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stitt29
2009-Feb-09, 01:28 PM
Hi

This is a question about CMB (I think that's Cosmic Microwave Background radiation)

As far as can gather this is seen as proof of the big bang because for Hydrogen to be created from the plasma stage the plasma only had to cool. But to convert 10% into Helium would produce CMB which is detected as being slightly stretched.

However what I am surmising is that this CMB might just as easily be coming from stars and is stretched because the Universe is expanding. Are there any reasons why this couldn't be so?

alainprice
2009-Feb-09, 01:31 PM
We all know the CMB radiation has its peak at an equivalent temperature of 2.7 Kelvin. The problem I have is that I do not know how many photons there are in this range.

I get the impression that there are a LOT of these redshifted photons compared to other frequencies, meaning that stars would have a hard time produsing the required intensity.

Just an idea.

ngc3314
2009-Feb-09, 01:47 PM
The strongest argument is that the CMB is a very exact blackbody (not only as regards spectral; shape but normalization; a Planck function has only one parameter). Single stars are at best approximate blackbodies (hot and cool ones aren't all that good an approximation). A mix of stars at different temperatures (or redshifts) gives a spectrum broader than a blackbody (for example, the spectrum of any galaxy or star cluster).

grant hutchison
2009-Feb-09, 02:30 PM
We all know the CMB radiation has its peak at an equivalent temperature of 2.7 Kelvin. The problem I have is that I do not know how many photons there are in this range.According to Paul Davies' Accidental Universe, there are about 108 times more CMB photons than starlight photons in the Universe at the present epoch.
The energy densities are closer, because the starlight photons average 103-104 times the energy of the CMB photons.

Grant Hutchison

alainprice
2009-Feb-09, 02:41 PM
According to Paul Davies' Accidental Universe, there are about 108 times more CMB photons than starlight photons in the Universe at the present epoch.
The energy densities are closer, because the starlight photons average 103-104 times the energy of the CMB photons.

Grant Hutchison


Just to be clear on the energy density. The starlight would need to be stronger as its CMB counterpart is very much redshifted and therefore, dimished energy content.

So we have two arguments already:
-the radiation curve is a nearly perfect blackbody
-the intensity of radiation is stronger than an equivalent fusion source(star) redshifted into the microwave region.

grant hutchison
2009-Feb-09, 03:05 PM
Just to be clear on the energy density. The starlight would need to be stronger as its CMB counterpart is very much redshifted and therefore, dimished energy content.Putting it another way, stars are several orders of magnitude hotter than the current cosmic microwave background, so their photons have (on average) several orders of magnitude more energy per photon. But the higher energy per photon in starlight is not enough to offset the much higher photon count of the cosmic microwave background. So (averaged across the Universe) the CMB contributes thousands of times more energy per unit volume than starlight does, and millions of times more photons.

Grant Hutchison

Cougar
2009-Feb-09, 04:46 PM
As far as can gather this [CMB] is seen as proof of the big bang....

It is one of several very strong independent observational supports for the theory.

StevenO
2009-Feb-21, 09:43 PM
It is one of several very strong independent observational supports for the theory.

Well...that is a theory. I have seen IMHO a far better theory for explaining most astronomical observations without the need for big bang, dark matter/energy and such. This theory explains that the CMBR is the radiation we receive from 'anti-matter' stars. Since these stars are aggregated in time they have no location in space that is why the radation is isotropic. Since anti-matter has reciprocal properties wrt. to regular matter, a high 'anti'-temperature will be received as low temperature in our matter universe which explains the 2.7K BlackBody.
This 'anti-universe' also gives a better explanation for cosmic rays and GRB. These are caused by anti-matter (supernova) explosions that push anti-matter back to into the matter zone with corresponding bursts of energy that are isotropically distributed over space.

PetersCreek
2009-Feb-21, 10:32 PM
StevenO,

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stitt29
2009-Feb-22, 12:31 AM
Putting it another way, stars are several orders of magnitude hotter than the current cosmic microwave background, so their photons have (on average) several orders of magnitude more energy per photon. But the higher energy per photon in starlight is not enough to offset the much higher photon count of the cosmic microwave background. So (averaged across the Universe) the CMB contributes thousands of times more energy per unit volume than starlight does, and millions of times more photons.

