Astronomers may be able to get an early warning about where to look for merging black holes and study gravitational waves, thanks to a new model created by Canadian and U.S. astrophysicists.
Two black holes coming together are expected to emit two jets of electromagnetic radiation just before merging, producing strong gravitational waves, Carlos Palenzuela, Luis Lehner and Steve Liebling predict in an article published online Thursday in Science.
By looking for both types of signals in the same area of sky, astronomers will be able to point their telescopes at the right spot in time "and be ready when the main show starts," said Lehner, a Guelph University physicist and affiliate faculty at the Perimeter Institute in Waterloo, Ont.
Einstein's general theory of relativity predicts that a black hole can form when a very large star runs out of nuclear fuel and is crushed by its own gravitational force. That force is so strong that the star collapses into a point called a singularity that is smaller than an atom's core.
Its powerful gravity attracts and pulls in nearby matter and even light, making it "black." Though the hole itself cannot be seen, it does bend light travelling past the object, causing lens-like distortions of objects behind it when looked at by distant observers such as humans on Earth.
Until now, no one has directly observed black holes or detected gravity waves, but they are both predicted by Einstein's theory of general relativity and highly sought after by physicists and astronomers.
Scientists hypothesize that every galaxy has a super massive black hole at its centre and that when galaxies merge, eventually their black holes will too, Lehner said.
Theory predicts that merging black holes will produce the strongest — and therefore most readily detected — gravity waves.
Researchers are excited about the possibility of detecting gravitational waves because unlike electromagnetic radiation such as visible light, they aren't scattered by objects between the source and the observer.
"Gravitational waves carry pristine information of the source," Lehner said.
Huge efforts have gone into looking for them, including the construction of instruments such as the $365-million Laser Interferometer Gravitational Wave Observatory, which includes detectors in Livingston, La., and at Hanford, Wash.
Massive moving objects such as stars and black holes are predicted by the general theory of relativity to produce ripples in space-time as they interact with one another. Those gravitational waves are predicted to get very weak as they travel long distances to places such as Earth. Nevertheless, they are expected to be detectable from Earth as small variations in space-time.
More sensitive instruments, including one slated to be launched into space, are currently being developed and are expected to detect gravity waves within the next decade.
"In order to understand them and help in the detection and analysis of those signals, we want accurate models that tell us what to expect … what to look for in data streams," Lehner said.
Even when gravity waves are detected, it will be difficult to pinpoint the source within a patch of sky that may contain 100,000 galaxies without some additional information, Lehner said. That's where the new model comes in.
Lehner created a mathematical model of two black holes merging with Panzuela, an astrophysicist affiliated with the Canadian Institute for Theoretical Physics in Toronto and Louisiana State University in Baton Rouge; and with Liebling, a physicist at Long Island University in Brookville, N.Y.
Role of plasma
A previous model created by other scientists considered only the two black holes themselves, even though black holes are actually surrounded by orbiting disks of dust and gas called accretion disks that generate a magnetic field and ultimately an electric field and a plasma of electrons and positrons.
"Our take was, 'Let's go after this, put this realism into it and more importantly, let's see what this plasma does in addition to gravitational waves that we can calculate,' " Lehner said.
The model showed that the plasma generates a jet of electromagnetic radiation from each black hole. They twist around each other and eventually merge into a single jet as the black holes themselves merge.
Astronomers have already observed in galaxies jets of electromagnetic radiation that may be generated by single black holes spinning — something that wouldn't produce gravitational waves.
Based on the results of the new model, it's possible that some of those single jets may in fact be dual jets that appear blurred into a single one, and may in fact come from two merging black holes that could be a source of gravitational waves.
The researchers are currently working on an even more advanced and realistic model in which the black holes have different instead of identical masses and are spinning.