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ACME - Multi-Messenger Inference with Bilby: From Gravitational Waves to the Hubble Constant
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Description
This hands-on tutorial explores how gravitational-wave and electromagnetic observations can be combined within a Bayesian framework to infer source properties and constrain the Hubble constant.
Building on gravitational-wave parameter estimation with Bilby (tutorial of 29 October), participants work through a guided analysis inspired by the binary neutron-star merger GW170817. The tutorial begins with the degeneracy between luminosity distance and binary inclination, examining how electromagnetic information can improve these measurements. Using prepared gravitational-wave posterior samples and simplified electromagnetic models, participants construct joint analyses, identify shared parameters, and account consistently for prior assumptions. Particular attention is given to the conditions under which observations can be combined, the avoidance of double-counting information, and the dependence of inferred constraints on assumptions about source geometry.
The analysis then extends to standard-siren cosmology: combining gravitational-wave distance information with the redshift of a host galaxy to infer the Hubble constant, including uncertainty from peculiar velocities. Through controlled variations of the electromagnetic constraints and modelling assumptions, participants investigate how statistical uncertainties and systematic effects propagate into the cosmological result, and how selection effects enter when many events are combined.
The goal is to provide a practical understanding of multi-messenger inference and enable participants to build, interpret, and critically assess joint analyses with Bilby.
- 09:30 Welcome, ACME introduction, environment check.
- 09:45 Lecture — combining observations, double counting, standard sirens.
- 10:30 Lab 1 — the distance–inclination degeneracy.
- 11:00 Lab 2 — an electromagnetic constraint on the viewing angle.
- 11:45–12:00 Break.
- 12:00 Lab 3 — the Hubble constant from one event.
- 12:55 Lab 4 — many sirens and the selection effect.
- 13:15 Wrap-up and closing discussion.
Note: times are indicative.
Preparation. Same environment as the tutorial of 29 October (uv, Python 3.11, bilby==2.8.0, lalsuite, gwpy, dynesty; on Windows use WSL). Scripts and the prepared posterior samples will be attached to this page. The connection link is sent by email to registered participants.
