A long-standing disparity between different measurements of the cosmic expansion rate might be explained, at least in part, by the existence of a “mirror world” containing copies of all known particles. That is the conclusion of three physicists in the US, who have shown how to reconcile contrasting values of the Hubble constant obtained from observations of the nearby and distant universe. Their scheme involves recalibrating the size of the universe without altering other cosmic parameters, but they caution that more research is needed to understand exactly how light and matter interacted at the dawn of time.
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