20Oct2013

A Journey Through Spacetime…

3D snapshot for an evolved Black Hole (BH) model. The disk and jet near the BH are aligned with the BH spin axis and point mostly in and out of the figure plane, whereas at larger distances the jet points roughly halfway between the BH spin axis and the disk’s rotational axis (pointing along the orange cylinder). Credit: TACC

Believed to be at the center of every galaxy, black holes have insatiable appetites and their presence is responsible for the evolution of surrounding stars.

Just what are these huge sources of gravitational pull and how are they the source of massive amounts of explosive energy?  “Over its lifetime, a black hole can release more energy than all the stars in a galaxy combined,” said Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology and a member of the U.S. National Academy of Science. “Black holes have a major impact on the formation of galaxies and the environmental growth and evolution of those galaxies.”

When it comes to a black hole, we’re talking about gravity so intense that not even light can escape. Because of this property, observing a black hole directly is impossible. However, astronomers are able to peer into the lives of black holes by taking a look at what is around them… for example the orbits of stars and any detectable energy. Through this information, scientists are able to create computer models to aid in understanding the data at hand and enable them to make predictions about the physics of black holes. However, models are only as accurate as their inferred materials make them.

“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,” explained Jonathan McKinney, an 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.”

Even though we can’t “see” a black hole, we can take a very good look at some of their characteristics, including accretion disks – the surrounding disk of superheated gases and dust that comprise a black hole’s event horizon. Also observable are relativistic jets – high powered streams of ionized gases that flow out over hundreds of thousands of light years of space. In a paper published in Science in January 2013, McKinney, Tchekhovskoy and Blandford predicted the formation of accretion disks and relativistic jets which are far more distorted than previously predicted. It would appear they are far more affected by the extreme gravity and magnetic forces than suspected. These new findings enable far more detailed models of environmental conditions of black holes and expand our knowledge into the field.

English: Black hole jet diagram. http://www.na...

Black hole jet diagram. (Photo credit: Wikipedia)

For some time now, astronomers held the option that accretion disks were relatively simple affairs and polar jets dominated the black hole scene. Their understanding put these jets expelling their materials straight out of the center of the accretion disk – a supposed “flat plate” environment. However, new 3D simulations taken on by today’s supercomputers of the National Science Foundation’s Extreme Science and Engineering Discovery Environment (XSEDE) and NASA has shown these previous assumptions to be “oversimplified”.

A new line of thinking – and simulations – show the jets to be aligned with the black hole’s spin when it is near the center, but becoming parallel from the disk’s rotational axis as it moves outward. This new information gives new insight, a strong theory that the interaction between the jet and disk is warping the accretion disk density.

“An important aspect that determines jet properties is the strength of the magnetic field threading the black hole,” said Alexander Tchekhovskoy, a post-doctoral fellow at the Princeton 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.”

“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.”

Seeking Answers

Just how does this affect our views of Einstein’s theory of general relativity? Astronomers are closer than ever to being able to see the details of the jets and accretion disks around black holes. In a September 2012 paper in Science Sheperd Doeleman of MIT revealed the initial images of the jet-launching structure near the supermassive black hole, M87.

Through the use of the Event Horizon Telescope, a very long baseline interferometry (VLBI) array composed of four telescopes at three geographical locations, we were able to take a more detailed look at known jet activity like never before.

An artist's conception of a supermassive black...

An artist’s conception of a supermassive black hole and accretion disk. (Photo credit: Wikipedia)

“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. These new observations will either validate the new models – or they won’t – but the answers will aid in furthering our understanding.

“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.”

Current Beliefs

Are the new models infallible? Like all models, they are based on assumptions, but this new information will help further refine our understanding of the physics of black holes and the “visual signals” picked up by our telescopes.”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.”

Original Story Source: Texas Advanced Computing Center News Release. Submitted by Tammy Plotner for “Dave Reneke’s World of Space and Astronomy News”.