“So any monkey business that we do in the early universe has to decay away.”Īlongside early dark energy, theorists have put forward other exotic forms of dark energy-such as quintessence and phantom dark energy-that also change as the universe ages. “The history of the universe since recombination is pretty consistent with the standard model,” said Kamionkowski, who coauthored a paper on early dark energy published in Physical Review Letters last June. But in all cases, the early dark energy must disappear after a few hundred thousand years, during an epoch known as recombination. Each of these additions to the standard model takes a different mathematical form-in some, the density of dark energy oscillates, or rocks, while in others it rolls down from a high value to zero. Randall and her collaborators devised what they call “rock ’n’ roll” solutions to the Hubble tension in a paper submitted to the Journal of High Energy Physics. “I would rather have two discrepancies explained by one parameter.”
DID THE UNIVERSE EXPAND FASTER THAN LIGHT FREE
“In this case, you add two free parameters in order to resolve one discrepancy-and I’m uneasy about that,” he said, comparing decaying dark matter to the epicycles in Ptolemy’s Earth-centric model of the universe. Yet he remains dissatisfied with the decaying dark matter idea, in part because it introduces two new uncertain quantities into the equations. “If you add this ingredient to the standard model of cosmology, everything holds together,” Loeb said. “It’s very easy to come up with all kinds of slight modifications,” said Marc Kamionkowski, a theoretical physicist at Johns Hopkins University-but it’s hard to do so, he said, without ruining the model’s perfect fit with a wealth of other astronomical observations.īy varying the decay rate and the amount of dark matter that is lost in each decay, Loeb and colleagues selected a model of decaying dark matter that they say still agrees with other astronomical observations.
As more and more dark matter decayed over time, they reasoned, its gravitational pull would have lessened, and thus the expansion of the universe would have sped up, relieving the Hubble tension.īut making small changes like this to the standard cosmological model can have unwanted knock-on effects. In a paper published last summer in Physical Review D, Loeb and two collaborators considered a form of dark matter that decays into a lighter particle and a massless particle known as a dark photon.
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