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PainterGuy

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- 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.

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).

What do you say about this?

Source #1:

"

Source #2:

"

Source #3:

"

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?

1: https://physics.stackexchange.com/q...ground-radiation-in-the-visible-spectrum?rq=1

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

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).

What do you say about this?

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._DickeSource #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_SandageSource #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?

**Helpful links:**1: https://physics.stackexchange.com/q...ground-radiation-in-the-visible-spectrum?rq=1

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