Edge of Universe: What's Beyond the Limit?

[um0123]

So there must be an edge, a point where matter stops and from there on in is a complete vacuum. The universe is supposedly expanding at an estimated 50-100 km/s/Mpc which is slower than light speed. So this means that light from the universe goes into a place (outside of the universe), that technically doesn't exist.

So...whats up with this?

 

[Dmitry67]

No, no, and no.
Start from reading this: https://www.physicsforums.com/showthread.php?t=261161

Big bang is not an explosion in space. There is no center of the universe. There is no edge of the universe.

 

[Chronos]

We are at the temporal edge of the universe, does it look any different in any direction?

 

[cbd1]

Still, where has the CMB photons that have already passed us (e.g. the ones a long time in the past)? Where have they gone? Can they travel forever without reaching some kind of border even though they are traveling faster than expansion?

 

[Dmitry67]

They go away from us.
Yes, they can travel forever.
They are not traveling "faster then expansion". At first, some objects in the Universe can recede faster then light (yes, surprise, surprise, you can not analyze the expansion from SR point of view, you need to use General relativity). But no expansion is needed for the photons to travel forever. It is possible in infinite universe or even finite (closed) universe (they will run in circles)

 

[edpell]

Dmitry67's point that is depend if the universe is open or closed determines if the photons travel forever or go around in circles forever.

My question is if the universe is closed does that mean the escape velocity is greater than c? And does that mean the universe is a black hole?

It is important to make the distinction between how far we can see out at this time (i.e the age of the universe times the speed of light), versus the extent of matter, versus the extend of space. For myself I am unsure if the last two are related.

 

[Dmitry67]

The escape velocity question does not make any sense.
Nothing under no conditions can 'escape' from the universe

 

[edpell]

If the universe is open can we ask how fast must an object be going to continue on forever?

 

[Dmitry67]

'open' means that universe is infinite
For example, flat universe (infinite, "Newtonian" 3D space) is an example of "open" Universe
Obviously, there are no problems for any photon to travel forever in infinite universe.

 

[russ_watters]

My question is if the universe is closed does that mean the escape velocity is greater than c? And does that mean the universe is a black hole?

This issue has nothing whatsoever to do with the concept of escape velocity.

 

[edpell]

What is the definition of a universe being closed?

 

[marcus]

What is the definition of a universe being closed?

Usage of the terms "closed", "flat", and "open" changed around year 1998 when it was realized that the universe could be spatially closed (in the sense of overall positive spatial curvature, like a hypersphere, or the idea finite spatial volume) and yet even though it was spatially closed it could continue expanding forever.

Before 1998 when you looked into pop cosmo books you would see a very neat and misleading triple package.

A:"closed" meant spatially closed and also the (wrong) idea that she has to end in a crunch
B:"flat" meant spatially flat, zero spatial curvature, and also the connotation that she keeps expanding but at a diminishing rate that is just barely fast enough to avoid re-collapse.
C:"open" meant negative spatial curvature, typically an infinite spatial volume, and also carried a connotation about future expansion

The explainers always locked you into associating some future expansion (or contraction) scenario with each separate spatial configuration. But it doesn't have to be that way.

After 1998 everybody recognized it wasn't that pat and simple---that with a positive cosmological constant, or equivalently with dark energy, you could have an expansion scenario more or less independent of which spatial configuration.

So closed has come to mean (at least for some of us) spatially closed. And it doesn't mean you have to end in a crunch.

The typical picture of a closed universe is the hypersphere, the 3d analog of a 2d surface of a sphere. The 2D toy model analogy can be pictured as the surface of an expanding balloon. But all existence is concentrated on the geometrical surface of the balloon. There is no rubber, there is no inside-the-balloon or outside-the-balloon. All that exists is 2d stars and 2d creatures on the 2d surface.
And then to get the real world you jack that picture up from 2 to 3d.

