# Universe size questions

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• 04-14-2019, 02:02 AM
chas53
Universe size questions
hey nFA,
I am in awe of the fact that we can see 13.3 billion light years in the visible spectrum.

but hearing your thoughts is very valuable to me.
here goes:
the calculation for the size of the Observable Universe is a calculation.
we do not observe anything 46 billion light years away.
its not possible for us to do that.
what we do instead, is, we know that the universe is expanding and has been since the Big Bang.
we take the rate of expansion and apply it to observations that we can make, and
viola (har har), 46 billion light years.

my next question is about the CMBR.
I used to think that it only existed very far away from here. (say, 46 billion light years away).
that is incorrect.
the CMBR exists everywhere in the universe.
it is equally distributed.
but the farthest away that it is, is 46 billion light years away.
now, how do we know that?
do we observe the CMBR 46 billion light years away, with radio telescopes (or some such method)?
or do we, once again, know this by calculating, based on the expansion rate of the universe?

and another question.
and I would love to hear your thoughts, if you feel like it.

it has to do with the idea that the universe is infinite and does not have a center and so on.
ok, picture this: our Observable Universe is 46 billion light years in every direction.
now lets imagine that 46 billion light years away, there is another observer.
that observer also has an Observable Universe of 46 billion light years.
but if that observer looks in the opposite direction from looking towards us,
she is looking at another chunk of space that is 46 billion light years in size.
but that is completely outside of our Observable Universe.
and lets extend that again, continuing 46 billion light years more, going away from us.
and then keep extending it over and over.
adding another 46 billion light years away from us, each time.
infinitely.
and,
this happens in every direction.

so the Universe has an infinite number of Observable Universes that
are outside of our ability to observe them.
and (and this is a key and big question), those UN- observable (to us), Universes
have a trillion galaxies in them, just like we do.

any thoughts about all of this, super appreciated.
thank you,
Charlie :)
• 04-14-2019, 04:06 AM
not_Fritz_Argelander
Re: Universe size questions
Quote:

Originally Posted by chas53
hey nFA,
I am in awe of the fact that we can see 13.3 billion light years in the visible spectrum.

but hearing your thoughts is very valuable to me.
here goes:
the calculation for the size of the Observable Universe is a calculation.
we do not observe anything 46 billion light years away.
its not possible for us to do that.
what we do instead, is, we know that the universe is expanding and has been since the Big Bang.
we take the rate of expansion and apply it to observations that we can make, and
viola (har har), 46 billion light years.

The source we see now from light emitted ~13 Billion years ago is now ~46 Billion light years away.

Quote:

my next question is about the CMBR.
I used to think that it only existed very far away from here. (say, 46 billion light years away).
that is incorrect.
the CMBR exists everywhere in the universe.
it is equally distributed.
but the farthest away that it is, is 46 billion light years away.
now, how do we know that?
Its source which has a ~13 Billion year flight to us is now 46 Billion light years away.

Quote:

do we observe the CMBR 46 billion light years away, with radio telescopes (or some such method)?
or do we, once again, know this by calculating, based on the expansion rate of the universe?
Every cubic centimeter of the universe, no matter where it is, contains about 200-300 photons that were emitted by a source. The photons traveled at the speed of light ~13 Billion years to get into any cubic centimeter they now occupy and that source is now 46 Billion light years away by calculation.

Quote:

and another question.
and I would love to hear your thoughts, if you feel like it.

it has to do with the idea that the universe is infinite and does not have a center and so on.
ok, picture this: our Observable Universe is 46 billion light years in every direction.
now lets imagine that 46 billion light years away, there is another observer.
that observer also has an Observable Universe of 46 billion light years.
but if that observer looks in the opposite direction from looking towards us,
she is looking at another chunk of space that is 46 billion light years in size.
but that is completely outside of our Observable Universe.
and lets extend that again, continuing 46 billion light years more, going away from us.
and then keep extending it over and over.
adding another 46 billion light years away from us, each time.
infinitely.
and,
this happens in every direction.
Yes.

Quote:

so the Universe has an infinite number of Observable Universes that
are outside of our ability to observe them.
and (and this is a key and big question), those UN- observable (to us), Universes
have a trillion galaxies in them, just like we do.
Yes all this follows from the homogeneity and isotropy of the universe on large enough scales.

Quote:

any thoughts about all of this, super appreciated.
thank you,
Charlie :)
Hope this helps. My answer is deliberately repetitious since I think there is one underlying concept that answers all the questions.
• 04-14-2019, 04:16 AM
chas53
Re: Universe size questions
thank you nFA.
you are super awesome and kind.
Peace and all good things,
Charlie
• 04-14-2019, 06:29 PM
chas53
Re: Universe size questions
nFA,
46 divided by 13 is ~ 3.5.
would you say I'm right if I say that that means that the Universe (our Observable Universe),
has expanded ~3.5 times since the source of the CMBR emitted the photons?
• 04-14-2019, 06:33 PM
not_Fritz_Argelander
Re: Universe size questions
Quote:

Originally Posted by chas53
nFA,
46 divided by 13 is ~ 3.5.
would you say I'm right if I say that that means that the Universe (our Observable Universe),
has expanded ~3.5 times since the source of the CMBR emitted the photons?

