Black Hole

I

Introduction

Black Hole, hypothetical body with a gravitational field that curves the space around it so strongly that nothing, including electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical boundary, called the event horizon, through which light can enter but not escape; it therefore appears totally black. Black holes can, in theory, have almost any mass, ranging from mini black holes with the mass of an asteroid that might have been formed in the early universe, up to supermassive black holes containing millions or even billions of solar masses that are thought to lie at the centres of most galaxies.

II

Properties

The modern black-hole concept originated in 1916, in the solutions of German astronomer Karl Schwarzschild to the equations of Albert Einstein's theory of general relativity. When the radius of a collapsing star reaches a certain value, now known as the Schwarzschild radius, the escape velocity equals the speed of light. Further collapse means that even light cannot escape from it. The Schwarzschild radius is the radius of the black hole’s event horizon, and its value depends only on the mass of the body. Neither the Sun nor the Earth is massive enough to collapse to within their Schwarzschild radii, which are, respectively, about 3 km (1.9 mi) and 9 mm (0.4 in). If a body is electrically charged or rotating, Schwarzschild's results are modified. An “ergosphere” forms outside the event horizon, and between the ergosphere and the event horizon, matter is forced to rotate with the black hole; in principle, energy can be emitted from the ergosphere.

According to general relativity, gravitation severely modifies space and time near a black hole. If astronauts were unlucky enough to approach a black hole, they would find themselves crossing the event horizon in a finite time, but to a distant observer they would appear to slow down as they approached the event horizon, never quite passing through it.

Once a body has contracted to within its Schwarzschild radius, it will theoretically collapse to a singularity, that is, a dimensionless object of infinite density. Many physicists and mathematicians have been dissatisfied with the idea of a singularity and suspect it to be an artefact arising from an incomplete description of gravity.

It is generally accepted that black holes have only three discernible features: mass, charge, and spin. There is no way of knowing anything about conditions inside the event horizon, because such information remains inaccessible to us. British cosmologist Stephen Hawking showed that a black hole of sufficiently small mass could capture one member of an electron-positron pair near the event horizon, the other escaping. The resulting radiation (since named Hawking radiation) would carry off energy, in a sense evaporating the black hole. Any primordial black holes (that is, those formed in the early universe) weighing less than a few billion tonnes would already have evaporated, but heavier ones might remain.

Hawking had maintained that the radiation could not provide any information, because information about the black hole is not merely unobtainable, but non-existent, having been destroyed by the extreme gravitational forces involved in the black hole’s creation. However, since quantum theory states that it is impossible for information to be destroyed, this created what is known as the black hole information paradox. It had been a matter of contention for nearly 30 years when in July 2004 Hawking himself announced that information does actually leak out as a black hole evaporates, but in a garbled form. He said that this occurs because quantum mechanical effects prevent a true event horizon from forming, but the issue remains controversial.

III

Formation

Gravitational collapse alone cannot form small black holes such as those postulated by Hawking, so if they exist, they could only have formed in the extreme conditions following the Big Bang (see Cosmology). However, black holes with masses from a few to a few tens of solar masses are thought to form when stars much more massive than the Sun collapse at the ends of their lives.

During a star’s life, its radius is determined by the balance between gravitational contraction and the outward pressure from the heat produced by nuclear fusion in its core. When the nuclear fuel in the core becomes exhausted, gravity can cause the core to contract to ever-higher densities. Two new types of pressure arise at densities 1 million and 1 million billion (10¹⁵) times that of water, respectively, and a white dwarf or a neutron star may then form, depending on the core’s mass. If the core mass exceeds a certain value, known as the Oppenheimer-Volkoff limit, however, neither of these types of pressure is sufficient to prevent collapse to a black hole. The exact value of the Oppenheimer-Volkoff limit is uncertain, but is calculated to be between 2 and 3 solar masses.

The mechanism for forming supermassive black holes of millions or billions of solar masses is not yet understood, although it probably involves the merger of smaller black holes and the intake of surrounding stars and gas.

IV

Finding Black Holes

There is, so far, no evidence for primordial black holes, but there are a number of good candidates for those of stellar mass. The best known is the binary star system Cygnus X-1, in which the primary is a normal star of about 30 solar masses. Doppler shifts in its spectrum show that a companion object of 10 to 15 solar masses is in orbit around it (see Doppler Effect). Although this companion is invisible at optical wavelengths, it is a strong emitter of X-rays, which are thought to come from an “accretion disc” of extremely hot gas, formed as the gas from the visible star spirals towards the companion. The companion in Cygnus X-1 is too massive to be a white dwarf or neutron star, and the only known alternative is a black hole.

Various other good candidates for black holes of stellar mass have been identified, including another in Cygnus (V404 Cygni), one in a satellite galaxy of our own Milky Way galaxy called the Large Magellanic Cloud (LMC X-3), and one in the constellation Monoceros (A0620-00).

It is now generally accepted that quasars and other types of galaxy with active nuclei are powered by supermassive central black holes, but it is also becoming recognized that many, if not all, galaxies of substantial size contain a supermassive black hole at their centre, including our own galaxy. The very centre of our galaxy is marked by a radio source called Sagittarius A* with a diameter smaller than the Solar System. From observations of gas and stars swirling around this central object, astronomers have calculated that the unseen mass must be around 3 million times that of the Sun.

Other likely examples have also been detected. In 1994 the Hubble Space Telescope provided strong evidence of a black hole at the centre of the galaxy M87. The high acceleration of gas in this region indicates that an object or group of objects of about 3 billion solar masses must be present.

There seemed to be nothing intermediate in size between the stellar mass black holes and the supermassive ones until 2002 when two separate teams of researchers using the Hubble Space Telescope found evidence of black holes with masses thousands of times that of the Sun. In November 2004 a black hole of 1,300 solar masses was identified in the Milky Way. Astrophysicists see these as important links between the two types of black holes previously discovered and should help to understand the formation of the supermassive ones.

The Chandra X-Ray Observatory launched in July 1999 has produced X-ray images of a number of highly active galactic nuclei, and looks likely to produce further evidence of the existence of a supermassive black hole at the centres of galaxies. In October 2002 the European Space Agency observatory INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) was launched to detect sources of gamma-rays along the galactic plane as part of its mission to identify new black holes, as well as studying already known ones.