I would like to study more about the notion of time in modern physics. In particular, I would like to understand better the following question: how does one measure the time in the early universe? To measure time one needs to have clocks. A natural choice to be used as clocks would be particles with mass. The problem is that just after the Big Bang particles with mass did not exist then the idea of use clocks cannot be applied and therefore the notion of time is lost. I read somewhere that there is an area of mathematics called conformal geometry that could be used to understand this problem. Does anyone know references on this topic? The application of conformal geometry in conjunction with the theory of general relativity has been studied at the moment? Any answer would be of great value!