How Can We See Light from the Early Universe?

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Astronomers have detected light from the early universe, specifically from around 500 million years after the Big Bang, raising questions about how we perceive this light. The discussion highlights that as the universe expands, the fabric of space stretches, which affects how light travels to us from distant galaxies. It is emphasized that while light from these galaxies has indeed traveled vast distances, the galaxies themselves were much closer when the light was emitted, complicating our understanding of cosmic distances and ages. The concept of redshift is crucial, as it indicates that many galaxies are receding faster than the speed of light, yet we can still observe their light due to the stretching of space. Ultimately, the universe's expansion allows us to see light from various epochs, despite the challenges in interpreting this information.
  • #31
Theoretical – Can the CMB go out?

Let’s forget about inflation and just say that the universe is finite, and the edge of the observable universe is 46.5 billion light-years away. And this is all there is.

In this scenario: Will a coming civilization in another galaxy have the risk of the CMB going out some day in x billion years from now (i.e. there are no more photons to 'deliver' from the CMB)?

And is there a possibility to calculate when this will happen in the theoretical scenario above?

(And will these poor 'losers' have any chance to really understand BB...? :smile:)
 
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  • #32
DevilsAvocado said:
Theoretical – Can the CMB go out?

Let’s forget about inflation and just say that the universe is finite, and the edge of the observable universe is 46.5 billion light-years away. And this is all there is.
Well, if that were literally true, then we would suddenly stop receiving CMB photons tomorrow (the part of the CMB that we observe is always coming from parts of the surface further and further away).
 
  • #33
Chalnoth said:
... we would suddenly stop receiving CMB photons tomorrow ...

Thanks, but why tomorrow (and not today or x billion years from now)?
 
  • #34
DevilsAvocado said:
And the answer is: Since the CMB is everywhere, we look further and further out in the CMB for every ticking moment, and the CMB will never end (I think...? :rolleyes:).

It will fade out eventually.
 
  • #35
S.Vasojevic said:
It will fade out eventually.

You mean like the 'light' will be 'stretched out' towards 0 K and 'zeroed'? Or there is no more photons to 'deliver' (even in a infinite universe)?
 
  • #36
Yes, it will be 'stretched' towards infinite wavelength.
 
  • #37
S.Vasojevic said:
Yes, it will be 'stretched' towards infinite wavelength.

Okay, thanks.

Just another thought: I guess this must be 'compatible' with the 'energy conservation law', but how does it work? Is the same energy still there at infinite wavelength, and if you 'reverse' the expansion, it will heat up again?

(Really stupid question since Chalnoth did answer this in #29, but I’ll just have to check... :redface:)
 
  • #38
Actually, it will never reach infinite wavelength, but it will be redshifted towards oblivion.

Yes, same energy is still there, and it would heat up if you reverse expansion, I think.
 
  • #39
S.Vasojevic said:
... same energy is still there ...

Okay, thanks.
 
  • #40
DevilsAvocado said:
Thanks, but why tomorrow (and not today or x billion years from now)?
If what we are seeing now were the edge (as in, nothing at all beyond the CMB we see today), then it would be gone tomorrow.
 
  • #41
Chalnoth said:
If what we are seeing now were the edge ...

Okay thanks.

But... (and now I’m taking a big risk to get lost in pure cosmic confusion) isn’t the Hubble volume 13.8 billion light years...? Wouldn’t there be a 'buffer' (like 46.5 - 13.8 = 32.7 bly) ...?

Edit: Of course this depends on the expansion rate (like DE influence), right?
Edit2: A finite universe could (must?) still expand, right?
 
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  • #42
DevilsAvocado said:
Okay thanks.

But... (and now I’m taking a big risk to get lost in pure cosmic confusion) isn’t the Hubble volume 13.8 billion light years...? Wouldn’t there be a 'buffer' (like 46.5 - 13.8 = 32.7 bly) ...?

Edit: Of course this depends on the expansion rate (like DE influence), right?
Edit2: A finite universe could (must?) still expand, right?
Well, basically, the matter we see right now as the CMB was, when the light was emitted, undergoing the phase change from a plasma to a gas. I suppose upon further reflection, this phase change probably will take a bit longer than a day. But once the phase change finishes, since you postulated no matter beyond the CMB, there simply wouldn't be any more matter out there to emit new CMB photons, and it would wink out.

Now, let me put a little caveat out there that if there actually were no matter beyond the current CMB, then that should have really significant effects upon the CMB that we see.
 
  • #43
Chalnoth said:
... if there actually were no matter beyond the current CMB, then that should have really significant effects upon the CMB that we see.

Aha! Very interesting!
So the CMB in fact tells us more about the (rest of) universe (> observable universe) then any IA Supernova!? A m a z i n g.
 
  • #44
DevilsAvocado said:
Aha! Very interesting!
So the CMB in fact tells us more about the (rest of) universe (> observable universe) then any IA Supernova!? A m a z i n g.
Well, that shouldn't be so surprising...the CMB is much much further away than supernovae are.
 
  • #45
Chalnoth said:
Well, that shouldn't be so surprising...the CMB is much much further away than supernovae are.

