GW data analysis

GW 1: A novel approach to discover unmodelled features and exotic GW sources 

Supervisors: Freija Beirnaert, Archisman Ghosh 

Current gravitational-wave (GW) searches rely heavily on modelling, in which General Relativity (GR) simulations are employed to construct gravitational waveforms. These parametrized waveforms encapsulate the most important features of a GW signal. The waveforms are then employed to identify GW events in the detector strain data, and subsequently to perform parameter estimation (PE) on the event. This approach works very well for detecting binary black hole (BBH), neutron star-black hole (NSBH) and binary neutron star (BNS) events. However, this methodology is not suitable for features that are not accounted for in the modelled waveforms, or to GW signals originating from sources of another nature, like core collapse supernovae or other exotic objects. 

Left panel: Gravitational “waveform” of the binary black hole merger GW150914 [1].

Right panel: An unmodelled Morlet-Gabor “wavelet” which can be used as a basis element to reconstruct the signal.

In this project, you will attempt to use this lack of modelling to your advantage. By deliberately utilizing waveforms that inadequately capture simulated GW signal features, you will investigate their impact on posterior samples obtained in the PE process. If discernible, these effects could be indicative of unmodelled features, prompting the development of a custom machine learning (ML) algorithm to detect such manifestations in real GW event data. 

You will first perform normal PE runs on simulated/real GW events. Once you are accustomed with the workflow, you will adjust the code to use wavelets to reconstruct the inadequately modelled features, establishing a setup to run on simulated GW events with all known features. Subsequently you will develop a general ML tool to identify the signatures of the unaccounted-for features in the resulting data.

The work involved will be computational. You will learn about statistical and machine learning methods. You will be involved in a large international collaboration in the forefront field of GW science.

[1] B. P. Abbott et al. “Observation of Gravitational Waves from a Binary Black Hole Merger” Phys.Rev.Lett. 116 (2016) 6, 061102, arXiv:1602.03837 [gr-qc].

GW 2: Hunting for exotic resonances in compact binary mergers 

Supervisors: Cezary Turski, Archisman Ghosh

Is it possible that the black hole-like compact objects which we are observing in gravitational waves (GWs) are not really black holes, but exotic compact objects mimicking black holes. Several such exotic alternatives have been proposed in literature. They include stars composed of exotic matter such as boson stars, dark matter stars, gravastars or else horizonless compact objects arising from quantum modifications to gravity, such as firewalls and fuzzballs [1]. The internal physics of such objects is expected to show up in the GW signal from their coalescence. However since their exact nature is unknown, a part of the GW signal from them cannot be modelled. In order to search for them, one needs to proceed with a combination of modelled and model-agnostic methods [2].

Figure: A classification of (exotic) compact objects. Figure from [3].

In this project, you will search for resonances in the internal fluid modes which can be triggered during the inspiral of a binary of such an exotic compact object. An overall “dephasing” can be expected due to the energy lost in the resonance. In addition, you will try to reconstruct the resonance part of the signal using an unmodelled basis of simplistic “wavelets”.

The work involved will be computational. You will learn about statistical methods (Bayesian statistics) and get exposed to state-of-the-art data analysis techniques. The work will be integrated into the activities of the Testing GR group of the LIGO-Virgo-KAGRA Collaboration. You will be involved in a large international collaboration in the forefront field of GW science.

[1] V. Cardoso and P. Pani, “Testing the nature of dark compact objects: a status report,” Living Rev. Rel. 22 (2019), arXiv:1904.05363 [gr-qc].

[2] K. W. Tsang et al., “A morphology-independent data analysis method for detecting and characterizing gravitational wave echoes,” Phys.Rev.D 98 (2018) 2, 024023, arXiv:1804.04877 [gr-qc].

[3] V. Cardoso and P. Pani, “Tests for the existence of black holes through gravitational wave echoes,” Nature Astron. 1 (2017) 9, 586-591, arXiv:1709.01525 [gr-qc].

GW3: Measuring the Hubble constant with LIGO-Virgo-KAGRA data 

Supervisors: Freija Beirnaert, Cezary Turski, Archisman Ghosh

The fourth observing run (O4) of the LIGO-Virgo-KAGRA (LVK) gravitational-wave (GW) detector network began in May 2023 and is expected to run until December 2024. 

After interesting candidate events have been identified and their parameters have been measured, it will be time to obtain science results out of the observations. A central role played by researchers in UGent is in the Cosmology working group of the LVK. A short-term goal of this group is to measure the Hubble constant, H0, the local expansion rate of the universe. One uses the GW distance measurement together with complementary redshift information (from possible electromagnetic counterparts, host galaxies, or galaxy clusters) to infer the cosmological parameters such as H0. Due to a tantalizing discrepancy between the local and early-universe measurements of H0, now dubbed as the “Hubble tension,” such an independent measurement of H0 from the GW sector can prove to be invaluable.

