In recent research, an international group of physicists from the H0LiCOW collaboration used NASA's Hubble Space Telescope and the Spitzer Space Telescope to estimate the universe's current expansion rate.
This newly calculated number — about 71.9 kilometers per second per megaparsec — is in agreement with last year's measurements by a research team led by Nobel laureate Adam Riess. But the new measurement does not match the rate estimated by the ESA's Planck satellite mission in 2015.
"You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right. But now the ends are not quite meeting in the middle and we want to know why," Riess said, at the time of the Hubble constant measurement last year.
H0LiCOW analyzed the way that light bends around the gravity wells of five distant galaxies, specifically chosen for their positions between us and more distant superbright galactic cores known as quasars.
The Planck number, however, is a projection based on the observation of the cosmic background radiation, the light left over from the Big Bang.
"The expansion rate of the universe is now starting to be measured in different ways with such high precision that actual discrepancies may possibly point towards new physics beyond our current knowledge of the universe," said lead author of the new study, Sherry Suyu, of the Max Planck Institute.
The shift in the estimate of the Hubble constant challenges our current understanding of the cosmos, indicating that the universe is likely much more complex than currently thought.