For decades, two independent methods of measuring the rate at which galaxies are flying apart, the Hubble constant, have given conflicting numbers. The situation resembled a room with two thermometers showing different temperatures. One method relied on observations of nearby supernovae, the other on analyzing ancient light, the cosmic microwave background left over from the big bang. Adam Riess, using supernovae as "standard candles," obtained a value of about 73 (in arbitrary units), while the Planck satellite, measuring tiny fluctuations in the cosmic microwave background, got 67. This discrepancy, known as the "Hubble tension," cast doubt on the accuracy of our cosmological models.

To resolve the conflict, astronomers brought in new data: observations of early galaxies from the JWST and a map of galaxy distribution from the DESI project, which records baryon acoustic oscillations — sound waves frozen into the matter of the early universe. To this they added the hypothesis of a brief flash of dark energy in the first moments after the big bang. This early dark energy is not the mysterious force accelerating the expansion of the universe today. It was a fleeting push of "anti-gravity" that, according to calculations, existed only for a short time and vanished long before the first stars appeared. The thermometer analogy helps explain the effect: imagine one of the thermometers briefly caught in a warm draft: its reading rises slightly, and the discrepancy between the devices nearly disappears. So here: the addition of early dark energy slightly alters the predicted picture, smoothing over the contradiction.

The mechanism is this: the flash of early dark energy left an imprint on the cosmic microwave background, making its patterns, temperature spots on the sky map, slightly more blurred (by a few percent). This effect, detected in Planck data, when recalculating the Hubble constant, reduces the gap with supernova measurements. Thus, the Standard Model of cosmology, augmented by this hypothesis, gets a fine-tuning, and the expansion of the universe is described without internal conflicts. It's important to understand: the thermometer analogy simplifies reality, because in cosmology we are not just calibrating instruments, but revising fundamental assumptions about the physics of the early universe.