Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos

by Scharf, Caleb

  Lewis Fry Richardson: A brief biography by Oliver M. Ashford is available online to subscribers to the Oxford Dictionary of National Biography Index (www.oxforddnb.com/view/article/35739).

  web-like cosmic brain: As maps of the positions of galaxies have grown, we have seen more and more of this “web” structure. It is also a defining characteristic of larger computer simulations that attempt to model the gravitational behavior of dark and normal matter and how structures form and grow on cosmic scales. The phrase “cosmic web,” coined in 1996 by University of Toronto astrophysicist Richard Bond, has become generally used by astronomers.

  total observable universe: Although I use this term loosely here, it does actually stem from a more rigorous definition. The “observable universe” is everything close enough to us for its light to have had time to reach us (though it requires specialized instruments to detect much of it). The accelerating expansion of the universe (our current understanding of the matter) will eventually limit how “far” we can see—there will be stars and galaxies that we simply won’t ever know about because their light will be stretched or redshifted too much.

  between light and dark: The complete answer to this question is also a resolution of what is known as Olbers’s Paradox. In 1823, the German scientist Heinrich Olbers first posed the question: If the universe is infinite (or at least very large), why is the sky not uniformly bright with the accumulated light of stars from every apparent direction? Many solutions have been proposed over the years, from steady-state cosmologies to opaque universes. The basic answer is that the universe is neither infinite in age nor static in terms of dynamics—its expansion diminishes the light from distant parts. But on more local scales, the absolute number of stars in any given region is critical in determining what we see as light and dark.

  3. ONE HUNDRED BILLION WAYS TO THE BOTTOM

  Hoover Dam: In addition to the U.S. Department of the Interior information online (www.usbr.gov/lc/hooverdam), see Michael Hiltzik, Colossus: Hoover Dam and the Making of the American Century (New York: Free Press, 2010).

  Bureau of Reclamation: Some excellent resources and short essays about the Hoover Dam are available at the bureau’s own website, www.usbr.gov/lc/hooverdam.

  Hydroelectricity in Norway: Many hydroelectric plants in Norway are all but invisible. Natural mountain lakes provide the needed reservoirs of elevated water, and often the only signs of a power station are a series of large pipes running down the side of a mountain or high cliff to connect to a turbine building.

  produced his work: See the notes from chapter 1 for references to Einstein’s papers on special and general relativity. See also the excellent discussion by Kip Thorne in Black Holes and Time Warps: Einstein’s Outrageous Legacy (New York: W. W. Norton & Company, 1994).

  work of many others: It is true that Einstein is the individual who ultimately succeeded in this great mental feat, but he certainly didn’t do it in complete isolation. For example, the German mathematician David Hilbert also arrived at a formulation of the field equations, as Einstein did, in late 1915. Hilbert gave Einstein full credit, but it does seem that Einstein benefited from the backdrop of others working on the problem.

  extremely rigid and stiff: Why is spacetime like this? It’s the same as asking why gravity is such a weak force compared to others like electromagnetism. We don’t really know, but theoretical physicists have some ideas. These include the Randall-Sundrum model, in which the universe is really five-dimensional and the weakness of gravity is due to the fact that we only experience a small part or projection of its properties into our dimensions. A good popular account is by Lisa Randall herself: Warped Passages: Unraveling the Mysteries of the Universe’s Hidden Dimensions (New York: Ecco, 2005).

  found a solution: The Kerr solution to the field equations is a more general version of Schwarzschild’s solution, and of course applies to any spherical mass.

  Roger Penrose: Penrose’s original paper that includes the extraction of black hole spin energy is “Gravitational Collapse: The Role of General Relativity,” Rivista del Nuovo Cimento, special issue I (1969): 252.

  nice geranium: The astute reader might note that neither a falling whale nor a falling pot of geraniums is an original invention. Douglas Adams, as so often is the case, got there first.

  a rocket almost thirty feet long: The vehicles used were Aerobees—small suborbital rockets just about twenty-five feet in height on the launch pad and capable of carrying a payload of about seventy kilograms (150 pounds) to an altitude of 250 kilometers (more than 150 miles).

