B Early Universe Radiation: Gamma to Microwave?

[Jimmy87]

Hi,

Never done much cosmology but reading around I have found several sources saying the early universe (pre roughly 300,000 years) the early universe was full of gamma radiation. Since the universe has expanded this gamma radiation has been red shifted into the microwave region. Other sources do not mention the specific type of EM radiation in the early universe but state the temperatures (like this https://en.wikipedia.org/wiki/Chronology_of_the_universe). If you use the peak wavelength equation then these temperatures correspond to to infrared around 300,000 years when light no longer interreacted with matter. Please could someone clarify the main correct theory for the type of radiation there was around 300,000 years. Is the first description of gamma red shifting into microwave correct?

Thanks

 

[PeroK]

Accoding to this, the temperature of the universe at 380,000 years was about 3000K:

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

The typical energy of a photon is given by $E_{mean} = 3k_B T$, where $k_B$ is the Bolzmann constant and $T$ is the temperature.

For $T = 3000K$ that gives $E = 0.774 eV$, which is infrared.

 

[Vanadium 50]

but reading around I have found several sources

Can you please provide them?

 

[Jimmy87]

Can you please provide them?

A Level Physics for OCR A Student Book (OCR A Level Sciences)
by Graham Bone, Nigel Saunders, Gurinder Chadha

I came to the exact conclusion PeroK did. I mean I haven't studied cosmology since high school but this is from my brother's A-Level textbook and the author has a doctorate in physics. The last paragraph says about gamma red shifting to microwave. I mean I know high school is simplified but surely this is just completely incorrect?

 

[PeroK]

View attachment 279838

I came to the exact conclusion PeroK did. I mean I haven't studied cosmology since high school but this is from my brother's A-Level textbook and the author has a doctorate in physics. The last paragraph says about gamma red shifting to microwave. I mean I know high school is simplified but surely this is just completely incorrect?

It is mistaken, yes. As the universe cooled, the typical photon energy reduced, but even in the visible spectrum, the photons would interact with hygrogen atoms trying to form. The gamma rays from the very early universe haven't survived.

Eventually, at about 380,000 years, the universe had expanded enough that photons had a good chance of evading interactions and relatively quickly the universe went from being opaque to visible light to being transparent to the now largely infrared radiation.

It is that infrared radiation that we can detect, although it has redshifted to microwave now. Radiation from before than time didn't survive as evidence of what the universe looked like. The light (in the shape of high energy photos) inevitably collided with and ionised hydrogen atoms, almost as quickly as they formed.

 

[Vanadium 50]

Well, now you know having a PhD doesn't make you immune to mistakes. The CMBR photons were, at the time of last scattering, more or less optical - certainly closer to optical than gamma rays.

 

[PeroK]

View attachment 279838

I came to the exact conclusion PeroK did. I mean I haven't studied cosmology since high school but this is from my brother's A-Level textbook and the author has a doctorate in physics. The last paragraph says about gamma red shifting to microwave. I mean I know high school is simplified but surely this is just completely incorrect?

Part of the problem may be the concept of teaching cosmology at A-level. Let's assume that whoever wrote that book knows at least as much cosmology as I do. But, it has to be packaged up for A-level students. The error may be one of simplification or omission, rather than actual misundestanding.

To explain the next level of detail, you need to go into hydrogen ionisation etc, which may be deemed beyond A-level. The author is then forced to a simplifying half-truth.

In my view, the A-level treatment of SR (Special Relativity) is much worse as it presents deep conceptual ideas badly - and gets students off on the wrong foot.

By contrast, the simplification above can easily be removed without any fundamental reworking of ideas - there's just another step in the process, another level of detail missing.

I wouldn't criticize the author in this case without knowing which simplifications were forced on the text.

 

[256bits]

The last paragraph says about gamma red shifting to microwave

The quote does not say the wavelength was that at re-combination,but for the young and extremely hot period of the universe, much earlier.

Note that the ionization energy of hydrogen is 13.6 eV - ultraviolet -
The majority of CBMR would had to have 'loose' energy to below that value to allow hydrogen atoms to persist.
So you are looking at something as you suspected - infrared, or around there, for peak wavelength.

