The discovery of the cosmic microwave background (CMB) radiation provided the most compelling evidence for the Big Bang theory, yet it also introduced a significant cosmological puzzle: the horizon problem. When astronomers map the CMB, which is the afterglow of the early universe from approximately 380,000 years after its inception, they find that its temperature is remarkably uniform across the entire sky. This uniformity suggests that the early universe reached a state of thermal equilibrium. However, according to the standard Big Bang model, distant regions of the sky were too far apart for light, and thus information, to have traveled between them in the time available. Without communication, these regions should not have been able to achieve the same temperature. To resolve this discrepancy, the theory of cosmic inflation was proposed in the early 1980s. Inflation suggests that for an infinitesimal fraction of a second, the universe underwent an exponential expansion, growing by a factor of at least 10 to the 26th power. This rapid expansion would have allowed a tiny, homogenous patch of the early universe, which had already achieved thermal equilibrium, to be stretched across the entire observable horizon. While inflation elegantly solves the horizon problem, it remains a subject of intense scrutiny because its underlying mechanism is not fully understood. Proponents point to the fact that inflation also explains the flatness of space and the origin of large-scale structures like galaxies. Critics, however, argue that some versions of inflation lead to the existence of a multiverse—a concept that is notoriously difficult to test empirically. Despite these debates, inflation remains the leading framework for understanding the first moments of the universe, and upcoming observations of gravitational waves from the early universe may finally provide the evidence needed to confirm or refine the theory.