  Riccardo Giacconi: Received the Nobel Prize in Physics in 2002 “for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources” (www.nobelprize.org/nobel_prizes/physics/laureates/2002/giacconi.html).

  fresh look at Cygnus X-1: The most recent observations (involving the Chandra X-ray Observatory) have found that the black hole in the Cygnus X-1 system is almost fifteen times the mass of the Sun, and is spinning eight hundred times a second. This makes it one of the largest “small” holes known in our galaxy—a possibly atypical object. See, for example, Jerome Orosz et al., “The Mass of the Black Hole in Cygnus X-1,” Astrophysical Journal 742, article id 84 (2011).

  Yakov Zel’dovich: Zel’dovich’s paper on energy release through accretion is “The Fate of a Star and the Evolution of Gravitational Energy Upon Accretion,” Soviet Physics Doklady 9 (1964): 195.

  Edwin Salpeter: Salpeter’s paper on energy release through accretion is “Accretion of Interstellar Matter by Massive Objects,” Astrophysical Journal 140 (1964): 796.

  Karl Jansky: Jansky’s results were published as “Radio Waves from Outside the Solar System,” Nature 132 (1933): 66.

  Finally, in 1962, a series: A key observation made use of lunar occultation of a distant radio source (a quasar) to pin down its location well enough for optical telescopes to target it. See C. Hazard et al., “Investigation of the Radio Source 3C 273 by the Method of Lunar Occultations,” Nature 197 (1963): 1037.

  Maarten Schmidt: The discovery of the distance (redshift) of the quasar 3C 273 was presented by Maarten Schmidt in “3C 273: A Star-like Object with Large RedShift,” Nature 197 (1963): 1040. Schmidt’s own recollection of the discovery in an interview contains more details; the transcript is held by the Center for History of Physics of the American Institute of Physics (www.aip.org/history/ohilist/4861.html).

  raging debate: This is a lengthy story in its own right. The greatest challenge for scientists was to try to explain the colossal energy output that was implied by objects like quasars and by the radio-bright structures being detected. Key figures included the English physicists Fred Hoyle (eventually Sir Fred Hoyle) and Geoffrey Burbidge, who realized that gravitational energy was probably behind these objects, although exactly how was not clear at that point.

  otherwise unremarkable galaxies: It was also known by this time that many galaxies have exceptionally bright centers (nuclei) that can be seen in visible light, as well as some curious spectral characteristics (for example, the so-called “Seyfert” galaxies). A generic name for all such phenomena became “Active Galactic Nuclei,” or AGN for short. While this is a term that is always used in modern astronomy, it can also be confusing, since it covers a multitude of situations. I have therefore avoided its use, preferring to be more explicit about the feeding states of black holes.

  Donald Lynden-Bell: In the interest of full disclosure, Donald was one of my advisors while I studied for a Ph.D., so my discussion is undoubtedly colored by that experience. However, I am not alone in my admiration: in 2008 Lynden-Bell and Maarten Schmidt were joint winners of the Kavli Prize in Astrophysics for their work on quasars and black holes. The original paper is by Donald Lynden-Bell, “Galactic Nuclei as Collapsed Old Quasars,” Nature 223 (1969): 690.

  4. THE FEEDING HABITS OF NONILLION-POUND GORILLAS

  flattened ring of gas: The central molecular ring is a quite complex structure. In addition to the ri
ng there are curved filamentary “spokes” emanating from the very center, seen in radio waves. These also appear to be in motion.

  colossal black hole: The object (black hole) and environment at the very center of our galaxy is also known as Sagittarius A*, often shortened to Sgr A*.

  technological prowess: The mass of the Milky Way’s central black hole has been estimated by Reinhard Genzel and his group at the Max-Planck-Institut für extraterrestrische Physik, Garching, Germany (the Max Planck Institute for Extraterrestrial Physics) and by the group led by Andrea Ghez at the University of California, Los Angeles. Both have obtained stellar motions at the galactic center that allow for the estimation of the mass and size of the central object. This is a tremendously challenging exercise, given the tiny size of the stellar orbits, from our perspective, and the faintness of the stars at this distance.