 

[PeroK]

The quote does not say the wavelength was that at re-combination,but for the young and extremely hot period of the universe, much earlier.

Note that the ionization energy of hydrogen is 13.6 eV - ultraviolet -
The majority of CBMR would had to have 'loose' energy to below that value to allow hydrogen atoms to persist.
So you are looking at something as you suspected - infrared, or around there, for peak wavelength.

Yes, and then it takes yet more physics to explain why the universe remained opaque even when the mean photon energy dropped below $13.6 eV$ and decoupling occurred at much lower mean photon energy.

 

[256bits]

Yes, and then it takes yet more physics to explain why the universe remained opaque even when the mean photon energy dropped below $13.6 eV$ and decoupling occurred at much lower mean photon energy.

Exactly. Fascinating subject.

 

[Ibix]

Also, minor point, the pigeons were not trapped. Most sources euphemise it as "kicked out", but at least one matches my recollection of an interview I saw many years ago with Wilson. Not that it matters to the science.

 

[Jimmy87]

Yes, and then it takes yet more physics to explain why the universe remained opaque even when the mean photon energy dropped below $13.6 eV$ and decoupling occurred at much lower mean photon energy.

That is interesting. Why did it remain opaque even though the photon energies were below hydrogen ionisation energies?

 

[PeroK]

That is interesting. Why did it remain opaque even though the photon energies were below hydrogen ionisation energies?

The distribution of photon energies goes beyond the mean (obviously). A proportion of these higher energy photons continue to ionise the hydrogen atoms that form, producing enough free electrons and protons to interact with the lower-energy photons.

The mean energy needs to drop well below $13.6 eV$, so that the proportion of higher energy photons is small enough to ionise too little hydrogen to soak up the radiation.

See the Saha equation:

https://en.wikipedia.org/wiki/Saha_ionization_equation

 

[Jimmy87]

The distribution of photon energies goes beyond the mean (obviously). A proportion of these higher energy photons continue to ionise the hydrogen atoms that form, producing enough free electrons and protons to interact with the lower-energy photons.

The mean energy needs to drop well below $13.6 eV$, so that the proportion of higher energy photons is small enough to ionise too little hydrogen to soak up the radiation.

See the Saha equation:

https://en.wikipedia.org/wiki/Saha_ionization_equation

Great thanks. So the matter is opaque with high energy photons through ionisation but what kind of interactions make the matter still opaque between the low energy photons and free electrons/protons?

 

[PeroK]

Great thanks. So the matter is opaque with high energy photons through ionisation but what kind of interactions make the matter still opaque between the low energy photons and free electrons/protons?

That would be Compton scattering of photons off free electrons (and protons).

 

[Jimmy87]

Part of the problem may be the concept of teaching cosmology at A-level. Let's assume that whoever wrote that book knows at least as much cosmology as I do. But, it has to be packaged up for A-level students. The error may be one of simplification or omission, rather than actual misundestanding.

To explain the next level of detail, you need to go into hydrogen ionisation etc, which may be deemed beyond A-level. The author is then forced to a simplifying half-truth.

In my view, the A-level treatment of SR (Special Relativity) is much worse as it presents deep conceptual ideas badly - and gets students off on the wrong foot.

By contrast, the simplification above can easily be removed without any fundamental reworking of ideas - there's just another step in the process, another level of detail missing.

I wouldn't criticize the author in this case without knowing which simplifications were forced on the text.

I appreciate some concepts are difficult to pitch to A-Level without giving the full picture but since they need to know the surface of last scattering is 300,000 years after the Big Bang they can simply say the average EM radiation at this point is infrared which has now been redshifted to microwave. I don't understand any benefit of the approach they took. It introduces a theory that doesn't even exist. I don't see how the true picture is any more complicated when the required level is purely qualitative anyway.

I am now even more concerned having searched through some past exam papers that questions ask for the origin of the CMBR and the answer in the mark scheme is gamma photons in the early universe.

 

[PeroK]

I am now even more concerned having searched through some past exam papers that questions ask for the origin of the CMBR and the answer in the mark scheme is gamma photons in the early universe.

That's very A-level-like.

 

[PeterDonis]

this is from my brother's A-Level textbook

What book? Please give specific information.

 

[Jimmy87]

What book? Please give specific information.

