Impact of the 3D source geometry on time-delay measurements of lensed type-Ia supernovae

It has recently been proposed that gravitationally lensed type-Ia supernovae can provide microlensing-free time-delay measurements provided that the measurement is taken during the achromatic expansion phase of the explosion and that color light curves are used rather than single-band light curves. If verified, this would provide both precise and accurate time-delay measurements, making lensed type-Ia supernovae a new golden standard for time-delay cosmography. However, the 3D geometry of the expanding shell can introduce an additional bias that has not yet been fully explored. In this work, we present and discuss the impact of this effect on time-delay cosmography with lensed supernovae and find that on average it leads to a bias of a few tenths of a day for individual lensed systems. This is negligible in view of the cosmological time delays predicted for typical lensed type-Ia supernovae but not for the specific case of the recently discovered type-Ia supernova iPTF16geu, whose time delays are expected to be smaller than a day.

H0LiCOW-XIII. A 2.4 per cent measurement of H-0 from lensed quasars: 5.3 sigma tension between early- and late-Universe probes

Vivien François Bonvin, Hung-Hsu Chan, Frédéric Courbin, Georges Meylan, Martin Raoul Robert Millon, Olga Tihhonova

We present a measurement of the Hubble constant (H-0) and other cosmological parameters from a joint analysis of six gravitationally lensed quasars with measured time delays. All lenses except the first are analysed blindly with respect to the cosmological parameters. In a flat Lambda cold dark matter (Lambda CDM) cosmology, we find H-0 = 73.3(-1.8)(+1.7) km s(-1) Mpc(-1), a 2.4 per cent precision measurement, in agreement with local measurements of H-0 from type Ia supernovae calibrated by the distance ladder, but in 3.1 sigma tension with Planck observations of the cosmic microwave background (CMB). This method is completely independent of both the supernovae and CMB analyses. A combination of time-delay cosmography and the distance ladder results is in 5.3 sigma tension with Planck CMB determinations of H-0 in flat Lambda CDM. We compute Bayes factors to verify that all lenses give statistically consistent results, showing that we are not underestimating our uncertainties and are able to control our systematics. We explore extensions to flat Lambda CDM using constraints from time-delay cosmography alone, as well as combinations with other cosmological probes, including CMB observations from Planck, baryon acoustic oscillations, and type Ia supernovae. Time-delay cosmography improves the precision of the other probes, demonstrating the strong complementarity. Allowing for spatial curvature does not resolve the tension with Planck. Using the distance constraints from time-delay cosmography to anchor the type Ia supernova distance scale, we reduce the sensitivity of our H-0 inference to cosmological model assumptions. For six different cosmological models, our combined inference on H-0 ranges from similar to 73 to 78 km s(-1) Mpc(-1), which is consistent with the local distance ladder constraints.

OXFORD UNIV PRESS2020

H0LiCOW-XIII. A 2.4 per cent measurement of H-0 from lensed quasars: 5.3 sigma tension between early- and late-Universe probes

Vivien François Bonvin, Hung-Hsu Chan, Frédéric Courbin, Georges Meylan, Martin Raoul Robert Millon, Olga Tihhonova

OXFORD UNIV PRESS2020