What do we talk about when we speak of cosmological redshift?

Abstract

From the first observations made by Slipher, our understanding and interpretation of the cosmological redshift was evolving until reaching the current consensus, through the expanding universe and the emergence of modern physical cosmology within the framework of General Relativity. The redshift is one of the most basic concepts of astronomy, and is one of the few observational parameters that can be measured directly. To refer to the temporal evolution of objects or cosmic structures in the universe, we often do so indistinctly through cosmic time or cosmological redshift. But repeatedly this connection ends up generating confusion not only among popular science communicators but also within the professional astronomical community. In this article, we will make a pedagogical approach to the link between cosmic time and cosmological redshift, and we will also clarify several common misunderstandings around this relation.