elitenoobboy said:
And this is because the cosmo. redshift is dynamic and changing? Is the current spatial expansion rate measurable in a lab? Would it be theoretically possible that the reason that light is redshifting be due to something else such as some unknown frictional force, rather than expanding space?
It is possible in principle, but only if theory is wrong. It is not possible theoretically, if by theory you mean consistent with present knowledge of physics.
You can postulate "unknown forces" to do anything you like. There's no evidence for them, however. There are known "forces" that will redshift light, but none of them fit the specific observations we have in this universe for the cosmological redshift.
But I thought that Einstein originally thought that the universe was static and only later changed his mind after Hubble's observations were published?
Yes, that is true. Einstein had to introduce a special cosmological constant term to avoid the expansion that was predicted from his theory, and he did this well before Hubble's observations, precisely because expanding space IS a prediction of general relativity. He just assumed that the rate of expansion was zero and derived as a consequence his "cosmological constant term".
(Addendum. And as Marcus points out: other physicists worked out the implications of GR in the absence of a presumed stability; all still well before Hubble.)
The text omitted from your wikipedia quote makes it a bit misleading. He was NOT prevented from predicting expanding universe by Hubble. He was prevented by his own choice to assume that there was no expansion, and introduce the cosmological constant term, all well before Hubble came along. What wikipedia says is as follows:
After Hubble's discovery was published, Albert Einstein abandoned his work on the cosmological constant (which he had designed to allow for a static solution to his equations). He later termed this work his "greatest blunder" since the assumption of a static universe had prevented him from predicting the expanding universe.
The full text is clear that it was Einstein's own assumptions that held him back from predicting what Hubble had later observed.
The full story is a bit complex, because in fact Hubble's original observations got the Hubble constant wrong by a factor of nearly 10. Hubble's initial estimates of the Hubble relation suggested that the universe was much too young. Hubble himself preferred a model that did not involve actual velocities -- either ballistic or an expansion of space term.
Cheers -- Sylas