Is Cosmic Microwave Background From old Galaxies?

How do we know the CMB isn't just extremely red-shifted light from very distant galaxies?
- ien cleary (age 53)
eastbourne, east sussex, uk

There are so many reasons that I probably will miss most of them.

Light from stars in galaxies has characteristic spectral features coming from atomic spectra. The frequency shift of these features is what tells us the redshift of the star. The CMB has no such features.

The total density of light (of all frequencies) from identifiable galaxies falls off in a regular way as a function of redshift. Extrapolating to greater redshifts would give much less CMB than found. 

The shape of the small variations in spectral density as a function of direction is predicted with amazing accuracy by the model of light coming from an almost homogeneous plasma, not yet with any major gravitational clumping. Galaxies by definition are already highly clumped up. 

Dynamical simulations of what would become of that almost homogeneous plasma over time show that the small density fluctuations found would grow into something very similar to the lumpy galaxies and clusters we now see. 

So there are many reasons why nobody even considers a model of the sort you suggest.

Mike W.

(published on 07/16/2015)