# Where Does Space End? It Must End Somewhere!

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It's hard to imagine because infinity is hard to imagine, but there is no reason it couldn't have started at an infinite size, but much more dense. It would have remained infinitely large, but the expansion would have then decreased the density.

But yes, it either is infinite and started infinite, or it is finite and started finite. (Or I suppose something really weird could have happened and it transitioned from one to the other, but you wouldn't get there from simple expansion).

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"Almost certainly many times larger"?

There is no such certainty. The entire universe could be exactly as large as the observable universe, by coincidence. It could be 50% larger than the observable universe. But it is almost certainly larger, and we don't have ANY clue how much larger.

Here is a good description of how we know this (with the obvious caveat that this is science and so we never know anything for certain):

http://scienceblogs.com/startswithabang/2012/07/18/how-big-is-the-entire-universe/

Explain HOW the universe can be infinite in size before saying "it could by infinite".

I'm not sure what "how" means. If it is infinite, is is just infinite.

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But yes, it either is infinite and started infinite, or it is finite and started finite.

Yes, that is excelant explanation.

If 13.8 Bly ago the Universe had started as finite than it can't be infinite today.

(Or I suppose something really weird could have happened and it transitioned from one to the other, but you wouldn't get there from simple expansion).

A finite universe can NEVER grow to an infinite size. For the universe to be infinite in size (and/or infinite in mass) it would have to ALREADY have been infinite in size at the first moment of the big bang. That is hard to imagine.

If the entire universe ever had a finite size, it could never grow to an infinite size at any finite rate. Even cosmic inflation was a finite rate of expansion. The difference between finite and infinite is so great that the difference is infinite.

Fully agree.

Somehow, we must offer better explanation for infinite Universe.

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Somehow, we must offer better explanation for infinite Universe.

The explanation is that if it is infinite, then it has always been infinite.

(Actually, it is possible to have a model where a finite universe becomes infinite, but I don't think anyone would consider it realistic.)

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The explanation is that if it is infinite, then it has always been infinite.

(Actually, it is possible to have a model where a finite universe becomes infinite, but I don't think anyone would consider it realistic.)

So, if it is infinite, why do we need the BBT?

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So, if it is infinite, why do we need the BBT?

That is a completely different question. We need the big bang theory to describe the evolution of the universe from an early, hot dense state, as indicated by the evidence.

Edited by Strange
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I'm quite confused. You have stated that:

The explanation is that if it is infinite, then it has always been infinite.

So, if it is infinite, then 13.8 Bly ago it was also infinite?

That is a completely different question. We need the big bang theory to describe the evolution of the universe from an early, hot dense state, as indicated by the evidence.

Hence, why do we need to set it (13.8 Bly ago) to finite hot dense state?

Edited by David Levy
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So, if it is infinite, why do we need the hot dense state?

Because that is what the evidence shows. How is it relevant if the universe being inifinite or not?

If the Universe is infinite, do you agree that 13.8 Bly ago it was also infinite?

Yes.

If so, then why do we need to set it (13.8 Bly ago) to finite hot dense state?

We don't. If it is infinite now, it was infinite then.

If you are wondering how something that is infinite can expand, then Hilbert's Grand Hotel might give you some insight.

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"Almost certainly many times larger"?

There is no such certainty. The entire universe could be exactly as large as the observable universe, by coincidence. It could be 50% larger than the observable universe. But it is almost certainly larger, and we don't have ANY clue how much larger.

Explain HOW the universe can be infinite in size before saying "it could by infinite".

A finite universe can NEVER grow to an infinite size. For the universe to be infinite in size (and/or infinite in mass) it would have to ALREADY have been infinite in size at the first moment of the big bang. That is hard to imagine.

If the entire universe ever had a finite size, it could never grow to an infinite size at any finite rate. Even cosmic inflation was a finite rate of expansion. The difference between finite and infinite is so great that the difference is infinite.

I said "what MOST would call infinite". There is no such thing as infinity. There is, however,such a distance that it would be impossible to travel completely before you and/or what ever you are using to travel is eroded away by the celestial elements, such as planets, moons, stars, comets, meteors, etc. etc.

Edited by The Think3r
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There is no such thing as infinity.

There is if the universe is infinite.

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IF there is such a thing as infinity then explain to me how it is possible with out breaking the "rules" of reality.

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IF there is such a thing as infinity then explain to me how it is possible with out breaking the "rules" of reality.

What rules would it break?

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I'm quite confused. You have stated that:

So, if it is infinite, then 13.8 Bly ago it was also infinite?

Hence, why do we need to set it (13.8 Bly ago) to finite hot dense state?

Our Observable Universe is finite. There may be a greater universe extending beyond the bounds of what we are capable of observing that goes to infinity, but the area that we can observe, everything we see when we look up in the sky, exists within a finite bubble of space.

