# Could the real size of the universe be infinite?

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14 hours ago, MigL said:

If the universe is not homogenous and isotropic, rewinding the clock back wll not lead to the Big Bang.
If the universe is not expanding equallyin all directions there is no Big Bang.

Please try to answer this one more time, because I can't find the first answer.  The universe is homogenous and isotropic for a volume less than 100 billion LY in diameter.  How do they know it must be that way to infinity assuming space does not curve?  Observations show that the universe is flat or so slightly curved that it is undetectable.

Interesting in the Hawking interview Youtube I posted above, Hawking never mentioned a flat universe.  He said it was always curved, but in a way unimaginable.  So if you travel in a straight line from Earth, you will necessarily return to Earth after curving around the universe.  That would mean the universe is not infinite in size.  The question is how far is the finite loop around the universe?  It must be further than the edge of the observable universe, because they have not detected the same configurations of galaxy clusters looking both directions.

Edited by Airbrush

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At some point it helps if one accepts they are an amateur trying to have an in depth discussion with experts. When that happens, one no longer tries to argue they are right, but instead tries to understand why they are wrong.

It's hard to understand how a Ferarri functions when your automotive knowledge is based upon playing with matchbox cars.

I'm speaking of myself of course.

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2 hours ago, Airbrush said:

Please try to answer this one more time, because I can't find the first answer.  The universe is homogenous and isotropic for a volume less than 100 billion LY in diameter.  How do they know it must be that way to infinity assuming space does not curve?  Observations show that the universe is flat or so slightly curved that it is undetectable.

Interesting in the Hawking interview Youtube I posted above, Hawking never mentioned a flat universe.  He said it was always curved, but in a way unimaginable.  So if you travel in a straight line from Earth, you will necessarily return to Earth after curving around the universe.  That would mean the universe is not infinite in size.  The question is how far is the finite loop around the universe?  It must be further than the edge of the observable universe, because they have not detected the same configurations of galaxy clusters looking both directions.

There are very small error bars in the WMAP data that gives us a likely flat universe. 100 billion L/Y's is a reasonable representation, but in essence, we can never really be sure.

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On 9/10/2021 at 1:10 PM, zapatos said:

At some point it helps if one accepts they are an amateur trying to have an in depth discussion with experts. When that happens, one no longer tries to argue they are right, but instead tries to understand why they are wrong.

It's hard to understand how a Ferarri functions when your automotive knowledge is based upon playing with matchbox cars.

I'm speaking of myself of course.

That means that since I am not an expert I should go away.  Thanks pal.

I'm not trying to argue I'm right.  I'm asking how do they know what they say they know?  They say "there can be no center or edge" to what we see as the big bang.   I still haven't seen a coherent answer.  Nobody is sure about anything beyond the observable universe.  I'm asking for your best guess.  I'm no expert, so I'm asking.  Can you answer my questions?  Einstein said something like "a good scientist should be able to explain their work to a child" basically.  At least lead the child in the right direction.  I have not seen the direction yet.

Edited by Airbrush
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1 hour ago, Airbrush said:

That means that since I am not an expert I should go away.  Thanks pal.

I'm not trying to argue I'm right.  I'm asking how do they know what they say they know?  They say "there can be no center or edge" to what we see as the big bang.   I still haven't seen a coherent answer.  Nobody is sure about anything beyond the observable universe.  I'm asking for your best guess.  I'm no expert, so I'm asking.  Can you answer my questions?  Einstein said something like "a good scientist should be able to explain their work to a child" basically.  At least lead the child in the right direction.  I have not seen the direction yet.

There are three possible types of universe...Open, Flat and closed.

An open universe is one with negative curvature much like a saddle. Such a  universe  would be infinite.

So far the evidence points to a flat universe, one where two parallel beams of light would remain parallel. This would also be infinite.

The other is a closed universe with positive curvature.

The evidence for the flat universe [WMAP] though still has small margins of error, and obviously those small margins could be parts of a larger curvature.

A finite [closed universe] would have spacetime shaped like a globe, with no centre or edge. If the universe is infinite, then again, we can have no centre or edge.

