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• PainterGuy
In summary, the cosmic microwave background radiation (CMBD) is a remnant of the hot, dense, rapidly expanding state of the universe that occurred about 379,000 years after the Big Bang. It currently has a radius of around 46.5 billion light-years and originated when the universe was 379,000 years old and almost 41 million light-years in size. Due to the expansion of the universe, the original spectrum of the CMBD has red-shifted and now corresponds to a blackbody at 2.74 K. This discovery was made accidentally by Arno Penzias and Robert Woodrow Wilson in 1964, but it was already predicted by cosmologists such as Robert H. Dicke
PainterGuy
TL;DR Summary
I understand that there are going to be many errors in what I'm saying about cosmic microwave background radiation so please bear with me. I'm only trying to grasp the basic picture. I have basically two questions. One is somewhat related to their discovery and the other one is about their journey from the edge of visible universe to get to earth. Thank you.
Hi,

I have few questions about cosmic microwave background radiation (CMBD) and trying to find simple answer at a basic level. I really appreciate your help and time!

The universe is almost 13.799 billion years old and currently has radius of around 46.5 billion light-years.

It is said that CMDB originated when the universe was 3,79,000 years old and almost 41 million light years and the temperature was around 3000 K. Every point in space became of source of radiation in all directions. The radiation spectrum was that of a blackbody at 3000 K. The blackbody spectrum at 3000 K is shown below.

Because the universe has expanded and is still expanding, the original spectrum has red-shifted and now corresponds to that of a blackbody at 2.74 K as shown below.

Please note that the first spectrum at 3000 K has 'wavelength' scale in nm and the one at 2.74 K has it in mm.

Question 1:
I think that before the discovery of CMDB in 1964, the cosmologists already understood that at what temperature recombination took place and the spectrum of CMDB radiation at the time of recombination. But I'm not able to understand that how the cosmologists knew that they must be looking out for microwaves as the excerpt below, Source #1, from Wikipedia tells and not for, say, EM waves in infrared region. How did they know that the original CMDB had been shifted to microwave region of spectrum? It'd would have been possible for them to estimate the region of red-shifted CMDB spectrum if they had an approximate idea of the age of universe. But the age of universe was not well established at that time. The closest estimate for age of universe could have been obtained using the estimate given by Sandage, Source #2, but he himself wasn't really sure of his estimate, Source #3. Anyway, using Sandage's Hubble constant value of '75' instead of currently known value of '67.8', gives the age of universe to be almost 13 billion years. Perhaps, they just had a rough idea that the shifted spectrum should be in radio region (microwaves are subset of radio waves).

Source #1:
"In the early 1960s, work on Brans–Dicke theory led Dicke to think about the early Universe, and with Jim Peebles he re-derived the prediction of a cosmic microwave background (having allegedly forgotten the earlier prediction of George Gamow and co-workers). Dicke, with David Todd Wilkinson and Peter G. Roll, immediately set about building a Dicke radiometer to search for the radiation, but they were scooped by the accidental detection made by Arno Penzias and Robert Woodrow Wilson (also using a Dicke radiometer), who were working at Bell Labs just a few miles from Princeton.[8][9]" - https://en.wikipedia.org/wiki/Robert_H._Dicke

Source #2:
"Sandage began working at the Palomar Observatory. In 1958 he published[5] the first good estimate for the Hubble constant, revising Hubble's value of 250 down to 75 km/s/Mpc, which is close to today's accepted value." - https://en.wikipedia.org/wiki/Allan_Sandage

Source #3:
"However Sandage, like Einstein, did not believe his own results at the time of discovery. His value for the age of the universe[further explanation needed] was too short to reconcile with the 25-billion-year age estimated at that time for the oldest known stars. Sandage and other astronomers repeated these measurements numerous times, attempting to reduce the Hubble constant and thus increase the resulting age for the universe." - https://en.wikipedia.org/wiki/Hubble's_law#Hubble_time

Question 2:
It is also said that the the CMB photon that reaches us today has traveled almost 13.8 billion light years in an infinite universe.

