Questions about cosmic background radiation

[HankDorsett]

TL;DR Summary

Understanding CBR

I'm trying to get a better understanding of CBR. Unfortunately the internet has limited information, I guess it's not a cool enough of a topic to discuss.

This is some of what I found, is it correct?
CBR is electromagnetic radiation.
Microwaves travel at the speed of light.
CBR was a result of Universe being superheated when it was created. ( there are a few post on this form that seem to contradict this. I've seen various claims that it was created hundreds of millions of years after the big bang and some that suggested it was created due to elements combining. )

I have more questions but I'm going to wait to hear about these before I continue.

 

[Bandersnatch]

This is some of what I found, is it correct?
CBR is electromagnetic radiation.
Microwaves travel at the speed of light.

All good.

CBR was a result of Universe being superheated when it was created. ( there are a few post on this form that seem to contradict this. I've seen various claims that it was created hundreds of millions of years after the big bang and some that suggested it was created due to elements combining. )

Surely nobody claimed millions of years?
Anyway, the basic thought process is like so:

The universe expands, hence dilutes and cools ;
-> so going into the past it was smaller, hence denser and hotter (also more uniform, as the primordial gas needs time to coalesce into stars and galaxies);
-> 1) hot gas glows 2) if you heat gas enough, it'll turn into plasma (= electrons and nuclei separate);
-> plasma is opaque to EM radiation;
-> no EM radiation could travel freely when the universe was in the plasma state, so we can't observe anything emitted in that period;
-> the moment when the universe expanded and cooled enough for the plasma to combine into neutral particles, is when it first became transparent = when the CMB was emitted;
-> this should have happened approx. 380 thousand years after the initial singularity.

So there's no contradiction between the reason for its existence being both the hot conditions in the early universe, and the particles combining (when it turned cool enough).

 

[HankDorsett]

All good.Surely nobody claimed millions of years?
Anyway, the basic thought process is like so:

The universe expands, hence dilutes and cools ;
-> so going into the past it was smaller, hence denser and hotter (also more uniform, as the primordial gas needs time to coalesce into stars and galaxies);
-> 1) hot gas glows 2) if you heat gas enough, it'll turn into plasma (= electrons and nuclei separate);
-> plasma is opaque to EM radiation;
-> no EM radiation could travel freely when the universe was in the plasma state, so we can't observe anything emitted in that period;
-> the moment when the universe expanded and cooled enough for the plasma to combine into neutral particles, is when it first became transparent = when the CMB was emitted;
-> this should have happened approx. 380 thousand years after the initial singularity.

So there's no contradiction between the reason for its existence being both the hot conditions in the early universe, and the particles combining (when it turned cool enough).

thanks for the detailed explanation of how it was created. I think this is the first time I've seen this level of detail.

 

[256bits]

Hi @HankDorsett
Yes, that was a good explanation.

Super heated universe - if you mean hot, really hot, like really hot
Superheated has a definition on its own for liquids and vapours.
Did someone use that term to describe the universe?

As good a site as any for reading.
https://ned.ipac.caltech.edu/level5/Sept02/Kinney/Kinney3.htmlThe time of the universe becoming transparent to photons due to formation of atoms CRB is called recombination.

 

[HankDorsett]

After a few hours of nerding out on this I am wondering if cosmic background radiation still exist. Are we only observing those microwaves that it emitted when it was still around? Would it be affected by radioactive decay?

 

[Bandersnatch]

Of course it exists, it's observable. The conditions that emitted it don't.
CMB is just old light (of certain wavelengths). It's radiation, it doesn't decay. It gets redshifted.

 

[HankDorsett]

Of course it exists, it's observable. The conditions that emitted it don't.
CMB is just old light (of certain wavelengths). It's radiation, it doesn't decay. It gets redshifted.

My brain is starting to hurt. What is the difference between CBR and CMB?

 

[Bandersnatch]

No difference. It's Cosmic Microwave Background Radiation. Usually abbreviated to CMB.
It's the light (Radiation) from the early universe (Cosmic) that has been redshifted by expansion into longer wavelengths (Microwave) and that we now observe all around us (Background).

 

[Vanadium 50]

CMB and CMBR are the common abbreviations. CBR is not common - you are the first person I have ever seen use it.

 

[HankDorsett]

No difference. It's Cosmic Microwave Background Radiation. Usually abbreviated to CMB.
It's the light (Radiation) from the early universe (Cosmic) that has been redshifted by expansion into longer wavelengths (Microwave) and that we now observe all around us (Background).

