How to Listen to the Background Hum of Gravitational Waves From all the Black Holes Colliding into Each Other

The first-ever detection of gravitational waves (which took place in September of 2015) triggered a revolution in astronomy. Not only did this event confirm a theory predicted by Einstein’s Theory of General Relativity a century before, it also ushered in a new era where the mergers of distant black holes, supernovae, and neutron stars could be studied by examining their resulting waves.

In addition, scientists have theorized that black hole mergers could actually be a lot more common than previously thought. According to a new study conducted by pair of researchers from Monash University, these mergers happen once every few minutes. By listening to the background noise of the Universe, they claim, we could find evidence of thousands of previously undetected events.

Their study, titled “Optimal Search for an Astrophysical Gravitational-Wave Background“, recently appeared in the journal Physical Review X. The study was conducted by Rory Smith and Eric Thrane, a senior lecturer and a research fellow at Monash University, respectively. Both researchers are also members of the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav).

Drs. Eric Thrane and Rory Smith. Credit: Monash University

As they state in their study, every 2 to 10 minutes, a pair of stellar-mass black holes merge somewhere in the Universe. A small fraction of these are large enough that the resulting gravitational wave event can be detected by advanced instruments like the Laser Interferometer Gravitational-Wave Observatory and Virgo observatory. The rest, however, contribute to a sort of stochastic background noise.

By measuring this noise, scientists may be able to study much more in the way of events and learn a great deal more about gravitational waves. As Dr Thrane explained in a Monash University press statement:

“Measuring the gravitational-wave background will allow us to study populations of black holes at vast distances. Someday, the technique may enable us to see gravitational waves from the Big Bang, hidden behind gravitational waves from black holes and neutron stars.”

Drs Smith and Thrane are no amateurs when it comes to the study of gravitational waves. Last year, they were both involved in a major breakthrough, where researchers from LIGO Scientific Collaboration (LSC) and the Virgo Collaboration measured gravitational waves from a pair of merging neutron stars. This was the first time that a neutron star merger (aka. a kilonova) was observed in both gravitational waves and visible light.

The pair were also part of the Advanced LIGO team that made the first detection of gravitational waves in September 2015. To date, six confirmed gravitational wave events have been confirmed by the LIGO and Virgo Collaborations. But according to Drs Thrane and Smith, there could be as many as 100,000 events happening every year that these detectors simply aren’t equipped to handle.

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.
Artist’s impression of merging binary black holes. Credit: LIGO/A. Simonnet.

These waves are what come together to create a gravitational wave background; and while the individual events are too subtle to be detected, researchers have been attempting to develop a method for detecting the general noise for years. Relying on a combination of computer simulations of faint black hole signals and masses of data from known events, Drs. Thrane and Smith claim to have done just that.

From this, the pair were able to produce a signal within the simulated data that they believe is evidence of faint black hole mergers. Looking ahead, Drs Thrane and Smith hope to apply their new method to real data, and are optimistic it will yield results. The researchers will also have access to the new OzSTAR supercomputer, which was installed last month at the Swinburne University of Technology to help scientists to look for gravitational waves in LIGO data.

This computer is different from those used by the LIGO community, which includes the supercomputers at CalTech and MIT. Rather than relying on more traditional central processing units (CPUs), OzGrav uses graphical processor units – which can be hundreds of times faster for some applications. According to Professor Matthew Bailes, the Director of the OzGRav supercomputer:

“It is 125,000 times more powerful than the first supercomputer I built at the institution in 1998… By harnessing the power of GPUs, OzStar has the potential to make big discoveries in gravitational-wave astronomy.”

What has been especially impressive about the study of gravitational waves is how it has progressed so quickly. From the initial detection in 2015, scientists from Advanced LIGO and Virgo have now confirmed six different events and anticipate detecting many more. On top of that, astrophysicists are even coming up with ways to use gravitational waves to learn more about the astronomical phenomena that cause them.

All of this was made possible thanks to improvements in instrumentation and growing collaboration between observatories. And with more sophisticated methods designed to sift through archival data for additional signals and background noise, we stand to learn a great deal more about this mysterious cosmic force.

Further Reading: Monash, Physical Review X

LIGO and Virgo Observatories Detect Black Holes Colliding

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.

On February 11th, 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the first detection of gravitational waves. This development, which confirmed a prediction made by Einstein’s Theory of General Relativity a century ago, has opened up new avenues of research for cosmologists and astrophysicists. Since that time, more detections have been made, all of which were said to be the result of black holes merging.

The latest detection took place on August 14th, 2017, when three observatories – the Advanced LIGO and the Advanced Virgo detectors – simultaneously detected the gravitational waves created by merging black holes. This was the first time that gravitational waves were detected by three different facilities from around the world, thus ushering in a new era of globally-networked research into this cosmic phenomena.

The study which detailed these observations was recently published online by the LIGO Scientific Collaboration and the Virgo Collaboration. Titled “GW170814 : A Three-Detector Observation of Gravitational Waves from a Binary Black Hole Coalescence“, this study has also been accepted for publication in the scientific journal Physical Review Letters.

