Frequently, people don't understand why expansion means that light gets redshifted. Here I link the two ideas. What causes the redshift of light, and how do represent it mathematically? What exactly gets stretched? Where does the energy go when it gets redshifted? It is critical to understand these principles because without this understanding the entire baseline of cosmology is hard to get. Everything is a measurable, and redshift is one of those things. For from a mystery or a trick of the math, it’s a real occurrence. As part of this, we study the expansion in great detail, outlining the Robertson-Walker Metric, a solution to the Einstein Field Equations of General Relativity. This is part of my complete intro Astronomy class that I taught at Willam Paterson University and CUNY Hunter.

Redshift: https://en.wikipedia.org/wiki/Redshift
Hubble's Law: https://en.wikipedia.org/wiki/Hubble%...
Cosmic Scale Factor: https://en.wikipedia.org/wiki/Scale_f...)
FLRW Metric: https://en.wikipedia.org/wiki/Friedma...