Cosmology began as a scientific discipline at the beginning of the 20th century, with the work of Albert Einstein and Edwin Hubble. It was Einstein that provided the theory within which it was possible to sensibly consider the entire Universe, and it was Hubble that provided some of the first observations that showed the Universe was expanding. Before the 20th century neither of these things had been possible, and cosmology had remained almost exclusively within the province of religion and philosophy. Since then it has flourished as a science, and it is currently in the process of becoming a precision science.
The gravitational interaction is fundamental to the study of cosmology, as gravity dominates over all other forces on large distance scales. Unfortunately, it is not possible to create consistent models of the Universe with Newton’s theory of gravity alone. This is because Newton assumed that his inverse square law of gravity is applicable to everything in the Universe, and that it is transmitted instantaneously. This means that, according to Newton, the gravitational field we experience on Earth should be a sum of the gravitational fields of every object that exists in the entire Universe. This isn’t necessarily a problem by itself, but it does become problematic if you try to add up the gravitational fields of infinitely many objects. In this case it turns out that Newton’s theory tells us that the total gravitational field at any given point in the Universe depends on the order in which we add up the gravitational fields of all of these objects. This is obviously not a very satisfactory situation.
Of course, we now know that Newton’s theory is only an approximation of the more complete theory developed by Einstein. Thankfully, the problem described above does not occur in Einstein’s theory. Instead, we get a rich set of models that are self-consistent, and that can be used to model the Universe we see around us. In fact, because of Einstein’s focus on space and time, we get a much deeper understanding of the Universe through his theory than we ever would have got from Newton’s. This is because, using Einstein’s theory, we are not only able to model how objects in the Universe move, with respect to each other, we are also able to create a model of how the space and time that make up the Universe behave. Let us now consider this in more detail.