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TDCOSMO V: strategies for precise and accurate measurements of the Hubble constant with strong lensing
Simon Birrer, Tommaso Treu
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Strong lensing time delays can measure the Hubble constant H₀ independent of any other probe. Assuming commonly used forms for the radial mass density profile of the lenses, a 2% precision has been achieved with 7 Time-Delay Cosmography (TDCOSMO) lenses, in tension with the H₀ from the cosmic microwave background. However, without assumptions on the radial mass density profile -- and relying exclusively on stellar kinematics to break the mass-sheet degeneracy -- the precision drops to 8% with the current data of the 7 TDCOSMO lenses, insufficient to resolve the H₀ tension. With the addition of external information from 33 Sloan Lens ACS (SLACS) lenses, the precision improves to 5%, {\it if} the deflectors of TDCOSMO and SLACS lenses are drawn from the same population. We investigate the prospects to improve the precision of time-delay cosmography without relying on mass profile assumptions to break the mass sheet degeneracy. Our forecasts are based on the hierarchical framework introduced by Birrer et al. (2020). With existing samples and technology, 3.3% precision on H₀ can be reached by adding spatially resolved kinematics of the 7 TDCOSMO lenses. The precision improves to 2.5% with the further addition of kinematics for 50 non-time-delay lenses from SLACS and the Strong Lensing Legacy Survey (SL2S). Expanding the samples to 40 time delay and 200 non-time delay lenses will improve the precision to 1.5% and 1.2%, respectively. Time-delay cosmography can reach sufficient precision to resolve the Hubble tension at 3-5σ, without assumptions on the radial mass profile of lens galaxies. By obtaining this precision with and without external datasets, we will test the consistency of the samples and enable further improvements based on even larger future samples of time delay and non-time-delay lenses (e.g. from the Rubin, Euclid, and Roman Observatories).
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