Rotating Question Curious About Astronomy? Ask an Astronomer

Why is the Cosmic Microwave Background not absorbed by interstellar hydrogen?

I'm a junior physics major at a small college and I've read about the importance of CMB anisotropies in popular literature. One thing has bugged me: as its wavelength stretched, at some point the CMB must have been at the right temperature for a measurable portion to be absorbed by interstellar hydrogen. I know there isn't much interstellar hydrogen, but then the anisotropies are extemely small. Is the absorption by interstellar matter negligible? If not, how would an astronomer separate the anisotropies caused by fluctuations in primordial universe from anisotropies caused (much later) by interstellar matter?

You are right that there will be appreciable absorption from interstellar hydrogen at certain wavelengths. However, these wavelengths do not correspond to those used for studying the CMB. As you know, the peak of the CMB is at a wavelength a little less than a millimeter, while interstellar absorption from neutral hydrogen in our galaxy occurs at 21 cm (and longer wavelengths for redshifted hydrogen). Most studies of the CMB are at low wavelengths where there is no absorption from neutral hydrogen.

August 2002, Jagadheep D. Pandian (more by Jagadheep D. Pandian) (Like this Answer)

Still Curious?

Get More 'Curious?' with Our New PODCAST:

More questions about Cosmology and the Big Bang: Previous | Next

How to ask a question:

If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist

This page has been accessed 20197 times since August 24, 2002.
Last modified: January 17, 2003 10:37:09 AM

Legal questions? See our copyright, disclaimer and privacy policy.
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.

Warning: Your browser is misbehaving! This page might look ugly. (Details)