  history of that effort: A fascinating but lengthy story. Interestingly, the research of the past fifty or so years into quasars/radio galaxies/“active” galactic centers has tended to be split off into wavelength regimes. Radio astronomers have studied the lobe-like structures and surveyed for bright radio sources across the cosmos. Astronomers who focus on visible light have pursued spectroscopic observations of quasars and galaxies, and so on. Part of the challenge was to somehow tie together the very different apparent behaviors that emanated from the centers of galaxies. Even confirming that an object like a quasar did indeed sit within a galaxy was difficult, since the quasar light swamped the starlight of the much, much fainter host galaxy. A large part of the answer is that it depends on whether you are looking “edge-on” or straight down toward the central objects. What has become known as the “unified” model or scheme is a physical arrangement thought to be common to most supermassive black holes. The hole itself is surrounded by both a thinner disk of accreting matter (described later in this chapter) and, outside this, a much thicker “donut” or torus of denser gas and dust. Above and below these structures are smaller clumps and clouds of hot gas that can be moving fast. Jets (as you will also see later in this chapter) can emerge from the center. Quasars are seen when the observer is looking almost straight down the central axis—inside the disk and torus.

  one-thousandth of the mass: This relationship is established by measuring the rate at which the central stars in a galaxy are moving around—their statistically typical velocities. Using Newtonian physics, this provides an estimate of the mass of stars in the bulge. A variety of techniques are then used to evaluate the central black hole mass, which is seen to obey the one-thousandth relationship. The astronomical tools and techniques needed to make this measurement really emerged at the start of the twenty-first century. Two key papers are Laura Ferrarese and David Merritt, “A Fundamental Relation Between Supermassive Black Holes and Their Host Galaxies,” Astrophysical Journal 539 (2000): L9, and Karl Gebhardt et al., “A Relationship Between Nuclear Black Hole Mass and Galaxy Velocity Dispersion,” Astrophysical Journal 539 (2000): L13.

  “static” surface: The phenomenon whereby matter within this distance of a spinning black hole can appear to be moving around faster than light is known as an extreme version of the Lense-Thirring effect, or frame-dragging, since it is the coordinate frame of spacetime that is being moved around. See, for example, Josef Lense and Hans Thirring, “On the Influence of the Proper Rotation of Central Bodies on the Motions of Planets and Moons According to Einstein’s Theory of Gravitation,” Physikalische Zeitschrift 19 (1918): 156.

  Werner Israel: Werner Israel’s discussion of the limiting spin on black holes is “Third Law of Black-hole Dynamics: A Formulation and Proof,” Physical Review Letters 57 (1986): 397.

  Roger Blandford and Roman Znajek: Blandford and Znajek described this mechanism in their paper “Electromagnetic Extraction of Energy from Kerr Black Holes,” Monthly Notices of the Royal Astronomical Society 179 (1977): 433.

  “synchrotron radiation”: The history of the discovery of synchrotron radiation is described by Herbert C. Pollock, “The Discovery of Synchrotron Radiation,” American Journal of Physics 51 (1983): 278. Although the discovery was made in 1947, it took astronomers a long time to recognize that the same mechanism was at play in the universe.

  Kip Thorne: Quote from the prologue of his book Black Holes and Time Warps: Einstein’s Outrageous Legacy (New York: W. W. Norton & Company, 1994, p. 23).

  5. BUBBLES

  apple pie from scratch: Quote from Cosmos, the television series by Carl Sagan, Ann Druyan, and Steven Soter, and from the book of the same name by Carl Sagan (New York: Random House, 1980).

  another fascinating story: The history of the investigation of the “lumpiness” of the early universe and its measurement through the study of the tiny variations in the cosmic microwave background radiation (together with the evaluation of the distribution of matter in our present-day universe) is indeed a great story. It’s one that is still playing out as we probe in ever more detail the physics of the very young universe. An excellent popular account of the first big breakthroughs with the COBE space mission is by John Mather (who later won the Nobel Prize for his work) and John Boslough, The Very First Light: The True Inside Story of the Scientific Journey Back to the Dawn of the Universe (New York: Basic Books, revised ed., 2008).

