However, it is known that by the end of the first second of time, the building blocks of matter had formed. At around one million years following the Big Bang, nuclei and electrons were at low enough temperatures to coalesce to form atoms. But the universe didn t start to look like it does today until small perturbations in the matter distribution were able to condense to form the stars and galaxies we know today. General relativity also implies that our expanding universe began from a singularity. A singularity marks a point where the curvature of space-time is infinite, or, in other words, it possesses zero volume and infinite density.
General relativity demands that singularities arise under two circumstances.
First, a singularity must form during the creation of a black hole. When a very massive star reaches the end of its life, its core, which was previously held up by the pressure of the nuclear fusion that was taking place, collapses and all the matter in the core gets crushed out of existence at the singularity. Second, general relativity shows that under certain reasonable assumptions, an expanding universe like ours must have begun as a singularity. In the early 1920s, Russian physicist and mathematician Alexander Friedmann became the first person to embrace the idea that the equations of Einstein s general theory of relativity called for a universe in motion. Einstein s equations, he developed the first model of a universe in motion.