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.
Mitch Begelman and his group conduct theoretical studies of the interaction of black holes with their surroundings. They are interested in determining how black holes swallow matter, release energy, create visible haloes and jets, and impact other objects they encounter in space. They'd like to understand how quasars give rise to massive black holes and how massive black holes regulate the structure of galaxies and the evolution of galaxy clusters. To this end, they develop sophisticated computer simulations of the behavior of hot and highly ionized gases around quasars, black holes, neutron stars, and galaxy clusters.
Perseus cluster sound waves
The group recently showed how supermassive black holes slow down the cooling of gas clouds surrounding them, reducing star formation and slowing the growth of the black hole itself. Once a supermassive black hole reaches a mass greater than a hundred million suns, it emits two high-energy jets traveling away from it in opposite directions. The emergence of the jets stops the growth of the supermassive black hole. The jets heat the gas they pass through and interact with the gas cloud to create a bubbling, buoyant mushroom cloud that spews energy out laterally. This explosive mushroom cloud produces sound waves that move away in all directions, heating everything in their path. Because of the heating they cause, these bubbling sound waves are nature's way of keeping galaxies and clusters from getting too big.
Begelman's group also studies the effect of photon bubbles on the evolution of black hole accretion disks. Photon bubbles are cavities of X-rays that form in the sea of electrically charged gas (plasma) found in accretion disks with strong magnetic fields. The researchers have discovered that photon bubbles grow fastest in response to gravitational acceleration, which is high in black hole accretion disks, and along magnetic field lines that are nearly vertical. Photon bubbles, which are often buoyant, can spontaneously evolve into bubble shockwave trains traveling toward the outer edge of an accretion disk. As photon bubbles rise up through an accretion disk, their behavior resembles convection patterns found in the Sun. The group posits that photon bubbles may be critical for cooling accretion disks dominated by high-energy radiation. It is currently looking at how photon bubble evolution relates to overall accretion disk dynamics.
Begelman's research has helped make JILA and the University of Colorado a leading research center on galaxy formation and the cosmology of the recent universe. For instance, he was part of the team that discovered the first evidence for energy extracted from a black hole. Like most of JILA's theoretical astrophysicists, Begelman works with observational astronomers to test his theories and to help them interpret their data.
Phil Armitage investigates the environments where black holes swallow gas. He wants to better understand the physics behind the variable and turbulent process that knocks perfectly stable gas orbiting a black hole out of orbit and into the singularity. It is just now becoming possible to calculate in detail how this might happen. However, so far astronomical observations of this process are only indirectly related to the theoretical calculations. The observations mainly detect low-density hot gases, which don't comprise the majority of the matter flowing into a black hole.
Armitage also studies the formation of supermassive black hole binaries during the merger of two galaxies. He recently determined that gravitational waves could provide evidence of the astrophysical processes that allow supermassive black hole binaries to coalesce. In the future, the Laser Interferometer Space Antenna (LISA), a joint mission being planned by NASA and the European Space Agency, will be able to gather this evidence.
The Laser Interferometer Space Antenna (LISA)
Prior investigations at JILA by Peter Bender have made important contributions to the design of LISA. Bender is now working with students and scientists at other institutions to improve the accuracy achievable by LISA and to simplify its mission. LISA's main objectives are to observe gravitational waves from massive black hole binaries formed in galaxy mergers and to study the formation and growth of massive black holes and the galaxies associated with them. To detect gravitational waves, precision laser measurements will be made between carefully protected test masses in three spacecraft that form an equilateral triangle in orbit around the Sun. Each side of the triangle will be approximately 5 million kilometers long.
One challenge during LISA's mission will be to maintain the constancy of the corner angles and side lengths of the spacecraft triangle. Because the spacecraft are at different distances from the Earth at any given time, each is affected slightly differently by Earth's gravity. Over time, this will result in variations in the triangular configuration, making accurate measurement of incoming signals more difficult.
