Introduction

Gravitational wave astronomy will open a new observational window on the universe. Since large masses concentrated in small volumes and moving at high velocities generate the strongest, and therefore most readily detectable waves, the final coalescence of blackhole binaries is expected to be one of the strongest sources. During the last century, the opening of the full electromagnetic spectrum to astronomical observation greatly expanded our understanding of the cosmos. In this new century, observations across the gravitational wave spectrum will provide a wealth of new knowledge, including accurate measurements of binary black-hole masses and spins.

The high frequency part of the gravitational wave spectrum, ~10 Hz ≲ f ≲ 103 Hz, is being opened today through the pioneering efforts of first-generation ground-based interferometers such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), currently operating at design sensitivity.

On 19 august 1972, Fred Hoyle sat in his office at the Institute of Astronomy in Cambridge for the last time. His summer had been busy. A record number of academic visitors had come to the institute to benefit from summer conferences, collaborations, lectures and discussions. He had fretted to make sure the institute would be financed securely for the next five years. Just three weeks earlier, the Institute of Astronomy had been born through a merger of two astronomy departments, after the university had decided to join the historic Observatories established in 1823 with the pioneering Institute of Theoretical Astronomy founded by Hoyle in 1965. Hoyle had been the head of Theoretical Astronomy for seven years, but now he had a new boss, because the university had not chosen him as the director of the combined institute.

On a sultry afternoon with a threat of thunder in the air, staff members who were in the old Observatories, including myself, made the short walk along the path through the parklike grounds to the building that had been the Institute of Theoretical Astronomy – IoTA for short – to take their afternoon tea in the library. This wonderful Cambridge tradition gave the researchers and their students an opportunity to exchange ideas, and maybe wish a departing visitor a safe trip back to California or India. But this afternoon, Hoyle would not be joining his colleagues for tea.

During the nineteenth century, savants in England continuously improved the science of astronomy, bringing it to a high professional level by the end of Queen Victoria's reign.

In January 1820, fourteen gentlemen and scholars, one of them the future computer pioneer Charles Babbage, had founded the Royal Astronomical Society, which received its Royal Charter from King William IV in 1831. Sir William Herschel, the discoverer of Uranus, the builder of giant telescopes and the most accomplished sidereal observer of his age, became the society's first president. In 1834, the British government provided the society with suitable premises free of charge, an arrangement that continued uninterrupted until 2004. The universities of Oxford and Cambridge had important observatories from 1794 and 1823 respectively, together with endowed professorships. At Greenwich, the Royal Observatory, one of the world's oldest scientific institutions, flourished in the age of Queen Victoria, and was noted for its accurate observations of the positions of stars. In 1884, an international conference in Washington, DC, convoked by President Chester Arthur of the United States, selected Greenwich as the world's prime meridian.

By the early twentieth century, British astronomy could hold its head high: a small community of professionals at the Royal Observatories and in the ancient universities conducted world-class research. Furthermore, they encouraged the development of astronomy in the dominions of the British Empire, with the establishment of observatories in Australia, Canada and South Africa, where the practitioners still looked to Greenwich for guidance.

Introduction

Deep surveys

Deep field observations are a suitable observational technique to probe AGN physics. In multi-wavelength campaigns, astronomers select a field in the sky and produce images from several pointings.

Introduction: Mass-estimation methods for active galaxies and quasars

The Hubble Space Telescope has played a key role in our ability to detect supermassive black holes in the centers of nearby galaxies and to determine their mass through its high angular resolution.

Fred hoyle applied his creative genius to an extraordinary range of problems and worked in a variety of genres. His research covered topics from the solar system to the entire universe. Everything interested him: the Sun, stars, interstellar dust, galaxies and cosmology. He even strayed into political and social science, climate change, archaeology, palaeontology and molecular biology. He exploited every medium available – research papers, monographs, science fiction, children's books and textbooks – to get his messages across. Any opportunity to make a radio or television broadcast delighted him, as did giving popular lectures, which attracted huge crowds. Most of the time he worked like a grasshopper in summer, jumping abruptly from one thing to another. In recounting his scientific career, it is inevitable that the narrative must by turns advance and backtrack. Take, for example, his work on accretion, on cosmology and on astrophysics: these all overlap in time, but they are largely disconnected in intellectual terms. For that reason, I have generally chosen to cover one area of research at a time, considering together clusters of related papers and books. We now turn to his investigations on the evolution of stars, which began just before the war and remained an important theme in his work until the mid-1960s.

Most productive scientists today write their papers on a computer. Hoyle's working method was no different from that of scholars the world over before the age of computers: pen and paper.

