# Where Does Space End? It Must End Somewhere!

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Seems to me that space is infinite and existence is eternal.

One might think that considering the vastness of our universe.

Not really.

Wait. What?

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Wait. What?

The observable universe doesn't look infinite, so why would it make it seem that space is infinite?

It's quite a mistake actually: The universe looks big, so space must be infinite.

Edited by Thorham
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The way I see things...

The universe is now aged at 13.7 billion years isn't it now??

But the universe is too big to have grown this much in that time, then they introduced this "inflation stage" of development and everything fitted into place again. Back then it could expand quicker than it does now.

What's outside the universe?? I LOVE this one!!

You can't say there is a void or just nothing outside our universe. Because even nothing not even a void could exist outside it.

So to be accurate you've got to say, there's nothing at all not even nothing outside out universe, haven't you??

What if there was a galaxy right on the edge of the universe??

I think if I was on a planet in that galaxy I'd be packing my bags and legging it as far away as possible. So maybe we'll find out about the edge of the universe when the fleet of alien ships come whizzing by, stopping to tell us we're going the wrong way.

Another theory I have is that any particles trying to become a part of this nothing that's not even nothing, will be repelled with more force than it went in with.

I have a similar point of view to this but I've always thought of it this way.

Imagine a universe of just you and me. Then the universe consists of the two of us and the space in between us because our presence defines that space as a two dimensional distance. We can measure it, watch it shrink or expand according to our movements, etc.

Add a third person and now we have just added another dimension to our space because we can now calculate the area defined between the three of us which we could not do before.

But if I turn around and look into the empty void behind me, it is completely lacking any point of reference, we have no appreciation of scale, no depth or width or height or or even motion to gauge time on for that matter.

There is nothing what so ever that we could use to define any single property of the void, therefore it must not exist.

Edited by TakenItSeriously
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.....if I turn around and look into the empty void behind me, it is completely lacking any point of reference, we have no appreciation of scale, no depth or width or height or or even motion to gauge time on for that matter.

There is nothing what so ever that we could use to define any single property of the void, therefore it must not exist.

Are you saying that nothing has never existed? Do you think the universe is more likely finite or infinite? If it is finite, do you think the big bang may be just one of many that exist within some higher order of structure? There might even be clusters and super-clusters of big bangs, beyond which is another higher structure.

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There is nothing what so ever that we could use to define any single property of the void, therefore it must not exist.

There are solutions to the Einstein Field Equations that describe an "empty" universe. The properties of such a universe can be studied (and compared to ours, for example). So the idea that you need content to define a universe appears to be false.

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For me at least, space ends a fingertip's thickness beyond the grey walls of my imagination. Perhaps I should learn to stretch my mind a bit, or perhaps not? Hmm.

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There are solutions to the Einstein Field Equations that describe an "empty" universe. The properties of such a universe can be studied (and compared to ours, for example). So the idea that you need content to define a universe appears to be false.

I'm having trouble understanding what an empty universe is. Or why they would try to distinguish it from nothing, was it done arbitrarily or were they trying to predict what would be left of a failed universe that recollapsed back in on itself.

Could we assume that an empty universe was an appropriate application for einsteins equations? Didn't they predict that space and time was created as a result of the first matter particles coming into existence?

I also can't imagine what the properties were that could be compared to ours. Was it flat, open, closed, expanding, contracting, homogeneous, isotopic, etc. does any property besides steady state even make sense without matter?

Can you tell us how they define an empty universe, or why they would want to define an empty universe. And what it's properties are?

Did it have a Higgs field?

Edited by TakenItSeriously
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One can use the FLRW metric to model any form of homogeneous and isotropic universe. Matter only, Lambda only, matter removed, radiation only, No dark matter or any other combinations.

The usage is to isolate the influence each has on the expansion and contraction rates. The results are surprising as it shows that even a universe comprised entirely of just matter will expand. Just as a universe of nothing but radiation albiet a different rate.

A good detailed coverage is Barbera Rydens "Introductory to Cosmology" she details every combination.

Each combination can be done on any curvature flat, positive or negative curved.

Edited by Mordred
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I'm having trouble understanding what an empty universe is. Or why they would try to distinguish it from nothing, was it done arbitrarily or were they trying to predict what would be left of a failed universe that recollapsed back in on itself.

There are various reasons people might do it.

The Milne Model was an early attempt to model the universe, but is contradicted by observations.

https://en.wikipedia.org/wiki/Milne_model

Could we assume that an empty universe was an appropriate application for einsteins equations?

Other vacuum solutions are just ways of exploring different cases. There are a limited number of exact solutions to the Einstein Field Equations so I think something can be learned from any of them, even if they don't describe the universe we live in.

