The Universe's expansion is accelerating — a discovery that earned the Nobel Prize for Saul Perlmutter, Brian Schmidt, and Adam Riess. The cause is called dark energy — an invisible force that pushes galaxies apart. But there is a hypothesis: in the distant past, it slowed the expansion down, like a thermostat in cooling mode, and later switched to heating. This transition might have been smooth.

Scientists tested whether the smoothness could be detected. They compared data on supernovae (stellar beacon explosions), redshifts of galaxies, and the growth of dark matter structures. It turned out the switch happened when the Universe was about 3.5 times younger, and dark matter was calling the shots — dark energy was just waking up. But current instruments cannot see the difference between a smooth and abrupt scenario: the calculations barely differ.

Future sky surveys like LSST and data on the cosmic microwave background will help resolve the details. Perhaps then we'll learn just how smoothly the cosmic thermostat kicks in.