In essence whether the universe is open, flat or closed, depends on the amount of matter/energy and consequently gravity, trying to close the universe, and the rate of expansion, and DE/ CC [cosmological constant] pushing things apart. This is called the "Hubble Constant" and at present is calculated at 74kms/second/mega parsec.

In essence, we are sure there is no edge to the universe that is flat, open or closed, and if there is no edge how can there be any centre to speak of?

This may explain better then I....

The other possibility also is some sort of exotic shape, like a torus for example...

Edited by beecee
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1 hour ago, Airbrush said:

That means that since I am not an expert I should go away.  Thanks pal.

Sounds like you have an inferiority complex.

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Don't go away Airbrush.

The Big Bang theory is based on certain observations, like galactic recession increasing linearly with increasing distance and the CMBR.
It is also predicated by GR, and the assumption that the universe is homogeneous and isotropic.

So yes, you are right, it is an assumption.; and at large enough distances, a fairly accurate assumption, for the observable universe.

Similarly GR does not allow for preferred frames, and a center of expansion certanly implies a preferred frame ( never mind the fact that the expansion with a center would look totally different unless we are the center; and what that implies for Religion ). So, unless drastic changes were made to GR, it would cease to be a viable theory.

You are then left with the problem of having to explain the observational evidence for isotropic universal expansion, and the homogenous/isotropic CMBR. These are actual physical observations, and cannot be assumed away; they require an explanation.

How would you do that ?
B

Edited by MigL
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18 hours ago, MigL said:

Don't go away Airbrush.

The Big Bang theory is based on certain observations, like galactic recession increasing linearly with increasing distance and the CMBR.
It is also predicated by GR, and the assumption that the universe is homogeneous and isotropic.

So yes, you are right, it is an assumption.; and at large enough distances, a fairly accurate assumption, for the observable universe.

Similarly GR does not allow for preferred frames, and a center of expansion certanly implies a preferred frame ( never mind the fact that the expansion with a center would look totally different unless we are the center; and what that implies for Religion ). So, unless drastic changes were made to GR, it would cease to be a viable theory.

You are then left with the problem of having to explain the observational evidence for isotropic universal expansion, and the homogenous/isotropic CMBR. These are actual physical observations, and cannot be assumed away; they require an explanation.

How would you do that ?
B

Thank you MigL.  That's what I call an answer!  You get +1 for that. 🙂

Edited by Airbrush
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• 2 months later...

In my mind Universe is just a circle. Do you guys remember the last episode of Interstellar, when they have finally survived? Every time you reach the end you reach its beginning.

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Exactly, the GR gives us fundamental assumptions, which are coming true with new observations and theories or waiting their time.

The observed universe is flat.
The CMB of the universe is more concentrated across one plane of the universe, forming a straight line in the universe. As the amount of radiation in one area is limited, it’s indicative of the size of the area in that direction.
In theory, it can have a positive curvature, negative curvature, flat curvature, or possibly a combination of all two or three.
A positive curvature corresponds to a closed universe. In a closed universe, the expansion of the universe comes to a stop due to gravity, all matter collapses back into a singularity.
A negative curvature corresponds to an open universe. In an open universe the density is not large enough for gravity to take over, instead the universe will expand forever. It may slow down approximating to a limit, but never stop.
A flat curvature corresponds to a flat universe. In a flat universe space expands forever but will either accelerate or decelerate, according to the amount of dark energy.
With a torus, the geometry of the universe would be a combination of positive and negative curvature. The three-torus model of the universe. Here, the shape of the universe is a three-dimensional torus, while a donut is a two-dimensional torus.

Infinity, singularity is a mathematical abstraction, which is being avoided by some mathematical branches even.

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3 hours ago, skydelph said:

Exactly, the GR gives us fundamental assumptions, which are coming true with new observations and theories or waiting their time.

