New Results from Simulations of the Complicated Physics Around Black Holes Point to a Possible Test of Einstein's Theory of Gravity

May 5, 2015

The only way to accurately predict the conditions near black holes is with extensive computer simulations of the complicated physics involved. While black holes are the quintessential manifestation of Einstein's General Relativity, very few precision tests of the theory have been based on actual observations of black holes. New simulation results point to an observable property of such systems that could be used as a precision test of Einstein's theory.

Many luminous systems in the Universe, from the cores of distant active galaxies to microquasars in our own Galactic backyard, involve matter falling onto a black hole.  The infalling matter takes the form of an accretion disk, in which the matter orbits in a shape like the rings of Saturn, as it gradually makes its way in to the location of the black hole itself.  The complicated interaction of the accreting matter, the intense local magnetic fields, and the spin of the black hole produce spectacular phenomena such as intense jets of particles and radiation squirting out from the center of the disk.

A physical model of the interaction processes involves complicated calculations in both General Relativity and Magnetohydrodynamics - the study of the interaction of charged particle plasmas with magnetic fields.  The most promising way to investigate these systems is with computer simulations in which the space is divided into fine cells, the particles, magnetic fields, and black hole spin are included, and the interactions within cells and at the cell boundaries are allowed to proceed according to the equations of General Relativity and Magnetohydrodynamics.  The properties of the resulting system, including how the matter is distributed, can then be 'observed'.  These astrophysical "GRMHD" simulations are one of the major challenges in computational physics.

Before today's computing power was available, previous simplified calculations by Novikov & Thorne in 1973 suggested that an accretion disk terminates at a specific radius near the black hole that is determined by the black hole's spin.  In order to perform the calculations at the time, the unrealistic assumption of a razor-thin accretion disk was made.  Now, a team including KIPAC's Jonathan McKinney and led by Robert Penna of Harvard University have recently conducted full GRMHD simulations and arrived at an intriguing conclusion.  They show that the result of a termination of the accretion disk at a radius determined by the black hole spin actually holds for a variety of realistic accretion disk scenarios.

McKinney explains that this opens up the possibility that the so-called truncation radius can be observed in black hole X-ray binary systems in our Galaxy, and checking whether the radius is at the location predicted by the simulations determined with General Relativity would be a new test of the theory itself.  Black hole X-ray binaries are systems where a normal star is orbiting and donating matter, in the form of an accretion disk, to a black hole.  The accretion disk itself is hot enough to glow in X-rays, which can be observed.

Precision tests of General Relativity as an accurate and complete theory of gravity are crucial for investigating dark energy, because all current probes of dark energy measure it through its gravitational effect on the course of cosmological evolution.

This work is based in part on a paper submitted to MNRAS, and is available from astro-ph at http://arxiv.org/abs/1003.0966

Science Contact:
Dr. Jonathan McKinney