Researchers from the University of Glasgow’s School of Physics & Astronomy are preparing for the next observing run of the international LIGO-Virgo-KAGRA (LVK) network of gravitational-wave detectors.
The LVK collaboration consists of scientists across the globe who use a network of observatories-LIGO in the United States, Virgo in Europe, and KAGRA in Japan-to search for gravitational waves, or ripples in space-time, generated by colliding black holes and other extreme cosmic events.
After three years of work to improve the performance of the network of gravitational wave detectors, Observing Run 4 (O4) is planned to start today (May 24th, 2023).
The enhancements are expected to bring a higher detection rate of gravitational-wave signals - faint ripples in spacetime which are caused by massive astronomical events like the merging of black holes.
Professor Sheila Rowan, Director of the University of Glasgow’s Institute for Gravitational Research and Chair of Natural Philosophy at the School of Physics & Astronomy, leads the University’s gravitational-wave research.
She said: "We’ve come a long way since the historic first detection of gravitational waves in 2015. The LIGO-Virgo-KAGRA network has built a catalogue of close to 100 detections over the course of the first three observing runs, expanding our understanding of the Universe.
"Glasgow researchers have been developing the data analysis that underpins each detection at every step. O4 is set to bring us a wealth of new detections, and we’re expecting to start receiving signals within the first day or two of the run commencing."
More detections will help gravitational wave astronomers build their understanding of the workings of the Universe. However, they will also bring new challenges for the network. Each detection requires a huge amount of computing power to analyse, and Glasgow researchers have been working to streamline the calculations.
Postdoctoral researcher Daniel Williams developed the Asimov framework, which will allow the team to set up and run analyses with minimal human intervention, improving the reliability of the analysis process and easing reproducibility.
He said: "Moving from having an event or two each week to having one or more a day will be a major challenge. Analysing each event requires lots of computing power, and to cope with this we spread the computing effort around the planet, automating and streamlining the analyses".
"Asimov will enable us to spend more time interpreting the results and thinking about the scientific payoff of our analysis."
PhD candidate Michael Williams has developed a second algorithm, called nessai (pronounced ’Nessie’). Nessai takes advantage of new developments in machine learning to speed up the process of computing the properties of a gravitational-wave source.
Nessai is currently undergoing final testing ahead of being used to analyse new signals with greatly improved speed, reducing the time taken to get results. This increased efficiency will help to reduce the overall carbon footprint of the network’s computer power.
Dr John Veitch, a senior lecturer in the School of Physics & Astronomy, said: "The enhanced sensitivity of the LIGO-Virgo-KAGRA network of gravitational-wave observatories in O4 promises to make this the most fruitful observing run yet for gravitational-wave astronomy: we expect to see hundreds of new collisions of black holes and neutron stars. With better sensitivity we will also get a clearer picture of the nature of these extreme objects, and be able to test Einstein’s theory of gravity with unprecedented precision."
The sources of gravitational waves discovered so far are binaries made up of black holes and neutron stars. These extreme objects are the remains of massive stars, and by studying them, we can learn more about these stars and how they shape our Universe.
Dr Christopher Berry, a lecturer in the School of Physics & Astronomy, said: "In our last observing run, we saw some truly exceptional sources. Black holes more massive than previously known; objects where we are not sure if they are black holes or neutron stars because their mass is exactly in the range where we don’t know what they are, and binaries where the two components have very unequal masses, which challenge our understanding of how these binaries form.
"O4 will reveal how common such things are-are they rare beasts, or do we need to rewrite our understand of astrophysics to account for them being common?"
Understanding how the measured properties of gravitational-wave sources, like the masses of black holes, relate to the properties of their parent stars requires detailed models of stellar evolution.
Dr Berry added: "We have been working on improving these models, to give a more accurate prediction of how black holes and neutron stars form. Gravitational waves are the perfect way to study black hole and neutron star binaries. However, we have many other pieces of information from other astronomical observations about how stars evolve. Putting these pieces together to solve the puzzle of stellar evolution requires accurate theoretical predictions."
Recent work has shown that the binaries observed by the network’s gravitational-wave detectors likely come from a mix of different formation channels, so there is a host of different physics to understand.
Combining the information from all the gravitational-wave observations allows gravitational-wave astronomers to reconstruct the astrophysical population of sources. Results from the last observing run indicated that black holes are more likely to form with some masses than others. These could be key hints to how they form. One of the priorities win the new observing run will be to confirm if these features are supported by the larger set of observations, and what new details can be discerned.
Another key astrophysical quantity the Glasgow team are excited to calculate from the population of observations is the Hubble constant, the cosmological parameter that describes how fast the Universe expands.
Dr Rachel Gray, a lecturer in the Institute for Gravitational Research, is co-chair of the LIGO-Virgo-KAGRA Collaboration’s Cosmology working group. Dr Gray explained: "One of the key science outcomes we’re excited for in O4 is measuring the Hubble constant, the speed at which the universe is expanding. Gravitational waves provide a new way of doing this which is independent of all other measurements which have been done previously. Given that those measurements disagree with each other, we’re hopeful that one day we’ll be able to provide additional insight-once we’ve detected enough gravitational waves.
"If we’re lucky we’ll see a gravitational wave with an electromagnetic counterpart during O4, which would provide us with a huge amount of information! But even if we don’t, we can use binary black holes and galaxy catalogues to measure the Hubble constant. We’ve made a lot of improvements to our analyses since the last observing run. Now we can easily include a far larger number of gravitational waves in our analyses. We’ve also put a lot of effort into ensuring that our results will be robust to current sources of uncertainty-things like not knowing the exact distribution of black hole masses - and we’re in a great place to start analysing the data which will soon arrive."
"Thanks to the work of more than a thousand people around the world over the last few years, we’ll get our deepest glimpse of the gravitational-wave Universe yet," said Jess McIver, the Deputy Spokesperson for the LIGO Scientific Collaboration (LSC). "A greater reach means we will learn more about black holes and neutron stars and increases the chances we find something new. We’re very excited to see what’s out there."
The first gravitational-wave signals were detected in 2015. Two years later, LIGO and Virgo detected a merger of two neutron stars, which caused an explosion called a kilonova, subsequently observed by dozens of telescopes around the world. So far, the global network has detected more than 80 black hole mergers, two probable neutron star mergers and a few events that were most likely black holes merging with neutron stars. During O4, researchers expect to observe even more energetic cosmic events and gain new insights into the nature of the universe.
O4 will last 20 months, including up to two months of commissioning breaks. The extended run time will increase the scientific output of O4 and allow additional time to prepare for the upgrades that will follow the run. University of Glasgow researchers are already investigating how to further enhance the sensitivity of detectors for future observing runs.
As in previous observing runs, alerts about gravitational-wave detection candidates will be distributed publicly during O4. Information about how to receive and interpret public alerts is available online.
The University of Glasgow’s contributions to the LIGO-Virgo-KAGRA Collaboration are funded by the Science and Technology Facilities Council (STFC), part of UKRI.
LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,500 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration.
The Virgo Collaboration is currently composed of approximately 850 members from 143 institutions in 15 different (mainly European) countries. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and the National Institute for Subatomic Physics (Nikhef) in the Netherlands.
KAGRA is the laser interferometer with 3 km arm-length in Kamioka, Gifu, Japan. The host institute is Institute for Cosmic Ray Research (ICRR), the University of Tokyo, and the project is co-hosted by National Astronomical Observatory of Japan (NAOJ) and High Energy Accelerator Research Organization (KEK). KAGRA collaboration is composed of over 480 members from 115 institutes in 17 countries/regions.