Speed of Gravity and Cosmology Constraints from Binary Neutron Stars using Time Delays between Gravitational Waves and Short Gamma-ray Bursts

Abstract The landmark detection of a gravitational wave (GW) from the Binary Neutron Star Merger (BNS) GW170817 and its electromagnetic counterparts allowed us to study the Universe in an entirely new way. Among the several discoveries made possible by GW170817, we can find the tightest constraints on the speed of gravity and the measurement of the Hubble constant H 0 . Both of these measurements were made thanks to several assumptions and conditions that might not hold for future detections. Specifically, the speed of gravity was measured using agnostic assumptions on the prompt time delay between the GW and the short Gamma-ray burst (sGRB), while the measurement of H 0 was made thanks to the identification of the source host galaxy. We will show that relaxing the assumptions on the GW-sGRB prompt time delay, as well as the need for the identification of the host galaxy, still allows us to infer the speed of gravity and H0 with populations of GW sources. By simulating populations of GW-sGRB detections with future observing runs, we find that: (i) it will be possible to jointly fit the GW-sGRB prompt time delay distribution together with the speed of gravity and (ii) it will be possible to measure the Universe expansion even when the source host galaxy is not observed. In particular, the latter result is made possible thanks to a new technique that we explored, which can assign a redshift to the source using the GW-sGRB observed time delay.