Didn't they predict that space and time was created as a result of the first matter particles coming into existence?

The presence (and distribution) of mass and energy affects the behaviour of space-time but doesn't cause it to exist.

Did it have a Higgs field?

No. 'Cos it was empty.

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One can use the FLRW metric to model any form of homogeneous and isotropic universe. Matter only, Lambda only, matter removed, radiation only, No dark matter or any other combinations.

The usage is to isolate the influence each has on the expansion and contraction rates. The results are surprising as it shows that even a universe comprised entirely of just matter will expand. Just as a universe of nothing but radiation albiet a different rate.

A good detailed coverage is Barbera Rydens "Introductory to Cosmology" she details every combination.

Each combination can be done on any curvature flat, positive or negative curved.

Thanks Mordred, modeling the universe as permutations of it's constituent parts makes a lot of sense.

...will expand. Just as a universe of nothing but radiation [would,] albeit at a different rate.

Radiation expansion brings up a question about the end of the universe.

In an ever expanding end of the Universe scenario after the last remnants of the last BH disappeared with a bang and we are only left with radiation, I've heard that it is supposed to eventually disperse into heat. But, without matter there doesn't seem to be a mechanism to absorb the radiation and turn it into heat.

So, is it correct to assume that the universe could never end and would expand forever as radiation in a steady state expansion?

The results are surprising as it shows that even a universe comprised entirely of just matter will expand.

Could you expand a bit on how it was surprising?

e.g.

Did matter alone expand at the same rate as our universe?

Or Did matter alone expand at a rate faster than what DE predicted?

There are various reasons people might do it.

The Milne Model was an early attempt to model the universe, but is contradicted by observations.

https://en.wikipedia.org/wiki/Milne_model

Other vacuum solutions are just ways of exploring different cases. There are a limited number of exact solutions to the Einstein Field Equations so I think something can be learned from any of them, even if they don't describe the universe we live in.

The presence (and distribution) of mass and energy affects the behaviour of space-time but doesn't cause it to exist.

No. 'Cos it was empty.

Ok, but to be clear, are we talking about a universe such as Mordred's example where at least one thing was still a part of the universe that was studied, be it radiation, lambda, matter, or DM.

Or are we talking about a universe that was truly empty of everything.

By the way, I mentioned the Higgs field as an after thought because, in theory, it is supposed to be everywhere. So I wondered if the Higgs field could define spacetime. It would be a single contiguous field that would contain all of the particles and all of the connections as FLRW defines them.

Also there are clearly problems with obtaining information about spacetime outside of our light cones, but it seems like their is a need to do just that. That's because there is a need for the universe to be homogeneous and isotropic for things like inertial frames to work.

To explain:

I assume that homogeneous and isotropic is to ensure all physical laws remain consistent no matter where we applied them. But what if an observable edge was very near to an actual edge of the universe or located next to an improbably large and dense cluster of galaxies. IDK, Whatever is large enough to break the local laws of physics. Then it seems like there is a potential for creating paradox where light speed is violated by information within our domain.

I see potential solutions using something like a Brane theory but that's getting into speculation. But I do see a definite need for a single field to be everywhere and the Higgs field seems to fit the bill. Also it imparts mass which could make it related to a quantum loop theory...

Sorry, I just had an epiphanic moment and I didn't want to loose my train of thought.

Edited by TakenItSeriously
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In an ever expanding end of the Universe scenario after the last remnants of the last BH disappeared with a bang and we are only left with radiation,

Black holes don't explode. So you would end up with a universe with black holes, the remains of stars and planets, etc separated by ever increasing distances.

Ok, but to be clear, are we talking about a universe such as Mordred's example where at least one thing was still a part of the universe that was studied, be it radiation, lambda, matter, or DM.

The Milne model, and others like it, are completely empty. You could create models that juts contain one or more of the components you mention, to see how they behave.

By the way, I mentioned the Higgs field as an after thought because, in theory, it is supposed to be everywhere. So I wondered if the Higgs field could define spacetime. It would be a single contiguous field that would contain all of the particles and all of the connections as FLRW defines them.

The same is true of all fields. There is nothing special about the Higgs field, in that respect.

I assume that homogeneous and isotropic is to ensure all physical laws remain consistent no matter where we applied them. But what if an observable edge was very near to an actual edge of the universe or located next to an improbably large and dense cluster of galaxies. IDK, Whatever is large enough to break the local laws of physics.

The assumption of homogeneous and isotropic is a good approximation to the universe at large scales, and allows a solution to the Einstein Field Equation to be derived. It has nothing to do with the laws of physics.