The observed universe is flat.
The CMB of the universe is more concentrated across one plane of the universe, forming a straight line in the universe. As the amount of radiation in one area is limited, it’s indicative of the size of the area in that direction.
In theory, it can have a positive curvature, negative curvature, flat curvature, or possibly a combination of all two or three.
A positive curvature corresponds to a closed universe. In a closed universe, the expansion of the universe comes to a stop due to gravity, all matter collapses back into a singularity.
A negative curvature corresponds to an open universe. In an open universe the density is not large enough for gravity to take over, instead the universe will expand forever. It may slow down approximating to a limit, but never stop.
A flat curvature corresponds to a flat universe. In a flat universe space expands forever but will either accelerate or decelerate, according to the amount of dark energy.
With a torus, the geometry of the universe would be a combination of positive and negative curvature. The three-torus model of the universe. Here, the shape of the universe is a three-dimensional torus, while a donut is a two-dimensional torus.

Infinity, singularity is a mathematical abstraction, which is being avoided by some mathematical branches even.

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2 hours ago, skydelph said:

Exactly, the GR gives us fundamental assumptions, which are coming true with new observations and theories or waiting their time.

The observed universe is flat.
The CMB of the universe is more concentrated across one plane of the universe, forming a straight line in the universe. As the amount of radiation in one area is limited, it’s indicative of the size of the area in that direction.
In theory, it can have a positive curvature, negative curvature, flat curvature, or possibly a combination of all two or three.
A positive curvature corresponds to a closed universe. In a closed universe, the expansion of the universe comes to a stop due to gravity, all matter collapses back into a singularity.
A negative curvature corresponds to an open universe. In an open universe the density is not large enough for gravity to take over, instead the universe will expand forever. It may slow down approximating to a limit, but never stop.
A flat curvature corresponds to a flat universe. In a flat universe space expands forever but will either accelerate or decelerate, according to the amount of dark energy.
With a torus, the geometry of the universe would be a combination of positive and negative curvature. The three-torus model of the universe. Here, the shape of the universe is a three-dimensional torus, while a donut is a two-dimensional torus.

Infinity, singularity is a mathematical abstraction, which is being avoided by some mathematical branches even.

Edit: quoted from credited text (it might help to look deeper to the subject as well).

Exactly, the GR gives us fundamental assumptions, which are coming true with new observations and theories or waiting their time.

'The observed universe is flat.
The CMB of the universe is more concentrated across one plane of the universe, forming a straight line in the universe. As the amount of radiation in one area is limited, it’s indicative of the size of the area in that direction.
In theory, it can have a positive curvature, negative curvature, flat curvature, or possibly a combination of all two or three.
A positive curvature corresponds to a closed universe. In a closed universe, the expansion of the universe comes to a stop due to gravity, all matter collapses back into a singularity.
A negative curvature corresponds to an open universe. In an open universe the density is not large enough for gravity to take over, instead the universe will expand forever. It may slow down approximating to a limit, but never stop.
A flat curvature corresponds to a flat universe. In a flat universe space expands forever but will either accelerate or decelerate, according to the amount of dark energy.
With a torus, the geometry of the universe would be a combination of positive and negative curvature. The three-torus model of the universe. Here, the shape of the universe is a three-dimensional torus, while a donut is a two-dimensional torus.
'

https://astronomyfacts.quora.com/What-do-you-think-is-beyond-our-observable-universe

Infinity, singularity is a mathematical abstraction, which is being avoided by some mathematical branches even.

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1 hour ago, skydelph said:

Edit: quoted from credited text (it might help to look deeper to the subject as well).

Especially if you don't know enough to critically appraise what you are quoting.

5 hours ago, skydelph said:

A positive curvature corresponds to a closed universe. In a closed universe, the expansion of the universe comes to a stop due to gravity, all matter collapses back into a singularity.

Not necessarily. It depends upon the 'size' of that universe.

1 hour ago, skydelph said:

A flat curvature

What, pray is a flat curvature ?

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On 12/3/2021 at 8:40 PM, studiot said:

Especially if you don't know enough to critically appraise what you are quoting.

I am not a judge of myself and the work done behind it.

On 12/3/2021 at 8:40 PM, studiot said:

What, pray is a flat curvature ?

