NOTES

1. DARK STAR

Chandra: One of NASA’s “Great Observatories,” ranked as the Hubble Space Telescope’s equal in ambition and cost. Launched on July 23, 1999. Information is available at the NASA/Chandra Science Center/Harvard site: http://chandra.harvard.edu.

Twelve billion years: Travel time for photons from this distant location, corresponding to a cosmological redshift of 3.8 (the ratio between the apparent recession velocity and the speed of light) and a co-moving distance (used in Hubble’s law) of about 23 billion light-years for a flat, vacuum energy–dominated cosmological model. In other words: a very long, long way away.

superclusters: Collections of galaxy clusters and galaxies spanning hundreds of millions of light-years.

Gondwana: Southern supercontinent believed to have existed from approximately 510 to 200 million years ago, which subsequently broke apart to form Africa, South America, Antarctica, Australia, and India. See, for example, Peter Cattermole, Building Planet Earth: Five Billion Years of Earth History (London: Cambridge University Press, 2000).

Bayeux Tapestry: The sixty-eight-meter-long tapestry narrating the Norman invasion of England in 1066. Halley’s comet is depicted, and was likely seen four months prior to the invasion. The seventy-five-to-seventy-six-year orbital period of the comet was first determined correctly by the English astronomer Edmond Halley in 1705.

past few decades: Two excellent further sources are Kip Thorne’s Black Holes and Time Warps: Einstein’s Outrageous Legacy (New York: W. W. Norton & Company, 1994), and the book by Mitchell Begelman and Martin Rees, Gravity’s Fatal Attraction: Black Holes in the Universe (Cambridge: Cambridge University Press, 2nd ed., 2010).

Thornhill: The Church of St. Michael and All Angels, Thornhill Parish, has both a memorial to John Michell in its tower and a more modern plaque to commemorate his accomplishments. The memorial is notable for its effusive descriptions of his tender and affectionate traits.

personal detail: More material is now coming to light about Michell, but I have drawn on numerous scraps of information from many different (often online) sources to produce a very modestly detailed portrait. Another source is Sir Archibald Geikie’s Memoir of John Michell, M.A., B.D., F.R.S., fellow of Queens’ college, Cambridge, 1749, Woodwardian professor of geology in the university 1762 (Nabu Press, 2010, reprint of original written in 1918), also available online at the University of California Libraries Digital Archives.

works on navigation and astronomy: One of Michell’s best-known contributions to physical science was his role in the invention of the torsion balance with Henry Cavendish. This wonderful device allows the gravitational force between two ball-like masses to be measured (an extraordinary accomplishment given the weakness of the forces involved), and hence for the gravitational force law to be calibrated by measuring the gravitational constant. Cavendish and Michell have each occasionally and interchangeably been referred to as “the man who weighed the world,” although Michell died in 1793, some four years before Cavendish made the actual measurement of the Earth’s density.

title of his paper: Michell’s presentation is published as a “Letter to Henry Cavendish” by John Michell, Philosophical Transactions of the Royal Society of London 74 (1784): 35–57.

Maxwell’s work: These four relationships were published in toto by Maxwell in 1865, following two earlier papers that had laid the groundwork. James Clerk Maxwell, “A Dynamical Theory of the Electromagnetic Field,” Philosophical Transactions of the Royal Society of London 155 (1865): 459.

Einstein would later write: This comment was made by Einstein in 1940 in the article “Considerations Concerning the Fundaments of Theoretical Physics” Science 91 (1940): 487.

Michelson-Morley experiment: Often cited (as it is here) as the best failed experiment ever. But of course it didn’t really fail, it was simply so well executed that it revealed the truth. Michelson and Morley’s original paper is quite excellent: Albert A. Michelson and Edward W. Morley, “On the Relative Motion of the Earth and the Luminiferous Ether,” American Journal of Science 34 (1887): 333–45.

special theory of relativity in 1905: Published by Albert Einstein as “Zur Elektrodynamik bewegter Körper,” which translates into English as “On the Electrodynamics of Moving Bodies,” Annalen der Physik 322, no. 10 (1905): 891.