(And here comes the regular layman-cosmic-confusion! :smile:)

So the edge of the observable universe, 46.5 billion light-years away, is actually where the CMB is right 'now'? But the light was emitted (13.8 byr - 377,000 yr =) 13,799,623,000 years ago, right?

And the most distant Supernova is 28 billion light-years away? Like in this picture:
300px-Embedded_LambdaCDM_geometry.png


Question (if this is correct): So the distance to the CMB is 'calculated', like the calculated position of the Supernova (orange line above)?

(OMG, stupid stupid stupid. How can the CMB be 'visible' 46.5 bly away in 13.8 billion years?? But I have to ask... :redface:)


Edit: (Albert Laymanstein is working overtime...) IF any of the above is correct, wouldn’t that mean we could get a 'hard proof' of inflation? The CMB photons was emitted 377,00 years after BB, and the IA Supernova photons was emitted 700 million years(?) after BB. That’s a difference of ~ 699 million light-years, which have now grown to (46.5 - 28 =) 18.5 billion light-years! = Inflation must be true! (or?)
 
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  • #46
SV gets it. You never leave a photon sphere, just redshift into obscurity.
 
  • #47
DevilsAvocado said:
So the edge of the observable universe, 46.5 billion light-years away, is actually where the CMB is right 'now'? But the light was emitted (13.8 byr - 377,000 yr =) 13,799,623,000 years ago, right?

And the most distant Supernova is 28 billion light-years away? Like in this picture:
300px-Embedded_LambdaCDM_geometry.png


Question (if this is correct): So the distance to the CMB is 'calculated', like the calculated position of the Supernova (orange line above)?
Well, you don't need to even go into this much detail. It is plenty sufficient to state that at the time the CMB was emitted, our universe was uniform to one part in one hundred thousand. That's uniform enough that stars simply were not possible, let alone supernovae. So all of the CMB photons that we see were already on their way by the time supernovae started to go off.
 
  • #48
Chronos said:
SV gets it. You never leave a photon sphere, just redshift into obscurity.
Well, that's the way it's going to happen in reality, because there's a lot more stuff out there beyond the CMB we see today. So as the part that we see today as the CMB becomes transparent and stops emitting new photons, there's always a new piece of the CMB just beyond that that is still emitting.
 
  • #49
Chalnoth said:
... That's uniform enough that stars simply were not possible, let alone supernovae. So all of the CMB photons that we see were already on their way by the time supernovae started to go off.

Thanks Chalnoth, for the elegant clarification.

And from this we can derive that 'working overtime' won’t do it if 'Albert Laymanstein' is not using his brain :rolleyes: – the inflation (of course) must have happen between BB and Recombination to get the uniform CMB (like stretching a wrinkled sheet), right...?

One thing still puzzles me: How can Saul Perlmutter and the other guys at the http://en.wikipedia.org/wiki/Supernova_Cosmology_Project" prove that the (DE) expansion is accelerating now, when the information is ~13 billion years old...? I get that is proven from the redshift, but how can one say it’s starting 'now', and not x billion years ago?


Edit: And I’m going to answer the question myself – If it started x billion years ago the redshift must be much much bigger! (And I will start thinking more and type less, promise... :biggrin:)
 
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  • #50
Chalnoth said:
... there's a lot more stuff out there beyond the CMB we see today ...

That’s what excited me in post #43. There are 'signs' in the current CMB of 'things' outside the observable universe, or?
 
  • #51
Chronos said:
... You never leave a photon sphere ...

But the photons leave us, don’t they? (Otherwise, where is the 'freeze button'? :rolleyes:)

Edit: And now things get complicated. Check out the link for 'photon sphere' – "A photon sphere is a spherical surface round a non-rotating black hole"... Mamma Mia! :biggrin:
 
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  • #52
DevilsAvocado said:
Thanks Chalnoth, for the elegant clarification.

And from this we can derive that 'working overtime' won’t do it if 'Albert Laymanstein' is not using his brain :rolleyes: – the inflation (of course) must have happen between BB and Recombination to get the uniform CMB (like stretching a wrinkled sheet), right...?
The big bang theory, without inflation, correctly predicts the amount of light elements that we measure. This means that it's basically correct, without modification, to very early times. Inflation proposes some changes to what happens at even earlier times.

It's not useful, I don't think, to talk about the "big bang" as if it were a singular event that spawned our universe, because that just adds confusion to the fact that the big bang theory describes what happens at later times, and has nothing at all to say about what happened at the earliest of times (or rather, it has some things in the theory, but we know they're completely wrong).

Instead, what we know is this: when we look at the past of our universe, the big bang theory describes things correctly back to a certain point. Before that, inflation describes things correctly. But we don't know how inflation started (other than it had to begin somehow). Perhaps if we discover precisely what inflation was, that theory will automatically come along with a method of generating an inflating patch, but we don't yet know.

DevilsAvocado said:
One thing still puzzles me: How can Saul Perlmutter and the other guys at the http://en.wikipedia.org/wiki/Supernova_Cosmology_Project" prove that the (DE) expansion is accelerating now, when the information is ~13 billion years old...? I get that is proven from the redshift, but how can one say it’s starting 'now', and not x billion years ago?
Because the expansion since then affects the relationship between redshift and brightness. Basically, distant supernovae are too dim compared to their redshifts.
 
