this matter and light interacting at?

A temporal edge to our universe using a photon as a ruler would be the difference between the durations of photons, oldest and youngest. The oldest from CMB and the youngest from matter, emission til reception or the travel time between the edges of our visible universe.
The CMB was emitted by matter, so your distinction between the CMB and other light is confusing. In any case the "age" of the photons, or the flight time, is what you could use for a ruler if you had a way of measuring how long that photon has existed. The CMB is indeed the "oldest" light there is.

Chronos
Gold Member
The temporal edge of the universe is a very 'real' effect experienced by any and all potential observers in the universe. It is a direct consequence of the finite speed of light and only confusing when you try to apply the concept of simultaneity. We humans have a logical weakness for that trap. One of the most important concepts of GR is the notion of 'simultaneity' is an illusion.

I don't understand this statement. What do you mean by "point" and "edge"? Are we talking about a horizon somewhere, or something else? Where is this matter and light interacting at?
Edge as in a line where an object begins or ends or a point near the beginning or the end, both would describe a photon with a emission/reception point. The youngest photons that I share the present moment with looks to be the outer edge of the universe I see. The other edge between the stars is the CMB or the oldest photons I see as the inner edge of my visible universe. The edge where matter ends and a photon begins or the edge where photon ends and matter begins both interactions make up the universe I see.

Simon Bridge
Homework Helper
Yes we know we can think of "edge" etc to be anything we like - surely what is important is how OP (n_kelthuzad) is thinking of it. OP has yet to return to clarify this issue. Until then we are all just relieving our bladders against the airflow.

Drakkith
Staff Emeritus
Edge as in a line where an object begins or ends or a point near the beginning or the end, both would describe a photon with a emission/reception point. The youngest photons that I share the present moment with looks to be the outer edge of the universe I see. The other edge between the stars is the CMB or the oldest photons I see as the inner edge of my visible universe. The edge where matter ends and a photon begins or the edge where photon ends and matter begins both interactions make up the universe I see.
The youngest are the outer edge, the oldest are the inner edge, matter beginning and ending? Do you realize how confusing you are making this? I think the other explanations are more than enough to satisfy the OP for now.

Chronos
Gold Member
None of this makes sense, so I am bailing from this thread.

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

I'm assuming the universe is euclidian (eucklidean? euclidean? my English sucks.) not hyperbolic or whatever geometry so there would be an reachable 'edge'.
Victor Lu
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.

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.

The temporal edge of the universe is a very 'real' effect experienced by any and all potential observers in the universe. It is a direct consequence of the finite speed of light and only confusing when you try to apply the concept of simultaneity. We humans have a logical weakness for that trap. One of the most important concepts of GR is the notion of 'simultaneity' is an illusion.
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.

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
Gold Member
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
Homework Helper
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?
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.

We already reside at the temporal edge of the observable universe. No matter where you look, the universe is younger in every direction. Do you notice anything peculiar? Any other observer in the universe would experience this same illusion, so, the concept of an edge is meaningless. We perceive a temporal edge that ends in the past [at the big bang], but, we cannot observe our future.
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'.

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.
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.