Exploring the calibration of cosmological probes used in gravitational-wave and multi-messenger astronomy

The field of gravitational wave astronomy has grown remarkably since the first direct detection of gravitational waves on 14th September 2015. The signal, originating from the merger of two black holes, was detected by the two US-based Advanced LIGO interferometers in Hanford (Washington State) and Livingston (Louisiana). The second observing run of the Advanced LIGO and Virgo detectors marked the first detection of a binary neutron star merger, along with its electromagnetic counterparts. The optical follow-up of the merger led to the first confirmed observations of a kilonova, an electromagnetic counterpart to binary neutron star and neutron star-black hole mergers whose existence was first predicted in 1970s. Following the multimessenger observations of the binary neutron star merger GW170817, constraints were put on the rate of expansion of the Universe using both gravitational wave and electromagnetic data. These measurements could help us understand the current tension between early-Universe and late-Universe measurements of the Hubble constant H0. The use of gravitational wave signals for measuring the rate of expansion of the Universe was proposed by Schutz in 1986. Compact binary coalescences can be used as distance markers, a gravitational wave analogue to standard candles: "Standard Sirens". Measurements of the Hubble constant from standard sirens are independent from previous methods of constraining H0. Bright sirens are gravitational wave signals that are detected coincidentally with electromagnetic signatures. These "bright" gravitational wave sirens are powerful cosmological probes, allowing us to extract information on both the distance and the redshift of the source. It is therefore important to maximise these coincident detections, and to carefully calibrate the data extracted from any standard siren. The work presented in this thesis can be divided into three main topics, all under the umbrella of maximising scientific returns from observations of compact binary coalescences. These three topics are: kilonova parameter estimation, cosmology with gravitational waves, and calibration of advanced gravitational wave detectors. We present work on inferring parameters from kilonova light curves. Ejecta parameters and information about the merging time of the progenitor is extracted from simulated kilonova light curves. We explore the consequence of neglecting some aspects of microphysics on the resulting parameter estimation. We also present new results on the inference of the Hubble constant through the application of a robust test of galaxy catalogue completeness to the current gravitational wave cosmology pipeline. We explore the impact of adopting a robust estimate of the apparent magnitude threshold mthr for the galaxy catalogues used in gravitational wave cosmology on the final inference of the Hubble constant H0 from standard sirens, and compare the results to those obtained when adopting a conservative estimate for mthr. Finally, we present the first results from the prototype of a Newtonian Calibrator at the LIGO Hanford detector. Calibrating the LIGO detectors is crucial to the extraction of the gravitational wave source parameters that are used in cosmology with standard sirens

Investigating the prospects for constraining the Hubble constant using compact binary coalescences as standard sirens

The use of gravitational wave observations from compact binary inspirals as standard sirens was first proposed by Schutz in 1986. Following the recent observations of compact binary coalescences by the Advanced LIGO detectors and the first standard siren measurement of the Hubble constant with the binary neutron star merger GW170817, and in anticipation of future detections during upcoming observing runs, it is useful to further investigate standard sirens, the gravitational wave analogues of standard candles, as an alternative way to measure the Hubble constant. Compact binary inspirals are well modelled, and their luminosity distance can be obtained from GW observations. From these distance measurements and using redshifts from EM galaxy catalogues and Bayesian inference, it is possible to assign a probability to each host galaxy, and a value for the Hubble constant can be obtained. While a redshift can sometimes be obtained from multi-messenger observations of binary neutron star coalescences, binary black hole mergers are not expected to produce electromagnetic signals, making statistical approaches an important tool in cosmology using gravitational waves. In this project, an investigation of statistical methods of measuring the Hubble constant with standard sirens is carried out using simulated data, to find out how well we can constrain the Hubble constant and to characterise the biases due to selection effects coming from the incompleteness of EM galaxy catalogues. Results are obtained for a range of aLIGO sensitivities, using both binary black hole and binary neutron star mergers as standard sirens. This constitutes an independent measurement of the Hubble constant that is competitive with other methods

Target-of-opportunity observations of gravitational-wave events with Vera C. Rubin Observatory

The discovery of the electromagnetic counterpart to the binary neutron star (NS) merger GW170817 has opened the era of gravitational-wave multimessenger astronomy. Rapid identification of the optical/infrared kilonova enabled a precise localization of the source, which paved the way to deep multiwavelength follow-up and its myriad of related science results. Fully exploiting this new territory of exploration requires the acquisition of electromagnetic data from samples of NS mergers and other gravitational-wave sources. After GW170817, the frontier is now to map the diversity of kilonova properties and provide more stringent constraints on the Hubble constant, and enable new tests of fundamental physics. The Vera C. Rubin Observatory's Legacy Survey of Space and Time can play a key role in this field in the 2020s, when an improved network of gravitational-wave detectors is expected to reach a sensitivity that will enable the discovery of a high rate of merger events involving NSs (∼tens per year) out to distances of several hundred megaparsecs. We design comprehensive target-of-opportunity observing strategies for follow-up of gravitational-wave triggers that will make the Rubin Observatory the premier instrument for discovery and early characterization of NS and other compact-object mergers, and yet unknown classes of gravitational-wave events