The universe around us (in the solar system or our galaxy) is not at all homogeneous or isotropic, but the laws of physics are exactly the same.

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You've gotten some good questions. First off our most likely end of death for the universe is heat death. If the universe continues to have accelerating expansion.

By heat death we mean extremely cold. How our universe expands is determined in a large part by thermodynamic interactions. As the universe expands its overall density and temperatures decreases.

What we refer to curvature directly relates to our overall density compared to a critical density. The critical density formula being.

$\rho_{crit} = \frac{3c^2H^2}{8\pi G}$

This however is prior to the cosmological constant. (Further detail on curvature including distance measure can be found here)

http://cosmology101.wikidot.com/universe-geometry

Page 2

http://cosmology101.wikidot.com/geometry-flrw-metric/

Those two pages will explain how curvature work with distance measures in the FLRW metric.

Now each contributor to total density has its own equation of state.

Further details here.

https://en.m.wikipedia.org/wiki/Equation_of_state_(cosmology)

Note radiation exerts a pressure influence but matter does not. The Higgs field can use the scalar modelling equation on that last link.

One of the key equations of the FLRW metric is the acceleration equation is given as

$\frac{\ddot{a}}{a}=-\frac{4\pi G\rho}{3c^2}(\rho c^2+3p)$

$H^2=\frac{\dot{a}}{a}=\frac{8\pi G\rho}{3c^2}-\frac{kc^2p}{R_c^2a^2}$

With above equations ( including links) you can calculate rate of expansion with whatever combination you desire.

There is another detail. As the universe expands the density of radiation,matter and the cosmological constant fall off at different rates. The region under the square root of this equation.

$H_z=H_o\sqrt{\Omega_m(1+z)^3+\Omega_{rad}(1+z)^4+\Omega_{\Lambda}}$

Show that matter and radiation density while decreases the cosmological constant does not. So even when neither matter nor radiation contribute to expansion the cosmological constant will continue to do so.

Now why is matter causing expansion a surprise. Well if you look at the equation of state it has a pressure contribution of zero.

So what keeps matter from collapsing under its own self gravity? Particularly in a matter only universe. Well the first thing we have to be clear on is pressure doesn't cause expansion. You require pressure gradients for that to happen. Which would entail a net flow. Which isnt isotropic nor homogenous. The fluid type we use in thermodynamic application is adiabatic and isentropic.

If you look at the last equation we see that the density of matter drops off. As the average density drops off the ability for gravity to cause compression decreases. Now think back to the critical density and the acceleration equation as to how that relation works with expansion

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Black holes don't explode. So you would end up with a universe with black holes, the remains of stars and planets, etc separated by ever increasing distances.

The Milne model, and others like it, are completely empty. You could create models that juts contain one or more of the components you mention, to see how they behave.

The same is true of all fields. There is nothing special about the Higgs field, in that respect.

The assumption of homogeneous and isotropic is a good approximation to the universe at large scales, and allows a solution to the Einstein Field Equation to be derived. It has nothing to do with the laws of physics.

The universe around us (in the solar system or our galaxy) is not at all homogeneous or isotropic, but the laws of physics are exactly the same.

Well, small enough black holes do go out with something of a pop, and over long enough periods, even larger black holes should evaporate, so there could very well be a situation in which the last black hole eventually goes out with a bang. At least as far as I know.

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Well, small enough black holes do go out with something of a pop, and over long enough periods, even larger black holes should evaporate, so there could very well be a situation in which the last black hole eventually goes out with a bang. At least as far as I know.

The trouble is that for a hole larger than about 1/100th the mass of the Earth, it will gain more mass from the cosmic microwave background than it will radiate, even if we ignore inflating gas and dust.

Such a black hole would be fractions of a millimetre in size and have a lifetime of billions of years (even if it didn't absorb and mass or energy.

So the only black holes that will explode are minute and almost certainly non-existent.

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The trouble is that for a hole larger than about 1/100th the mass of the Earth, it will gain more mass from the cosmic microwave background than it will radiate, even if we ignore inflating gas and dust.

Such a black hole would be fractions of a millimetre in size and have a lifetime of billions of years (even if it didn't absorb and mass or energy.

So the only black holes that will explode are minute and almost certainly non-existent.

Currently. I'm assuming the background temp of the universe is going to continue dropping as it expands. Eventually it should, presumably, reach a point where black holes are no longer being fed faster than they can radiate.

Granted, that's not going to be happening anytime soon, but it should eventually happen, shouldn't it?