Berkeman asked me to go back and insert a reference but for some reason I can't edit that post:

A Level Physics for OCR A Student Book (OCR A Level Sciences) Paperback
by Graham Bone , Nigel Saunders, Gurinder Chadha.

 

[PeterDonis]

surely this is just completely incorrect?

No. It does not say anywhere that the radiation was gamma rays at the time the universe became transparent to radiation (i.e., at about 380,000 years).

the temperature of the universe at 380,000 years was about 3000K

Yes, but the book being referred to does not say gamma rays were present at that time. It says they were present "when the Universe was young and extremely hot". It does not say when that was.

I don't understand any benefit of the approach they took. It introduces a theory that doesn't even exist.

This is not correct. The book, or at least the excerpt you give, is indeed leaving out a significant event (the universe becoming transparent to radiation, i.e., the "surface of last scattering" event), but what the book is saying is based on perfectly standard cosmology, a theory that certainly does exist. The book is describing what that theory is saying about a time much earlier than the time of last scattering: a time less than a second after the Big Bang, when the universe was filled with gamma rays. The theory then describes how the temperature of that radiation has changed from then to now.

In other words, the surface of last scattering did not produce radiation in a universe that didn't have any before; it just marks the point at which the universe became transparent to the radiation that was already present (because the hydrogen stopped being ionized and became neutral atoms). So the surface of last scattering does not explain the origin of the radiation.

I agree it would have been nice if the book had also mentioned the "last scattering" event at roughly the point where your excerpt comes from; but the fact that it didn't does not make what the book does say wrong.

 

[PeterDonis]

A Level Physics for OCR A Student Book (OCR A Level Sciences)

For readers who might be confused by the term "A Level" here, note that it does not mean what Physics Forums means by the "A" thread level. The "A" thread level here at PF assumes graduate-level (i.e., Masters or Doctorate) background knowledge. "A Level" in the textbook title above refers to the UK educational system's "A Level" qualification, which is required for university entrance, i.e., it is the equivalent of high school level in the US, and a high school level thread here at PF is level "B" (for "Basic").

 

[anorlunda]

I don't see any mystery here. With radiation exactly at the ionization energy, the probability of ionization events balances the probability of deionization events. As things cool, the probability of deionization increases and ionization decreases. It is not a binary choice, it is a proportional difference.

It will take time for (almost) all the free electrons and free photons to find each other and combine. One could define a half life where half the free particles combine to form atoms. The half life would be a function of the mean temperature of the radiation and the density of particles. The temperature and density are changing all the time. Calculating that half life is beyond me, but I wager that others could do it.

The transparency of the universe is a function of the number of free electrons, and it to is an analog quantity, not binary.

 

[Vanadium 50]

Part of the problem may be the concept of teaching cosmology at A-level.

True.

es, but the book being referred to does not say gamma rays were present at that time. It says they were present "when the Universe was young and extremely hot". It does not say when that was.

That is also true, but it makes the strong implication that this is immediately before the moment in question. For a textbook, I believe it is better to make things clear.

 

[Jimmy87]

True.
That is also true, but it makes the strong implication that this is immediately before the moment in question. For a textbook, I believe it is better to make things clear.

As I stated before PeterDonis' thread in post #16 it says this explicitly in the mark schemes for questions on the origin of the CMBR which supports the implication I outlined earlier (i.e. the CMBR is red shifted gamma radiation). Here is such a question:

And here is the exam board's official mark scheme for the question:

Reference: this exam questions was taken from: https://www.ocr.org.uk/qualifications/as-and-a-level/physics-a-h156-h556-from-2015/assessment/

Even in the guidance section it explicitly says 'allow gamma waves in the early universe are red shifted to microwaves'. This is why I don't see why they can't just re-write their course to say 'infrared radiation from 380,000 years has been redshifted to microwaves'. That seems no more complicated to me. Also I was flicking through the aforementioned textbook and they need to know all the key events after the big bang including the surface of last scattering at 380,000 years where they need to explicitly know that EM radiation no longer interacted with matter. So keeping this incorrect notion is in complete contradiction with the information they need to know about the surface of last scattering.

