B Early Universe Radiation: Gamma to Microwave?

[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.