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[sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
Section - I.14. Can the CMB be redshifted starlight?

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Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
Previous Document: I.13. Why haven't the CMB photons outrun the galaxies in the Big Bang?
Next Document: I.15. Why is the sky dark at night? (Olbers' paradox)
See reader questions & answers on this topic! - Help others by sharing your knowledge
No!  The CMB radiation is such a perfect fit to a blackbody that it
cannot be made by stars.  There are two reasons for this.

First, stars themselves are at best only pretty good blackbodies, and
the usual absorption lines and band edges make them pretty bad
blackbodies.  In order for a star to radiate at all, the outer layers
of the star must have a temperature gradient, with the outermost
layers of the star being the coolest and the temperature increasing
with depth inside the star.  Because of this temperature gradient, the
light we see is a mixture of radiation from the hotter lower levels
(blue) and the cooler outer levels (red).  When blackbodies with these
temperatures are mixed, the result is close to, but not exactly equal
to a blackbody.  The absorption lines in a star's spectrum further
distort its spectrum from a blackbody.
                                                                        
One might imagine that by having stars visible from different
redshifts that the absorption lines could become smoothed out.
However, these stars will be, in general, different temperature
blackbodies, and we've already seen from above that it is the mixing
of different apparent temperatures that causes the deviation from a
blackbody.  Hence more mixing will make things worse.
                                                                               
How does the Big Bang produce a nearly perfect blackbody CMB?  In the
Big Bang model there are no temperature gradients because the Universe
is homogeneous.  While the temperature varies with time, this
variation is exactly canceled by the redshift.  The apparent
temperature of radiation from redshift z is given by T(z)/(1+z), which
is equal to the CMB temperature T(CMB) for all redshifts that
contribute to the CMB.
       

User Contributions:

1
Keith Phemister
Sep 13, 2024 @ 11:23 pm
Copied from above: If the Universe were infinitely old, infinite in extent, and filled
with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP

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