Introduction

Most of the radiation from luminous accreting black holes is released within the innermost 20 gravitational radii (i.e., 20rg = 20 GM/c2). In such an energetic environment, iron is a major source of x-ray line emission, with strong emission lines in the 6.4–6.9 keV band. Observations of such line emission provides us with a diagnostic of the accretion flow and the behavior of matter and radiation in the strong gravity regime very close to the black hole (Fabian et al. 2000; Reynolds & Nowak 2003; Fabian & Miniutti 2009; Miller 2007).

The rapid x-ray variability found in many Seyfert galaxies is strong evidence for the emission orginating at small radii.

I am often asked if I had a mentor who propelled me towards a life in science. There was no personal mentor as such, but Fred Hoyle, more than anybody, was my role model, and he strongly influenced my career in several ways. I was one of many youngsters deeply influenced by Frontiers of Astronomy, which I read whilst in the Sixth Form at Woodhouse Grammar School in North Finchley. About the same time I was presented at Speech Day with Norton's Star Atlas by our local Member of Parliament, one Margaret Thatcher. These events set me squarely on the path of theoretical astronomy and cosmology.

Fred's science fiction also influenced me. I have vivid memories of the television series A for Andromeda, featuring the ravishing Julie Christie. His masterful book The Black Cloud continues to colour my thinking about the nature of life and consciousness. I was thrilled to discover that a professional scientist could combine fundamental research with fiction writing, bringing to both challenging new concepts and ideas.

The first Hoyle lecture I attended was delivered at the Royal Society in 1967, when I was a beginning PhD student at University College London. Fred spoke about the arrow of time, and the Wheeler-Feynman theory of electrodynamics. I can definitely trace my lifelong interest in the nature of time to this lecture. Indeed, I soon thereafter abandoned my research into atomic astrophysics and took up the problem of providing a quantum description of the Wheeler-Feynman theory for the remainder of my PhD thesis.

Introduction

The M• ~ 4 × 106M⊙ MBH in the GC and the stars around it are the closest and observationally most accessible of such systems (Eisenhauer et al. 2005; Ghez et al. 2005). Observations of the GC thus offer a unique opportunity to study in great detail the effects of the MBH and its extreme environment on star formation, stellar evolution and stellar dynamics, and the interactions of stars and compact remnants with the MBH.

Here the focus is stellar relaxation processes. Relaxation plays an important role in a wide range of phenomena that involve close interactions with an MBH (the “losscone problem,” Section 1.1).

Introduction

Black holes are ‘sighted’ everywhere in the universe these days. Originally located in compact binary x-ray sources in the 1970s, the cosmic presence of black holes has expanded considerably in recent decades. They now are believed to be the central engines that power quasars, active galactic nuclei (AGNs) and gamma-ray bursts (GRBs). They are identified in the cores of all bulge galaxies. They are presumed to form significant populations of compact binaries, including black hole–black hole binaries (BHBHs) and black hole–neutron star binaries (BHNSs). Black holes may even show up soon in the Large Hadron Collider!

Gravitationally, black holes are strong-field objects whose properties are governed by Einstein's theory of relativistic gravitation—general relativity.

Introduction

One of the most important results of the last few years has been the realization that most, if not all, galaxies harbor supermassive black holes (BH) in their centers. The presence of a supermassive BH can manifest itself as luminous quasar “activity,” powered by accretion of matter onto the BH itself. Such a quasar phase, which peaks somewhere around redshift 2, is likely to play an important role in the build-up of these BHs.

Early one morning in October 1933, Fred's mother and father, and his twelve-year-old sister Joan, gave the young scholar a hearty send-off. He began the long journey to Cambridge at Bingley railway station. As the local train chugged along the valley, Hoyle looked back at the forest of mill chimneys clouding the air with smoke. On the hillsides the trees were already showing their autumnal colours. His travelling companions included half a dozen of his classmates from the grammar school. At the first stop, Shipley, they hauled their bags and suitcases to another local train, which brought them to Leeds. Here they bought tickets for the express to Peterborough, about thirty-five miles north of Cambridge. From there a local train clattered across the bleak fenland. Hoyle must have found this flat landscape strikingly different from the moors and dales of Yorkshire. From his window seat he could see farmers ploughing with horses, an extensive drainage system of ditches and dykes, and the fourteenth-century octagon tower of Ely Cathedral soaring over the fens. After some eight hours of travel, Fred and a throng of students descended from the packed train at Cambridge, where they found themselves on the longest railway platform in the country.

The returning undergraduates and the freshers dispersed either to colleges or to lodging houses. Emmanuel College had assigned young Fred a room in a shared house about a mile from both the college and the railway station. Of course, he had no money to spare for a cab.