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Black Holes & Galaxies

Black holes are some of the most mysterious and intriguing objects in the Universe. Gravitational forces near them are so strong that nothing—not even light—can escape their fatal attraction. Small- and intermediate-sized black holes are formed in the cataclysmic explosions that mark the death of large-to-massive stars. Supermassive black holes (millions to billions times more massive than our Sun) exist in the center of galaxies, including the Milky Way. How they form and their role in the creation and maintenance of galactic structures is under study.

Phil Armitage conducts theoretical studies of the interaction of black holes with their surroundings. He is interested in theories that explain the causes of star formation within massive accretion disks orbiting black holes and the coalescence of planetary systems from disks of gas and dust orbiting new stars. He has evaluated a "gravitational collapse" theory, which described gravitational interactions in an accretion disk that cause clumps of gas to form. These clumps then attract more matter to them, eventually forming stars (around black holes) or planets (around stars).

Many scientists believe this mechanism is more likely to occur in the outer regions of a supermassive black hole than in a planetary accretion disk. If so, the gravitational collapse theory may explain why there are often a multitude of stars near black holes. For instance, two egg-shaped necklaces of magnificent stars were created about six million years ago around the black hole at the center of the Milky Way Galaxy. A similar cluster of massive stars orbits Andromeda’s central black hole.

Armitage’s simulation (which is based on the gravitational collapse theory) shows brilliant stars like these being created when gaseous disks in the process of being pulled into orbit around a black hole fragment into stars. The black-hole-mediated process is altogether different from star creation in cold gas clouds elsewhere in a galaxy. However, the accretion disks around black holes exhibit similar dynamics to the disks around stars that form planets.

Mitch Begelman investigates the origins of massive black holes in the center of galaxies. He wants to understand how massive black holes regulate the structure of galaxies and the evolution of galaxy clusters. He undertakes many different research projects that probe aspects of these important processes, including studies of jets created by black holes, the release of energy from black holes, and black-hole mergers that occur when galaxies collide. He uses sophisticated computer simulations to model such complex processes.

Begelman works with Armitage on a sophisticated simulation to test Begelman’s ground-breaking hypothesis that explains the origin of the supermassive black holes found at galactic centers. Begelman’s idea is that the seeds for colossal black holes were sown during the initial formation of galaxies about a billion years after the Big Bang. The black holes formed at what had been the centers of huge, dense reservoirs of dark matter coupled to ordinary matter (mostly hydrogen gas). This coupling broke down when the dense clouds of hydrogen gas cooled sufficiently to begin falling into the center under their own gravity. The in-fall process eventually led to the creation of a dense, self-gravitating core supported by intense radiation pressure.

The cores resembled gigantic stars, except that they never quite reached equilibrium. The in-falling gas continued to compress these cores until they ran out of nuclear fuel and lost the ability, after a couple of million years, to resist the intense gravitational pressures. At this point, the cores collapsed into black holes with masses ranging from tens of thousands to several million suns.

The black holes then began sucking in the leftover mass of their envelopes at the incredible rate of about 10 solar masses per year. Hot matter escaping the black holes’ gravitational pull puffed up these envelopes into gigantic yellow quasi stars resembling red giants, expanding them at least a hundredfold in less than a million years. Eventually these yellow giant stars evaporated, leaving behind black holes with masses between 100,000 and 10,000,000 suns. Begelman believes these primordial black holes were the seeds that gave rise to the supermassive black holes (ranging in size from 100,000 to 10 billion suns) seen today in every galaxy in the Universe that has a bulging center.

Begelman has discovered that if these seed black holes were not too massive, it would be possible to force-feed them from a relatively small envelope of gas around them. This envelope would be regularly replenished as more gas moved into the galactic center. During a period of force-feeding, the black hole could grow rapidly as the gravity of an entire galaxy forced matter into it. The feeding process would stop when energy in the form of jets spewed out of the black hole and blew away the envelope. Eventually, however, gas would fall back towards the black hole, reforming the envelope and initiating another cycle of force-feeding. These force-feeding cycles would stop once the black hole grew large enough to fling the envelope completely out of the galaxy.

