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: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) Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
|
Comment about this article, ask questions, or add new information about this topic: