My very elementary knowledge of astronomy tells me that the distance to Galaxies is measured based on identifying Cepheid variables and then making the assumption that all variables of the same period have the same intrinsic magnitude, and using this coupled with apparent magnitude to estimate distances to remote galaxies.

Are all our assumptions and conclusions on the size of the Universe based on this fundamental assumption? And, (if I have understood this correctly), then it bothers me quite a bit. ..what if this fundamental assumption is wrong?

What if Cepheid variables in the Andromeda Galaxy for example have the same period as a variable in the Small Magellanic Cloud but have a different intrinsic magnitude? What if the relationshop does not hold universally?

How does that change our beliefs regarding the size of the universe and the distance of galaxies?

I seem to recall at least once before astronomers came to the conclusion there are are not one but two types of Cepheid variables, based on which there was a significant re-evaluation of distances...what if there are are not two but many more types?

or have I completely missed the point????

Seeker