# Is the Universe infinite?

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I thought we were speaking hypothetically. If we are talking about two universes that expand like the Big Bang in all directions then I agree it cannot happen.

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You fool!   You don't even know how this all started, its because strange said the big bang did not imply the universe has an origin there. I said that the big bang implies there was so

No one knows. Except for me, and I'm not saying.

The non bolded points have conveniently been dealt with while I took a break. The finite observable universe originated (probably) in a volume smaller than a proton but because of the lack of obse

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The Universe is not expanding into something, Airbrush.
It is not expanding into a 3dimensional space, which if occupied by another universe, will then overlap, or be prevented from expanding.

Think of it as a number line, where the infinite numbers of universes occupies the 'space' between each adjacent numbers.
Each of those 'spaces is composed of an infinity of points.
Now expand the spacing between each of those points.

The space between each number expands, but still has an infinite number of points.
As a result, if all the 'spaces' ( between each number ) expand, then the number line expands, but still has  infinite numbers.

The number 3 is not preventing the space between 1 and 2 from expanding, nor is there any overlapping numbers.

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22 hours ago, MigL said:

Think of it as a number line, where the infinite numbers of universes occupies the 'space' between each adjacent number.

…….The number 3 is not preventing the space between 1 and 2 from expanding, nor is there any overlapping numbers.

Yes but the space between numbers can never grow to an infinite size.

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But it can hold an infinite amount of points.
There are orders of infinities.

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If there is no special point in the universe, then every point in our observable universe should have a past light cone that is indistinguishable from our point of view. Building from this logic, a point at the edge of our observable universe should have points on it's observable universe's edge which we can't observe, which have their own indistinguishable past light cones. Building on the principle of universal isotropy, therefore, the universe should be infinite in size.

What about time? If the universe is cyclical in nature, then it should extend infinitely in time as well. If it isn't, then the universe has a definite beginning, the Big Bang. Does that mean it has a definite "end"? Not necessarily, though the most convincing evolutionary scenario to my mind is the heat death, when entropy can no longer increase, which can be interpreted as the ending of time, which would make time finite towards the future.

Maybe(?) at this point in Time Temperature will become "negative" and suddenly a new Big Bang will spark anew, which would make the whole thing cyclical, but this is pure speculation.

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7 hours ago, MigL said:

There are orders of infinities.

An infinite number of them!

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6 hours ago, Strange said:

An infinite number of them!

Theoretically, mathematically, you can have an infinite number of infinities, you cannot have more than one universe with an infinite size.  Unless someone can explain how you can?  Universes are not shaped like number lines.  They are 3 dimensional structures.

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1 hour ago, Airbrush said:

Theoretically, mathematically, you can have an infinite number of infinities, you cannot have more than one universe with an infinite size.  Unless someone can explain how you can?  Universes are not shaped like number lines.  They are 3 dimensional structures.

So you didn't believe anything I said?

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1 hour ago, zapatos said:

So you didn't believe anything I said?

No, I agreed with you completely.

Zapatos:  "I thought we were speaking hypothetically. If we are talking about two universes that expand like the Big Bang in all directions then I agree it cannot happen."

There you agreed with me.  Now let's not speak hypothetically for a moment.  Let's speak physical feasibility.  You cannot have 2 universes, that have infinite sizes (in 3 dimensions) co-existing.  If you can, then explain how you can.  "Vector universes" are infinite in only one direction, but that can't happen physically, it seems to me.  And I just made up the term "vector universes."

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If there is no special point in the universe, then every point in our observable universe should have a past light cone that is indistinguishable from our point of view. Building from this logic, a point at the edge of our observable universe should have points on it's observable universe's edge which we can't observe, which have their own indistinguishable past light cones. Building on the principle of universal isotropy, therefore, the universe should be infinite in size.

That does not follow. If the universe is a 3D torus, then you might be able to look in some direction and see yourself from the back, and if you look to your straight left, you might see the right side of your body. Or look straight up, and probably get dizzy. The same things repeat in all directions, and in total the universe has only finite content.

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22 minutes ago, Airbrush said:

There you agreed with me.  Now let's not speak hypothetically for a moment.  Let's speak physical feasibility.  You cannot have 2 universes, that have infinite sizes (in 3 dimensions) co-existing.

What if the building blocks of the universes don't interact?

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9 minutes ago, zapatos said:

What if the building blocks of the universes don't interact?

You could have a  universe within this one populated with particles that have weaker than neutrino-levels of interaction with this universe.

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10 hours ago, taeto said:

That does not follow. If the universe is a 3D torus, then you might be able to look in some direction and see yourself from the back, and if you look to your straight left, you might see the right side of your body. Or look straight up, and probably get dizzy. The same things repeat in all directions, and in total the universe has only finite content.

Obviously nobody knows if the Universe is a torus. If it isn't, my reasoning applies. If it is, that would be a limiting condition

9 hours ago, StringJunky said:

You could have a  universe within this one populated with particles that have weaker than neutrino-levels of interaction with this universe.

That kind of sounds like Star Trek's depiction of parallel universes which exist in the same space but have different 'quantum phases' so they don't interact with each other - or only gravitationally, which would explain dark matter, if we hadn't discovered galaxies that have a different ratio of light/dark matter, since a reasonable assumption would be that large scale evolution would be the same in gravitationally bound parallel universes

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NASA's Hubble Space Telescope took an image of a bizarre, ghostly looking galaxy called NGC 1052-DF2 that astronomers calculate to have little to no dark matter. This is the first galaxy astronomers have discovered to be so lacking in dark matter, which is thought to comprise 85% of our universe's mass.

