What happened to the range of frequencies after recombination?

[artis]

So as the hot plasma of a universe expanded it grew colder and at "recombination" it reached a state where it isn't opaque to em radiation any more unlike dense plasma which is.
So we say this is the moment the CMB started.
But this got me thinking, after recombination the matter was still some 3000 K hot and I would assume it did not cool momentarily but it was a gradual process. The universe still being much smaller than today and rather dense , why don't we see a range of frequencies as the CMB?
In other words the matter after recombination was still giving off radiation for some time (and continues to this day) so shouldn't there be a wide range of frequencies seen as the CMB?

Say for example the radiation given off after recombination was of ever lower frequency adding redshift due to expansion it could now be in the Mhz range and some maybe even lower?

 

[Vanadium 50]

why don't we see a range of frequencies as the CMB?

We do.

 

[artis]

@Vanadium 50 so are you saying that because of the large redshift that the EM has encountered it is all now down right next to the very limit of temp. for EM radiation?
So basically in this graph there is everything starting from what was once very hot (right after recombination) down to what was once "not that hot" long after recombination?

Would the logic go like radiation emitted long after recombination had a lower blackbody temp originally at emission so even though it was emitted later due to it's original lower frequency it now after expansion of space is right next to the radiation that was once of much higher frequency but endured a longer/higher redshift ?

 

[Vanadium 50]

You asksed why it wasn't a range of frequencies. I showed that it in fact is. No more no less.

 

[mathman]

What is your idea for time scale for "long after recombination" ?

 

[artis]

@mathman Well approximately a time scale until which the universe was still a rather hot and homogeneously dense place.
I don't know the exact time that such a condition lasted but I assume it wasn't mere few years.

 

[Ibix]

Is the question you are asking not "why do we see a single frequency in the CMB", but rather "why doesn't the CMB have a range of temperatures corresponding to the primordial plasma at various stages of cooling"?

 

[artis]

@Ibix yes that is what I'm asking, you happened to put it in better writing.

 

[mathman]

From 370000 yrs to 1 billion yrs the temperature went from 4000 K to 60 K.

 

[256bits]

In other words the matter after recombination was still giving off radiation for some time (and continues to this day) so shouldn't there be a wide range of frequencies seen as the CMB?

That would be an incorrect statement of what recombination is all about.
Recombination is when the electrons and hydrogen nuclei combined to from atoms, and the photons hence decoupled from the matter. It wasn't a distinct event in time but lasted for some 60,000 years or so.
Once the ratio of free electrons to protons was diminished enough, the photons could travel large distances (such as from that time to now ) without encountering any matter, or if they did, the neutral hydrogen atom was not re-ionized due to the decrease in energy of the photon due to expansion.

The CMB is from the hot plasma from that time.
The photons produced after recombination are from different processes.

 

[artis]

@256bits I get your point but why would my statement be wrong?
Matter radiates, my room heater for example does it as we speak.

The way I see it is that the CMB started after recombination or should we say that when the conditions were right for the photons to decouple, but where did it end? As far as I know it did not end instead the matter kept on radiating only as it cooled down the emitted radiation was and is of decreasing frequency, also expansion of space decreases it's frequency/redshifts it.

 

[mathman]

@256bits I get your point but why would my statement be wrong?
Matter radiates, my room heater for example does it as we speak.

The way I see it is that the CMB started after recombination or should we say that when the conditions were right for the photons to decouple, but where did it end? As far as I know it did not end instead the matter kept on radiating only as it cooled down the emitted radiation was and is of decreasing frequency, also expansion of space decreases it's frequency/redshifts it.

Qualitatively, you are right. Quantitatively, emissions after recombination radiation is much less and appears mainly as starlight.

 

[256bits]

Matter radiates, my room heater for example does it as we speak.

That's a condensed matter state.
I don't think that in the early stages after recombination the gas had enough time to be clumped, but instead was transparent to the CMB photons.

