Previously, cosmologists imagined dark energy as a steady breeze, uniformly inflating the Universe like a balloon. The discovery of accelerated expansion, made by Adam Riess and Saul Perlmutter at the end of the 20th century, only reinforced this picture.
But a summary of fresh data, from supernova explosions to galaxy maps and echoes of the Big Bang, tells a different story. Studying the light of millions of galaxies (the DESI instrument, spectroscopy) revealed: the force inflating the balloon was pulsing. In the past, the pressure was slightly weaker, then strengthened, but now it may be weakening again.
If dark energy is indeed inconstant, the Universe's finale could be different: expansion isn't eternal — after trillions of years it might slow down, and galaxies will start approaching each other, triggering a reverse collapse.