What short, extremely rapid expansion in the very early universe explains why the universe looks so flat and smooth on the largest scales today?

The short, extremely rapid expansion in the very early universe that explains why the universe looks so flat and smooth on the largest scales today is called Cosmic Inflation. It was an epoch of incredibly fast, exponential expansion of spacetime that occurred immediately after the Big Bang, lasting from approximately 10^-36 seconds to 10^-32 seconds after the universe's birth. During this fleeting period, the universe expanded by a factor of at least 10^26, growing from subatomic scales to roughly the size of a grapefruit. This immense expansion addresses two major puzzles: the flatness problem and the horizon problem, which explain the universe's observed large-scale flatness and smoothness. The energy driving inflation is theorized to have come from a hypothetical scalar field called the 'inflaton field.'

Flatness: The 'flatness' of the universe refers to its overall geometry. A flat universe is one where the geometry of space is Euclidean, meaning parallel lines never meet, much like on a flat sheet of paper. Observations of the Cosmic Microwave Background (CMB) indicate that the universe's geometry is extremely close to flat. Without inflation, the universe's initial energy density would have needed to be fine-tuned to an almost impossibly precise degree for it to appear flat today. Inflation solves this by dramatically stretching spacetime. Imagine a small, wrinkled balloon representing a curved universe. If you inflate it to an enormous size, any small patch of its surface would appear virtually flat. Similarly, inflation stretched a tiny, potentially curved region of the early universe so vastly that any local curvature within the observable universe was effectively diluted to near zero, making it appear flat on cosmic scales.

Smoothness: The 'smoothness' of the universe on the largest scales refers to its observed homogeneity (looking the same in all places) and isotropy (looking the same in all directions) when averaged over vast distances. This uniformity is most strikingly seen in the Cosmic Microwave Background (CMB) radiation, which shows the early universe had an almost perfectly uniform temperature across the entire sky. The 'horizon problem' highlights that without inflation, regions of the CMB currently visible from opposite sides of the sky would not have been in causal contact (meaning light or information couldn't travel between them) since the Big Bang. Therefore, they shouldn't have had enough time to equalize their temperatures. Inflation resolves this by proposing that the entire observable universe originated from a very tiny region that *was* in causal contact and could thermalize (reach a uniform temperature) *before* inflation began. This small, uniform patch then underwent extreme exponential expansion, stretching its inherent smoothness and uniformity across the immense distances we observe today. Any initial irregularities or inhomogeneities within this pre-inflationary patch were stretched to such large scales that they are now imperceptible over the vast cosmological distances, resulting in the observed large-scale smoothness.