  James Jeans: The work describing the calculations of gravitational instability by James Jeans is “The Stability of a Spherical Nebula,” Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character 199 (1902).

  emerged in the late 1960s: First discussion by James Felten et al.: “X-rays from the Coma Cluster of Galaxies,” Astrophysical Journal 146 (1966): 955.

  in the mid-1970s: A key paper discussing cooling gas was based on observations with the Uhuru satellite: Susan M. Lea et al., “Thermal-Bremsstrahlung Interpretation of Cluster X-ray Sources,” Astrophysical Journal 184 (1973): L105.

  “cooling flow”: The theoretical and observational interpretations of gas cooling in galaxy clusters came together from three main studies: Len Cowie and James Binney, “Radiative Regulation of Gas Flow Within Clusters of Galaxies—A Model for Cluster X-ray Sources,” Astrophysical Journal 215 (1977): 723; Andrew Fabian and Paul Nulsen, “Subsonic Accretion of Cooling Gas in Clusters of Galaxies,” Monthly Notices of the Royal Astronomical Society 180 (1977): 479; and William Mathews and Joel Bregman, “Radiative Accretion Flow onto Giant Galaxies in Clusters,” Astrophysical Journal 224 (1978): 308.

  In 1994: Andrew Fabian, “Cooling Flows in Clusters of Galaxies,” Annual Reviews of Astronomy and Astrophysics 32 (1994): 277.

  10 million degrees: As data accumulated, the full picture emerged. A good overview is presented by John Peterson et al., “High-Resolution X-ray Spectroscopic Constraints on Cooling-Flow Models for Clusters of Galaxies,” Astrophysical Journal 590 (2003): 207.

  Boehringer used: This study used the high-resolution imager on the mission known as ROSAT to study Perseus. Hans Boehringer et al., “A ROSAT HRI Study of the Interaction of the X-ray-Emitting Gas and Radio Lobes of NGC 1275,” Monthly Notices of the Royal Astronomical Society 264 (1993): L25.

  a million seconds altogether: This extraordinary set of data is described by Andrew Fabian et al., “A Very Deep Chandra Observation of the Perseus Cluster: Shocks, Ripples and Conduction,” Monthly Notices of the Royal Astronomical Society 366 (2006): 417. Since then Fabian and his colleagues have obtained even more data that extend their X-ray map outward across the cluster, revealing more structures: Andrew Fabian et al., “A Wide Chandra View of the Core of the Perseus Cluster” (forthcoming in Monthly Notices of the Royal Astronomical Society; available as a preprint: http://arxiv.org/abs/1105.5025).

  Perseus is not the only: Work by astronomers such as Brian McNamara has shown many other clusters with bubbles and activity. See, for example, Brian McNamara et al., “The Heating of Gas in a Galaxy Cluster by X-ray Cavities and Large-scale Shock Fronts,” Nature 433 (2005): 45.

  “flyba
ll” governor: The method of attachment of this system is sometimes called a conical pendulum, since instead of swinging back and forth, the pendulum mass moves in a circle at the end of its stiff arm.

  converted from gas into stars: Evidence exists that the observed (low) rate of gas cooling implied by X-ray observations is in accord with the number of new stars forming in at least some galaxy cluster cores if the star formation efficiency from cooling cluster gas is 14 percent. This would match the universal fraction of normal matter that is in cluster stars. Michael McDonald et al., “Star Formation Efficiency in the Cool Cores of Galaxy Clusters” (forthcoming in Monthly Notices of the Royal Astronomical Society; available as a preprint: http://arxiv.org/abs/1104.0665).

  6. A DISTANT SIREN

  John Lennon: Paraphrasing lyrics from Lennon/McCartney, “Across the Universe” (from the Beatles’ charity album for the World Wildlife Fund, No One’s Gonna Change Our World, London, Apple Records, 1969).

  produced overgrown galaxies: Also known as the “overcooling problem.” See for example A. J. Benson et al., “What Shapes the Luminosity Function of Galaxies?,” Astrophysical Journal 599 (2003): 38.

  ROSAT: The Roentgen Satellite was developed by Germany, the United States, and the United Kingdom. It was launched in 1990 and turned off in 1999. Like many space-borne instruments, ROSAT had several detectors attached to the end of one main telescope. These included X-ray imaging devices that exploited the electrostatic characteristics of X-ray photons interacting with atoms—either in gases or in solids. In this way, X-ray photons could be converted to electrical signals that could then be used to construct an image.