HiSET Science: Theories Relating to the Origin of the Universe

Model of the Inflationary Universe
Hubble's law states that the speed at which a galaxy appears to be moving away from the Earth is proportional to its distance from
Earth. This relatively simple formula (v = Hr, where v is the velocity of a receding galaxy, r is its distance from Earth, and H is the Hubble constant) had an important implication at the time that it was developed—the universe is expanding. This fact, in turn, implies that the universe began at a specific point in the past. This model suggests that a random conglomeration of quarks and leptons, along with the strong force (all the forces in the universe unified as one), existed in the very dense, very hot, early universe. When the universe was a certain age (about 10–35 seconds old), the strong force separated out from the mass. This enabled the rapid expansion of the particles that formed the universe.

Big Bang Theory
The theory of the big bang expands upon the model of the inflationary universe. This theory hypothesizes that the early universe consisted of elementary particles, high energy density and high levels of pressure and heat. This single mass experienced a phase change (similar to that of freezing water) when it cooled and expanded. This transition caused the early universe to expand exponentially; this period of growth is called cosmic inflation. As it continued to grow, the temperature continued to fall. At some point, baryogenesis (an unknown process in which quarks and gluons become baryons, such as protons and neutrons) occurred, somehow creating the distinction between matter and antimatter. As the universe continued to cool, the elementary forces reached their present form, and elementary particles engaged in big bang nucleosynthesis (a process that produced helium and deuterium nuclei). Gravity became the predominant force governing interactions between particles; this enabled increasing accretion of particles of matter, which eventually formed the universal constituents as we recognize them today.