That bubble of space, specifically, used to be much smaller, and consequently much hotter and denser as all of the matter we can observe in the universe was packed into a very small space.

But if the universe is infinite, we would not be talking about a very small universe getting very big, but of an infinitely large and uniformly hot and dense universe becoming an infinitely large universe that was much cooler and less dense through the metric expansion of space.

Our local pocket of the universe then would have gone from being very small to very large, but the overall universe would have always been infinite if it is now.

Expansion/the Big Bang is not the universe "growing." It is the distance between any two given points in space increasing. If distance increases (and see the Hotel example linked above for how something infinitely large could expand) and the amount of matter/energy remains the same, th density will increase, and you will get a situation that looks exactly like what we can see right now when we look out into space.

Edited by Delta1212
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If it is infinite now, it was infinite then.

Thanks

How is it relevant if the universe being inifinite or not?

This is the most important question.

Based on the BBT, 13.8 BY ago, the universe was finite hot dense state.

If you are wondering how something that is infinite can expand, then Hilbert's Grand Hotel might give you some insight.

The question is: How could it be that something that was FINITE hot dense state could expand to infinite?

As it is stated:

A finite universe can NEVER grow to an infinite size. For the universe to be infinite in size (and/or infinite in mass) it would have to ALREADY have been infinite in size at the first moment of the big bang. That is hard to imagine.

If the entire universe ever had a finite size, it could never grow to an infinite size at any finite rate. Even cosmic inflation was a finite rate of expansion. The difference between finite and infinite is so great that the difference is infinite.

It is quite clear that a finite Universe can never expand into Infinite Universe in a 13.8 By time frame.

So by definition - an infinite Universe contradicts the basic Idea of finite hot dense state.

Never the less - let's see the following explanation:

But if the universe is infinite, we would not be talking about a very small universe getting very big, but of an infinitely large and uniformly hot and dense universe becoming an infinitely large universe that was much cooler and less dense through the metric expansion of space.

You claim that if the Universe is infinite, than 13.8 BY ago it was an infinitely large and uniformly hot and dense universe

However, how could it be that an infinite hot dense Universe could get cooler?

Let's assume that in that oven we have set an infinite no. of cakes and its temp. is very high (6000 K).

If we "expand" the distance between the cakes, is it going to have any effect on the oven temp?

I do believe that the answer is no.

If so, the expansion couldn't decrease the infinite Universe temp.

Therefore, do you agree that an infinite Universe contradicts the idea of finite or infinite hot dense state.

Edited by David Levy
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Thanks

This is the most important question.

Based on the BBT, 13.8 BY ago, the universe was finite hot dense state.

The question is: How could it be that something that was was FINITE hot dense state could expand to infinite?

As it is stated:

It is quite clear that a finite Universe can never expand into Infinite Universe in a 13.8 By time frame.

So by definition - an infinite Universe contradicts the basic Idea of finite hot dense state.

Never the less - let's see the following explanation:

You claim that if the Universe is infinite, than 13.8 BY ago it was an infinitely large and uniformly hot and dense universe

However, how could it be that an infinite hot dense Universe could get cooler?

Let's assume that in that oven we have set an infinite no. of cakes and its temp. is very high (6000 K).

If we "expand" the distance between the cakes, is it going to have any effect of the oven temp?

I do believe that the answer is no.

If so, the expansion couldn't decrease the infinite Universe temp.

Therefore, do you agree that an infinite Universe contradicts the idea of finite or infinite hot dense state.

But you're not just expanding the distance between the cakes. You're expanding the distance between the molecules of air as well. And that being the case, yes, the temperature will decrease.

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But you're not just expanding the distance between the cakes. You're expanding the distance between the molecules of air as well. And that being the case, yes, the temperature will decrease.

Sorry, I disagree.

The expansion of molecules of air can't decrease the temp!

In order to get it cooler we need to mix it with cooler environment.

Unfortunately, in an infinite hot dense universe there is no extra room for cooler environment.

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Based on the BBT, 13.8 BY ago, the universe was finite hot dense state.

Not necessarily. Only if the universe is finite in size. Otherwise it was infinite.

So by definition - an infinite Universe contradicts the basic Idea of finite hot dense state.

Nonsense.

If we "expand" the distance between the cakes, is it going to have any effect on the oven temp?

The gas in the oven will get cooler. Have you noticed that when you let the gas out of an aerosol can it gets cold? Same thing.

I do believe that the answer is no.

Which is why you should rely on physics, rather than beliefs.

Therefore, do you agree that an infinite Universe contradicts the idea of finite or infinite hot dense state.

Of course not.

Remember, when you say "do you agree" it really means "I made this up because I don't understand science".

Edited by Strange
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The gas in the oven will get cooler. Have you noticed that when you let the gas out of an aerosol can it gets cold? Same thing.

