On Wednesday, October 28, 2020, LIGO and Virgo—an international collaboration of gravitational-wave detectors—released data from the first-half of their third observing run. Between April 1 and October 1, 2019, 39 new gravitational-wave events were added to the 11 previously confirmed events, bringing the total number of gravitational-wave events to 50. The discoveries break new ground by encoding a wealth of information on the history and formation of black holes and neutron stars throughout the universe and span a wide range of masses, distances, and spins of companion stars. They represent cosmic sources consistent with the coalescences of binary black holes, binary neutron stars, and neutron star–black hole binaries.
We asked the LIGO group at Penn State to help us understand what these new discoveries mean:
Q: Briefly, what is LIGO and what does it do?
PSU LIGO: LIGO stands for Laser Interferometer Gravitational-wave Observatory. As the name describes, the two LIGO detectors—one in Hanford, Washington and one in Livingston, Louisiana—observe astrophysical gravitational waves, which are ripples in spacetime that were predicted over 100 years ago by Einstein and first observed by LIGO in 2016. The LIGO Scientific Collaboration is an international group of scientists which analyze the LIGO data and findings, at the same time releasing the analyzed data. The detections in this data release were reported jointly with the European Virgo Collaboration, which operates a similar gravitational-wave detector based in Italy.
Q: What aspects of the LIGO project does the Penn State group focus on?
PSU LIGO: The Penn State group focuses on all aspects of gravitational wave astronomy: detection of gravitational-wave signals, estimation of parameters of the source, and exploiting the detected events to understand the cosmos. Our group developed and ran a flagship analysis pipeline that analyzes the data from the observatories in real time and alerts scientists when a gravitational wave is detected. This helps in observing the source that produced the waves using optical, infra-red, radio, and x-ray telescopes. We also conduct a further analysis to determine properties of the sources, make new discoveries in astrophysics and fundamental physics, and plan for the next generation of gravitational-wave observatories.
Q: Why was LIGO/Virgo able to detect so many more events on this run?
PSU LIGO: In short, both our ability to collect data and analyze it have significantly improved. The LIGO and Virgo detectors received major upgrades between the previous observing run and the latest observing run which improved their sensitivity and ability to collect data. Additionally, the analysis pipelines that search the data for gravitational wave signals were enhanced to improve the detection efficiency.
Q: What are the highlights so far of the detections from this run?
PSU LIGO: We added 39 new events to the total event count – more than tripling the previous number of detections! Among these, we detected our most massive binary black hole yet (GW190521), the most distant event (GW190706_222641), as well as the most asymmetric system—with the biggest difference between the masses of the two objects—we’ve ever observed (GW190814). We also observed three mergers where one component was smaller than three solar masses, suggesting that we may have detected our first neutron star and black hole merger.
Q: What do these discoveries tell us about our universe?
PSU LIGO: The universe does not appear to be the same when observed with optical, infrared, and radio telescopes. Gravitational waves provide us yet another window to observe the universe and reveal objects and phenomena that are not accessible to other telescopes. For example, optical and infrared telescopes can observe matter around black holes but not the black holes themselves. Although isolated black holes don’t emit any waves, binary black holes emit gravitational waves that are accessible to LIGO and Virgo. As we detect more events, we enhance our ability to make inferences about the astrophysical populations of black hole and neutron stars and we deploy these detections to understand the fundamental laws of nature such as Einstein’s theory of gravity, the nature of black holes, the structure of spacetime, the expansion rate of the universe, etc.
Q: What is next for LIGO?
PSU LIGO: The three current observatories (2 LIGO and 1 Virgo) and their detection pipelines are undergoing even more improvements between now and our next observing run. A fourth detector known as KAGRA in Japan joined the third observing run briefly and will be back with far greater sensitivity for the next observing run. Beyond this, the next generation of detectors, both on the ground and in space, are currently under development. These new detectors will be more sensitive and able to examine a wider range of the gravitational-wave spectrum, which we do not currently detect such as supermassive black hole binaries! Between current and proposed detectors, we hope to see a large population of neutron star mergers to better understand the physics of particles at very high densities, and hopefully we will also detect more exotic compact objects, like primordial black holes or dark matter black holes, to expand our knowledge of the dark sector as well.
The members of the LIGO group at Penn State we spoke with includes: Chad Hanna, associate professor of physics and of astronomy and astrophysics; B.S. Sathyaprakash, Elsbach Professor of Physics and Professor of Astronomy and Astrophysics; and graduate students Rebecca Ewing, Rachael Huxford, and Divya Singh.
Find more information about Penn State’s LIGO research at the Center for Multimessenger Astrophysics which is part of the Institute for Gravitation and the Cosmos and at Chad Hanna’s lab webpage. You can also follow PSU LIGO on twitter @psuLIGO.
A free preprint of a scientific article detailing the data release is available.