This simple fact: These have been discussed over the past century in many excellent accounts beyond those of Einstein himself. To my mind, the flexibility of time is still perhaps the most amazing and bewildering aspect. Little wonder that relativity has also been a topic for philosophers to mull over.

general theory of relativity: I explore this topic in significantly more detail in chapter 3. To note here, however: there is some confusion in popular accounts about when Einstein published this theory. Although he presented the correct theoretical ideas in 1915, they were in a series of papers that included retractions and corrections of his earlier efforts. In 1916 he finally presented his better-known and more complete discussion and review article, “The Foundation of the General Theory of Relativity,” Annalen der Physik 49 (1916).

In 1927, Heisenberg: His paper was “Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik,” translated roughly into English as “On the visualizable [or “intuitive”] content of quantum theoretical kinematics and mechanics,” Zeitschrift für Physik 43 (1927): 172. The word anschaulichen apparently defies easy translation into English.

In the 1920s astronomers: Throughout this period the great English physicist Sir Arthur Eddington played numerous roles. Not least was his devastating (and ultimately incorrect) critique later on of Chandrasekhar’s 1935 presentation on white dwarfs.

in 1935 Chandrasekhar presented: Key papers were “The Highly Collapsed Configurations of a Stellar Mass,” Monthly Notices of the Royal Astronomical Society 95 (1935): 207, and “Stellar Configurations with Degenerate Cores,” ibid., 226.

extreme states of matter: Both nuclear bombs and stellar interiors involve environments where nuclear constituents (protons and neutrons) can become dissociated from their usual place within atomic nuclei. For objects like neutron stars, the need to use general relativity to understand their structure presents an additional challenge.

John Wheeler: John Archibald Wheeler (1911–2008) was one of the great American theoretical physicists who worked on general relativity. He also worked on the Manhattan Project and mentored many extraordinary scientists, including Richard Feynman and Kip Thorne. Generations of students know him as one of the authors of the seminal textbook titled Gravitation with Charles Misner and Kip Thorne (San Francisco: W. H. Freeman, 1973)—all 1,200 pages of it.

NASA Goddard Institute for Space Studies: One of the least-known NASA outposts, home to some of the best planetary and climate science research going on today. Tom’s (the restaurant) is still there, and tourists are often lined up taking pictures of it, because its exterior is used in the opening sequence for the TV show Seinfeld.

2. A MAP OF FOREVER

oldest recognizable: I have taken a small liberty here in making this a statement of fact. It seems that not everyone agrees on the celestial interpretation of these carvings and paintings from tens of thousands of years ago (during the Paleolithic). One interesting reference is the discussion by Amelia Sparavigna, “The Pleiades: The Celestial Herd of Ancient Timekeepers” (online in the physics preprint archives at http://arxiv.org/abs/0810.1592).

range of photon wavelengths: Sources differ a little on the actual sensitivity range of human eyes, but between about 380 and 750 nanometers is typical. Sensitivity is not uniform, but peaks at around 550 nanometers (green light) and is a combination of the different cone and rod receptor sensitivities in the human retina. For comparison, bees have orange to blue sensitivity along with some ultraviolet sensitivity—spanning wavelengths of approximately 300 to 600 nanometers.

Harlow Shapley: Many sources exist on Shapley’s life and long career. A detailed obituary was published in Nature: Z. Kopal, “Great Debate,” Nature 240 (1972): 429. The American Institute of Physics holds the transcript of an interview with Shapley in 1966 (www.aip.org/history/ohilist/4888_1.html). The Franklin Institute holds details of his life and family (www.fi.edu/learn/case—files/shapley/index.html).

“The Sun is…” Shapley quote is taken from the published results of his globular cluster survey, “Globular Clusters and the Structure of the Galactic System,” Publications of the Astronomical Society of the Pacific 30 (1918): 42.

modern mapping of the cosmos: Edwin Hubble, also using the Mount Wilson Observatory, found in the early 1920s that many nebulae were actually galaxies in their own right, separate and very distant from ours—something that Harlow Shapley did not initially agree with. By 1929, Hubble had also shown that they are all moving away from one another—the first direct evidence of the expansion of the universe. Edwin Hubble, “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae,” Proceedings of the National Academy of Sciences of the United States of America 15 (1929): 168.

