Before observational evidence was gathered, theorists developed frameworks based on what they understood to be the most general features of physics and philosophical assumptions about the universe.
When Albert Einstein developed his general theory of relativity in 1915, this was used as a mathematical starting point for most cosmological theories including the Big Bang and the Steady State theories.
In order to arrive at a cosmological model, however, theoreticians needed to make assumptions about the nature of the largest scales of the universe. The assumptions that the Big Bang relied upon are:
- the universality of physical laws – that the laws of physics don't change from one place and time to another,
- the cosmological principle – that the universe is roughly homogeneous and isotropic in space though not necessarily in time, and
- the Copernican principle – that we are not observing the universe from a preferred locale.
These assumptions when applied to the Einstein field equations naturally result in a universe which has the following features:
- an expansion of the universe,
- the universe emerging from a hot, dense state at a finite time in the past,
- the lightest elements were created in the first moments that time existed as we know it, and
- a cosmic microwave background pervading the entire universe should exist, which is a record of a phase transition that occurred when the atoms of the universe first formed.
These features were derived by numerous individuals over a period of years; indeed it was not until the middle of the twentieth century that accurate predictions of the last feature and observations confirming its existence were made. Non-standard theories developed either by starting from different assumptions or by contradicting the features predicted by the Big Bang.