We discuss the status of the black hole entropy formula SBH = AH/4G in low energy effective field theory. The low energy expansion of the black hole entropy is studied in a non-equilibrium situation: the semiclassical decay of hot flat space by black hole nucleation. In this context the entropy can be defined as an enhancement factor in the semiclassical decay rate, which is dominated by a spha...

Using the inverse scattering method we construct an exact stationary asymptotically flat 4+1-dimensional vacuum solution describing “black saturn”: a spherical black hole surrounded by a black ring. Angular momentum keeps the configuration in equilibrium. Black saturn reveals a number of interesting gravitational phenomena: (1) The balanced solution exhibits 2-fold continuous non-uniqueness for...

We discuss some of the drawbacks of using event horizons to define black holes and suggest ways in which black holes can be described without event horizons, using trapping horizons. We show that these trapping horizons give rise to thermodynamic behavior and possibly Hawking radiation too. This raises the issue of whether the event horizon or the trapping horizon should be seen as the true bou...

There are at least two formation scenarios consistent with the first gravitational-wave observations of binary black hole mergers. In field models, black hole binaries are formed from stellar binaries that may undergo common envelope evolution. In dynamic models, black hole binaries are formed through capture events in globular clusters. Both classes of models are subject to significant theoret...

Using the inverse scattering method we construct an exact stationary asymptotically flat 4+1-dimensional vacuum solution describing “black saturn”: a spherical black hole surrounded by a black ring. Angular momentum keeps the configuration in equilibrium. Black saturn reveals a number of interesting gravitational phenomena: (1) The balanced solution exhibits 2-fold continuous non-uniqueness for...

If the stellar population of the bulge contains black holes formed in the final core collapse of ordinary stars with M ∼> 30M⊙, then about 25,000 stellar mass black holes should have migrated by dynamical friction into the central parsec of the Milky Way, forming a black hole cluster around the central supermassive black hole. These black holes can be captured by the central black hole when the...

The topological structure of the event horizon has been considered in terms of the Morse theory. The elementary process of topology changes can be understood as a handle attachment. It has been found that there are certain constraints on the black hole topology changes: (i) There are n kinds of the handle attachment in the (n+1)-dimensional black hole space-times. (ii) The handle is further cla...

Radio emission from early–type (E and S0) galaxies usually signals the presence of a central supermassive black hole. If radio luminosity and black–hole mass are correlated, then we can use the large sample of radio galaxies observed in the 2dF Galaxy Redshift Survey (2dFGRS) to estimate the local (z < 0.1) black–hole mass function for early–type galaxies. Integrating over this mass function yi...

Recently, we derived an improved universal upper bound to the entropy of a charged system S ≤ π(2Eb − q 2)/¯ h. There was, however, some uncertainty in the value of the numerical factor which multiply the q 2 term. In this paper we remove this uncertainty; we rederive this upper bound from an application of the generalized second law of thermodynamics to a gedanken experiment in which an entrop...

We generalize the discussion of [1] to charged black holes. For the two dimensional charged black hole, which is described by an exactly solvable worldsheet theory, a transition from the black hole to the string phase occurs when the Hawking temperature of the black hole reaches a limiting value, the temperature of free strings with the same mass and charge. At this point a tachyon winding arou...