Speaker
Description
The time delay between the images of gravitationally lensed quasars probes a combination of angular diameter distances and the mass profile of the lens galaxy. Some observational campaigns to measure such systems have reported a determination of the Hubble parameter H0 in tension with determinations based on the cosmic microwave background and large scale structure. I will discuss challenges and opportunities associated with the lensing H0 measurements. A central challenge facing the method is the mass sheet degeneracy (MSD). To mitigate the MSD, lensing collaborations must study and constrain cosmological weak lensing distortions to the strong lensing system, which can bias the inferred value of H0 at the few percent level. I will show that in current H0 measurements, the weak lensing corrections have not been dealt with in full. Another related challenge is due to uncertainties in the intrinsic density profile of the lens galaxy, notably the possibility of an extended core feature, which could act as an approximate MSD. I will review how a careful account of this possible systematic effect in the observational data analysis had alleviated a previous claim for a lensing H0 tension. The core-MSD effect is also an interesting opportunity: it could suggest a non-minimal model of dark matter, perhaps a clue for a small relic abundance of ultralight axions.
