"Both the ACT and the South Pole Telescope are designed to map the cosmic microwave background (CMB), primordial radiation sometimes described as the afterglow of the Big Bang. The CMB is one of the pillars of cosmologists’ understanding of the Universe.
By mapping subtle variations in the CMB across the sky, researchers have found compelling evidence for the ‘standard model of cosmology’. This model describes the evolution of a Universe containing three primary ingredients: dark energy; the equally mysterious dark matter, which is the primary cause of the formation of galaxies; and ordinary matter, which accounts for less than 5% of the Universe’s total mass and energy.
Current state-of-the-art CMB maps were provided by the European Space Agency’s Planck mission, which was active between 2009 and 2013.
Calculations based on Planck data predict — assuming that the standard model of cosmology is correct — exactly how fast the Universe should be expanding now. But for the past decade or so, increasingly accurate measurements of that expansion, based on observations of supernova explosions and other techniques, have found it to be 5–10% faster.
Theorists have suggested a plethora of modifications to the standard model that could explain this difference. Two years ago, cosmologist Marc Kamionkowski at Johns Hopkins University in Baltimore, Maryland, and his collaborators suggested an extra ingredient for the standard model4. Their ‘early dark energy’ — which made more precise an idea that they and other teams had been working on for several years — would be a sort of fluid that permeated the Universe before withering away within a few hundred thousand years of the Big Bang. “It’s not a compelling argument, but it’s the only model we can get to work,” says Kamionkowski." [1]
1. Nature 597, 460-461 (2021)
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