Casertano et al. have used Gaia to provide a small but important update in the debate over the value of the Hubble Constant. The ESA Gaia mission is measuring parallaxes for billions of stars. This is fundamental data that will advance astronomy in many ways, no doubt settling long standing problems but also raising new ones – or complicating existing ones.

Traditional measurements of the H0 are built on the distance scale ladder, in which distances to nearby objects are used to bootstrap outwards to more distant ones. This works, but is also an invitation to the propagation of error. A mistake in the first step affects all others. This is a long-standing problem that informs the assumption that the tension between H0 = 67 km/s/Mpc from Planck and H0 = 73 km/s/Mpc from local measurements will be resolved by some systematic error – presumably in the calibration of the distance ladder.

Well, not so far. Gaia has now measured enough Cepheids in our own Milky Way to test the calibration used to measure the distances of external galaxies via Cepheids. This was one of the shaky steps where things seemed most likely to go off. But no – the scales are consistent at the 0.3% level. For now, direct measurement of the expansion rate remains H0 = 73 km/s/Mpc.

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2 thoughts on “Cepheids & Gaia: No Systematic in the Hubble Constant

  1. So, what would happen to the Standard Model of Cosmology if one assumed that the Planck measure is wrong and we used 73 km/s/Mpc as the value for H0? What problem(s) would this cause?

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