Begelman and his group also study the dissipation of magnetic fields in high-energy environments in space. They have discovered a new type of beaming known as kinetic beaming. The behavior of kinetic beams, which include beams of gamma rays and x-rays, is determined by the amount of energy present. The group has explained how particles can be accelerated to near light speeds inside kinetic beams. It figured out that linear acceleration processes throughout the Crab Nebula boost electrons to relativistic speeds. In one tiny area of the nebula, beams of electrons traveling at nearly the speed of light are also gyrating in a magnetic field. This process regularly generates flares consisting of intense and focused beams of gamma rays. The group uses the Janus supercomputer in its investigation of these flares and similar current-driven instabilities driven by flows of electrons (in space) that produce magnetic fields.

In another project, the Begelman group uses the Janus supercomputer to explore Kelvin-Helmholtz (K–H) instabilities in jets emitted by black holes or other astrophysical objects. This kind of instability is the reason flags wave. It occurs when two fluids flow past each other and something “tickles” the interface, causing waves to form. Both fluids become very turbulent. What intrigues the researchers is the fact that jets are propelled incredible distances through space without breaking up—even though they are likely subject to K­–H instabilities.

In related work, the group is studying the formation and dissipation of gamma-ray flares inside jets produced by black holes. The researchers want to better understand how jets are able to carry large amounts of concentrated energy over great distances in space.

In addition to investigating flares and jets, Begelman is working with Armitage on modeling the merger of black holes when galaxies collide. In 1980, Begelman (then at the University of California at Berkeley) and colleagues from Caltech and the United Kingdom’s Institute of Astronomy partially explained how these mergers occur: As two black holes move toward the center of the new galaxy, they “bump” into stars, knocking them out of the way. The black holes lose energy and move closer together until they are about 0.1–0.01 light years apart. At this point, there aren’t any stars left to lob out of the galactic center.

In their early work on mergers, Begelman and Armitage used high-speed supercomputers to show that if the two black holes have masses of less than 10 million suns, additional interactions with gas clouds would then bring them close enough to merge. However, this mechanism failed to promote mergers between larger black holes. The simulation required so much gas to bring the bigger black holes together that stars formed in the gas cloud instead. Thus, a third process must be involved in the merger of more massive black holes, possibly their interaction with the new stars formed during the fragmentation of a large gas disk.

Andrew Hamilton uses Einstein's theory of relativity to model the internal structure and behavior of rotating black holes.  He believes that the solutions to Einstein’s equations hold the key to unlocking what really happens inside a black hole.

Working with JILA member Gavin Polhemus, a physics teacher at Poudre High School in Fort Collins, Colorado, Hamilton considered the amount of entropy (or disorder) that might be created inside a black hole as it accreted more mass. At first it appeared that far more entropy was being created inside black holes than should be possible according to the laws of physics.

The two researchers developed an explanation for the excess entropy that was rooted in string theory and quantum gravity: complementarity. Complementarity accounts for the fact that different observers can experience different quantum realities even though they are, in fact, part of the same event. This idea is consistent with one of the central paradigms of string theory; it predicts that over a black hole’s lifetime of billions of years, observers (who have miraculously managed to survive falling into a black hole) could experience as many as 10 quadrillion alternate quantum realities—which are all actually the same experience!

Hamilton and Polhemus have had some difficulty convincing astrophysicists and string theorists of the validity of their new hybrid theory. “Our priority is to understand the interior structure of rotating black holes,” Hamilton says. “And string theory has important ideas to offer about the high-energy processes occurring there.”

Hamilton is now exploring the structure of quantum mechanical waves inside a black hole that is violently unstable. He is also investigating the characteristics of Hawking radiation inside a black hole’s event horizon, which itself appears to be an optical illusion. Nevertheless, this illusory horizon has physical properties that can be described by a combination classical physics and quantum mechanics. In the process of studying the illusory horizon, Hamilton says he’s getting a better understanding of quantum gravity. He’s even begun interpreting the illusory horizon as a holographic projection of the interior of the black hole itself.

Hamilton has used his understanding of black-hole structure and behavior to create a Black Hole Flight Simulator, which he used in his role as science advisor to multimedia black hole productions by the Denver Museum of Nature and Science, NOVA, and the National Geographic Channel.