 

[PeterDonis]

it makes the strong implication that this is immediately before the moment in question

I don't see how, since the excerpt shown from the textbook in post #4 does not mention the surface of last scattering or the universe becoming transparent to radiation at all. The only reason the OP is confused is that he has read somewhere else that there was a surface of last scattering 380,000 years after the Big Bang. That's not the fault of the writers of the textbook.

In order to even judge the textbook, I would want to know whether they mention the surface of last scattering/recombination/universe becoming transparent to radiation elsewhere, and if so, how they relate it to what is stated in the excerpt given in the OP. Without that information I don't think we have enough data to judge.

 

[PeterDonis]

I don't see why they can't just re-write their course to say 'infrared radiation from 380,000 years has been redshifted to microwaves'.

Because they are trying to explain where, originally, the radiation comes from. The radiation does not originally come from 380,000 years after the Big Bang; it existed long before then. All that happened 380,000 years after the Big Bang is that the universe became transparent to radiation.

What would be incorrect would be if the textbook said that the radiation 380,000 years after the Big Bang was gamma radiation. But, as I have already explained, the textbook does not say that. That is an incorrect inference that you drew based on things you had read somewhere else besides the textbook.

 

[Jimmy87]

Because they are trying to explain where, originally, the radiation comes from. The radiation does not originally come from 380,000 years after the Big Bang; it existed long before then. All that happened 380,000 years after the Big Bang is that the universe became transparent to radiation.

What would be incorrect would be if the textbook said that the radiation 380,000 years after the Big Bang was gamma radiation. But, as I have already explained, the textbook does not say that. That is an incorrect inference that you drew based on things you had read somewhere else besides the textbook.

Ok now I am confused. From what PeroK and others said earlier I thought the radiation we see today as microwave radiation did come from the surface of last scattering (380,000 years ago). The early high energy radiation that existed in the very early universe did not make it to the surface of last scattering as I have understood it? So are you saying the early gamma radiation got red shifted to radiation present at decoupling? When you look at the CMB you are seeing the EM radiation at the time of decoupling - you are not looking at the high energy gamma radiation from the early universe. Did you not see my last post where the mark scheme gives the credit for CMB being 'early gamma radiation that has red shifted to microwave radiation'. I see zero ambiguity here.

What would be incorrect would be if the textbook said that the radiation 380,000 years after the Big Bang was gamma radiation. But, as I have already explained, the textbook does not say that. That is an incorrect inference that you drew based on things you had read somewhere else besides the textbook.
[/QUOTE]

So if you take 'early gamma radiation is red shifted to microwave radiation' and this excerpt from the textbook:

A Level Physics for OCR A Student Book (OCR A Level Sciences)
by Graham Bone, Nigel Saunders, Gurinder Chadha

How can these two compliment each other in any way?

 

[PeroK]

I don't think I'd lose any sleep over what it says about cosmology in an A-level text-book. Nothing's grossly wrong - it's just not the whole story. It's not like they're teaching creationism.

 

[PeterDonis]

The early high energy radiation that existed in the very early universe did not make it to the surface of last scattering as I have understood it?

You have understood it wrong. The radiation was there from the very early universe on, and has been redshifted ever since then, but the universe did not become transparent to it until the surface of last scattering. (Note the name: surface of last scattering. "Scattering" is not the same as "emission".)

 

[PeterDonis]

So if you take 'early gamma radiation is red shifted to microwave radiation' and this excerpt from the textbook:
View attachment 279895
A Level Physics for OCR A Student Book (OCR A Level Sciences)
by Graham Bone, Nigel Saunders, Gurinder Chadha

Yes, that does look inconsistent with what is said in the earlier excerpt you quoted. (And wrong; I would have said this time is when the universe became transparent to radiation.) Is this from earlier in the textbook than your previous excerpt, or later?

 

[Jimmy87]

[

You have understood it wrong. The radiation was there from the very early universe on, and has been redshifted ever since then, but the universe did not become transparent to it until the surface of last scattering. (Note the name: surface of last scattering. "Scattering" is not the same as "emission".)

Ok so PeroK said 'Eventually, at about 380,000 years, the universe had expanded enough that photons had a good chance of evading interactions and relatively quickly the universe went from being opaque to visible light to being transparent to the now largely infrared radiation.

