Black holes from prediction to detection
On 14 September 2015, the LIGO/VIRGO instruments detected the first statistically significant signal of a gravitational wave passing through Earth, one hundred years after the publication of the theory of General Relativity. The signal was identified as originating from the merger of two stellar black holes orbiting each other. Black holes, since their theoretical conception from Albert Einstein and Karl Schwarzschild, have played an integral role both in theoretical physics and later on in astronomy and astrophysics.
Currently, black holes are thought to be important, both as probes of the most extreme regime of gravity, but also as regulators of their environments both on stellar and galactic scales. Nature journals have published key results relating to black holes, including some of the most highly cited papers in astronomy. This collection showcases the presence of black holes in scientific research over the last 50 years and the evolution of our understanding of black holes in astronomy and astrophysics.
In the Research tab, you will find a collection of some of the most important discoveries relating to black holes that were published in Nature journals. From the association of black holes to the most luminous galaxies in the Universe, to Hawking radiation, and quantum black holes, this tab offers a slice of some of the most exciting research in astronomy.
In the News, Views and Comments tab, you will find the reaction of the community to what today is considered rather commonplace knowledge. What would happen if one lowers a rope with a mass tied to its end inside a black hole? Why is the detection of gravitational waves such an important breakthrough? Click through to find more.
In the Theory and Predictions tab, you will find papers that have dealt with black holes on a theoretical level, making predictions about both the observational signatures of black holes and the properties of the black holes themselves. Despite their blackness, black holes power some of the most luminous objects in our Universe through the accretion of matter and the gravitational release of energy.
Black holes come in all sizes (and potentially shapes). In the Quantum to stellar black holes tab, we take a closer look at black holes with small masses (up to a few hundred or thousand solar masses). These can be found scattered around galaxies, the end product of stellar evolution, and even presumably in the primordial Universe, products of quantum fluctuations and extremely high-energy processes.
Milky Way and beyond takes a step back and looks at the most massive black holes we currently know. From the black hole hiding at the centre of our very own galaxy to the giant black holes found in other galaxies in our local Universe and beyond. In this tab, you will find papers about stars being destroyed by black holes, the evidence for a black hole at the centre of the Milky Way, the missing intermediate-mass black holes and even systems of binary supermassive black holes whose gravitational wave signature will eventually be observed by LISA.
We now know that quasars are some of the most luminous — and often some of the most distant — objects in the Universe, powered by massive black holes that are actively accreting matter. However, this was not always common knowledge among astronomers. Fred Hoyle, Geoffrey Burbidge and Wallace Sargent in a series of papers in 1966 investigated the question of whether quasi-stellar objects are local to our Galaxy or are indeed far away from us. Considering its flux variability, they arrived at the puzzling conclusion that if the object lies at cosmological distances, then relativistic particles should be produced in situ! By what process that could happen was unclear at the time. Image credit: ESA/Hubble & NASA
By 1972, there were indications that quasi-stellar objects were indeed powered by accreting black holes. Nonetheless, this scenario was still considered controversial, and unambiguous evidence for the existence of black holes was still far off in the horizon. Roger Penrose wrote an article to familiarize black holes with Nature readers. He covered topics that included the fact that light cannot escape a black hole, some black holes may be rotating and naked singularities may exist. However, the energy conversion efficiency apparently associated with black holes still remained a mystery. Accretion of matter had yet to assume its place in our paradigm of quasars.
Martin Rees discussed stars being disrupted and swallowed whole by massive black holes residing at the centres of galaxies. This study was motivated by the apparent discrepancy presented by the measured stellar orbits in nearby galaxies and the relative nuclear quiescence of these galaxies. The former implied that a central massive object lies there whereas the latter indicated that even though stars were within reach of the black hole and could potentially be disrupted, no appreciable observational signature was produced. He showed that the two are indeed not mutually exclusive. Further studies have now revealed the relative rarity of such tidal disruption events (one such event every ten to a hundred thousand years per galaxy).
How black holes grow, especially those found at the edge of the observable Universe, has been a persistent question since we observed quasars only less than a billion years after the Big Bang. In 2003, Laura Ferrarese discussed a study by Barkana and Loeb predicting that quasars are fed by hydrogen collapsing towards the black hole at the centre of massive dark matter haloes. This study was motivated by the double-peaked Lyman-alpha emission lines in the spectra of distant quasars. Only a couple of years later, cosmological simulations would reveal that massive quasars sit at the intersection of cosmic filaments and populate some of the most massive dark matter haloes at those epochs.
The turn of the millennium saw the emergence of a new paradigm of the role that black holes play in regulating not only the matter around them but also their host galaxies in their entirety. Following a number of scaling relations linking the mass of black holes with the mass of their hosts, Tiziana Di Matteo, Volker Springel and Lars Hernquist showed that actively accreting black holes can expel large amounts of energy that both quench ongoing star-formation in their hosts and stop their own growth. A key element of this scenario were galactic mergers that provided the necessary dynamic instabilities and gas reservoirs for both star formation and black hole activity to commence.