I have been trying to understand the statement above. Please have a look on this attachment below. CMDB has always been with us and it will theoretically will be always be there but as time passes its spectrum would become more red-shifted and its intensity would also decrease. The figure on left shows the visible universe when the recombination took place. The CMDB photons from locations A, B, and C have already been received by the earth. Since the locations D and E have expanded to the distance of 46.5 billion light years over the time of almost 13.7 billion years, therefore photons from locations D and E are being received now.

Since the distance became 46.5 billion light years from 41 million light years over the period of almost 13.7 billion years (I think we would need to subtract the age of universe at the time recombination). The light had to travel almost 1134 times more distance to reach us now.

Do I make any sense?

2: https://en.wikipedia.org/wiki/Draper_point
3: https://physics.stackexchange.com/q...ngth-was-the-cmb-originally?noredirect=1&lq=1
4: https://qr.ae/TSDCX9
5: https://qr.ae/TSDCyp
6: XBr4GkRnY04 (insert "www.youtube.com/" in the front)
7: https://forum.cosmoquest.org/archive/index.php/t-109221.html
9: https://en.wikipedia.org/wiki/Cosmic_age_problem
10: https://en.wikipedia.org/wiki/Cosmic_microwave_background#History
11: https://en.wikipedia.org/wiki/Age_of_the_universe#History
12: https://en.wikipedia.org/wiki/Hubble's_law#Hubble_time

PainterGuy said:
The universe is almost 13.799 billion years old

More precisely, it has been that time since the hot, dense, rapidly expanding state that is the earliest state for which we have good evidence, and which occurred about 379,000 years before the event of the CMB emission.

PainterGuy said:
and currently has radius of around 46.5 billion light-years

What the "radius" of the universe is depends on how you define that term; there are multiple possible distances that that term could mean, and pop science sources in cosmology are notoriously vague about which one they are using. However, the actual physical parameter in this case is the ratio of scale factors from CMB emission to now, which can be thought of as the ratio by which the universe has expanded from CMB emission to now. Any distance you pick will have expanded by the same ratio, so it's really the ratio that is the number to keep in mind. See further comments below.

PainterGuy said:
It is said that CMDB originated when the universe was 379,000 years old and almost 41 million light years and the temperature was around 3000 K.

Yes. (Note that the 41 million light years is not directly observed, it's just calculated from the ratio of scale factors. As I've said, that ratio is really the key number.)

PainterGuy said:
Because the universe has expanded and is still expanding, the original spectrum has red-shifted and now corresponds to that of a blackbody at 2.74 K

Yes. The ratio of temperatures is the same as the ratio of scale factors, and in turn is derived from the observed redshift of the CMB; the redshift is the actual observed quantity that is the basis for all the other numbers. The scale factor ratio is ##1 + z## for redshift ##z##.

PainterGuy said:
I think that before the discovery of CMDB in 1964, the cosmologists already understood that at what temperature recombination took place and the spectrum of CMDB radiation at the time of recombination.

More precisely, cosmologists knew that the Big Bang model of the universe predicted that there would be a time of recombination (actually a misnomer since ions and electrons had never before been combined into atoms in the universe, so it was really "first combination") and that there would be radiation at a temperature of around 3000 K (roughly the ionization temperature of hydrogen gas) emitted at that time, which should have been traveling through the universe and redshifting ever since.

PainterGuy said:
How did they know that the original CMDB had been shifted to microwave region of spectrum?

They didn't "know" since they didn't know the exact redshift (scale factor ratio) to expect, but they knew enough about its rough order of magnitude, based on a rough estimate of the age of the universe and a rough estimate of the time of recombination, to know that the predicted temperature of the radiation now would be a few degrees K (i.e., a redshift of roughly 1000). That puts it in the microwave region.

PainterGuy said:
the age of universe was not well established at that time.

Not "well established" in the sense of knowing the number to several significant figures, but its rough order of magnitude (about 10 billion years) was known, and that is already enough to give the rough order of magnitude of the CMB temperature today (see above).

PainterGuy said:
It is also said that the the CMB photon that reaches us today has traveled almost 13.8 billion light years in an infinite universe.