Caption: The latest measurement of the Hubble constant by the LIGO-Virgo-KAGRA Collaboration; figure from [1].

In this project you will work with researchers who have developed the LVK codebase gwcosmo+ for cosmology inference and have put together the upGLADE galaxy catalogue to go along with it. You will be a part of the team that carries out the first cosmology analyses on O4 data. If we are lucky, we may observe a multimessenger signal in O4 and get to analyze the data from it. Until now, we have seen only one such multimessenger signal, namely GW170817.

The work involved will be computational. You will learn about statistical methods and get exposed to state-of-the-art data analysis techniques. You will be involved in a large international collaboration in the forefront field of GW science.

[1] B. P. Abbott et al. “Constraints on the cosmic expansion history from GWTC-3,” arXiv: 2111.03604 [astro-ph.CO].

GW4: Observational science challenges with the Einstein Telescope

Supervisors: Freija Beirnaert, Cezary Turski, Archisman Ghosh

The Einstein Telescope (ET) will have more than ten times the sensitivity of the current LIGO-Virgo-KAGRA detector network. As a result we will see merging black holes which are much farther away (out to the edge of the known universe), as well as heavier black holes, (thousands of times the mass of the sun), which merge at lower frequencies made accessible by the ET. Typical signals observed by the ET will also be much longer – minutes to hours or even longer (as opposed to a fraction of a second for the current detector network). This is exciting. However it brings forth several computational challenges. How will we carry out data analysis for such long signals? Signals will certainly overlap – how will we disentangle them from each other? The ET Collaboration Observational Science Board was recently established to answer such questions.

In this project, you will develop prospective techniques for ET data analysis. In particular you will attempt to bring together traditional methods such as Bayesian Markov Chain Monte Carlo (MCMC) together with more recently developed Convolutional Neural Network (CNN) algorithms with the goal of efficiently analyzing multiple signals present simultaneously in the data.

The work involved will be computational. You will learn about statistical methods and get exposed to state-of-the-art data analysis techniques. You will be involved in a large international collaboration in the forefront field of GW science.

GW 5: Optical layout of the ETpathfinder

Supervisors: Daniela Pascucci, Archisman Ghosh

Ghent University has been involved in the ETpathfinder project since its beginning. The facility in Maastricht is currently still under construction, with the aim to have the interferometer up and running by the end of this year. 

Although the layout of ETpathfinder is broadly defined, optical simulations are needed to define position and specifications of all auxiliary optics used to have the proper beam parameters at each step of the path. You will be actively involved in the development of ETpathfinder, working on optical simulations to define the final layout. 

You will be a member of the Einstein Telescope Collaboration. You will get involved with the technology and challenges of interferometric detectors in the active field of gravitational-wave science. The project will be done in close coordination with the ETpathfinder community. Travel to Maastricht, where the facility is located, may be required.

Left panel: Simplified layout of ETpathfinder in its first phase [1]. Right panel: picture of the ETpathfinder hall with all the vacuum towers assembled [2].

GW 6: Optical characterization of 2μm photodetectors 

Supervisors: Hans Van Havermaet, Daniela Pascucci, Archisman Ghosh 

Next generation GW observatories such as the Einstein Telescope (ET) will use silicon mirrors as test masses to detect low frequency GW with unprecedented sensitivities. This implies the use of new laser wavelengths at 1550nm or 2090nm. The performance of such optics will be first tested at the ETpathfinder facility in Maastricht. To detect the main interferometer signal, coming from the GW strain, photodiodes with a high quantum efficiency (HQE) and a large dynamic range for these new wavelengths are needed. At 1550nm HQE InGaAs sensors exist, however at 2090nm more development is needed and custom made extended InGaAs photodiodes with optimized quantum efficiency, especially for 2090 nm, will need to be acquired and tested. In this project we will contribute to the development of such novel 2090nm HQE photodiodes by performing optical characterizations of available photodiodes. 

UGent has recently set up an optical lab for characterization of optical components for 2090nm. You will set up a measurement system at UGent optical lab with existing readout electronics (available in UAntwerpen) to test the sensor for e.g. dark noise, efficiency, and optical power levels.

The work involved will be hands-on instrumentational, with focus on optics and mechanics. The project will be under joint supervision with Prof. Hans Van Haevermaet (UAntwerpen). You will also get a chance to work at the ETpathfinder facility in Maastricht and be a part of the exciting activity in the Belgian-Dutch region centred around the Einstein Telescope.