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  • #53
DevilsAvocado said:
That’s what excited me in post #43. There are 'signs' in the current CMB of 'things' outside the observable universe, or?
Very very indirectly. Basically we expect that any sort of boundary can't be sudden: there's going to be some effects going on near the edge that would be detectable. So in a way, the fact that we see no significant deviations from smoothness and uniformity out to the limits of our vision (the CMB), we know that this smoothness and uniformity must continue for a while beyond the limits of our vision.
 
  • #54
Chalnoth said:
It's not useful, I don't think, to talk about the "big bang" as if it were a singular event that spawned our universe, because that just adds confusion to the fact that the big bang theory describes what happens at later times, and has nothing at all to say about what happened at the earliest of times (or rather, it has some things in the theory, but we know they're completely wrong).

I read somewhere that the expression "Big Bang" actually came from one of the "steady state” proponents (Fred Hoyle?), as a 'patronizing' joke... and then it became the official name of the theory. A more describing name is perhaps The Universe Evolution Theory...

Chalnoth said:
Instead, what we know is this: when we look at the past of our universe, the big bang theory describes things correctly back to a certain point. Before that, inflation describes things correctly. But we don't know how inflation started (other than it had to begin somehow). Perhaps if we discover precisely what inflation was, that theory will automatically come along with a method of generating an inflating patch, but we don't yet know.

Interesting, I watched Sean Carroll (Caltech) & Mark Trodden (UPenn) on Bloggingheads.tv, and in one section http://bloggingheads.tv/diavlogs/21709?in=23:39&out=40:25". At 27:00 Sean state – "You’re attempted to explain why the early universe is so special, by imagine it started out even more special …!"

There’s clearly 'some' work to do... :wink:

Chalnoth said:
Because the expansion since then affects the relationship between redshift and brightness. Basically, distant supernovae are too dim compared to their redshifts.

Thanks, for the explanation. I read that the photons actually arrive at a 'lower rate' (like a machinegun slowing down) due to the expansion of space. I guess this is causing the 'dusky effect'...

Thanks for taking the time.
 
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  • #55
Chalnoth said:
... there's going to be some effects going on near the edge that would be detectable ...

Fantastic. And I guess that Planck will 'reveal' even more on this topic. (I’m not lurking for 'insider info'! :wink:)
 
  • #56
DevilsAvocado said:
I read somewhere that the expression "Big Bang" actually came from one of the "steady state” proponents (Fred Hoyle?), as a 'patronizing' joke... and then it became the official name of the theory. A more describing name is perhaps The Universe Evolution Theory...
Yes, basically. It was a derogatory term for the theory. I don't really like "Universe Evolution" either, as Evolution is too closely associated with biological evolution, which is a completely unrelated process.

Perhaps "uniform expansion theory" or simply go by the names of the people that came up with the metric: "Friedman-Walker-Robinson theory," or FRW theory for short.

DevilsAvocado said:
Interesting, I watched Sean Carroll (Caltech) & Mark Trodden (UPenn) on Bloggingheads.tv, and in one section http://bloggingheads.tv/diavlogs/21709?in=23:39&out=40:25". At 27:00 Sean state – "You’re attempted to explain why the early universe is so special, by imagine it started out even more special …!"

There’s clearly 'some' work to do... :wink:
Most definitely. The fact that the earliest stage of our universe had such vastly lower entropy than our current stage really needs explaining. Sean's got some really good popular articles that go into why this is, such as this one:
http://www.scientificamerican.com/article.cfm?id=the-cosmic-origins-of-times-arrow

DevilsAvocado said:
Thanks, for the explanation. I read that the photons actually arrive at a 'lower rate' (like a machinegun slowing down) due to the expansion of space. I guess this is causing the 'dusky effect'...

Thanks for taking the time.
Well, another aspect of the redshift is time dilation. Basically, if it takes twice as long for the next peak of the electromagnetic wave to arrive, then it also takes twice as long for the next photon to arrive.
 
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  • #57
Chalnoth said:
Most of the stuff we see out there (stuff at very roughly redshift greater than one) is now and always has been receding faster than the speed of light, according to the simplest definition of recession velocity (the definition of recession velocity is actually rather arbitrary).

What is the simplest definition of recession velocity, and how it yields recession faster than light for z > 1, and why is Marcus saying that it is safer to say that it is true for z > 1.7 ?
 
  • #58
S.Vasojevic said:
What is the simplest definition of recession velocity, and how it yields recession faster than light for z > 1, and why is Marcus saying that it is safer to say that it is true for z > 1.7 ?
First, you construct a distance by imagining what would happen if we froze the universe's expansion right now, and timed some light signals between different places. Then you ask how rapidly this distance changes with time. That gives you a relative velocity.

And Marcus is just plugging in the numbers and showing that it's really only true for things around that far away. I was just pulling a number off the top of my head because I didn't remember exactly and didn't feel like looking it up.
 
  • #59
Thanks Chalnoth.
 

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