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Astronomy

Using the Cosmic Microwave Background to Constrain the Hubble Constant

Quick fact

The Planck satellite's measurement of the cosmic microwave background gives a Hubble constant of about 67.4 km/s/Mpc, which is about 8% smaller than the 73 km/s/Mpc value inferred from local distance measurements—a discrepancy known as the Hubble tension.

Why this is interesting

The oldest light in the universe carries a hidden measurement of how fast it is expanding today. But that measurement disagrees with what we see in our cosmic backyard.