James' World 2

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Imagine sitting in the center of a firework that has just exploded. After the first flash of light and heat, sparks fly off in all directions, with some streaming together into fiery filaments and others fading quickly into cold, ashy oblivion. After a moment more, the smoke is all that remains—the echo, if you will, of the firework’s big bang.

Now imagine the firework is the universe, which scientists think began with a similar explosion. Where the firework’s expansion is propelled by a chemical reaction, the expansion of the cosmos comes from the energy of empty space itself. From where we sit, it seems that the universe is expanding in all directions, faster and faster at every moment.

This spring scientists announced that something is wrong with the fireworks. For the first time since the discovery of dark energy—the mysterious force that is accelerating our cosmic fireworks show—cosmologists think we may be on the cusp of something new. Two prominent dark energy surveys seeking to measure the nature of this force found evidence that dark energy seems to have weakened over time.

“If it is true, it is a big deal,” says Licia Verde, a theoretical cosmologist at the Institute of Cosmos Sciences of the University of Barcelona in Spain and a member of the collaboration reporting the oddity. “But as usual, extraordinary claims require extraordinary proofs.”

Dark energy was assumed to be a constant force in the universe, as unchanging and reliable as the forward march of time. If the new results are right, it is changeable after all. “It’s mega important,” says Paul J. Steinhardt, a cosmologist at Princeton University, who did not work on the data, adding that this is only true if the results hold up to scrutiny. “But it’s still early days.”

The news is based on a combination of two dark energy studies, called the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI), with a third set of preexisting data. DES measures distant supernovae, and the DESI experiment measures galaxies and sound waves from the early universe. The third component measures the cosmic microwave background (CMB)—the smoke ring of the cosmic firework.

DES yielded new findings back in February, and DESI came out with novel results in April. The DESI data produced a detailed three-dimensional map of the universe. It showed that galaxies appear to be spread apart less than they should be if dark energy’s role was unchanging through cosmic time.

The DESI telescope is perched on Kitt Peak in Arizona and measures the positions of millions of galaxies as they existed between 12 billion and two billion years ago. Astronomers compared these observed galactic locales against where galaxies are expected to be based on dark energy predictions and saw the lack of expected spreading.

A bigger surprise came when cosmologists combined the DESI galaxies, DES’s supernovae, and the cosmic microwave background. The map of reality began to drift apart from theory.

Theorists have been buzzing since if the results are true, a bedrock assumption of cosmology is incorrect. Scientists might have to throw out the widely held idea that dark energy is a “cosmological constant”—a static element of the universe.

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Vitalii Pasichnyk/Getty Images

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