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 was one of those who 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 Red-Shift,” 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 ring 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.

“flyball” 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.

Wilhelm Roentgen: In 1895 he discovered that something still emerged after cathode rays (electrons) passed through a thin film of aluminum with a cardboard backing. He noticed that this unknown phenomenon produced fluorescence in material some distance away, and correctly surmised that it represented a new type of ray or radiation.

a process that wrapped up: The project that became known as the Wide Angle ROSAT Pointed Survey (WARPS for short) started in 1995 and resulted in seven major scientific papers, the most recent in 2009. Along the way we had help from Matt Malkan at UCLA and others. The first paper was Scharf et al., “The Wide-Angle ROSAT Pointed X-ray Survey of Galaxies, Groups, and Clusters. I. Method and First Results,” Astrophysical Journal 477 (1997): 79.

produced a camera: The submillimeter camera used in Hawaii was called the Submillimeter Common User Bolometer Array, or SCUBA for short, built by a team at what was then the Royal Observatory in Edinburgh, Scotland.

uninspiring name of 4C41.17: As with many astronomical objects, this dull name indicates the source of its first detection and its location, 4C being the fourth Cambridge radio survey and 41.17 indicating the angular declination of the object in the Earth’s northern sky. The first inklings that this object might represent a baby galaxy cluster were given by Rob Ivison and colleagues: “An Excess of Submillimeter Sources near 4C 41.17: A Candidate Protocluster at Z = 3.8?,” Astrophysical Journal 452 (2000): 27.

Dan Schwartz: Schwartz’s paper was “X-ray Jets as Cosmic Beacons,” Astrophysical Journal Letters 569 (2002): 23.

Jim Felten and Philip Morrison: Felten and Morrison’s paper was “Omnidirectional Inverse Compton and Synchrotron Radiation from Cosmic Distributions of Fast Electrons and Thermal Photons,” Astrophysical Journal 146 (1966): 686.

Arthur Compton: Winner of the Nobel Prize in Physics in 1927. Biography available from the Nobel Foundation: www.nobelprize.org/nobel_prizes/physics/laureates/1927/compton-bio.html.

Wil van Breugel: The results that Wil showed us came out of his team’s more extensive program of observing distant objects. See for example Michiel Reuland et al., “Giant Lyα Nebulae Associated with High-Redshift Radio Galaxies,” Astrophysical Journal 592 (2003): 755.

report our findings: We did, and the paper is by Caleb Scharf, Ian Smail, Rob Ivison, Richard Bower, Wil van Breugel, and Michiel Reuland: “Extended X-ray Emission Around 4C41.17 at z = 3.8,” Astrophysical Journal 596 (2003): 105. The “z = 3.8” in the title refers to the cosmological redshift (a surrogate for distance) of the light from this object, which in turn indicates an apparent velocity away from us that is 3.8 times the speed of light. Of course, it is the universe itself that is expanding and stretching the wavelength of the photons to give this impression.

Only some 4 percent of galaxies: This statistic has been derived from survey data of the local universe. See for example Xin et al., “Active Galactic Nucleus Pairs from the Sloan Digital Sky Survey. I. The Frequency on ~ 5–100 kpc Scales” (in preprint form at http://arxiv.org/abs/1104.0950, 2011). Also, a related work discusses how the gravitational interactions between other galaxies may encourage the feeding of supermassive black holes: Xin et al., “Active Galactic Nucleus Pairs from the Sloan Digital Sky Survey. II. Evidence for Tidally Enhanced Star Formation and Black Hole Accretion” (also as a preprint, http://arxiv.org/abs/1104.0951, 2011).