 

[JimJCW]

But this got me thinking, after recombination the matter was still some 3000 K hot and I would assume it did not cool momentarily but it was a gradual process. The universe still being much smaller than today and rather dense , why don't we see a range of frequencies as the CMB?

My understanding is that before the recombination era, the CMB photons already existed (approximately one billion photons for every nucleon). During the recombination era, these photons were freed to fill the space with a blackbody photon gas, the CMB, at a temperature of about 3000 K. It was a one-time event, not a continuous photon-emission process by hot matter.

 

[timmdeeg]

My understanding is that before the recombination era, the CMB photons already existed (approximately one billion photons for every nucleon). During the recombination era, these photons were freed to fill the space with a blackbody photon gas, the CMB, at a temperature of about 3000 K. It was a one-time event, not a continuous photon-emission process by hot matter.

Yes it was a "one time event" because after recombination the then neutral hydrogen atoms didn't radiate.

 

[PeterDonis]

My understanding is that before the recombination era, the CMB photons already existed

Photons don't have distinct identitities that can be tracked over time, so this statement, as you state it, is incorrect.

A correct statement would be that the universe was filled with radiation before the recombination era; the radiation was just constantly interacting with matter. The recombination era was when that constant interaction between radiation and matter stopped: the universe became transparent to radiation and the radiation started propagating freely instead of constantly interacting with matter. What we call the CMB is the freely propagating radiation from that time. But there is no way to identify particular CMB photons as having "existed" before recombination.

It was a one-time event, not a continuous photon-emission process by hot matter.

More precisely, the emission of the CMB we see today was a "last emission" of radiation by matter as matter became transparent to radiation. Before recombination, matter was constantly emitting radiation, which was immediately absorbed again by matter (that's the constant interaction between radiation and matter that I described above).

 

[JimJCW]

More precisely, the emission of the CMB we see today was a "last emission" of radiation by matter as matter became transparent to radiation. Before recombination, matter was constantly emitting radiation, which was immediately absorbed again by matter (that's the constant interaction between radiation and matter that I described above).

The photon-to-baryon ratio in the universe is about billion to one. How did the “last emission” emit so many CMB photons during the recombination period?

 

[256bits]

The photon-to-baryon ratio in the universe is about billion to one. How did the “last emission” emit so many CMB photons during the recombination period?

Semantics perhaps, If one considers photon/electron scattering as photon emission due to the exchange of energy .

 

[PeterDonis]

The photon-to-baryon ratio in the universe is about billion to one.

That was true long before the time of CMB emission. That ratio was established when the universe cooled down enough for quark-antiquark pairs to annihilate each other and produce photons, leaving an excess of about one part per billion of quarks to form the baryons in our current universe.

How did the “last emission” emit so many CMB photons during the recombination period?

It didn't have to emit any more photons than already existed. I have already explained that the universe was filled with radiation before the time of CMB emission; all that happened at the time of CMB emission was that that radiation stopped interacting with matter and began propagating freely through the universe.

 

[PeterDonis]

Semantics perhaps, If one considers photon/electron scattering as photon emission due to the exchange of energy .

What happened at recombination was not photon-electron scattering (that is Compton scattering, which is dominant at much higher temperatures; at the temperature at recombination, a few thousand degrees K, Compton scattering is negligible). Recombination was photons being emitted by electrons as they combined with protons to form hydrogen atoms. Although it is often talked about as though it were a single, instantaneous event, in fact recombination was a process that took time; basically, the fraction of hydrogen atoms formed by recombination (electrons combining with protons and emitting photons) that survived without being broken apart again by absorbing photons went from essentially zero to essentially one over a fairly short interval of time ("short" in cosmological terms, anyway) as the universe expanded and the average temperature decreased below the ionization energy of hydrogen.

 

[PeterDonis]

why don't we see a range of frequencies as the CMB?

