Cosmic optics

As a branch of physical sciences, cosmology is quite peculiar; unlike in condensed-matter physics, quantum chemistry, or molecular biology, cosmology does not allow us to make controlled experiments on the object under study. We can only see what the Universe allows us to see, and strive to correctly interpret its messages. Thus, in order to understand anything about the history, the content,  and the origin of the Universe, it is crucial to understand how light propagates through it. I like to call this general topic “cosmic optics”.