Hubble's Law states that the velocity at which a galaxy recedes from an observer is directly proportional to its distance from the observer, implying that the universe is expanding. This relationship is fundamental to cosmology and provides evidence for the Big Bang theory, as it suggests that galaxies were once closer together in the past.
The Inflationary Universe theory posits that the universe underwent an exponential expansion in the first tiny fraction of a second after the Big Bang, solving several cosmological puzzles such as the horizon and flatness problems. This rapid expansion set the initial conditions for the large-scale structure of the cosmos we observe today.