Because of this:

the fraction of hydrogen atoms formed by recombination (electrons combining with protons and emitting photons) that survived without being broken apart again by absorbing photons went from essentially zero to essentially one over a fairly short interval of time ("short" in cosmological terms, anyway)

In other words, the interval of time during which recombination happened (i.e., during which the fraction of hydrogen atoms that were formed and not broken apart again went from essentially zero to essentially one) was so short, by cosmological standards, that we cannot observe any "spread" in the temperature of the CMB.

 

[PeterDonis]

the matter after recombination was still giving off radiation for some time

No, it wasn't. The matter after recombination was neutral hydrogen atoms in their ground state. Those atoms could not emit any radiation because that would have required them to go to a lower energy state, and there were no lower energy states available (that's what "ground state" means).

Much later, the hydrogen atoms combined into molecules, which allowed further emission of radiation (since the energy of an H2 molecule in its ground state is less than twice the energy of an H atom in its ground state), and then matter started clumping into larger bound states, which in turn allowed more emission of radiation.

 

[256bits]

Because of this:In other words, the interval of time during which recombination happened (i.e., during which the fraction of hydrogen atoms that were formed and not broken apart again went from essentially zero to essentially one) was so short, by cosmological standards, that we cannot observe any "spread" in the temperature of the CMB.

It has to have a thickness of last scattering comparable to much less than the measured μK anistopy of the hot and cold regions of the cmb.

 

[Drakkith]

No, it wasn't. The matter after recombination was neutral hydrogen atoms in their ground state. Those atoms could not emit any radiation because that would have required them to go to a lower energy state, and there were no lower energy states available (that's what "ground state" means).

Much later, the hydrogen atoms combined into molecules, which allowed further emission of radiation (since the energy of an H2 molecule in its ground state is less than twice the energy of an H atom in its ground state), and then matter started clumping into larger bound states, which in turn allowed more emission of radiation.

What prevents collisions between hydrogen atoms from exciting each other (1s to 2s for example) and then radiating that energy away as the atoms transition back to the ground state? Or did this actually happen, but have negligible effect on things?

 

[PeterDonis]

Or did this actually happen, but have negligible effect on things?

This is what I would think, yes.

 

[JimJCW]

Recombination was photons being emitted by electrons as they combined with protons to form hydrogen atoms.

While the recombination was photons being emitted by electrons as they combined with protons to form hydrogen atoms, these emitted photons had an energy of 13.6 eV and were not directly related to the CMB spectrum at the time (photon energy near the peak of the spectrum was about 1 eV). Some people are talking about the hydrogen line, for example, https://www.physicsforums.com/file:///C%3A/Users/jcwang01/Favorites/Downloads/astro-ph0502571.pdf. Maybe we just find the common ground of the following statements:

Post #14:

My understanding is that before the recombination era, the CMB photons already existed (approximately one billion photons for every nucleon). During the recombination era, these photons were freed to fill the space with a blackbody photon gas, the CMB, at a temperature of about 3000 K.​

Post #19:

I have already explained that the universe was filled with radiation before the time of CMB emission; all that happened at the time of CMB emission was that that radiation stopped interacting with matter and began propagating freely through the universe.​

 

[PeterDonis]

While the recombination was photons being emitted by electrons as they combined with protons to form hydrogen atoms, these emitted photons had an energy of 13.6 eV and were not directly related to the CMB spectrum at the time (photon energy near the peak of the spectrum was about 1 eV).

The peak of the spectrum at recombination is not expected to be equal to the ionization energy of hydrogen from the ground state, because the hydrogen atoms formed at recombination were not formed immediately in the ground state. Also, the actual process was more complicated than my simple statement that you quoted. The complications do not affect the basic answer to the OP's question in this thread, which is why I did not go into them.

The Wikipedia article on recombination gives a decent overview:

https://en.wikipedia.org/wiki/Recombination_(cosmology)

 

[PeterDonis]

Maybe we just find the common ground of the following statements:

I already rephrased your statement to address the objection I made, which is that photons do not have distinct identities so you can't pick out particular "CMB photons" that existed before recombination and match them up with CMB photons after recombination. That is why the more general term "radiation" is a better term. I have already agreed that the universe was filled with radiation before recombination.