Science a GoGo's Home Page Forums General Discussion Physics Forum Interpreting the CMB

If galaxies in the observable Universe are evenly distributed in every direction, couldn’t what we perceive as the CMB, and interpret as the remnant of the BB, just be radiation from the stars in all these galaxies.

I think the one of the problems with interpreting the CMB as "just radiation from the stars" is its spectrum. The spectrum matches the theoretical pattern of a low temperature black body. Stars are not low temperature black bodies, they are high temperature bodies. And it was predicted before it was found. Actually it wasn't found by anybody who was looking for it. When Penzias and Wilson found it they worked for AT&T and were just looking at ways to reduce the noise in microwave communications. They tried to find the source every way they could, but found that it was just there. They mentioned it to somebody they knew and were told, "Why don't you talk to so-and-so at Princeton. When they did the people they talked to were just discussing trying to find it. That's how you go about just missing a Nobel.

Another thing is that if it is the radiation from stars, why doesn't it get higher when we point our antennas at individual stars and galaxies. It doesn't, which is why it is thought to be not associated with the current crop of astronomical bodies.

Bill Gill

Thanks Bill, that makes sense.

However, I believe that back in the 1920s Arthur Eddington calculated that the minimum temperature to which any object in space would cool, because of the radiation from stars, was 2.8K, and that this was later extrapolated to include any temperature that would be measured in intergalactic space.

A more up to date measurement of the temperature of space is 2.725K (remarkably close to Eddington's), which is the temperature of the CMB. What happened to Eddington’s calculations?

Well, you have a good question. Unfortunately I don't have a good answer. I don't recall hearing bout that calculation, and I have no idea what assumptions were made in making the calculation.

However, I don't believe in letting a lack of knowledge stop me from speaking up about something I don't understand. So I will make a couple of WAGs (wild ass guesses). One is that the he may not have included the expansion of the universe into his calculations. Another is that he may have been talking about the temperature of a body which is warmed by the radiation from the stars. And then of course there may be some relation between Eddington's calculation and the CMB. And then it may just be that he used some inaccurate assumptions. That has happened may times. A good example is Lord Kelvin's estimate of the life time of the sun (about 20,000 years) which was a good example of theory based on current knowledge. Kelvin didn't know about nuclear reactions, so came up with an impossibly short lifetime.

Keep in mind that these are WAGs, and may have nothing to do with reality.

Bill Gill

Obviously, modern calculations and interpretations are more likely to be accurate than are their counterparts from the 1920s. It just struck me that if Eddington's figure was a WAG, it was remarkably close to the current figure.

Anyway, it got me away from the WAGs of a couple of other threads and back to something approaching science.

You may wish to take a look at this research.

Pecker, J & Narlikar, J.V. Stellar and Extra-Galactic Radiation at the Earth's Surface J.Astrophys. Astr. 2006 27 p 1-6.

Abstract:
Reviving a calculation made by Eddington in the 1920s, and using the most recent and comprehensive data bases available on stars and galaxies, including more than 2,500,000stars and around 20,000 galaxies we have computed their total radiation received at the Earth just outside its atmosphere. This radiation density, if thermalized, would be equivalent to a temperature of 4.212K. The comparability of this temperature to that of the cosmic microwave background(2.723K)may either be a pure coincidence or may hold a key to some as yet unknown ,aspect of the universe.

Thanks Oph, the maths look a bit scary, but I shall have a better look when time permits.

You should keep in mind that the co-author Narlikar worked with Hoyle and the Burbridges on Steady State theory and so has a vested interest in the CMBR not being an 'echo of the Big Bang'. That explains the "may hold a key to someas yet unknown aspect of the universe". It's a coded message understood, I would imagine, by everyone in the field.

As usual with this sort of thing, I struggled with the maths. Apart from the obviously updated data, I'm not at all sure that their argument greatly improves on Eddington's contribution. However, it might be interesting to see where they go with their proposed further research.