Black hole – is the region in space-time, and the gravitational attraction is so large that it can not leave even objects moving at the speed of light (including photons of light itself). Boundary of this region is called the event horizon, and its characteristic size – is the gravitational radius.
Theoretically, the possible existence of such regions of space-time should be of some exact solutions of Einstein’s equations, the first of which was obtained by Karl Schwarzschild in 1915. Exact inventor of the term is unknown, but the name was popularized by John Archibald Wheeler and the first time publicly consumed in a popular lecture, “Our universe: the known and unknown” on December 29, 1967. Previously, such astrophysical objects were called “collapsed star” or “collapsars” (Collapsed stars), as well as the “frozen star”, as described in Black holes.
The question of the real existence of black holes is closely linked to the true theory of gravity, which implies their existence. In modern physics the standard theory of gravity, is best confirmed experimentally, and it is the theory of general relativity (GR), confidently predicting the possibility of the formation of black holes, but their existence is possible, and in other (not all) models. Therefore, observational data are analyzed and interpreted, especially in the context of general relativity, although, strictly speaking, this theory is not experimentally confirmed for the conditions corresponding to the region of space-time in the immediate vicinity of the stellar mass black holes. Therefore, the allegations of direct evidence for the existence of black holes, including in this article below, strictly speaking, can be understood in the sense of confirming the existence of astronomical objects, such dense and massive, as well as with some other observable properties that can be interpreted as black holes in general relativity, as stated in Introduction to Black holes.
In addition, black holes are often called objects that are not strictly relevant to the above definition, but only approaching of their properties to a black hole – is or example, that it could be a collapsing star at the late stages of collapse. In modern astrophysics, to this distinction is not given much importance, as well as observational manifestations “almost collapsed” (“Frozen”) stars and the “real” (“eternal”) black hole is almost identical. This is because the differences between physical fields around the collapsar from those of the “eternal” black hole decreases as a power law with a characteristic time of the order of the gravitational radius is divided by the speed of light.
Since the theoretical prediction of black holes remains an open question about their existence, since the existence of solutions of the “black hole” is not a guarantee that there are mechanisms for the formation of such objects in the universe. There are known, however, the mechanisms that may lead to the fact that a region of space-time will have the same properties (same geometry) as the corresponding region of a black hole. For example, as a result of collapse of a star can form the space- time.
Depicted with a dark color it is filled with the substance of its stars and the metric is determined by the properties of this substance. But the light gray area coincides with the corresponding region of Schwarzschild space. It is about such situations in astrophysics, which are referred to as the formation of black holes, which from a formal point of view in a certain freedom of speech. Outside, however, very soon this facility will be virtually indistinguishable from the black hole in all of its properties, so the term is applied to obtain the configuration with a very high degree of accuracy, as described in Properties of Black holes.
In reality, due to accretion on the one hand, and (possibly) the Hawking radiation, on the other hand, space-time around a collapsar deviates from the above exact solutions of Einstein’s equations. And though any small area (except in the neighborhood of the singularity), the metric is distorted slightly, and the global causal structure of space-time may vary dramatically. In particular, the present space- time can, in some theories, no longer has an event horizon.
According to modern ideas, there are four scenarios for formation of a black hole:
gravitational collapse (the catastrophic compression) is a sufficiently massive star (more than 3.6 solar masses) at the final stage of its evolution; and the collapse of the central part of the galaxy or right-galactic gas. Modern ideas put enormous black hole in the center of many, if not all, spiral and elliptical galaxies. For example, in the center of our galaxy there is located black hole Sagittarius A* mass, around which there is the smaller black hole. The formation of black holes at the Big Bang happens as a result of fluctuations in the gravitational field and / or matter. Such black holes are called primary. The appearance of black holes in high energy nuclear reactions – are the Quantum black holes, as described in What is a Black hole?
The existence of black holes, stellar and galactic scale is considered by most scientists proved reliable astronomical observations. Astronomers have found that the mass of super massive black holes may be significantly underestimated. Researchers found that in order to move the stars in the galaxy M87 (which is located at a distance of about 50 million light-years from Earth) as is the case now, the mass of the central black hole should be at least 6.4 billion solar masses, for example, twice the current estimates of core M87, which are 3 billion solar masses.
Black holes. 2011. 11 March 2011. <http://www.bbc.co.uk/science/space/universe/sights/black_holes/>
Introduction to Black holes. 2011. 11 March 2011. <http://imagine.gsfc.nasa.gov/docs/science/know_l1/black_holes.html>
Properties of Black holes. 2010. 11 March 2011. <http://superstringtheory.com/blackh/blackh2.html>
What is a Black hole? 2011. 11 March 2011. <http://science.howstuffworks.com/dictionary/astronomy-terms/black-hole1.htm>