Consider it is 'flat'.
Thank you for the points.

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2 hours ago, skydelph said:

I am not a judge of myself and the work done behind it.

Consider it is 'flat'.
Thank you for the points.

How can I consider curvature flat ?

What does this mean?

I asked because I am questioning the quote from Quora and your understanding/use of it.

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'Flat curvature' here is 'zero curvature', where the local geometry is flat. 'Flat' is usually understood in 2D. But for the universe we are talking about local 3D flatness.

There is still skepticism and no full agreement on what flatness should mean due to CMB measurement results, local gravitational lensing and curvatures.

Flat universe ( lambda cold dark matter - ΛCDM) is an accepted model. If it is removed, the detected and observed phenomenon will not fit to CMB measurements and conclusions.

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I realize it is not your assertion ( see Phi's Quora link ), but I have a problem with this statement

On 12/3/2021 at 7:21 AM, skydelph said:

The CMB of the universe is more concentrated across one plane of the universe, forming a straight line in the universe. As the amount of radiation in one area is limited, it’s indicative of the size of the area in that direction.

The CMB radiation is homogenous to 1 part in 100,000 for RMS variations of 18 uK ( after subtracting out dipole anisotropy ). See here ...

So I have to ask ...
What concentration ?
What plane, or line ?

If anything, they are evidence of quantum fluctuations in the pre-inflation era, and in no way indicative of any universal extent in a particular direction.

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52 minutes ago, skydelph said:

'Flat curvature' here is 'zero curvature', where the local geometry is flat. 'Flat' is usually understood in 2D. But for the universe we are talking about local 3D flatness.

I thought you didn't understand it.

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The observable universe is isotropic and homogeneous. This was the main assumption at the start of calculations, observations and theories.

The CMB map is the oldest detected radiation that was created after the big bang. The European Space Agency's Planck satellite produced the CMB map (that is also present in the Wikipedia article), showing measurements of the intensity and the orientation of the CMB all over the sky.

‘It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and galaxies of today.’ - quote

That is what is meant by intensity.

The calculated intensity odds are assumed to be caused by galaxy formation after the matter was cooled enough. Calculation of the odds of intensity show that the universe prefers one direction over another at 1 in 121,000.

Planck Satellite, COBE and Hubble Ultra Deep Field combined show that the universe is isotropic.

Every valuable article is a part of valuable knowledge on the subject.

Thank you for the comprehensive dialogue and every contribution.

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1 hour ago, skydelph said:

Calculation of the odds of intensity show that the universe prefers one direction over another at 1 in 121,000.

No, that's not quite right. The number is a measure of how isotropic you can say the universe is, limited by experimental error. The universe could be even more isotropic than that, but we can't  tell, because of limitations of the experiment.

It is different from claiming that the universe is anisotropic at that level. That is the best-case scenario (or worst case, depending on how you look at it), but nobody is claiming that this is the actual anisotropy

IOW, the error bars include zero. It's entirely different, and erroneous, to argue that they exclude zero.

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On 12/3/2021 at 10:28 PM, Kevin_Hall said:

In my mind Universe is just a circle. Do you guys remember the last episode of Interstellar, when they have finally survived? Every time you reach the end you reach its beginning.

Our observable universe, is obviously a sphere centered on our position. We still do not have evidence convincing enough to say whether the universe is finite or infinite.

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16 hours ago, swansont said:

No, that's not quite right. The number is a measure of how isotropic you can say the universe is, limited by experimental error. The universe could be even more isotropic than that, but we can't  tell, because of limitations of the experiment.

It is different from claiming that the universe is anisotropic at that level. That is the best-case scenario (or worst case, depending on how you look at it), but nobody is claiming that this is the actual anisotropy

IOW, the error bars include zero. It's entirely different, and erroneous, to argue that they exclude zero.

Totally right, thank you.
- The numbers are duplicated and commented like that. And they really mean that there might be error, so there is no 100% result to be sure.
- And this is totally different from anisotropic and is not making anisotropic conclusion in any word.

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