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Clocking the universe’s expansion

Conflicting estimates of the Hubble constant — a measure of the speed of the universe’s expansion — have sparked debate among cosmologists. The discrepancy in data from the Planck satellite with data from other methods may be just a mistake, or it may require adjusting the cosmological model of the universe, as Tom Siegfried explained in “Cosmic question mark” (SN: 4/5/14, p.