"It is said" is not a good reference. You don't give any specific links for this item; and I strongly suspect that whatever sources you are getting it from are not textbooks or peer-reviewed papers.

As I said above, there are various "distances" that could be used, and none of them really mean what the average lay person expects them to mean. Also none of them are directly observed. What is directly observed is the redshift ##z## of the CMB; and the key number that is calculated from that is the ratio of scale factors, ##1 + z##. That is the number to focus on. Asking "how far the photons traveled" is (a) vague and doesn't have a single well-defined answer, and (b) irrelevant to the physics anyway since there is no direct observable that corresponds to it.

PainterGuy said:
The light had to travel almost 1134 times more distance to reach us now.

No, it didn't, because the light was traveling during the intervening time; it didn't cover the full 46 billion light years because it wasn't emitted "now", it was emitted 13.799 billion minus 379,000 years ago, and your points D and E weren't 46 billion light years away then. They were, if we use the distance measure you are implicitly using, about 41 million light years away then; but that doesn't mean the light covered 41 million light years either, because the universe was expanding while the light was traveling. So the "distance that the light actually traveled" seems intuitively to be somewhere in between the 41 million light years and the 46 billion light years; but that in itself doesn't tell us what that distance actually is, or even if the question we are asking is well-defined.

Sources that say the light traveled 13.8 billion years (roughly) are punting on the whole question by using a different concept of "distance", namely "light travel time distance", where the distance any light ray has traveled to reach you is simply the time it took to travel. (This number also fits nicely between the 41 million light years and 46 billion light years, so it seems like a reasonable answer to a non-expert.) The problem is that equating these two things is only valid in flat spacetime, and the spacetime of our universe isn't flat. So really these sources are implicitly telling you something more like: "The question of how far the light actually traveled isn't really well-defined or meaningful, but if you insist on an answer, we'll just quote you the light travel time distance and leave it at that." It would be even better, IMO, to just say straight out that the question isn't well-defined and focus attention instead on the actual direct observable, the redshift, and the ratio of scale factors that is derived from it.

PainterGuy, berkeman, pinball1970 and 1 other person
Thank you very much! It was really helpful.

PeterDonis said:
and I strongly suspect that whatever sources you are getting it from are not textbooks or peer-reviewed papers.

The first problem is that English is not my first language so I have trouble expressing myself properly and precisely.

I haven't used any textbooks or peer-reviewed papers/articles. Don't you think that peer-reviewed articles are way too much technical for a person who is just trying to understand something at a basic level just out of curiosity? For example, Google Scholar has a huge database of peer-reviewed articles but I have always found them too technical and have assumed that they are written for other professionals or academics in the same field. Are there any peer-reviewed journals which are not that technical and provide an online searchable database?

PeterDonis said:
No, it didn't, because the light was traveling during the intervening time; it didn't cover the full 46 billion light years because it wasn't emitted "now", it was emitted 13.799 billion minus 379,000 years ago, and your points D and E weren't 46 billion light years away then. They were, if we use the distance measure you are implicitly using, about 41 million light years away then; but that doesn't mean the light covered 41 million light years either, because the universe was expanding while the light was traveling. So the "distance that the light actually traveled" seems intuitively to be somewhere in between the 41 million light years and the 46 billion light years; but that in itself doesn't tell us what that distance actually is, or even if the question we are asking is well-defined.

Sources that say the light traveled 13.8 billion years (roughly) are punting on the whole question by using a different concept of "distance", namely "light travel time distance", where the distance any light ray has traveled to reach you is simply the time it took to travel.

I wanted to clarify few points.

CMDB being received was emitted when the universe was almost 3,79,000 years old after the Big Bang and the radius of visible universe was almost 41 million light years. Currently, the universe is considered to be 13.799 billion years old and the radius of visible has become 46.5 billion light years. The points D and E shown in my last post have moved away to the radius 46.5 billion light years as a result of expansion of the universe. Do you agree with what I'm saying in this para, at least to some extent?