7. ORIGINS: PART I

helps control the production of stars: Numerous reviews have now been written in the scientific literature about the relationship of black holes to star and galaxy properties. One useful article is by Andrea Cattaneo et al: “The Role of Black Holes in Galaxy Formation and Evolution,” Nature 460 (2009): 213.

some galaxies lack: The nature of the central stellar bulges of galaxies and their black holes is very much at the forefront of current research—and controversy. In particular, why some galaxies, such as our own, have so little central bulge is a bit of a mystery. A good starting point for this discussion is a short summary by Jim Peebles, “How Galaxies Got Their Black Holes,” Nature 469 (2011): 305.

can change a planet: This is not an idle comment. It is now clear that the evolution of life (particularly single-celled microbial life, the bacteria and the archaea) is completely intertwined with the surface evolution of the Earth—from chemistry to climate—over the past 4 billion years. An excellent and provocative discussion is by Paul Falkowski, Tom Fenchel, and Edward DeLong, “The Microbial Engines That Drive Earth’s Biogeochemical Cycles,” Science 320 (2008): 1034.

under the thrall: Although this research on chemistry around stars of different masses is still very new, it is quite compelling, because the physical explanation makes a lot of sense. For more on this, see Pascucci et al., “The Different Evolution of Gas and Dust in Disks Around Sun-like and Cool Stars,” Astrophysical Journal 696 (2009): 143.

but not unreasonable: Indeed it’s not. The jury is still very much out on how the surface chemistry developed on the young planet Earth. We do know, however, that the planet was being pelted by a lot of meteoritic material containing a rich mixture of organic and inorganic molecules. Some fraction of the early chemistry must have been due to this extraterrestrial material—the tail end of planet formation itself.

stellar giants: For the idea that the first stars in the universe were huge, and would give rise to large black hole remains, see, for example, Piero Madau and Martin Rees, “Massive Black Holes as Population III Remnants,” Astrophysical Journal 551 (2001): L27.

sufficiently huge blob: Skipping over any true stellar object and going directly to a large black hole: see, for example, Begelman et al., “Formation of Supermassive Black Holes by Direct Collapse in Pregalactic Halos,” Monthly Notices of the Royal Astronomical Society 370 (2006): 289.

enormous whirlpools: These simulations and their implications are reported by L. Mayer et al., “Direct Formation of Supermassive Black Holes via Multi-scale Gas Inflows in Galaxy Mergers,” Nature 466 (2010): 1082. These results and their broader implications are also discussed by Marta Volonteri: “Astrophysics: Making Black Holes from Scratch,” Nature 466 (2010): 1049.

“dark ages” of the cosmos: This is a large area of research. One expert is the astronomer Zoltán Haiman of Columbia University, who also gives an excellent overview and discussion of the possible role of smaller black holes in “Cosmology: A Smoother End to the Dark Ages,” Nature 472 (2011): 47.

Felix Mirabel: Led the study that is reported by Mirabel et al., “Stellar Black Holes at the Dawn of the Universe,” Astronomy & Astrophysics 528 (2011).

molecular hydrogen cools much faster: This is likely critically important in the very young universe. An excellent reference is Zoltán Haiman, Martin Rees, and Abraham Loeb, “H2 Cooling of Primordial Gas Triggered by UV Irradiation,” Astrophysical Journal 467 (1996): 522.

8. ORIGINS: PART II

the Andromeda galaxy: Also often known by its more “official” astronomical name of Messier 31, or M31 for short.

something resembling an elliptical: Gravitational simulations of the Andromeda/Milky Way collision suggest that this is a possibility. The largest source of uncertainty in what will happen is actually due to our lack of very high-precision measurements of the relative motion of the two galaxies—we can measure Andromeda’s velocity toward us very well, but measuring transverse motion is difficult, so we cannot be certain that it is approaching us precisely head-on.

