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21st century measurements

More recent measurements from the Planck mission published in 2018 indicate a lower value of 67.66 ± 0.42 (km/s)/Mpc, although, even more recently, in March 2019, a higher value of 74.03 ± 1.42 (km/s)/Mpc has been determined using an improved procedure involving the Hubble Space Telescope. The two measurements disagree at the 4.4 σ level, beyond a plausible level of chance. The resolution to this disagreement is an ongoing area of active research.

In October 2018, scientists presented a new third way (two earlier methods, one based on redshifts and another on the cosmic distance ladder, gave results that do not agree), using information from gravitational wave events (especially those involving the merger of neutron stars, like GW170817), of determining the Hubble constant.

In July 2019, astronomers reported that a new method to determine the Hubble constant, and resolve the discrepancy of earlier methods, has been proposed based on the mergers of pairs of neutron stars, following the detection of the neutron star merger of GW170817, an event known as a dark siren. Their measurement of the Hubble constant is 73.3 +5.3 −5.0 (km/s)/Mpc.

Also in July 2019, astronomers reported another new method, using data from the Hubble Space Telescope and based on distances to red giant stars calculated using the tip of the red-giant branch (TRGB) distance indicator. Their measurement of the Hubble constant is 69.8 +1.9 −1.9 (km/s)/Mpc.

In February 2020, the Megamaser Cosmology Project published independent results that confirmed the distance ladder results and differed from the early-universe results at a statistical significance level of 95%. In July 2020, measurements of the cosmic background radiation by the Atacama Cosmology Telescope predict that the Universe should be expanding more slowly than is currently observed.

In July 2023, an independent estimate of the Hubble constant was derived from the optical counterpart of a neutron-star merger, a so-called kilonova. Due to the blackbody nature of early kilonova spectra, such systems provide strongly constraining estimators of cosmic distance. Using the kilonova AT2017gfo, these measurements indicate a local-estimate of the Hubble constant of 67.0 ± 3.6 (km/s)/Mpc.

In August, 2024. Wendy Freedman et al. prepublished a meaurement of the Hubble constant using data from the James Webb Space Telescope to re-measure the second rung of the cosmic distance ladder, using standard candle stars to measure the distances to ten nearby galaxies which have hosted type Ia supernovae, the standard candle used for the third rung. They used three different types of standard candle: tip of the red-giant branch stars, J-region asymptotic giant branch stars (carbon stars particularly bright in the J band), and the traditional Cepheid variable stars. The TRGB amd JAGB distances agreed with each other within 1%, while the Cepheid-measured distances are a few percent shorter, resulting in computed Hubble constant values of 69.85 ± 2.33, 67.96 ± 2.65, and 72.05 ± 3.62 km/s/Mpc, respectively. While not yet conclusive, this hints at the posssibility that the lower H values may be correct and that higher measurements could be due to some unknown systematic error in the measurement of Cepheid variable stars.