The magnetic universe

Where did the black hole energy go?

Researchers have estimated that a total gravitational energy equivalent to nearly ten billion supernovae is released during a typical formation, garnering the prize of being the largest energy production process in the present universe. Modern astronomical observations suggest that giant black holes were more active in the past, when the universe was only a fraction of its current age.

So where did all that black-hole energy go? Intense radiation, powerful winds and enigmatic magnetic fields are three of the most important channels for transporting this energy away from the black holes. Some models suggest that the radiation released when black-hole systems formed in the early universe is responsible for re-ionizing the universe after recombination. But to a large extent, radiation has very little dynamic impact once the matter becomes very dilute. Similarly, kinetic winds tend not to propagate very far before losing most of their energy within the galaxy.

But enigmatic magnetic fields are a different story. Working with the University of Toronto, Hui Li of Plasma Physics and Stirling Colgate of the Theoretical Astrophysics group have accounted for a significant fraction of a black hole ’s energy in magnetic fields. The magnetic energy is carried away in the form of neatly lined-up columns of magnetic fields that propagated to a distance slightly larger than the average separation distance between galaxies. The field ’s unique nature of containing a large amount of energy while occupying a limited volume causes magnetic fields to remain dynamically important for a long time, perhaps as long as the age of the universe, according to Li.

Li and a team of other researchers, including Burt Wendroff of Mathematical Modeling and Analysis and John Finn of Plasma Theory, have developed a comprehensive theory of the accretion process – an increase in the mass of a celestial object by the collection of surrounding inter- stellar gases and objects by gravity – and have confirmed the theory by extensive hydrodynamic simulations.

The illustration shows the formation of large-scale vortices in an accretion disk around the black hole. Pressure is overlaid with velocity arrows.

The vortices are anti-cylones enclosing the high-pressure region.

Large-scale waves are also produced in connection with the vortices.

Li says that researchers are just beginning to understand the pieces of the picture and the results are encouraging.

There is increasing evidence that we should view the evolution of the universe as a magnetohydrodynamic phenomenon rather than a process dominated only by gravity and hydrodynamics.