Sloan Digital Sky Survey: The SDSS finally began in 2000; its genesis was quite prolonged. A primary leader and advocate for the project (which at the time was a very new concept) was the Princeton astronomer Jim Gunn. The SDSS uses a technique known as drift scanning: the telescope remains fixed, and as the Earth rotates, a strip of the sky the width of the instrument’s cameras passes by. Data are continually acquired.

project called Galaxy Zoo: The project has an excellent website (http://zoo1.galaxyzoo.org/), and a great discussion of the history of the project has been written by Forston et al., “Galaxy Zoo: Morphological Classification and Citizen Science” (available as a preprint, http://arxiv.org/abs/1104.5513, 2011).

duty cycle is related: The specific results are presented by Schawinski et al., “Galaxy Zoo: The Fundamentally Different Co-evolution of Supermassive Black Holes and Their Early- and Late-type Host Galaxies,” Astrophysical Journal 711 (2010): 284.

astronomers have recently realized: Several groups of researchers have stated that the Milky Way seems to be a green valley galaxy. It is possible that Andromeda is one as well, albeit a bit more red than green. A nice discussion is by Mutch et al., “The Mid-life Crisis of the Milky Way and M31” (available as a preprint, http://arxiv.org/abs/1105.2564, 2011).

zones of X-ray light: The center of our galaxy produces all sorts of X-ray emissions, coming from both small and large structures, making it very hard to peel apart the layers. For example, see Snowden et al., “ROSAT Survey Diffuse X-ray Background Maps. Part II.,” Astrophysical Journal 485 (1997): 125.

In 2010: The results that revealed the gamma-ray structure in our galaxy are reported by Meng Su, Tracey Slatyer, and Doug Finkbeiner, “Giant Gamma-ray Bubbles from FERMI-LAT: Active Galactic Nucleus Activity or Bipolar Galactic Wind?,” Astrophysical Journal 724 (2010): 1044.

the X-rays we see are echoes: There are several lines of evidence for activity from our central black hole, and I have used the X-ray evidence for discussion. One example of this type of reflection observation is given by Ponti et al., “Discovery of a Superluminal Fe K Echo at the Galactic Center: The Glorious Past of Sgr A* Preserved by Molecular Clouds,” Astrophysical Journal 714 (2010): 732.

9. THERE IS GRANDEUR

maximum size for black holes: There may indeed be a maximum (excluding the possibility of the merger of two or more already supermassive holes). In our cosmic neighborhood it’s around 10 billion times the mass of our Sun, 2,500 times the size of the Milky Way’s central black hole. See, for example, Priya Natarajan and Ezequiel Treister, “Is There an Upper Limit to Black Hole Masses?,” Monthly Notices of the Royal Astronomical Society 393 (2009): 838.

stars to be born: There is evidence of rings of young blue stars orbiting within three light-years of the central black hole of the Milky Way galaxy, as well as in Andromeda. For example, see Paumard et al., “The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics, and Formation,” Astrophysical Journal 643 (2006): 1011. Theoretical models seem to concur with the possibility of stars forming out of disks around the black holes; see, for example, Bonnell and Rice, “Star Formation Around Supermassive Black Holes,” Science 321 (2008): 1060.

flung out: It’s really still speculative, but the Chandra X-ray Observatory may have caught just such a thing. See Jonker et al., “A Bright Off-nuclear X-ray Source: A Type IIn Supernova, a Bright ULX or a Recoiling Supermassive Black Hole in CXOJ122518.6+144545,” Monthly Notices of the Royal Astronomical Society 407 (2010): 645.

known as gravity waves: These gravitational ripples produce strain on spacetime. Waves expected from astrophysical sources (such as merging black holes) will have polarizations, and will produce a very particular strain pattern that moves free masses back and forth. The frequency of the waves can be quite high, causing perhaps thousands of oscillations a second. The strength of the wave (its amplitude) also drops off with distance from the source.

Black Hole Imager: At this stage BHI is still a concept, albeit with a number of laboratory test-bed experiments being carried out to investigate the necessary techniques. Two thorough descriptions written by Keith Gendreau and colleagues were submitted to the United States Astronomy Decadal Review in 2010: “The Science Enabled by Ultrahigh Angular Resolution X-ray and Gamma-ray Imaging of Black Holes” (http://maxim.gsfc.nasa.gov/documents/Astro2010/Gendreau_BlackHoleImager_CFP_GAN_GCT.pdf), and “Black Hole Imager: What Happens at the Edge of a Black Hole?” (http://maxim.gsfc.nasa.gov/documents/Astro2010/Gendreau_BHI.pdf).