  In classes on cosmology, students are often told that photons stretch as
space expands, but just how physical is this picture? Does space really expand?
In this article, we explore the notion of the redshift of light with Einstein's
general theory of relativity, showing that the core underpinning principles
reveal that redshifts are both simpler and more complex than you might naively
think. This has significant implications for the observed redshifting of
photons as they travel across the universe, often refereed to as the
cosmological redshift, and for the idea of expanding space.
