Roberto Emparan

A Rotating Black Ring Solution in Five Dimensions

The vacuum Einstein equations in five dimensions are shown to admit a solution describing a stationary asymptotically flat spacetime regular on and outside an event horizon of topology S1xS2. It describes a rotating black ring. This is the first example of a stationary asymptotically flat vacuum solution with an event horizon of nonspherical topology. The existence of this solution implies that the uniqueness theorems valid in four dimensions do not have simple five-dimensional generalizations. It is suggested that increasing the spin of a spherical black hole beyond a critical value results in a transition to a black ring, which can have an arbitrarily large angular momentum for a given mass.

Black holes radiate mainly on the brane

We examine the evaporation of a small black hole on a brane in a world with large extra dimensions. Since the masses of many Kaluza-Klein modes are much smaller than the Hawking temperature of the black hole, it has been claimed that most of the energy is radiated into these modes. We show that this is incorrect. Most of the energy goes into the modes on the brane. This raises the possibility of observing Hawking radiation in future high energy colliders if there are large extra dimensions.

Generalized Weyl solutions

It was shown by Weyl that the general static axisymmetric solution of the vacuum Einstein equations in four dimensions is given in terms of a single axisymmetric solution of the Laplace equation in three-dimensional flat space. Weyl’s construction is generalized here to arbitrary dimension D>4. The general solution of the D-dimensional vacuum Einstein equations that admits D22 orthogonal commuting non-null Killing vector fields is given either in terms of D23 independent axisymmetric solutions of Laplace’s equation in threedimensional flat space or by D24 independent solutions of Laplace’s equation in two-dimensional flat space. Explicit examples of new solutions are given. These include a five-dimensional asymptotically flat ‘‘black ring’’ with an event horizon of topology S 1 3S 2 held in equilibrium by a conical singularity in the form of a disk.

Cosmic Rays as Probes of Large Extra Dimensions and TeV Gravity

If there are large extra dimensions and the fundamental Planck scale is at the TeV scale, then the question arises of whether ultrahigh energy cosmic rays might probe them. We study the neutrino-nucleon cross section in these models. The elastic forward scattering is analyzed in some detail, hoping to clarify earlier discussions. We also estimate the black hole production rate. We study energy loss from graviton mediated interactions and conclude that they cannot explain the cosmic ray events above the GZK energy limit. However, these interactions could start horizontal air showers with characteristic profile and at a rate higher than in the standard model.

Quantum Black Holes as Holograms in AdS Braneworlds

We propose a new approach for using the AdS/CFT correspondence to study quantum black hole physics. The black holes on a brane in an AdSD+1 braneworld that solve the classical bulk equations are interpreted as duals of quantum-corrected D-dimensional black holes, rather than classical ones, of a conformal field theory coupled to gravity. We check this explicitly in D = 3 and D = 4. In D = 3 we reinterpret the existing exact solutions on a flat membrane as states of the dual 2+1 CFT. We show that states with a sufficiently large mass really are 2+1 black holes where the quantum corrections dress the classical conical singularity with a horizon and censor it from the outside. On a negatively curved membrane, we reinterpret the classical bulk solutions as quantum-corrected BTZ black holes. In D = 4 we argue that the bulk solution for the brane black hole should include a radiation component in order to describe a quantum-corrected black hole in the 3+1 dual. Hawking radiation of the conformal field is then dual to classical gravitational bremsstrahlung in the AdS5 bulk.

String balls at the LHC and beyond

In string theory, black holes have a minimum mass below which they transition into highly excited long and jagged strings --- ``string balls. These are the stringy progenitors of black holes; because they are lighter, in theories of TeV-gravity, they may be more accessible to the LHC or the VLHC. They share some of the characteristics of black holes, such as large production cross sections. Furthermore, they evaporate thermally at the Hagedorn temperature and give rise to high-multiplicity events containing hard primary photons and charged leptons, which have negligible standard-model background.

Tubular Branes in Fluxbranes

We describe the construction of new configurations of self-gravitating p-branes with worldvolume geometries of the form R1,p−s×Ss, with 1⩽s⩽p, i.e., tubular branes. Since such branes are typically unstable against collapse of the sphere, they must be held in equilibrium by a fluxbrane. We present solutions for string loops with non-singular horizons, as well as M5-branes intersecting over such loops. We also construct tubular branes which carry in their worldvolume a dissolved, lower-dimensional brane (as in the dielectric effect), or an F-string. However, the connection between these solutions and related configurations that have been studied earlier in the absence of brane self-gravity, is unclear. It is argued that, at least in some instances, the self-gravitating solutions do not appear to be able to reproduce stable configurations of tubular branes.

Black hole astrophysics in AdS braneworlds

We consider astrophysics of large black holes localized on the brane in the infinite RandallSundrum model. Using their description in terms of a conformal field theory (CFT) coupled to gravity, deduced in Ref. [1], we show that they undergo a period of rapid decay via Hawking radiation of CFT modes. For example, a black hole of mass few ×M⊙ would shed most of its mass in ∼ 10 4 − 10 5 years if the AdS radius is L ∼ 10 −1 mm, currently the upper bound from table-top experiments. Since this is within the mass range of X-ray binary systems containing a black hole, the evaporation enhanced by the hidden sector CFT modes could cause the disappearance of X-ray sources on the sky. This would be a striking signature of RS2 with a large AdS radius. Alternatively, for shorter AdS radii, the evaporation would be slower. In such cases, the persistence of X-ray binaries with black holes already implies an upper bound on the AdS radius of L < 10 −2 mm, an order of magnitude better than the bounds from table-top experiments. The observation of primordial black holes with a mass in the MACHO range M ∼ 0.1 − 0.5 M⊙ and an age comparable to the age of the universe would further strengthen the bound on the AdS radius to L < few × 10 −6 mm.

From p-branes to fluxbranes and back

In this note we study aspects of the interplay between ∞uxbranes and p-branes. We describe how a ∞uxbrane can be physically realized as a limit of a brane-antibrane conflguration, in a manner similar to the way a uniform electric fleld appears in between the plates of a capacitor. We also study the evolution of a ∞uxbrane after nucleation of p-branes. We flnd that Kaluza-Klein ∞uxbranes do relax by forming brane-antibrane pairs or spherical branes, but we also flnd that for ∞uxtubes with dilaton coupling in a difierent range, the fleld strength does not relax, instead it becomes stronger after each nucleation bounce. We speculate on a possible runaway instability of such ∞uxtubes and an eventual breakdown of their classical description.

Exact gravitational shockwaves and Planckian scattering on branes

We obtain a solution describing a gravitational shockwave propagating along a Randall-Sundrum brane. The interest of such a solution is twofold: on the one hand, it is the rst exact solution for a localized source on a Randall-Sundrum three-brane. On the other hand, one can use it to study forward scattering at Planckian energies, including the eects of the continuum of Kaluza-Klein modes. We map out the dierent regimes for the scattering obtained by varying the center-of-mass energy and the impact parameter. We also discuss exact shockwaves in ADD scenarios with compact extra dimensions.