Bender recently evaluated the possibility of canceling out the differential gravitational forces on the spacecraft by continually applying tiny forces to the test masses inside each of the spacecraft. The tiny forces would be different for each spacecraft and would exactly counterbalance the differential effects of Earth's gravity. He concluded that this approach should work. The main challenge will be to ensure that the applied forces have very low noise to avoid unwanted perturbations of the test-mass separation.
Bender is now working with colleagues around the world to prepare for the LISA mission. A LISA Pathfinder mission is scheduled for 2009, with the main mission scheduled for launch in 2017. The Pathfinder's goal will be to verify the (expected) extremely low level of residual test-mass accelerations.
Images of galaxies (taken by the Hubble Space Telescope) emitting X-ray flashes. The galaxies appear very small - about the size of a penny viewed from 5 miles away - because of their distance. The bottom panels show a close up of the galaxies with circles representing the Chandra X-ray positions of the "afterglow" light from the flashes.
Rosalba Perna studies gamma-ray bursts and their afterglows at longer wavelengths. Gamma-ray bursts, which occur billions of light years from Earth, are among the most intriguing astronomical phenomena of modern times. They emit a hundred billion times more energy in a few seconds than our Sun produces in a year. These luminous events signal the formation of a black hole during a supernova. Perna uses gamma ray bursts and their afterglows to probe star and galaxy formation in the early (high red-shift) universe. Because they are more luminous than quasars, gamma-ray bursts can penetrate the dense and dusty stellar nurseries that produced the universe's earliest generations of stars.
Perna and her colleagues at Stanford and Princeton recently figured out the relationship between X-ray flashes, X-ray rich gamma-ray bursts, and gamma-ray bursts detected by different space-based observatories. X-ray flashes are transient astronomical X-ray sources that last from several seconds to a few minutes. High-energy gamma-ray bursts signal the formation of a black hole during the collapse of a massive star. X-ray rich gamma-ray bursts share characteristics with both gamma-ray bursts and X-ray flashes. After studying the afterglows of these three phenomena, the researchers concluded that all three emanate from similar structures and differ only in their orientation relative to our line of sight from Earth.
Perna is a collaborator on teams using the Swift satellite, launched in November of 2004. The teams have (1) discovered the oldest, most distant gamma-ray burst to date, (2) identified the optical afterglow of a short gamma-ray burst of the type that is due to mergers between two neutron stars or a neutron star and a black hole, and (3) detected the presence of X-ray flares in the early afterglows of gamma-ray bursts. She recently collaborated with Phil Armitage on a model proposing that fragmenting accretion disks give rise to these flares. Perna also studies anomalous X-ray pulsars (a type of neutron star) whose X-ray emission may be powered by such sources as accretion disks around neutron stars and black holes and black-hole remnants of the universe's earliest massive stars.
Cosmologist Andrew Hamilton explores the implications of Einstein's theory of relativity for black-hole structure and behavior. He has proposed a new model for the flow of matter into a black hole, in which space flows like a river through a flat background. Objects like light rays that move through the river of space abide by the rules of special relativity. When the river of space falls into a stationary black hole, it flows toward the singularity faster than light, bringing along everything inside it. In a rotating black hole, the river of space also twists. As objects move in the river, they are buffeted by tidal changes in the speed and rotation of the river of space. A photon in the river would experience these changes as an evolving curvature of space-time.
Hamilton has created a new model suggesting that black holes are not mostly empty, as previously thought. According to his analysis, the space between a black hole's outer edge, or event horizon, and the edge of its inner core, or inner horizon, is mostly empty. However, the core is filled with hot relativistic plasma generated by the violently unstable inner horizon. The singularity lies nestled inside the dense, hot plasma.
Hamilton's new model confirms a 1919 prediction by Poisson and Israel of an unstable inner horizon due to relativistic waves of baryons streaming toward the inner horizon and waves of dark matter counter-streaming back toward the outer horizon. The model also has implications for how a person falling into a black hole would be killed: Tidal forces on a river of space flowing into a stellar-sized black hole would stretch out a human being until she was as thin as spaghetti, tearing her apart in the process. In contrast, a person falling into a supermassive black hole, where tidal forces are weak, would be fried by the plasma.