It is that infrared radiation that we can detect, although it has redshifted to microwave now. Radiation from before than time didn't survive as evidence of what the universe looked like'

The radiation prior to 380,000 years didn't survive to see today as Perok said. That radiation from the early universe has already interacted with matter. If a gamma photon ionises a hydrogen atom you can't see it today as it has gone. An infrared photon from 380,000 years after the Big Bang has decoupled with matter and can still be seen today.

 

[PeterDonis]

I would have said this time is when the universe became transparent to radiation.

To expand on this: the significance of the universe becoming transparent to radiation is the information we can deduce from the properties of the radiation as we observe it today. Since before the time of last scattering, 380,000 years after the Big Bang, the universe was opaque to radiation, any information about the detailed state of the universe from before that time that could have been contained in the radiation got destroyed by the continual random interaction of the radiation with matter. But after the universe became transparent to radiation, the interaction between that radiation and the matter in the universe was negligible, so the information contained in the radiation has been preserved, essentially unchanged except for the redshift, from then to now. So we can directly deduce details of what the universe was like 380,000 years after the Big Bang from observed properties of the CMBR (for example, how much deviation there was from perfect homogeneity and isotropy), but we cannot directly deduce such details about the universe at any earlier time from the CMBR as we observe it now; we have to make indirect inferences to extrapolate further backward.

 

[Jimmy87]

Yes, that does look inconsistent with what is said in the earlier excerpt you quoted. (And wrong; I would have said this time is when the universe became transparent to radiation.) Is this from earlier in the textbook than your previous excerpt, or later?

Later. This is from page 392 and the OP on the CMB was page 390.

 

[PeterDonis]

The radiation prior to 380,000 years didn't survive to see today as Perok said. That radiation from the early universe has already interacted with matter. If a gamma photon ionises a hydrogen atom you can't see it today as it has gone. An infrared photon from 380,000 years after the Big Bang has decoupled with matter and can still be seen today.

You're assuming that individual photons have a distinct "identity" between interaction events with matter. That's not the case.

You are also assuming that all of the interaction events between radiation and matter prior to 380,000 years after the Big Bang were "absorption" events followed by "emission" events (where a "different photon" was emitted). That's not the case either. (Note again the word "scattering". "Scattering" does not mean "absorption" any more than it means "emission".)

 

[PeterDonis]

Later. This is from page 392 and the OP on the CMB was page 390.

Hm, that's pretty close together. I would expect at least some students to pick up on the inconsistency and ask questions about it.

 

[Jimmy87]

You're assuming that individual photons have a distinct "identity" between interaction events with matter. That's not the case.

You are also assuming that all of the interaction events between radiation and matter prior to 380,000 years after the Big Bang were "absorption" events followed by "emission" events (where a "different photon" was emitted). That's not the case either. (Note again the word "scattering". "Scattering" does not mean "absorption" any more than it means "emission".)

Right ok thanks. So to check I now have the correct picture. Gamma radiation from the very early universe has been red shifted to microwave radiation but the only information that can be extracted is the red shift from 380,000 years to today? So since it is not emission and absorption dominated are you saying that the cause of the radiation being predominantly infrared at 380,000 years rather than gamma is through red shift?

 

[PeterDonis]

Gamma radiation from the very early universe has been red shifted to microwave radiation but the only information that can be extracted is the red shift from 380,000 years to today?

No. We can extract more information from the CMBR we observe today than just its redshift. We can, and have, measured in detail the variation in the observed temperature of the CMBR all over the sky; small variations in that temperature (about 1 part in 100,000 or less) carry information about the detailed structure of the universe 380,000 years after the Big Bang. But those variations don't carry information about the detailed structure of the universe any earlier; the radiation was there earlier, and was redshifting earlier, but the continual interaction with matter prevented the radiation from preserving any information about the detailed structure of the universe.

 

[PeterDonis]

since it is not emission and absorption dominated are you saying that the cause of the radiation being predominantly infrared at 380,000 years rather than gamma is through red shift?

