Black holes have been the subject of intense interest ever since scientists began speculating about their existence. Originally proposed in the early 20th century as a consequence of Einstein’s Theory of General Relativity, black holes became a mainstream subject a few decades later. By 1971, the first physical evidence of black holes was found and by 2016, the existence of gravitational waves was confirmed for the first time.
This discovery touched off a new era in astrophysics, letting people know collision between massive objects (black holes and/or neutron stars) creates ripples in spacetime that can be detected light-years away. To give people a sense of how profound these events are, Álvaro Díez created the Black Hole Collision Calculator (BHCC) – a tool that lets you see what the outcome of a collision between a black hole and any astronomical object would be!
On the 12th of April, 2019, the LIGO and Virgo gravitational wave observatories detected the merger of two black holes. Named GW190412, one of the black holes was eight solar masses, while the other was 30 solar masses. On the 14th of August that year, an even more extreme merger was observed, when a 2.5 solar mass object merged with a black hole nearly ten times more massive. These mergers raise fundamental questions about the way black hole mergers happen.
Black hole merger events are some of the most energetic, fearsomely energetic events in all the cosmos. When black holes merge, they’re entirely invisible, the only evidence of the cataclysm some faint whisper of gravitational waves. Until now.
One of the most pressing questions in astronomy concerns black holes. We know that massive stars that explode as supernovae can leave stellar mass black holes as remnants. And astrophysicists understand that process. But what about the supermassive black holes (SMBHs) like Sagittarius A-star (Sgr A*,) at the heart of the Milky Way?
SMBHs can have a billion solar masses. How do they get so big?
Einstein’s Theory of General Relativity predicted that black holes would form and eventually collide. It also predicted the creation of gravitational waves from the collision. But how often does this happen, and can we calculate how many stars this will happen to?
A new study from a physicist at Vanderbilt University sought to answer these questions.
Astronomers have spotted a 40 billion solar mass black hole in the Abell 85 cluster of galaxies. They found the behemoth using spectral observations with the Very Large Telescope (VLT.) There are only a few direct mass measurements for black holes, and at about 700 million light years from Earth, this is the most distant one.
Astronomers have spotted three supermassive black holes (SMBHs) at the center of three colliding galaxies a billion light years away from Earth. That alone is unusual, but the three black holes are also glowing in x-ray emissions. This is evidence that all three are also active galactic nuclei (AGN,) gobbling up material and flaring brightly.
This discovery may shed some light on the “final parsec problem,” a long-standing issue in astrophysics and black hole mergers.
If contemplating the vast size of astronomical objects makes you feel rather puny and insignificant, then this new discovery will make you feel positively infinitesimal.
It’s almost impossible to imagine an object this large: a super massive black hole that’s 40 billion times more massive than our Sun. But there it is, sitting in the center of a super-giant elliptical galaxy called Holmberg 15A. Holmberg 15A is about 700 million light years away, in the center of the Abell 85 galaxy cluster.
A little over a year ago, LIGO was taken offline so that upgrades could be made to its instruments, which would allow for detections to take place “weekly or even more often.” After completing the upgrades on April 1st, the observatory went back online and performed as expected, detecting two probable gravitational wave events in the space of two weeks.
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.
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.
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.