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Currently. I'm assuming the background temp of the universe is going to continue dropping as it expands. Eventually it should, presumably, reach a point where black holes are no longer being fed faster than they can radiate.

Granted, that's not going to be happening anytime soon, but it should eventually happen, shouldn't it?

Good point. Eventually the universe will be cold and sparse enough that even large black holes will evaporate slowly.

Really, really slowly. Have you heard the Buddha's description of eternity, where he asks you to imagine a large mountain that is wiped gently with a soft cloth once every thousand years until it disappears? That is nothing compared to the lifetime of a black hole!

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Eventually BHs will radiate assuming Hawking radiation is correct. Even then though a bh will go out in a whimper rather than an explosion.

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This is a handy calculator for finding out the characteristics of black holes: http://xaonon.dyndns.org/hawking/

You can enter any parameter (mass, temperature, etc) and find out all the others. (I was planning to create a page like this, then found this one.)

Fun calculator

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You've gotten some good questions. First off our most likely end of death for the universe is heat death. If the universe continues to have accelerating expansion.

By heat death we mean extremely cold. How our universe expands is determined in a large part by thermodynamic interactions. As the universe expands its overall density and temperatures decreases.

What we refer to curvature directly relates to our overall density compared to a critical density. The critical density formula being.

$\rho_{crit} = \frac{3c^2H^2}{8\pi G}$

This however is prior to the cosmological constant. (Further detail on curvature including distance measure can be found here)

http://cosmology101.wikidot.com/universe-geometry

Page 2

http://cosmology101.wikidot.com/geometry-flrw-metric/

Those two pages will explain how curvature work with distance measures in the FLRW metric.

Now each contributor to total density has its own equation of state.

Further details here.

https://en.m.wikipedia.org/wiki/Equation_of_state_(cosmology)

Note radiation exerts a pressure influence but matter does not. The Higgs field can use the scalar modelling equation on that last link.

One of the key equations of the FLRW metric is the acceleration equation is given as

$\frac{\ddot{a}}{a}=-\frac{4\pi G\rho}{3c^2}(\rho c^2+3p)$

$H^2=\frac{\dot{a}}{a}=\frac{8\pi G\rho}{3c^2}-\frac{kc^2p}{R_c^2a^2}$

With above equations ( including links) you can calculate rate of expansion with whatever combination you desire.

There is another detail. As the universe expands the density of radiation,matter and the cosmological constant fall off at different rates. The region under the square root of this equation.

$H_z=H_o\sqrt{\Omega_m(1+z)^3+\Omega_{rad}(1+z)^4+\Omega_{\Lambda}}$

Show that matter and radiation density while decreases the cosmological constant does not. So even when neither matter nor radiation contribute to expansion the cosmological constant will continue to do so.

Ok but my question about radiation is not so much about the rate of expansion but if there was some other mechanism I am not aware of where radiation can dissipate without any mass in the universe. Because, based on what we know it seems like radiation is always going to be around and I don't understand why the current end of the universe models don't account for perpetual ever-expanding radiation?

Previously, I used to think of the universe ending when all matter was gone. Now I don't know what to think.

Edited by TakenItSeriously
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No radiation sticks around but as the universe expands its influence decreases. Due to lower average density. In the very far future radiation will have negligant influence. Same with matter

Edited by Mordred
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• 2 months later...

Very simple theory can solve this( edge of the space)my theory is this universe has no start and no end

There are only two things exist in this universe >matter and space <matter takes its place in space .so no matter only space remains

Is no space

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• 3 weeks later...

Ever since I was a young boy, I have wrestled with trying to understand space. In particular, I have never really understood how space is supposed to never end. I really don't see how that's possible. Everything ends somewhere. Where one thing ends the next begins.

Can people please provide thoughts on this?

Is it... Nothing what is nothing, u can't wrap your head around it.... Nothing

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Is it... Nothing what is nothing, u can't wrap your head around it.... Nothing

So-called "nothing" is not nothing, but rather a complicated medium of potential from which a big bang may arise. "Nothing" is in fact something.

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• 1 month later...

I am new to this forum, particularly this subject matter on.... the universe ending.

I read with interest some of the ideas put forward from people with clear and logical minds and,

also some not so scientific or logical thoughts as well.

I have studied about the universe, black holes etc. for most of my life and listened to many experts

watched every documentary I could get my hand on, but I still have different thoughts on these subjects that

I would love to discuss here if possible.

First thing I would love to hear about is people thoughts on is what is out side the known universe and, what is

inside the smallest known objects such as atom nucleus or Quarks etc.

My idea is that they are both simular and after hearing from people in the know or have a few good ideas

I would like to start putting some of my ideas up for discussion.

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