Black hole simulations explore limits of spacetime
Washington: Black holes are voracious absences at the center of galaxies that shape the growth and death of the stars around them through their powerful gravitational pull and explosive ejections of energy.
Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology and a member of the US National Academy of Sciences said that over its lifetime, a black hole can release more energy than all the stars in a galaxy combined.
"Black holes have a major impact on the formation of galaxies and the environmental growth and evolution of those galaxies," he said.
Gravitational forces grow so strong close to a black hole that even light cannot escape from within, hence the difficulty in observing them directly.
Scientists infer facts about black holes by their influence on the astronomical objects around them: the orbit of stars and clumps of detectable energy.
With this information in hand, scientists create computer models to understand the data and to make predictions about the physics of distant regions of space. However, models 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," Jonathan McKinney, an assistant professor of physics at the University of Maryland at College Park said.
"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," he said.
This makes black holes the ultimate experimental testing grounds for Einstein's general theory of relativity.
While black holes cannot be observed, they are typically accompanied by other objects with distinctive features that can be seen, including accretion disks, which are circling disks of superhot matter on our 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.
McKinney, Alexander Tchekhovskoy, and Blandford predicted that 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. Their highly detailed models of the black hole environment contribute new knowledge to the field.
For decades, a simplistic view of the accretion disks and polar jets reigned. It was widely believed that accretion disks sat like flat plates along the outer edges of black holes and that jets shot straight out perpendicularly.
However, new 3D simulations performed on the powerful supercomputers of the National Science Foundation's Extreme Science and Engineering Discovery Environment (XSEDE) and NASA overturned this oversimplified view of jets and disks.
The simulations show that the jet is aligned with the black hole's spin near the black hole but that it gradually gets pushed by the disk material and becomes parallel to (but offset from) the disk's rotational axis at large distances.
The interaction between the jet and disk leaves a warp in the accretion disk density.
"An important aspect that determines jet properties is the strength of the magnetic field threading the black hole," Tchekhovskoy, a post-doctoral fellow at the Princeton Center for Theoretical Science said.
"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," he said.
In the simulations, the twisting energy grows so strong that it actually powers the jet. In fact, the jet can reorient the accretion disk, rather than the other way around, as was thought previously.
The research is published in the journal Science.