Sharpening the constraint

Significant reductions in the error budget for the Key Project can be achieved through infrared measurements of the Cepheids. This work is in progress with NICMOS (Freedman etal.2000) and stands to improve our knowledge of the chemical composition sensitivity of the Cepheid period luminosity relation and the extinction correction of the Cepheid distances. Extension of the range of the Cerro Calan / CTIO supernova survey (Hamuy etal.1996) and the Mount Stromlo Abell cluster supernova survey (Reiss etal. 1998) will ensure that H₀ measured locally is as close as needs be to the global value. Improved photometric calibration will reduce a most unwelcome contribution to the Key Project's error budget (Mould etal.1999).

In the longer term constraints on H₀ will emerge from the study of the surface of last scattering of the cosmic microwave background (CMB). The CBI and MAP missions expect to achieve results comparable in precision to the Key Project, but PLANCK plans to reach 1%. More conventionally, SIM will measure such accurate Cepheid parallaxes that the Key Project's dependence on the LMC for the basic reference distance will be lifted. Additional supernova calibrators can be expected to halve the uncertainty in the Hubble Constant over HST's lifetime. And SIM's accurate globular cluster parallaxes will halve the uncertainty in their ages.

Although future work on the CMB promises to constrain all the cosmological parameters to great accuracy, the surface of last scattering features at least as much interesting physics as the interiors of globular cluster stars. It will therefore be useful to pursue the conventional constraints on H₀t₀ as a parallel consistency check.

© Copyright Astronomical Society of Australia 1997