Aerial view of the Virgo Observatory. Credit: The Virgo collaboration/CCO 1.0

The event, designated as GW170814, was observed at 10:30:43 UTC (06:30:43 EDT; 03:30:43 PDT) on August 14th, 2017. The event was detected by the National Science Foundation‘s two LIGO detectors (located in Livingston, Louisiana, and Hanford, Washington) and the Virgo detector located near Pisa, Italy – which is maintained by the National Center for Scientific Research (CNRS) and the National Institute for Nuclear Physics (INFN).

Though not the first instance of gravitational waves being detected, this was the first time that an event was detected by three observatories simultaneously. As France Córdova, the director of the NSF, said in a recent LIGO press release:

“Little more than a year and a half ago, NSF announced that its Laser Interferometer Gravitational Wave Observatory had made the first-ever detection of gravitational waves, which resulted from the collision of two black holes in a galaxy a billion light-years away. Today, we are delighted to announce the first discovery made in partnership between the Virgo gravitational-wave observatory and the LIGO Scientific Collaboration, the first time a gravitational wave detection was observed by these observatories, located thousands of miles apart. This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our universe.”

Based on the waves detected, the LIGO Scientific Collaboration (LSC) and Virgo collaboration were able to determine the type of event, as well as the mass of the objects involved. According to their study, the event was triggered by the merger of two black holes – which were 31 and 25 Solar Masses, respectively. The event took place about 1.8 billion light years from Earth, and resulted in the formation of a spinning black hole with about 53 Solar Masses.

LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu

What this means is that about three Solar Masses were converted into gravitational-wave energy during the merger, which was then detected by LIGO and Virgo. While impressive on its own, this latest detection is merely a taste of what gravitational wave detectors like the LIGO and Virgo collaborations can do now that they have entered their advanced stages, and into cooperation with each other.

Both Advanced LIGO and Advanced Virgo are second-generation gravitational-wave detectors that have taken over from previous ones. The LIGO facilities, which were conceived, built, and are operated by Caltech and MIT, collected data unsuccessfully between 2002 and 2010. However, as of September of 2015, Advanced LIGO went online and began conducting two observing runs – O1 and O2.

Meanwhile, the original Virgo detector conducted observations between 2003 and October of 2011, once again without success. By February of 2017, the integration of the Advanced Virgo detector began, and the instruments went online by the following April. In 2007, Virgo and LIGO also partnered to share and jointly analyze the data recorded by their respective detectors.

In August of 2017, the Virgo detector joined the O2 run, and the first-ever simultaneous detection took place on August 14th, with data being gathered by all three LIGO and Virgo instruments. As LSC spokesperson David Shoemaker – a researcher with the Massachusetts Institute of Technology (MIT) – indicated, this detection is just the first of many anticipated events.

Artist’s impression of two merging black holes, which has been theorized to be a source of gravitational waves. Credit: Bohn, Throwe, Hébert, Henriksson, Bunandar, Taylor, Scheel/SXS

“This is just the beginning of observations with the network enabled by Virgo and LIGO working together,” he said. “With the next observing run planned for fall 2018, we can expect such detections weekly or even more often.”

Not only will this mean that scientists have a better shot of detecting future events, but they will also be able to pinpoint them with far greater accuracy. In fact, the transition from a two- to a three-detector network is expected to increase the likelihood of pinpointing the source of GW170814 by a factory of 20. The sky region for GW170814 is just 60 square degrees – more than 10 times smaller than with data from LIGO’s interferometers alone.

In addition, the accuracy with which the distance to the source is measured has also benefited from this partnership. As Laura Cadonati, a Georgia Tech professor and the deputy spokesperson of the LSC, explained:

“This increased precision will allow the entire astrophysical community to eventually make even more exciting discoveries, including multi-messenger observations. A smaller search area enables follow-up observations with telescopes and satellites for cosmic events that produce gravitational waves and emissions of light, such as the collision of neutron stars.”

Artist’s impression of gravitational waves. Credit: NASA

In the end, bringing more detectors into the gravitational-wave network will also allow for more detailed test’s of Einstein’s theory of General Relativity. Caltech’s David H. Reitze, the executive director of the LIGO Laboratory, also praised the new partnership and what it will allow for.

“With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos,” he said. “Virgo brings a powerful new capability to detect and better locate gravitational-wave sources, one that will undoubtedly lead to exciting and unanticipated results in the future.”

The study of gravitational waves is a testament to the growing capability of the world’s science teams and the science of interferometry. For decades, the existence of gravitational waves was merely a theory; and by the turn of the century, all attempts to detect them had yielded nothing. But in just the past eighteen months, multiple detections have been made, and dozens more are expected in the coming years.

What’s more, thanks to the new global network and the improved instruments and methods, these events are sure to tell us volumes about our Universe and the physics that govern it.

Further Reading: NSF, LIGO-Caltech, LIGO DD