The Impact of Dust on Cepheid and Type Ia Supernova Distances

Milky-Way and intergalactic dust extinction and reddening must be accounted for in measurements of distances throughout the universe. This work provides a comprehensive review of the various impacts of cosmic dust focusing specifically on its effects on two key distance indicators used in the distance ladder: Cepheid variable stars and Type Ia supernovae. We review the formalism used for computing and accounting for dust extinction and reddening as a function of wavelength. We also detail the current state of the art knowledge of dust properties in the Milky Way and in host galaxies. We discuss how dust has been accounted for in both the Cepheid and SN distance measurements. Finally, we show how current uncertainties on dust modeling impact the inferred luminosities and distances, but that measurements of the Hubble constant remain robust to these uncertainties.Comment: Invited chapter for the edited book {\it Hubble Constant Tension} (Eds. E. Di Valentino and D. Brout, Springer Singapore, expected in 2024

A test of Gaia Data Release 1 parallaxes: implications for the local distance scale

We present a comparison of Gaia Data Release 1 (DR1) parallaxes with photometric parallaxes for a sample of 212 Galactic Cepheids at a median distance of 2~kpc, and explore their implications on the distance scale and the local value of the Hubble constant H_0. The Cepheid distances are estimated from a recent calibration of the near-infrared Period-Luminosity P-L relation. The comparison is carried out in parallax space, where the DR1 parallax errors, with a median value of half the median parallax, are expected to be well-behaved. With the exception of one outlier, the DR1 parallaxes are in remarkably good global agreement with the predictions, and the published errors may be conservatively overestimated by about 20%. The parallaxes of 9 Cepheids brighter than G = 6 may be systematically underestimated, trigonometric parallaxes measured with the HST FGS for three of these objects confirm this trend. If interpreted as an independent calibration of the Cepheid luminosities and assumed to be otherwise free of systematic uncertainties, DR1 parallaxes would imply a decrease of 0.3% in the current estimate of the local Hubble constant, well within their statistical uncertainty, and corresponding to a value 2.5 sigma (3.5 sigma if the errors are scaled) higher than the value inferred from Planck CMB data used in conjunction with Lambda-CDM. We also test for a zeropoint error in Gaia parallaxes and find none to a precision of ~20 muas. We caution however that with this early release, the complete systematic properties of the measurements may not be fully understood at the statistical level of the Cepheid sample mean, a level an order of magnitude below the individual uncertainties. The early results from DR1 demonstrate again the enormous impact that the full mission will likely have on fundamental questions in astrophysics and cosmology.Comment: A&A, submitted, 6 pages, 3 figure

Type Ia Supernova Distances at Redshift >1.5 from the Hubble Space Telescope Multicycle Treasury Programs: The Early Expansion Rate

We present an analysis of 15 Type Ia supernovae (SNe Ia) at redshift z 〉 1 (9 at 1.5 〈 z 〈 2.3) recently discovered in the CANDELS and CLASH Multi-Cycle Treasury programs using WFC3 on the Hubble Space Telescope. We combine these SNe Ia with a new compilation of ~1050 SNe Ia, jointly calibrated and corrected for simulated survey biases to produce accurate distance measurements. We present unbiased constraints on the expansion rate at six redshifts in the range 0.07 〈 z 〈 1.5 based only on this combined SN Ia sample. The added leverage of our new sample at z 〉 1.5 leads to a factor of ~3 improvement in the determination of the expansion rate at z = 1.5, reducing its uncertainty to ~20%, a measurement of H(z=1.5)/H_0 = 2.69_(-0.52)^(+0.86). We then demonstrate that these six derived expansion rate measurements alone provide a nearly identical characterization of dark energy as the full SN sample, making them an efficient compression of the SN Ia data. The new sample of SNe Ia at z 〉 1.5 usefully distinguishes between alternative cosmological models and unmodeled evolution of the SN Ia distance indicators, placing empirical limits on the latter. Finally, employing a realistic simulation of a potential Wide-Field Infrared Survey Telescope SN survey observing strategy, we forecast optimistic future constraints on the expansion rate from SNe Ia