Anyone have time to help me with my particular mental block in cosmology?
When my conceptual astro. course gets to the obligatory last couple of
lectures on cosmology, I get my usual feeling of doom - because I know
very little cosmology (and believe very little of what I know!)
In particular, I have a conceptual problem with the cosmological
redshift, not helped by any of the explanations I read in the intro.
texts (they often make it worse).
If the universe is expanding as a cosmic microwave background photon is
travelling through space-time, then I am happy to accept that the
photon is "stretched" (if not, would it be blue
shifted?) However as space is being stretched, why isn't the metric
being stretched too, in which case a photon whose wavelength was
once one centimetre, for example, would still measure a centimeter now?
(Because today's 1 cm would be bigger than the original 1 cm by the same
factor as the amount by which the photon has been stretched.)
Analogies such as the expanding balloon don't help, because a grid on the
balloon stretches the way I imagine the metric to stretch - wrongly, no
doubt.
Feel free to tell me that I am being dim, as long as you enlighten me in
the process!