From the Big Bang to the Present: Gravity Is Why the Universe Is So Uniform
The temporal evolution of the universe, from the Big Bang to the present, is described by Einstein’s field equations of general relativity. However, there are still a number of open questions about cosmological dynamics, whose origins lie in supposed discrepancies between theory and observation. One of these open questions is: Why is the universe in its present state so homogeneous on large scales?
From the Big Bang to the present
It is assumed that the universe was in an extreme state shortly after the Big Bang, characterized in particular by strong fluctuations in the curvature of spacetime. During the long process of expansion, the universe then evolved towards its present state, which is homogeneous and isotropic on large scales — in simple terms: the cosmos looks the same everywhere. This is inferred, among other things, from the measurement of the so-called background radiation, which appears highly uniform in every direction of observation. This homogeneity is surprising in that even two regions of the universe that were causally decoupled from each other — i.e., they could not exchange information — still exhibit identical values of background radiation.
To resolve this supposed contradiction, the so-called inflation theory was developed, which postulates a phase of extremely rapid expansion immediately after the Big Bang, which in turn can explain the homogeneity in the background radiation.
However, how this phase can be explained in the context of Einstein’s theory requires a number of modifications of the theory, which seem artificial and cannot be verified directly.