Design of the Output Mode Cleaner for the ETpathfinder

To push the sensitivity beyond the current limit some fundamental changes are required for next-generation gravitational wave detectors. These changes include the use of a cryogenic system to keep the test masses at low temperatures, which implies the use of a different material for the mirrors (silicon instead of silica) and different wavelengths of the laser beams (1550 nm and 2090 nm instead of 1064 nm). ETpathfinder will serve as a test facility to develop these new technologies, but some preliminary tests are needed. The Gravity Group of Ghent University will take charge on the development of the Output Mode Cleaner (OMC) for ETpathfinder. 

A mode cleaner is an optical device that allows us to select a preferred mode for the light beam and removes any other unwanted modes. It is basically an optical cavity with resonance frequency equal to the one of the selected mode, which will then be enhanced while the others are reduced. Even if it is one of the last components in the detector, the OMC is fundamental to improve the read out signal.

Layout of Advanced LIGO OMC. All the optical components are bonded on a single breadboard made of fused Silica.
Example of simulations with Finite Element Analysis software done for Advanced LIGO OMC.

The student will work on the design of the OMC, which will be defined through simulations with specific simulation softwares. This is a fundamental step which will be carried out in order to define and evaluate the following aspects:

  • Optical layout (cavity shape, length, etc.)
  • Mirrors parameters (size, radius of curvature, etc.)
  • OMC performance (shot noise, transmission, etc.)

Searching for gravitational waves from supernovae with the Einstein Telescope

Supernovae (SNe) are among the GW sources which we expect to see but have not detected yet. While detecting GWs from an SN going off would be a big discovery in itself, it is also possible that we have a multimessenger observation of an SN. The proposed third-generation of GW detectors such as the Einstein Telescope (ET) will be much more sensitive than the current LIGO-Virgo-KAGRA network and there is an even better chance to have a historic supernova detection with ET. One of the proposed sites of ET is on the Belgian-Dutch border, and currently, several teams are investigating the scientific case behind building ET. In this work, we are going to explore how sensitive ET will really be to GWs from SNe, how well the detector will be able to reconstruct the GW signal, and estimate how many such observations will ET be able to make. In order to do that we will use state-of-the-art 3D SN models and model-independent GW detection algorithms. A successful realization of the project would be a strong argument in favour of building ET itself. From a practical point of view, the project would mainly involve coding in python and learning how to use some LIGO-Virgo-KAGRA algorithms.

By the end of the project, the student would have gained insight into the data analysis tools for a state-of-the-art experiment. They would also have learned to work in a large international collaboration on the frontier of fundamental research.

The Ghent Gravity Group has already been actively involved in analysing GW signals from SNe; this figure shows one of our machine learning algorithms differentiating between signals produced by various different SN models.
Example of a simulated GW signal from an SN. The figure on the left shows the waveform in the time domain while the figure on the right shows it in the frequency domain. The green lines show the simulated signal and the purple lines show what could be reconstructed from the detection just with one of the LIGO detectors.

The student would learn about the analysis of data, statistical methods, and astrophysics, and get involved with a large international collaboration in the active field of gravitational-wave science.


Probing cosmology with gravitational waves

With the current compact binary detections, we are able to independently measure the local expansion rate of the universe or the Hubble constant H0. A part of the method relies on the association of the GW observations with information from galaxy catalogues. As we get more observations in the future, we should also be able to also probe the parameters that govern the acceleration of the universe, namely dark matter and dark energy. The next generation of GW detectors would dramatically reduce the measurement uncertainties and additionally bring in information about the large scale structure of matter in the universe. In this project, we will investigate how well different detector network configurations of the third generation of GW detectors (CE and ET) will help measure the parameters that quantify cosmology and the large scale structure of the universe. 

The work involved will be coding and computational. The student will learn to use some existing codebase and develop new algorithms. Some of the work performed by the student will contribute to data analysis for LIGO-Virgo-KAGRA and building the data analysis infrastructure for the ET detector network.

The state‐of‐the‐art GW measurement of H0 using the observations of the first and second observing runs of Advanced LIGO‐Virgo. The “dark standard sirens” used together with galaxy catalogues lead to a remarkable improvement over H0 from GW170817.
Reconstructed two-point correlation function of the underlying large scale structure from a simulated universe filled with GW sources.

The student would learn about the analysis of data, and statistical methods, and get involved with a large international collaboration in the active field of gravitational-wave science.