On the morning of April 4, physicists filed into a third-floor meeting room at Harvard University’s Jefferson Laboratory. Word had gotten out that there would be a big announcement from the Dark Energy Spectroscopic Instrument (DESI) collaboration, a group of physicists who are investigating dark energy — a mysterious, repulsive form of energy that permeates the universe. The meeting room at Harvard got so crowded with people wanting to watch the livestream that some sat on the floor. Everyone eventually decided to move to the bigger lecture hall downstairs.
DESI’s announcement lived up to the hype. The group’s results indicated that dark energy, which most physicists have long held to be unchanging, may in fact be weakening.
Since 1998, we’ve known that the expansion of the universe is accelerating. Dark energy is the name given to the accelerant. In the standard theoretical model of the cosmos, dark energy has a simple form: It is spread uniformly in space, maintaining a constant density at all times. Dark energy of this type, known as the cosmological constant, would not get diluted as the universe expands. Rather, as space grows, the amount of dark energy grows with it. And so this accumulating energy expands the universe with ever-increasing gusto.
But the constancy of dark energy is merely a hypothesis, one that the DESI experiment set out to check. The collaboration reported that they had so far mapped and analyzed the locations of 6.4 million galaxies to determine how fast the universe has expanded as a function of time. (More distant galaxies reveal the younger universe.) If dark energy is a cosmological constant, then the acceleration should hold steady. DESI’s data set wasn’t big enough to test this by itself, though the team will be able to do so after mapping a total of 40 million galaxies during the five-year survey. But when the scientists pooled their data with other astronomical observations, the combined data sets favored an evolving dark energy.
The findings fall short of the level of statistical certainty needed to claim a discovery. Right now, DESI is calling it a “hint” that dark energy might be weakening in strength. But according to Nathalie Palanque-Delabrouille, a cosmologist at Lawrence Berkeley National Laboratory and one of the leaders of DESI, “that hint gets stronger as we start combining different data sets. And all these data sets seem to be pointing in the same direction.”
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