# CMBR Origins

• B
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
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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.

vanhees71
Staff Emeritus
but reading around I have found several sources

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?

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vanhees71
PeroK
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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.

vanhees71 and Jimmy87
Staff Emeritus
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.

vanhees71
PeroK
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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 criticise the author in this case without knowing which simplifications were forced on the text.

256bits
Gold Member
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
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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
Gold Member
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
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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.

diogenesNY and 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
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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
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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 criticise 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
PeroK
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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
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this is from my brother's A-Level textbook

What book? Please give specific information.

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
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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
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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").

Klystron and russ_watters
anorlunda
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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.

Staff Emeritus
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.

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
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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.