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New type of dark energy could solve Universe expansion mystery

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EXCERPTS: Cosmologists have found signs that a second type of dark energy — the ubiquitous but enigmatic substance that is pushing the current Universe’s expansion to accelerate — might have existed in the first 300,000 years after the Big Bang.

Two separate studies — both posted on the arXiv preprint server in the past week — have detected a tentative first trace of this ‘early dark energy’ in data collected between 2013 and 2016 by the Atacama Cosmology Telescope (ACT) in Chile. If the findings are confirmed, they could help to solve a long-standing conundrum surrounding data about the early Universe, which seem to be incompatible with the rate of cosmic expansion measured today. But the data are preliminary and don’t show definitively whether this form of dark energy really existed.

“There are a number of reasons to be careful to take this as a discovery of new physics,” says Silvia Galli, a cosmologist at the Paris Institute of Astrophysics.

The authors of both preprints — one posted by the ACT team, and the other by an independent group — admit that the data are not yet strong enough to detect early dark energy with high confidence. But they say that further observations from the ACT and another observatory, the South Pole Telescope in Antarctica, could provide a more stringent test soon. “If this really is true — if the early Universe really did feature early dark energy — then we should see a strong signal,” says Colin Hill, a co-author of the ACT team’s paper who is a cosmologist at Columbia University in New York City.

[...] Hill says that he was previously sceptical about early dark energy, and that his team’s findings surprised him. Vivian Poulin, an astrophysicist at the University of Montpellier in France and a co-author of the second study2 based on ACT data, says it was reassuring that his team’s analysis agreed with the ACT team’s own. “The lead authors are very, very hard-nosed, conservative people, who really understand the data and the measurements,” Kamionkowski says.

But Galli warns that the ACT data seem to be inconsistent with calculations by the Planck team, which she was part of. And although the ACT’s polarization data might favour early dark energy, it is unclear whether its other major set of data — its map of CMB temperatures — shows such a preference. For these reasons, she adds, it will be crucial to cross-check the results using the South Pole Telescope, an experiment she is part of.

Wendy Freedman, an astronomer at the University of Chicago in Illinois who has contributed to some of the most precise measurements of cosmic expansion, says that the ACT-based results are interesting, if preliminary. “It is important to pursue different models” and compare them with the standard one, she says... (MORE - missing details)


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