What would be surprising if inflation did not occur regarding the cosmic microwave background?

The cosmic microwave background (CMB) radiation provides a snapshot of the early universe, and one of the key aspects of inflation is that it leads to a nearly homogeneous and isotropic universe. If inflation did not occur, we would expect that regions of the universe could not have shared information or exchanged light or matter in the early moments after the Big Bang.

During inflation, the rapid expansion of space pushes different regions apart so quickly that they could no longer influence each other. This would create uniformity in the CMB because all parts of the universe would have been smoothed out, regardless of their location. If inflation did not happen, then we would see considerable differences across the CMB due to regions having distinct histories and temperatures, since they would not have interacted. Thus, areas of the sky would not exhibit the same conditions, and you would not have a consistent background signal as observed today. The uniformity of the CMB implies that, despite the vast distances involved, regions have reached a thermal equilibrium due to their interactions before inflation took place.

Therefore, the expectation that locations could exchange light or matter is essential for understanding the uniformity of the CMB, making it surprising if that exchange did not occur, as it would lead to significant