February 22, 2013
Using Black Hole Disks And Jets To Explore The Limits Of Spacetime
Watch the video "Magneto-Spin Alignment Effect Movie (Black Hole Jet)"
April Flowers for redOrbit.com - Your Universe OnlineBlack holes are voracious monsters at the center of galaxies that shape the growth and death of the stars around them with their tremendous gravitational pull and explosive ejections of energy. "Over its lifetime, a black hole can release more energy than all the stars in a galaxy combined," explains“¯Roger Blandford, Stanford professor, director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) and a member of the National Academy of Science.
"Black holes have a major impact on the formation of galaxies and the environmental growth and evolution of those galaxies."
Close to a black hole, gravitational forces become so strong that even light cannot escape from within, making them very difficult to observe directly. Instead, scientists infer facts about black holes by their influences on the astronomical objects around them, for example, the orbit of stars and clumps of detectable energy. With this inferred information, scientists can create computer models to understand the data and make predictions about the physics of distant regions of space. Models, however, are only as good as their assumptions.
“All tests of general relativity in the weak gravity field limit, like in our solar system, fall directly along the lines of what Einstein predicted," explains“¯Jonathan McKinney, assistant professor of physics at the University of Maryland at College Park. "But there is another regime — which has yet to be tested, and which is the hardest to test — that represents the strong gravitational field limit. And according to Einstein, gravity is strongest near black holes."
Because of this, black holes are the ultimate experimental testing grounds for Einstein's theory of general relativity. While black holes cannot be directly observed, other objects with distinctive features typically accompany them. These include accretion disks, which are circling disks of superhot matter on the side of the black hole's event horizon, and relativistic jets, high-powered streams of ionized gases that shoot hundreds of thousands of light years across the sky.
In a new study published this week in the journal Science, McKinney, Blandford and their colleague Alexander Tchekhovskoy of Princeton predicted the formation of accretion disks and relativistic jets that warp and bend more than previously thought, shaped both by the extreme gravity of the black hole and by powerful magnetic forces generated by its spin. The highly detailed models the team developed represent a significant contribution to our understanding of black holes.
REPLACING A SIMPLISTIC VIEW OF DISKS AND JETS
For years a relatively simplistic view of these accretion disks and polar jets has reined in astronomical research. Scientists believed that accretion disks sat like flat plates along the outer edges of black holes and that jets shot straight out perpendicular to the event horizon. However, new 3D simulations performed on the powerful supercomputers at the National Science Foundation's Extreme Science and Engineering Discovery Environment (XSEDE) and NASA overturned this facile view of jets and disks.
The jet is aligned with the black hole's spin near the black hole, but the simulations show that it gradually gets pushed by the disk material and becomes parallel to (but offset from) the disk's rotational axis at large distances. And the density of the accretion disk is warped by the interaction between the jet and the disk.
"An important aspect that determines jet properties is the strength of the magnetic field threading the black hole," explained Tchekhovskoy, a post-doctoral fellow at the Princeton´s Center for Theoretical Science. "While in previous works it was a free parameter, in our series of works the field is maximum: it is as strong as a black hole's gravity pull on the disk."
The twisting energy in the simulations grows so strong that it powers the jet. In reality, the jet can reorient the accretion disk rather than the other way around as was thought previously.
"People had thought that the disk was the dominant aspect," McKinney said. "It was the dog and the jet was the wagging tail. But we found that the magnetic field builds up to become stronger than gravity, and then the jet becomes the dog and the disk becomes the wagging tail. Or, one can say the dog is chasing its own tail, because the disk and jet are quite balanced, with the disk following the jet — it's the inverse situation to what people thought."
Researchers are closer than ever to begin able to see the details of jets and accretion disks around the black holes. Sheperd Doeleman of MIT recently reported the first images of the jet-launching structure near the supermassive black hole M87 at the center of a neighboring galaxy. The researchers captured the event using the Event Horizon Telescope, a very long baseline interferometry (VLBI) array composed of four telescopes at three geographical locations.“¯While those images only constituted a small sliver of a vast skyscape, the results have given astronomers like the current team the hope that they will get their first comprehensive glimpse into the black hole's neighborhood in the next three to five years.
"We'll see the gases swirl around the black hole and other optical effects that will be signatures of a black holes in spacetime that one can look out for," said Blandford. Whether the results match their models or not, the scientists expect to learn a lot from either outcome.
"If you don't have an accurate model and anything can happen as far as you understand, then you're not going to be able to make any constraints and prove one way or another whether Einstein was right," McKinney explained. "But if you have an accurate model using Einstein's equations, and you observe a black hole that is very different from what you expected, then you can begin to say that he may be wrong."
The model developed by Blandford and his colleagues will help serve that comparative role, though they say that they still need one crucial bit of information to make the observations meaningful. According to the team, they must find a way to translate the physics of the black hole system into a visual signal as it would be seen from the vantage point of our telescopes billions of light years away.
"We're in the process of making our simulations shine, so they can be compared with observations," McKinney said, "not only to test our ideas of how these disks and jets work, but ultimately to test general relativity."