Göttingen 2025 – scientific programme
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GR: Fachverband Gravitation und Relativitätstheorie
GR 15: Cosmo III
GR 15.3: Talk
Friday, April 4, 2025, 11:40–12:00, ZHG008
Orientation systematics in the multi-messenger inference of the Hubble constant — •Michael Müller — Universität Greifswald, Greifswald, Germany
Multi-messenger observations of coalescing binary neutron stars are a direct probe of the expansion history of the universe and carry the potential to shed light on the disparity between low- and high-redshift measurements of the Hubble constant H0. To measure the value of H0 with such observations requires pristine inference of the luminosity distance and the true source redshift with minimal impact from systematics. A significant uncertainty in the measurement of the former with gravitational waves (GWs) arises from the poorly constrained orientation of the merging binary system relative to the observer. However, observations of the electromagnetic (EM) counterpart emission from the highly collimated relativistic jet, present in the post-merger phase, can provide strong constraints on the orientation of the source and thus inform the distance inference from the GW data. In arXiv:2406.11965, we investigate the consequences of a potential disparity between system orientations obtained from the EM and GW data, which, if not carefully treated when combining observations, can bias the inferred value of H0. Already small misalignments of 3∘−6∘ between the inherent system orientations for the GW and EM emission can bias the inference by O(1 − 2σ) if not taken into account. I will discuss complications with the interpretation of the system orientation in the post-merger phase and present a summary of the core findings of this investigation.
Keywords: compact binary mergers; cosmology; gravitational waves; relativistic jets