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Two methods of measuring the Universe's expansion rate yield conflicting results. Measurements from supernova explosions, carried out by Adam Riess's team, show faster expansion than data from the cosmic microwave background — the light left over from the Big Bang. The discrepancy is so large that it cannot be explained by chance — it's a real crisis in cosmology.

A team of scientists spent twenty years monitoring quasars — the blazing hearts of distant galaxies, home to giant black holes. Their light isn't just bright; it constantly flickers like a flame in the wind. Moreover, massive quasars twinkle slower than smaller ones, much like a large bonfire burns more steadily than a tiny candle. By capturing this pattern with precise brightness measurements, astronomers learned to gauge distance to quasars by the rhythm of their flickering, turning them into a sort of cosmic candle.

When this new expansion map was compared with supernova data previously obtained by Saul Perlmutter and his colleagues, a striking finding emerged: dark energy — the mysterious force that pushes space apart — was not always constant. Its density seems to have changed along with the expansion of the Universe. This upends the standard model of unchanging dark energy and hints at an unexpected fate: expansion could either speed up or suddenly slow down.