Inflation theory explains universe's rapid expansion from "summary" of A Brief History of Time by Stephen W. Hawking
Inflation theory proposes that in the early moments after the Big Bang, the universe underwent a period of extremely rapid expansion. This expansion was driven by a repulsive form of gravity generated by a hypothetical particle known as the inflaton. The inflaton field caused the universe to expand exponentially, doubling in size every fraction of a second. This rapid expansion helps to explain certain puzzling aspects of the universe that would otherwise be difficult to account for. One of the key problems that inflation theory addresses is the uniformity of the cosmic microwave background radiation. This radiation is essentially the afterglow of the Big Bang and is remarkably consistent in all directions of the sky. Without inflation, it would be difficult to explain how regions of the universe that have never been in contact with each other could have such similar properties. Inflation theory posits that these regions were once close together but were pushed apart by the rapid expansion of the universe during the inflationary period. Another issue that inflation theory helps to resolve is the flatness of the universe. According to the theory of general relativity, the overall geometry of the universe is determined by the amount of matter and energy it contains. In a flat universe, the total energy density is precisely balanced, leading to a geometrically flat universe. Inflation theory suggests that the rapid expansion during the inflationary period would have smoothed out any deviations from flatness, leaving us with the nearly flat universe we observe today. Additionally, inflation theory provides a mechanism for the generation of primordial density perturbations, which are thought to be the seeds of structure formation in the universe. These tiny fluctuations in the early universe eventually grew into galaxies, clusters of galaxies, and large-scale cosmic structures. Without inflation, it is challenging to explain how these density perturbations could have arisen on the scales observed today.- Inflation theory offers a compelling explanation for the rapid expansion of the universe in its early moments after the Big Bang. By invoking the concept of an inflaton field and exponential expansion, inflation theory helps to address several key cosmological puzzles, including the uniformity of the cosmic microwave background radiation, the flatness of the universe, and the generation of primordial density perturbations.
