No, it’s not constant! More

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Nick James

No, it’s not constant! More correctly it should be called the Hubble parameter and it evolves with time but the odd thin is that we have two discordant measurements at the current epoch.

These are two very accurate measurements made in two different ways that don’t agree at a significant statistical level. Planck gives 67.7 km/s/Mpc from observations of the Cosmic Microwave Background and Gaia/HST gives 73.5 km/s/Mpc from more direct observations of Supernova redshifts. The two are very different measurements. I think the Planck measurement assumes a particular cosmological model and fits a power spectrum to the variations in the CMB and then infers H from that. The Gaia/HST measurements use a conventional redshift v distance approach but, with Gaia’s parallax measurements and HST’s Cepheid measurements, we now have a much better distance ladder to the distant SNe.

All interesting stuff. Cosmologists and astronomers will never be out of a job.