A map of microwave radiation from 379,000 years after the Big Bang. [Credit: NASA Goddard Space Flight Center]
The term “Big Bang” conjures the image of an explosion at the beginning of the universe. But how did the entire universe grow from one bang?
This primal moment is better understood not as an expansion in time and space but rather as an expansion of time and space. In a matter of seconds after this metaphorical bang, the four fundamental forces (gravitational, strong nuclear, weak nuclear and electromagnetic) and the most basic and common elements—hydrogen and helium—arose.
About 300,000 years later, these forces formed dense clouds of matter. This lumpy dispersal of matter, confirmed in 1992 by NASA’s Cosmic Background Explorer satellite, represented the embryonic stage of the billions of galaxies that emerged from one to five billion years after the bang. These galaxies are now scattered throughout the universe.
As hydrogen and helium condensed in these galaxies, they formed the nuclear furnaces known as stars. These stars exploded and burned out as supernovae, converting hydrogen and helium into larger atoms like carbon, oxygen and nitrogen.
And as temperatures cooled, the gases produced compounds that condensed into liquid droplets or solid grains. Metals and rock-forming silicates, the types of rocks that now form most of the Earth’s mantle and crust, were the first grains produced. Decreasing temperatures led to further solid combinations, this time with sulfur compounds and carbon- and water-rich silicates. The grains condensed to form larger aggregates known as planetesimals. Some of these still survive today as comets and asteroids. Others were large enough to experience a strong gravitational attraction with each other, forming moon-sized objects called protoplanets that would later combine further to form planets. Impacts during this stage likely stripped Mercury of part of its mantle and crust, reversed Venus’ rotation and broke the Earth apart to create the Moon.
The gravity of the larger gas giants could redirect the orbits of the planetesimals still scattered throughout the forming solar system. Icy material entered planets, like Earth, from other regions of the solar system. This process introduced water and the organic compounds essential for the rise of life that fed off chemicals in the ocean, such as single-celled algae and bacteria. And the rest, as they say, is history.