Matter is roughly uniformly distributed over the whole of space---back in the toy model case that means over the whole 2d surface of the balloon. There is no outside-the-universe. No edge. No boundary. A finite 3d volume, just like in the 2d toy model there is a finite 2d area (of the surface of the balloon).

You asked what does "closed universe" mean. I think that is about it. Spatial closure, edgelessness, finite volume, finite circumference. I'd like to hear Dmitry's definition, though. In my experience he's reliably informed clear and concise.

If you have problems with anything I or anybody have said, keep asking questions.

 

[pywakit]

'open' means that universe is infinite
For example, flat universe (infinite, "Newtonian" 3D space) is an example of "open" Universe
Obviously, there are no problems for any photon to travel forever in infinite universe.

If what you say is correct then we can deduce a couple of things.

Ours is the 'only' universe in existence. Past and present.

(Or there would be detectable photons leaking in from every direction )

The medium of space stops at our borders. And therefore either deflects photons back on themselves or they are being absorbed by this 'no space'.

( Contradicted by Einstein's Uniformity of Space ( unless disturbed by mass ) math.

There are other universes, but none of them operate under the same conditions of space, nor the same particle physics.

( Out of infinite and eternal space, and therefore an infinite number of 'local' universes, this would be the height of ego ... and totally illogical )

Realistically, if space is infinite ... then the properties of space are the same everywhere. And the interaction between 'local' universes and space itself would demand the same operating physics. Unless there is 'magic' going on ...

Something is preventing photons from other universes from ever reaching us. I'm thinking each local universe functions exactly the same as ours, and it is a closed loop. No photons escape. Otherwise, photons would in fact 'live forever' traveling freely in the mediun of space.

Again ... there would be evidence of them.

Glaring flaws in my logic, guys?

 

[Dmitry67]

Marcus definition is more accurate.
I provided an oversimplified version ignoring Dark Energy

P.S
And you see? I was right, there are muc h more serious issues

 

[Dmitry67]

If what you say is correct then we can deduce a couple of things.

1 Ours is the 'only' universe in existence. Past and present.
(Or there would be detectable photons leaking in from every direction )

2 The medium of space stops at our borders. And therefore either deflects photons back on themselves or they are being absorbed by this 'no space'.

(Contradicted by Einstein's Uniformity of Space ( unless disturbed by mass ) math.

3 There are other universes, but none of them operate under the same conditions of space, nor the same particle physics.

( Out of infinite and eternal space, and therefore an infinite number of 'local' universes, this would be the height of ego ... and totally illogical )

I don't see how anything of that can be "deduced" from what I said

 

[Chronos]

How does any of this make sense if we reside in the 'oldest' region of the universe?

 

[pywakit]

I don't see how anything of that can be "deduced" from what I said

If there are other 'finite' universes like ours out there, there must be an 'infinite' number of such universes past and present. Can't be just 2 or 3 now can there? And if the photons can travel unimpeded forever ... then we would detect these photons leaking into our local universe from any direction.

Unless there was a property of infinite space we are not aware of that would prevent photons from ever making it to the great voids between universes.

 

[pywakit]

I don't see how anything of that can be "deduced" from what I said

I'm too tired to cover the rest tonight. But I will tomorrow.

 

[Dmitry67]

If there are other 'finite' universes like ours out there, there must be an 'infinite' number of such universes past and present. Can't be just 2 or 3 now can there? And if the photons can travel unimpeded forever ... then we would detect these photons leaking into our local universe from any direction.

Unless there was a property of infinite space we are not aware of that would prevent photons from ever making it to the great voids between universes.

You use the assumption that if there are more than 1 infinite universe they must somehow intersect. You assume that these universes share the same physical space. This assumption is wrong. But definition, Universe is mathematically complete, so nothing can 'leak' to or from another universe.

Mathematically any number of infinite Universes can coexist. If you ask: do they exists in the same space? the answer is N/A

 

[pywakit]

I don't see how anything of that can be "deduced" from what I said

You use the assumption that if there are more than 1 infinite universe they must somehow intersect. You assume that these universes share the same physical space. This assumption is wrong. But definition, Universe is mathematically complete, so nothing can 'leak' to or from another universe.

