Intuition for CMB: Exploring Radiation & Expansion

[Mr Davis 97]

I am having conceptual issues about what the CMB actually is. I read that it is the remnant radiation at the moment of recombination roughly 380,000 years after the Big Bang. But what about this statement implies that we would be able to observe this radiation today? To put it naively, when I look up into the night sky, where exactly is this radiation coming from? As a parallel, when I see a star I know that I am seeing the light that it emitted possibly millions of years ago relative to me. How can I really make a parallel statement when it comes to the CMB radiation, when looking out into the void of deep space, when at the time that radiation was present everywhere in this universe at the same time? I guess my main concern is how radiation encompassing the entire universe changes over time in an expanding universe. Also, if at the time 380,000 years after the big bang, this light was free to roam all over the universe, why can't it emitting from inside my house or a patch of land nearby? Why do have to look into deep space?

 

[Bandersnatch]

To put it naively, when I look up into the night sky, where exactly is this radiation coming from?

It comes from the gas that filled the universe back then. Imagine you're immersed in a large body of water, and then every particle of H2O sends a sound or light signal (somehow, magically) in every direction. You'll see it coming at you from all directions, with signals from progressively further afar arriving progressively later.
Same with CMBR, only instead of water you have plasma emitting light, which then travels in every direction.

I guess my main concern is how radiation encompassing the entire universe changes over time in an expanding universe.

It doesn't 'encompass' the universe, it fills the universe. I'm not sure if how it changes is relevant - in any case, it gets redshifted.

Also, if at the time 380,000 years after the big bang, this light was free to roam all over the universe, why can't it emitting from inside my house or a patch of land nearby? Why do have to look into deep space?

Because it's not being emitted now in your house. It was emitted long ago, and to see it you need to look in directions where there are no opaque objects obstructing its path.

 

[Mr Davis 97]

It comes from the gas that filled the universe back then. Imagine you're immersed in a large body of water, and then every particle of H2O sends a sound or light signal (somehow, magically) in every direction. You'll see it coming at you from all directions, with signals from progressively further afar arriving progressively later.
Same with CMBR, only instead of water you have plasma emitting light, which then travels in every direction.

So with this pool analogy, would it be at all correct to say that we (the Earth) in the pool have already absorbed the light signals in the near vicinity, and that what the CMB is is the light was emitted from water far away from us that we are just now receiving?

 

[Bandersnatch]

So with this pool analogy, would it be at all correct to say that we (the Earth) in the pool have already absorbed the light signals in the near vicinity, and that what the CMB is is the light was emitted from water far away from us that we are just now receiving?

Yes. All the radiation emitted nearby has already passed us by, or impinged on some disinterested dinosaurs etc. We're only receiving the light emitted from sufficiently far away, so that it had to travel for the past 13.7 Gy to get here. Tomorrow we'll see light from even further away.

 

[Mr Davis 97]

Yes. All the radiation emitted nearby has already passed us by, or impinged on some disinterested dinosaurs etc. We're only receiving the light emitted from sufficiently far away, so that it had to travel for the past 13.7 Gy to get here. Tomorrow we'll see light from even further away.

So doesn't that imply that the CMB radiation that scientists measure in 1,000,000 years (if we're still here) will be different than the radiation we are measuring now? If that is so then what makes the CMB useful if it can change over time?

Also, what makes the CMB useful if we only have partial information regarding the measurement, since much of that light we have already absorbed?

 

[Bandersnatch]

The valuable thing about CMBR is that its initial characteristics are known due to how plasma physics works, and there's just one major thing that affects these characteristics as it travels - namely the expansion. So if some future scientist observes CMBR, she'll know exactly how much expansion has happened in the intervening time by measuring how much the radiation differs from the calculated initial state.

Furthermore, expansion affects just one of many characteristics of this radiation (wavelength). So, even as CMBR will differ in this one aspect at various times in the history of the universe, the remaining characteristics will still be visible in an unchanged state, giving the same information of the state of the universe at its emission.

 

[Mr Davis 97]

The valuable thing about CMBR is that its initial characteristics are known due to how plasma physics works, and there's just one major thing that affects these characteristics as it travels - namely the expansion. So if some future scientist observes CMBR, she'll know exactly how much expansion has happened in the intervening time by measuring how much the radiation differs from the calculated initial state.

Furthermore, expansion affects just one of many characteristics of this radiation (wavelength). So, even as CMBR will differ in this one aspect at various times in the history of the universe, the remaining characteristics will still be visible in an unchanged state, giving the same information of the state of the universe at its emission.

One more question. Coming from the pool analogy, since the pool is finite, all of that initial radiation would reach us. How does this differ from the actual universe? Is it possible that all of the radiation from recombination would eventually reach us? Or does the expansion of the universe somehow preclude that?

 

[Bandersnatch]

1. It's possible that the universe is infinite,
2. Or, that it is finite and closed (instead of a pool, imagine a planet-wide ocean), in which case it'd be in principle possible for CMBR to 'circumnavigate' the universe.
3. Regardless of the above considerations, in an accelerating universe there exists a maximum distance from beyond which no signal can reach us (i.e. the cosmological event horizon). This simply means that light emitted from sufficiently far away won't be able to make headway against the (eventually) exponentially expanding universe. The closer the light was emitted to this horizon, the longer it'll take it to reach us, with light emitted at the horizon reaching us after infinite time. Even though the horizon is at a finite distance, it takes infinite time for light to get to us from it.

Since we don't know whether the universe is infinite of not, but it does appear that we're living in an accelerating one, this means that we're limited to a patch of the universe from which we may observe CMBR. We'll be always able to observe it, albeit in an ever-increasingly redshifted and dimmed form, as CMBR from even just a bit farther away will take ever longer and longer to get here.