The magnetic universe
Where did the black hole energy go?
A typical giant black hole forms when 100 million solar
masses are packed into a region the size of the solar
system, creating an extraordinary deep potential well.
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
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.