Mathematically any number of infinite Universes can coexist. If you ask: do they exists in the same space? the answer is N/A

I make no such assumption. In my model they CAN'T intersect. Where did you get this idea? The distance between these infinite in number 'finite, local universes' ... like ours ... is equivalent to the distances between materializing sub-planck particles. They are nowhere near each other.

'THE' universe is SPACE. It is infinite. OUR universe occupies but one tiny, finite 'geographic' location in infinite space.

I must be a terrible communicator ... sorry.

Did you actually read my model? You seem to have missed all the important points.

Oh. I'm sorry again. Forgot which forum I was on. No, you haven't read my model. Sorry for the confusion. It's posted on Science Forums. SFN. Also under pywakit, if you are interested.

 

[Dmitry67]

'THE' universe is SPACE. It is infinite. OUR universe occupies but one tiny, finite 'geographic' location in infinite space.

This is not called 'Universe'. It called 'Visible Universe' or 'Hubble volume'.
That 'visible' makes a huge difference.

I am interested in Mathematical Multiverse by Max Tegmark. In the Multiverse context 'OUR Universe' is ambigous: for you it means a part of the Unvierse while in Mutltiverse it means the WHOLE infinite Universe, but... "ours"

 

[pywakit]

I use the terms too loosely I suppose. Our 'universe' isn't an actual universe. It's just the space occupied by our expanding volume of matter/energy.

The universe ... the actual universe ... would appear to be truly infinite. Our 'Hubble volume' is equivalent to materializing sub-planck particles comparitively against the backdrop of infinity. Hardly noticeable at all when looked at on scales this big.

 

[edpell]

Marcus, what you said makes sense to me. My question would be before 1998 we were told that the shape of the hypersphere depended on the density of matter (I think?) now we say the shape is one given and the density is another given and they are independent? What the **** happened in 1998? Who gets credit for this idea?

 

[Dmitry67]

Dark Energy
Universe can be closed, but because of DE it can expand forever.

 

[cbd1]

Two papers were released in 1998 and 1999 that measured the redshifts of ultra-long distance supernovae [Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, by the High-Z Supernova Research team; and MEASUREMENTS OF AND  FROM 42 HIGH-REDSHIFT SUPERNOVAE, by the Supernova Cosmology Project]

The data from both teams shows that the redishifts at increasing distance are less than expected by Hubble's constant. This has been interpretted to say that when those supernovas happened (with increasing time in the past), they were moving away from us a little less velocity than local galaxies are moving away from us now. We have been unable to figure out any other explanation, so we believe the observational data to show that expansion has accelerated since then. This is the primary evidence for dark energy.


However, I personally believe that it is possible that there is another explanation for the decreased expected redshifts with distance. If we can figure out why the expansion of space may have cause the redshifts to be skewed, it could be interpreted to show that it is possible expansion has, in fact, been slowing all this time.

- I would love to here retorts about this particular possibility. Say that *if* the redshifts have been skewed and do not represent the true recessional speed of those galaxies at the time the supernovae exploded (this effect could have actually reversed the redshifts that were higher than expected by Hubble's constant) , *then* it is possible that the expansion of the universe has not accelerated (and could actually have decelerated since then).

 

[edpell]

so you are saying the rate of expansion is increasing and expected to continue to increase. In other words the universe is exploding? No crunch, no heat death, but another big bang?

 

[cbd1]

Edpell, I believe you must have misread my post.

 

[edpell]

cbd1, sorry. I was thinking of a talk given by Charles Baltay at Columbia in about 2004 at the retirement celebration for Won Yong Lee. His point (I think) went like this non-zero cosmological constant pressure outward with pressure proportional to the size of the universe. So pressure leads to larger universe leads to more pressure leads to... In other words a positive feedback loop that ends in a bang!