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Obviously nobody knows if the Universe is a torus. If it isn't, my reasoning applies. If it is, that would be a limiting condition

I think that I wrote it wrong to you, by missing that you said that isotropy is your assumption. Clearly the torus is anisotropic, just from the description of what happens if you observe different directions. My idea is better explained by suggesting that the Universe is flat and finite, but very large, so much that we cannot detect anisotropy by looking at the observable part.

It seems quite reasonable to say that an absolutely flat and isotropic Universe has to be infinite. Interesting.

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12 minutes ago, taeto said:

I think that I wrote it wrong to you, by missing that you said that isotropy is your assumption. Clearly the torus is anisotropic, just from the description of what happens if you observe different directions. My idea is better explained by suggesting that the Universe is flat and finite, but very large, so much that we cannot detect anisotropy by looking at the observable part.

It seems quite reasonable to say that an absolutely flat and isotropic Universe has to be infinite. Interesting.

To be fair, we couldn't determine that just by looking out because we are looking back in time. To be sure, we would have to take a snapshot of what we see now and compare it with what we see over the next 14 bn years to see if other parts of the universe developed to be exactly what they are today. Also considering the accelerating expansion of the universe we might never be able to determine that because these regions would recede beyond our cosmic horizon by then...

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18 hours ago, Airbrush said:

Universes are not shaped like number lines.  They are 3 dimensional structures.

So that would three orthogonal number lines.

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Here is a plot (courtesy Lawden) of the Friedmann solutions to the Einstein Field equations showing the time evolution of universes with different input cosmological constant (k)

With negative k the universe expands without bound

With zero k the universe becoems asymptotic.

With positive k the universe expans and contracts cyclically.

So the real question is not

Is the uninverse infinite, but

Will the universe ever be infinite, and if so when?

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The Universe spreads over Space AND over Time. It was, it is, it will be.

When you ask the question about what IS the universe, you will only get as an answer a slice of what the universe (will) look(ed).

And when you look at the sky, some stuff you see is in the past, the distance determining how long in the past.

Some closer stuff is roughly in the present.

None is in the future.

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1 hour ago, studiot said:

Here is a plot (courtesy Lawden) of the Friedmann solutions to the Einstein Field equations showing the time evolution of universes with different input cosmological constant (k)

With negative k the universe expands without bound

With zero k the universe becoems asymptotic.

With positive k the universe expans and contracts cyclically.

So the real question is not

Is the uninverse infinite, but

Will the universe ever be infinite, and if so when?

you also have the lambda term in there that Einstein introduced to make the Universe static if k turned out to be different from 0, because he didn't like the idea of an expanding or shrinking universe. Now k is pretty certainly 0, with very small error bars, but it is expanding at an accelerated rate, so it's lambda we're looking at for dark energy and the continued and accelerating expansion of the universe.

I've speculated in another thread that even at the Big Bang, the volume of the Universe might have been infinite, but that our observable universe originated from a single point of that big bang. Why shouldn't the isotropic principle extend to the Big Bang?

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On ‎7‎/‎19‎/‎2018 at 10:24 AM, zapatos said:

What if the building blocks of the universes don't interact?

That's a different case.  My special case is IF the building blocks interact.  Dark matter doesn't interact with anything, except gravitationally, and DM is not considered another universe.

10 hours ago, Strange said:

So that would three orthogonal number lines.

Interesting, could you please elaborate on this and how orthogonal number lines can represent physical reality where several universes that are infinite in size can co-exist?  I'm trying hard to visualize.

I've speculated in another thread that even at the Big Bang, the volume of the Universe might have been infinite, but that our observable universe originated from a single point of that big bang. Why shouldn't the isotropic principle extend to the Big Bang?

Yes because how can something finite in size become infinite in size?  No matter how fast the expansion rate, something with a finite size, in 3 dimensions, cannot "grow" into something infinite in size in a finite time period.  Over an infinite period of time would it approach infinity?

Edited by Airbrush
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40 minutes ago, Airbrush said:

Interesting, could you please elaborate on this and how orthogonal number lines can represent physical reality where several universes that are infinite in size can co-exist?  I'm trying hard to visualize.

I was just thinking of the three spatial coordinates. Nothing more than that.

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1 hour ago, Airbrush said:

.  Over an infinite period of time would it approach infinity?

You wouldn't have a reference time to measure at so rather meaningless when comparing geometry change. The minute you choose a time to take a measurement it becomes a finite point in time. Obviously time is never ending under physics. The finite time is when you take a measurement.

Edited by Mordred
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1 hour ago, Airbrush said:

That's a different case.  My special case is IF the building blocks interact.  Dark matter doesn't interact with anything, except gravitationally, and DM is not considered another universe.

Your special case seems to be so narrow that you are claiming two things in our universe cannot occupy the same space at the same time. You can also claim that an electron cannot at the same time be a photon.

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On 7/20/2018 at 11:43 AM, studiot said:

With negative k the universe expands without bound

Or it crunches back, depending on the value of lambda, no?

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19 hours ago, zapatos said:

Your special case seems to be so narrow that you are claiming two things in our universe cannot occupy the same space at the same time. You can also claim that an electron cannot at the same time be a photon.

How can two things occupy the same space at the same time?

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