Cosmological Inflation: a Simple Qualitative Explanation

Cosmological in ation: a brief reminder. In large-scale, the Universe is reasonably uniform. This observed uniformity poses a problem to cosmology; see, e.g., [3]. Indeed, it is easy to explain the uniformity of gas inside a given volume, where molecules interact with each other, as a result of which, inhomogeneities disappear. However, in cosmology, the Universe's expansion goes so fast that di erent parts of the Universe have no time to interact with each other: we observe distant quasars billions of years after they shine, so by the time our signal goes back, the Universe may have collapsed again. To explain the observed uniformity, modern cosmology has to go beyond the usual non-quantum equations, and take into account that right after the Big Bang, quantum e ects led to an exponential expansion ( cosmological in ation ); see, e.g., [2]. As a result of this expansion, spatially separated areas of the observed Universe come from the same micro-region and thus, have similar quantities.