Just after the Big Bang, the Universe's dimensions may have been completely different to the four-dimensional space-time we know and love today.
Shortly after the Big Bang, the Universe possessed only one dimension of space and one dimension of time. It was basically a straight line. As the Universe began to cool, and expanded, this one dimension of space became “wrapped up” in such a way to create two dimensions of space and one of time — a plane, like a sheet of flat paper.
The transition from one to two dimensions of space was calculated by the researchers to occur when the Universe “cooled” to an energy level of 100 TeV (tera-electron volts, a measurement of energy commonly used in particle physics). A period of time after that, the Universe continued to expand and cool until it reached an energy of 1 TeV. At this point, the Universe got promoted to a higher dimension; three dimensions of space and one dimension of time, i.e., the Universe we live in today.
Mureika and Stojkovic think the Universe will eventually be promoted again, to a five-dimensional state, at some point in the future.