If you agree that the light which was emitted almost when the universe was 3,79,000 years old is being received now, why would it be wrong to say that it took the light 13.799 billion years minus 3,79,000 years to get here? In other words, it took the light "13.799 billion minus 3,79,000" light years to get here. Would you consider it a satisfactory statement for a non-expert?

Thanks a lot for your help and time!

PainterGuy said:
Don't you think that peer-reviewed articles are way too much technical for a person who is just trying to understand something at a basic level just out of curiosity?

Peer-reviewed papers might be, but textbooks aren't. A basic textbook on cosmology will enable you to get a basic understanding; you read the parts you want to and leave aside the other parts for a later date.

PainterGuy said:
CMDB being received was emitted when the universe was almost 3,79,000 years old after the Big Bang

Yes. (Btw, why do you keep putting a comma after the 3? It should just be 379,000.)

PainterGuy said:
and the radius of visible universe was almost 41 million light years

For one definition of "visible universe", yes. But "visible" is a misnomer; see below.

PainterGuy said:
Currently, the universe is considered to be 13.799 billion years old

Yes.

PainterGuy said:
and the radius of visible has become 46.5 billion light years

The radius of what you are calling the "visible universe" is, yes--the part that was 41 million light-years in radius when the CMB was emitted. But not all of that 46.5 billion light-years is visible to us now, because of the finite speed of light. The CMB radiation that is just reaching us now was emitted from a point that is now 46.5 billion light years away from us; but we aren't seeing that point as it is now, but as it was when the CMB was emitted, (13.799 billion minus 379,000) years ago.

PainterGuy said:
If you agree that the light which was emitted almost when the universe was 3,79,000 years old is being received now, why would it be wrong to say that it took the light 13.799 billion years minus 3,79,000 years to get here?

It wouldn't be, and I never said it was. The time the light took to travel is not an issue.

PainterGuy
PeterDonis said:
Peer-reviewed papers might be, but textbooks aren't. A basic textbook on cosmology will enable you to get a basic understanding; you read the parts you want to and leave aside the other parts for a later date.

Thank you very much! No doubt a textbook is always the best source to learn any topic properly and coherently. But as they say, "Better than a thousand days of diligent study is one day with a great teacher". I was able to clarify many important points with your and other members' help.

PeterDonis said:
(Btw, why do you keep putting a comma after the 3? It should just be 379,000.)

Sorry! I mostly think of numbers in a different numbering system and many a time forget to arrange them in American numbering style.

Tom.G

## 1. What is the cosmic microwave background radiation (CMB)?

The cosmic microwave background radiation is a type of electromagnetic radiation that is present throughout the entire universe. It was first discovered in 1964 and is believed to be the remnant heat from the Big Bang, the event that is thought to have created the universe.

## 2. How was the CMB discovered?

The CMB was discovered by two scientists, Arno Penzias and Robert Wilson, in 1964. They were conducting experiments using a large radio telescope and noticed a persistent background noise that they could not explain. After ruling out all possible sources, they realized that they had stumbled upon the CMB.

## 3. What does the CMB tell us about the early universe?

The CMB provides valuable information about the early universe. It shows that the universe was once much hotter and denser, and it also supports the theory of the Big Bang. It also helps us understand the structure and composition of the universe, as well as the expansion rate of the universe.

## 4. How does the CMB support the Big Bang theory?

The CMB supports the Big Bang theory in several ways. First, it shows that the universe was once much hotter and denser, which is consistent with the idea of a rapid expansion from a single point. Second, the CMB has a very specific pattern, known as the blackbody spectrum, which is predicted by the Big Bang theory. Lastly, the CMB provides evidence for the existence of dark matter and dark energy, which are crucial components of the Big Bang theory.

## 5. How is the CMB used in cosmology research?

The CMB is used in cosmology research in a variety of ways. It is used to study the structure and composition of the universe, as well as to measure the expansion rate of the universe. It is also used to test and refine theories such as the Big Bang theory and to search for evidence of inflation, a period of rapid expansion in the early universe. Additionally, the CMB is used to study the effects of dark matter and dark energy on the evolution of the universe.

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