Yes. More precisely, redshift due to expansion of the universe. If the universe had not expanded by an enormous factor between the Big Bang and 380,000 years after the Big Bang, the radiation would not have changed from gamma radiation just after the Big Bang to infrared at 380,000 years. Emission, absorption, and scattering in a non-expanding universe (more precisely, a non-expanding universe in thermal equilibrium, as the early universe was) would not cause any redshift.

 

[Vanadium 50]

The text talks about gamma rays. The photons we see in CMBR were never gamma ways. They were, however, once visible or near-visible. I think that the book is misleading in that regard.

 

[Vanadium 50]

Let me give you a similar thing in another context: "The Apache were known as fierce warriors. Crazy Horse was victorious against the US Army at the Battle of the Little Bighorn." Both sentences are true, and one might sensibly infer that Crazy Horse was an Apache. But he wasn't.

 

[PeterDonis]

The photons we see in CMBR were never gamma ways.

As I have already pointed out in response to the OP, photons don't really have distinct identities to begin with, so "the photons we see in CMBR" is not even a well-defined concept. And even if we allow ourselves, as a heuristic approximation, to talk about particular "photons" as distinct entities, we cannot say for sure that none of the ones we observe in the CMBR were ever gamma rays, because, as I also pointed out to the OP, not all interactions between light and matter are absorption/re-emission; the very word "scattering", as in "surface of last scattering", illustrates that.

I don't think the book is misleading in what it says about gamma rays in the early universe, or about radiation redshifting from then to now. The issue I see is that what it says in the first excerpt given is inconsistent with what it says in the second excerpt given. I have already described what I would have said instead of the second excerpt.

 

[Jimmy87]

As I have already pointed out in response to the OP, photons don't really have distinct identities to begin with, so "the photons we see in CMBR" is not even a well-defined concept. And even if we allow ourselves, as a heuristic approximation, to talk about particular "photons" as distinct entities, we cannot say for sure that none of the ones we observe in the CMBR were ever gamma rays, because, as I also pointed out to the OP, not all interactions between light and matter are absorption/re-emission; the very word "scattering", as in "surface of last scattering", illustrates that.

I don't think the book is misleading in what it says about gamma rays in the early universe, or about radiation redshifting from then to now. The issue I see is that what it says in the first excerpt given is inconsistent with what it says in the second excerpt given. I have already described what I would have said instead of the second excerpt.

Thank you for your insights. I was just wanting for some further clarification on the early gamma radiation. You said it gets predominantly scattered. I have been doing a bit of research about when EM radiation interacts wit free electrons in a plasma they seem to always get scattered and never absorbed due to come conservation laws. I found this post on Physics StackExchange:

https://physics.stackexchange.com/questions/225522/free-electron-cant-absorb-a-photon

The first main response from ProfRob outlines this. So when the early universe was a plasma is it right to think that there were no absorptions as an absorption would break conservation laws if it is an unbound electron? Or I guess maybe some electrons momentarily get bound as an atom and then an absorption event can occur?

I can't seem to find an answer to another question about the neutrino decoupling. It says very early on neutrinos decoupled from matter so hypothetically there should be CMB for neutrinos (CnB). I was just wondering how neutrinos managed to interact with matter in the early universe? No sources seem to mention the mechanism for how neutrinos interacted with matter very early on?

Thanks for any insights offered.

 

[PeterDonis]

You said it gets predominantly scattered.

I didn't say "predominantly". I just said scattering is one of the possible interactions. However, the reference you give is correct that a free electron cannot absorb or emit a single photon, since there is no way for such a process to occur without violating conservation laws. That indicates that, at least to a first approximation, we should expect scattering to be the only possible interaction between electrons and photons in a plasma.

I guess maybe some electrons momentarily get bound as an atom and then an absorption event can occur?

This is possible, yes; basically, the electron would emit a photon and become bound to an atom (the fact that it becomes bound means that the nucleus of the atom--probably a proton since the atom is probably hydrogen--can absorb some of the energy and/or momentum and allow conservation laws to be satisfied), and then soon after that it would absorb a photon and get kicked out of the atom (and again the presence of the proton would allow the process to take place without violating conservation laws). How often this takes place as compared with scattering will depend on the temperature; the higher the temperature, the less likely it is that an electron will emit a photon of just the right energy to allow it to become bound to a proton in a hydrogen atom, at just the same time that a proton happens to be near enough to bind it.