Yes. That's the right idea roughly. But only roughly. There is a formula:

1 + z = a(now)/a(then) where a is the size scale of the universe. Precisely the z for the CMB is about 1000 since the photons were emitted at 3000 K and are now at 3 K.

I think this will take some additional untangling?

The 3.5 factor is due to acceleration in the expansion. Let me check.
• 04-14-2019, 06:38 PM
chas53
Re: Universe size questions
awesome. :)
thank you.

nFA,

"so the Universe has an infinite number of Observable Universes that
are outside of our ability to observe them.
and (and this is a key and big question), those UN- observable (to us), Universes
have a trillion galaxies in them, just like we do."

I know you said yes to this.
but, I just want to say:
isn't it soooooo astonishing to think this?
an infinite number of UN- Observable Universes with a trillion galaxies in them?
and, how many people do you think know this?
I would guess, not many.
any thoughts on my musings?
• 04-14-2019, 07:10 PM
not_Fritz_Argelander
Re: Universe size questions
Quote:

Originally Posted by chas53
nFA,
46 divided by 13 is ~ 3.5.
would you say I'm right if I say that that means that the Universe (our Observable Universe),
has expanded ~3.5 times since the source of the CMBR emitted the photons?

Quote:

Originally Posted by not_Fritz_Argelander
Yes. That's the right idea roughly. But only roughly. There is a formula:

1 + z = a(now)/a(then) where a is the size scale of the universe. Precisely the z for the CMB is about 1000 since the photons were emitted at 3000 K and are now at 3 K.

I think this will take some additional untangling?

The 3.5 factor is due to acceleration in the expansion. Let me check.

The resolution of the problem, this contradiction between 3.5 factor by one method and 1000 by another, is realizing that that the 13 vs 46 Bly is not a meter stick. It is a calculation done in a comoving reference frame so it isn't quite the same as a linear expansion factor. The redshift method, where the wavelength of the CMB provides a handy meter stick is a better representation of linear distance measure.

So in comoving coordinates the universe has expanded by a factor of 3.5, but measured with a meter stick it has expanded by a factor of 1000 since the CMB was emitted.
• 04-14-2019, 07:12 PM
chas53
Re: Universe size questions
Quote:

Originally Posted by not_Fritz_Argelander
The resolution of the problem, this contradiction between 3.5 factor by one method and 1000 by another, is realizing that that the 13 vs 46 Bly is not a meter stick. It is a calculation done in a comoving reference frame so it isn't quite the same as a linear expansion factor. The redshift method, where the wavelength of the CMB provides a handy meter stick is a better representation of linear distance measure.

So in comoving coordinates the universe has expanded by a factor of 3.5, but measured with a meter stick it has expanded by a factor of 1000 since the CMB was emitted.

wow!!!
fantastic amazing.
thank you nFA!!!
• 04-14-2019, 07:17 PM
not_Fritz_Argelander
Re: Universe size questions
Quote:

Originally Posted by chas53
awesome. :)
thank you.

nFA,

"so the Universe has an infinite number of Observable Universes that
are outside of our ability to observe them.
and (and this is a key and big question), those UN- observable (to us), Universes
have a trillion galaxies in them, just like we do."

Speaking loosely yes. I'm a little uncomfortable with "other observable universes" or "universes observable to others" since the language gets close to multiverses. Rather there is our Observable Universe and the Nonobservable Universe. Other observers have their observable universes which may or may not overlap with ours. Each sees the same observable universe in terms of number of galaxies, temperature of CMB, mean density.

This is different from the multiverse vision in which the physical constants take on different values, the speed of light, charge on the electron, Planck's constant can have different values and the conservation laws of physics can no longer apply to the universe as a whole.

Quote:

I know you said yes to this.
but, I just want to say:
isn't it soooooo astonishing to think this?
an infinite number of UN- Observable Universes with a trillion galaxies in them?
and, how many people do you think know this?
I would guess, not many.
any thoughts on my musings?
I think that if you look in ancient religious and philosophical literature you will find many sources that echo this vision. But that discussion would derail the thread. Much literature, but few people. :)
• 04-14-2019, 07:22 PM
not_Fritz_Argelander
Re: Universe size questions
Quote:

Originally Posted by chas53
wow!!!
fantastic amazing.
thank you nFA!!!

Welcome. So the 3.5 factor really represents the additional expansion due to Dark Energy I believe and not the total expansion.
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