Grant Hutchison

Got you. But let me raise another point. Visible light from stars doesn't always get through to us e.g they twinkle because of matter coming between us, the observer and the star. With the naked eye we see stars at about 2 billion light years away max, i think approx. Using telescopes we can see stars up to 12 billion light years. So my thinking goes couldn't the microwaves be coming from an infinite amount of stars beyond our visible light horizon. this would explain the higher photon count of cmb. This would solve olbers paradox i.e the night sky is "white" with microwaves. but not visible light as visible light gets blocked out, microwaves go through solids.

does anyone think this is a possibility?

grant hutchison
2009-Feb-22, 12:54 AM
This would solve olbers paradox i.e the night sky is "white" with microwaves. but not visible light as visible light gets blocked out, microwaves go through solids.So how does dust filter the spectra of many stars of different temperatures so that all the transmitted wavelengths precisely match a 2.7K black-body spectrum?
(As for Olbers, dust doesn't solve his paradox: given an infinite Universe full of stars, the interstellar dust soon heats up and begins to glow at the same temperature as the stars.)

Grant Hutchison

Spaceman Spiff
2009-Feb-22, 01:26 AM
Here is a nice explanation (http://www.astro.ucla.edu/%7Ewright/stars_vs_cmb.html) of why the CMB cannot have its origin in stars (or anything else other than a single-temperature-Planck-spectrum-emitting matter to a few parts in 100,000).

And here (http://homepages.wmich.edu/~korista/sun-images/solar_specbb.jpg) is a nice spectral plot demonstrating how a star's spectrum deviates from a single temperature blackbody -- and our Sun's (G2) spectrum is one of the closer matches due in part to the dominant continuous opacity source being photodissociation of the H- (negative) ion whose wavelength dependence is smooth.

alainprice
2009-Feb-22, 06:59 PM
The fact that the shape of the radiation curve is a blackbody automatically makes me think of a surface. More precisely, a single surface emitting this radiation for that instant. What we are seeing is a bubble around us, which would match the surface of an imaginary bubble around us which represents the point at which the universe went from primarily dark to mostly see-through. The starting temperature is about 3000K.

If you start with a smaller bubble, you get a higher temp but that one already went by us. A bigger imaginary bubble can be made as the origin but will measure a lower temperature. We'll have to wait to measure it.

If the CMBR is from stars, then we have problems matching it to the current microwave power distribution.

Anybody know the size of the visible universe when the surface of last scattering happened(380,000 years from the BB)?

Just curious.

Spaceman Spiff
2009-Feb-22, 07:42 PM
1) The universe was not dark before the recombination event (electrons combining with He then H as the temperature dropped in the expanding universe). Photons outnumbered the baryons by about 2 billion to 1. It's just that the photon mean free path was limited in size (much less than the Hubble distance before that time of recombination).

2) This statement:

If you start with a smaller bubble, you get a higher temp but that one already went by us. A bigger imaginary bubble can be made as the origin but will measure a lower temperature. We'll have to wait to measure it.is apparently illustrating a misconception. The cmb photons do not outrun the galaxies. (Read this (http://www.astro.ucla.edu/%7Ewright/photons_outrun.html).) This radiation field was everywhere then, and it remains everywhere now. The only difference is that the temperature and energy density of this radiation field have been dramatically reduced due to the expansion that has occurred in the interim time.

3) Your idea of a "surface" is on the right track. This is a "surface" of last scattering (between free electrons and the photons). Try here (http://www.astro.ucla.edu/%7Ewright/BBhistory.html) and here (http://map.gsfc.nasa.gov/universe/bb_tests_cmb.html).

4) Our current particle horizon (http://en.wikipedia.org/wiki/Observable_universe) is something like 47 billion light years away. Such was (1+z) smaller at a time (380,000 years after the bb) corresponding to the redshift we presently measure. For the cmb, z ~ 1080. This isn't the same thing as "size of the universe", which is unknown (it's beyond our horizon).