No.

In this case you have two different environments: different temp, different pressure, different gas mix...

Therefore, as the gas gets out the aerosol it also gets cold.

In an infinite Universe there is only one environment.

Think about an aerosol inside an aerosol.

If you try it you should find that there is no change in the temp.

Edited by David Levy
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It is basic thermodynamics. As the pressure drops, so does the temperature. Did you miss this at school?

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Sorry, I disagree.

The expansion of molecules of air can't decrease the temp!

In order to get it cooler we need to mix it with cooler environment.

Unfortunately, in an infinite hot dense universe there is no extra room for cooler environment.

As pressure increases, temperature increases. As pressure decreases, temperature decreases. If you could only cool something by mixing it with an even cooler environment, your refrigerator wouldn't work without having to be constantly refilled with ice. It works on the exact same temperature-pressure relationship we're talking about now:

https://en.m.wikipedia.org/wiki/Heat_pump_and_refrigeration_cycle

Simplified: You take a liquid, put it under pressure, pump it into the insulated box of the refrigerator, drop the pressure which flash cools it, it absorbs the heat inside the refrigerator, then you pump it back out where you can repressurization it, expel the waste heat and restart the cycle.

You literally rely on this principle that you don't think exists to keep your food cold.

Edited by Delta1212
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...Based on the BBT, 13.8 BY ago, the universe was finite hot dense state....

You tend to keep mixing The (Entire) Universe with The Observable Universe.

Try this for visualisation:

Currently our Observable Universe seems to be this big:

--> O

--> .

(i.e. the Observable Universe went from "." to "O").

However, we don't expect that The Observable Universe (O) is The (Entire) Universe. From the evidence (see previous in this thread) it's probably many times bigger:

--> OOOOOOOOOO

Which would mean it was probably once like:

--> ..........

But, The Universe might actually be infinite:

--> --- OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO ---

Which expanded from an infinite:

--> --- .................................................. ---

... and how an infinite dense thing can expand to an infinite less-dense thing, see Hilbert Hotel (previous in this thread).

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It is basic thermodynamics. As the pressure drops, so does the temperature.

Yes and no

Yes - as the pressure drops, the temperature drops.

That is correct.

No - that is just the starting point.

Somehow you need to get rid of the extra heat.

So, let's look at a refrigerator.

There are two isolated arias: Inside and outside.

The engine is actually using the idea of different pressures to generate cold gas.

However, this activity creates extra heat.

In an ideal engine the total "cold energy" which it generates should be identical to a total "heat energy".

So, we must find a way to get rid of that extra heat energy.

Therefore, we have to set an isolated environment in order to keep the coldness inside the refrigerator.

However, in the Universe there is only one environment.

It is similar to a refrigerator without its walls, or like an engine working in an open environment.

Technically, that ideal engine generates "heat energy" which is identical to the "cold energy".

If we can't move the extra heat energy to different environment, than the total energy of that ideal engine should be zero.

Therefore, the universe which is considered as a homogenous environment can't cool itself by itself.

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The extra heat is distributed over a greater volume.

Please study the following entry thermodynamic formula.

$pV=nRt$

p=pressure

V=volume

N number of moles (particles)

t =temperature.

R is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant.

Until you understand This equation there is no point in explaining how it works for an adiabatic fluid.

Here is a half decent coverage.

As the Volume increases BOTH the temperature and pressure drops

The universe is modelled as an adiabatic and isentropic fluid.

Meaning there is no net outflow of energy. The isentropic term refers to adiabatic and reversible.

Edited by Mordred
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The extra heat is distributed over a greater volume.

Infinite universe has an infinite volume.

So what does it mean - a greater volume over an infinite volume?

With regards to the example which I have used -

Do you agree that : ideal engine generates "heat energy" which is identical to the "cold energy"?

Do you also agree that: If we can't move the extra heat energy to different environment, than the total energy of that ideal engine should be zero?

If so, technically if the universe generates cold energy due to the expansion, it also creates identical heat energy.

Those energies should eliminate each other.

In any case, with regards to your statement - "The extra heat is distributed over a greater volume."

Why the "extra cold" can't be distributed over a greater volume in an infinite volume Universe?

Edited by David Levy
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Doesn't matter if the Universe is finite or infinite.

The same gas laws are applicable to both.

In the case of modelling our OBSERVABLE portion its finite.

However there is no rule that states an infinite universe cannot increase in size either.

The change in volume in the infinite case applies in the same way as the finite case. The distance from any measurements increases as the Volume increases.

You can only measure a finite portion of an infinite quantity.

However infinity can increase in size

There is no limit to infinity.

In terms of the ideal gas laws though we can't model infinite density too well. But compression not expansion.

Edited by Mordred
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