Thanks for the info. This is just another opportunity to unlearn past beliefs. I did a quick internet search after your post and I was able to find somewhat different definitions between them. Internet discrepancies was the main reason I came to this form.

@Vanadium 50 it's opposite for me. I mean they come across cosmic background radiation or CBR.

 

[russ_watters]

After a few hours of nerding out on this I am wondering if cosmic background radiation still exist. Are we only observing those microwaves that it emitted when it was still around?

Note that light will travel forever until/unless it hits something. The CMB is what we see from a flash originating everywhere in the universe, early in its life.

 

[russ_watters]

After a few hours of nerding out on this I am wondering if cosmic background radiation still exist. Are we only observing those microwaves that it emitted when it was still around? Would it be affected by radioactive decay?

Wait...maybe the real problem here is you think "radiation" = "radioactive"? Radioactive decay produces radiation, but not all radiation comes from radioactive decay. Radioactive decay is named for radiation, not the other way around.

 

[diogenesNY]

@HankDorsett ,

If you want to see the CMBR, here is all you have to do:

Get an old TV set... I mean a _really_ old one, like maybe a 1970s vintage jobbie... that is to say, one with not only a CRT, but analog tuning controls and a functional set of 'rabbit ears' (remember those?).

Turn it on, and let it warm up if necessary (that is if it is a vacuum tube set)... and turn the channel selector to an off channel, that is a channel that is not being broadcast on... which nowadays is pretty much all of them, as analog TV is broadcast only by amateurs and low power stations, although that part of the spectrum is now also used for other things. Anyway, tune the dial to a channel that is nominally free from broadcast traffic.

The 'snow' that you will see and hear, that is the black and white spotty chaos on the screen and the white noise that you hear, is a pretty reasonable AV representation of the CMBR, some of it anyway. There is probably no shortage of local noise as well, but at least you are into the right neighborhood.

diogenesNY

 

[kimbyd]

@HankDorsett ,

If you want to see the CMBR, here is all you have to do:

Get an old TV set... I mean a _really_ old one, like maybe a 1970s vintage jobbie... that is to say, one with not only a CRT, but analog tuning controls and a functional set of 'rabbit ears' (remember those?).

Turn it on, and let it warm up if necessary (that is if it is a vacuum tube set)... and turn the channel selector to an off channel, that is a channel that is not being broadcast on... which nowadays is pretty much all of them, as analog TV is broadcast only by amateurs and low power stations, although that part of the spectrum is now also used for other things. Anyway, tune the dial to a channel that is nominally free from broadcast traffic.

The 'snow' that you will see and hear, that is the black and white spotty chaos on the screen and the white noise that you hear, is a pretty reasonable AV representation of the CMBR, some of it anyway. There is probably no shortage of local noise as well, but at least you are into the right neighborhood.

diogenesNY

Note that it has to be a low channel for a significant fraction of the signal to be from the CMB. Ideally 2 or 3.

 

[kimbyd]

Summary: Understanding CBR

I'm trying to get a better understanding of CBR. Unfortunately the internet has limited information, I guess it's not a cool enough of a topic to discuss.

This is some of what I found, is it correct?
CBR is electromagnetic radiation.
Microwaves travel at the speed of light.
CBR was a result of Universe being superheated when it was created. ( there are a few post on this form that seem to contradict this. I've seen various claims that it was created hundreds of millions of years after the big bang and some that suggested it was created due to elements combining. )

I have more questions but I'm going to wait to hear about these before I continue.

This resource may be useful:
http://background.uchicago.edu/~whu/beginners/introduction.html

The super short version is that when our universe was, if I recall, a few hundred thousand years old, it cooled to below around 3000K, at which time it cooled from a plasma to a gas. The roughly 3000K thermal radiation that was bouncing around in the plasma was then able to stream freely through the newly-transparent universe. In effect, as the matter became a transparent gas, the photons became a photon gas (before then, photons and matter were tightly-coupled, which is what a plasma is). Our universe has expanded by a little over a factor of 1000 since then, which has decreased the temperature of this photon gas has dropped to below 3K.

There are tiny variations in the temperature of this roughly 3k photon gas from place to place which represent photons streaming from slightly hotter or slightly colder regions, which were slightly hotter or colder because they were slightly more or less dense. The more dense bits later became galaxy clusters and larger structures, with the less dense bits becoming voids with few galaxies. These variations in temperature are tens of microkelvins (i.e., most being within ~0.00005K hotter or colder than the surrounding CMB).