Astrolunch: Suhail Dhawan (Stockholm)

Date: 
Tue, 07/07/2020 - 12:30 to 13:30

Title: The Hubble constant with explosive transients

Abstract: The recent discrepancy between local measurements of the Hubble constant and the inferred, global value from the cosmic microwave background could indicate new physics, e.g. additional neutrino species, phantom dark energy. However, the explanation could be more mundane, e.g. unknown systematic uncertainties. Hence, it is critical to measure H₀ from completely independent cosmological probes to resolve the cause of the tension.

In this talk, I will summarise my previous work on two novel probes of H₀, namely gravitationally lensed Type Ia supernovae, and gravitational wave standard sirens. I will present the time-delay, lensing magnification and extinction estimates for the first ever resolved, strongly lensed Type Ia supernova, iPTF16geu. I will also present the impact of combining the electromagnetic signal from the kilonova associated with GW170817 on the inferred value of H₀. This technique using the kilonova colours is important for future EMGW events, since LIGO O3 is finding events at significantly larger distances than GW170817, making radio observations challenging.

Finally, in my talk, I will look into unaccounted-for source of systematic uncertainty in the local distance ladder estimate of H₀, e.g. the assumption of the cosmological model and covariance between supernovae in the Cepheid calibrator galaxies and Hubble flow. We find that the systematic shift in the inferred H₀ depending on the assumed cosmological model is very small and cannot account for the observed tension.

Publications:

Magnification, dust and time-delay constraints from the first resolved strongly lensed Type Ia supernova
https://arxiv.org/abs/1907.06756

Constraining the observer angle of the kilonova AT2017gfo associated with GW170817: Implications for the Hubble constant
https://arxiv.org/abs/1909.13810

Cosmological model insensitivity of local H0 from the Cepheid distance ladder
https://arxiv.org/abs/2001.09260