Title: Tidal effects around higher-dimensional black holes Authors: Richard Brito, Vitor Cardoso, Paolo Pani

In four-dimensional spacetime, moons around black holes generate low-amplitude tides, and the energy extracted from the hole's rotation is always smaller than the gravitational radiation lost to infinity. Thus, moons orbiting a black hole inspiral and eventually merge. However, it has been conjectured that in higher-dimensional spacetimes orbiting bodies generate much stronger tides, which backreact by tidally accelerating the body outwards. This effect, analogous to the tidal acceleration experienced by the Earth-Moon system, would determine the evolution of the binary. Here, we put this conjecture to the test, by studying matter coupled to a massless scalar field in orbit around a singly-spinning rotating black hole in higher dimensions. We show that in dimensions larger than five the energy extracted from the black hole through superradiance is larger than the energy carried out to infinity. Our numerical results are in excellent agreement with analytic approximations and lend strong support to the conjecture that tidal acceleration is the rule, rather than the exception, in higher dimensions. Superradiance dominates the energy budget and moons "outspiral"; for some particular orbital frequency, the energy extracted at the horizon equals the energy emitted to infinity and "floating orbits" generically occur. We give an interpretation of this phenomenon in terms of the membrane paradigm and of tidal acceleration due to energy dissipation across the horizon.

Black holes are extremely powerful and efficient engines that not only swallow up matter, but also return a lot of energy to the Universe in exchange for the mass they eat. When black holes attract mass they also trigger the release of intense X-ray radiation and power strong jets. But not all black holes do this the same way. This has long baffled astronomers. By studying two active black holes researchers at the SRON Netherlands Institute for Space Research have now gathered evidence that suggests that each black hole can change between two different regimes, like changing the gears of an engine. The team's findings will be published in two papers in the journal Monthly Notices of the Royal Astronomical Society. Read more

We can all rest easy. Small black holes that may be roaming space undetected would leave Earth unscathed if they hit us. Various models suggest matter may have collapsed into black holes soon after the big bang. The smallest of these so-called primordial black holes would have evaporated through a process called Hawking radiation long ago. But those weighing a billion tonnes or more could still be around, and many of these black holes would be hard to detect - unless they hit us, says Katherine Mack of the University of Cambridge. Read more

Title: Detectable seismic consequences of the interaction of a primordial black hole with Earth Authors: Yang Luo, Shravan Hanasoge, Jeroen Tromp, Frans Pretorius

Galaxies observed today are likely to have evolved from density perturbations in the early universe. Perturbations that exceeded some critical threshold are conjectured to have undergone gravitational collapse to form primordial black holes (PBHs) at a range of masses. Such PBHs serve as candidates for cold dark matter and their detection would shed light on conditions in the early universe. Here we propose a mechanism to search for transits of PBHs through/nearby Earth by studying the associated seismic waves. Using a spectral-element method, we simulate and visualize this seismic wave field in Earth's interior. We predict the emergence of two unique signatures, namely, a wave that would arrive almost simultaneously everywhere on Earth's free surface and the excitation of unusual spheroidal modes with a characteristic frequency-spacing in free oscillation spectra. These qualitative characteristics are unaffected by the speed or proximity of the PBH trajectory. The seismic energy deposited by a proximal {M^{PBH} = 10^{15}} g PBH is comparable to a magnitude M_w=4 earthquake. The non-seismic collateral damage due to the actual impact of such small PBHs with Earth would be negligible. Unfortunately, the expected collision rate is very low even if PBHs constituted all of dark matter, at {~ 10^{-7} {yr}^{-1}}, and since the rate scales as {1/M^{PBH}}, fortunately encounters with larger, Earth-threatening PBHs are exceedingly unlikely. However, the rate at which non-colliding close encounters of PBHs could be detected by seismic activity alone is roughly two orders of magnitude larger --- that is once every hundred thousand years --- than the direct collision rate.

Black holes are one of the most destructive forces in the universe, capable of tearing a planet apart and swallowing an entire star. Yet scientists now believe they could hold the key to answering the ultimate question - what was there before the Big Bang? The trouble is that researching them is next to impossible. Black holes are by definition invisible and there's no scientific theory able to explain them. Despite these obvious obstacles, Horizon meets the astronomers attempting to image a black hole for the very first time and the theoretical physicists getting ever closer to unlocking their mysteries. It's a story that takes us into the heart of a black hole and to the very edge of what we think we know about the universe.

Naked black-hole hearts live in the fifth dimension

Luis Lehner of the Perimeter Institute in Ontario, Canada, has proposed a situation where naked singularities might exist: in the extra dimensions proposed by string theory. Black holes would not just be points in the four dimensions we experience - three of space and one of time. They would become "black strings" which extend into a fifth dimension of space.

Title: The fastest way to circle a black hole Authors: Shahar Hod

Black-hole spacetimes with a "photonsphere", a hypersurface on which massless particles can orbit the black hole on circular null geodesics, are studied. We prove that among all possible trajectories (both geodesic and non-geodesic) which circle the central black hole, the null circular geodesic is characterised by the shortest possible orbital period as measured by asymptotic observers. Thus, null circular geodesics provide the fastest way to circle black holes. In addition, we conjecture the existence of a universal lower bound for orbital periods around compact objects (as measured by flat-space asymptotic observers): T_{\infty}\geq 4\pi M, where M is the mass of the central object. This bound is saturated by the null circular geodesic of the maximally rotating Kerr black hole.

Title: Conformal Symmetry for Black Holes in Four Dimensions Authors: Mirjam Cvetic, Finn Larsen

We show that the asymptotic boundary conditions of general asymptotically flat black holes in four dimensions can be modified such that a conformal symmetry emerges. The black holes with the asymptotic geometry removed in this manner satisfy the equations of motion of minimal supergravity. We develop evidence that a two dimensional CFT dual of general black holes in four dimensions account for their black hole entropy.

ESA's Integral gamma-ray observatory has spotted extremely hot matter just a millisecond before it plunges into the oblivion of a black hole. But is it really doomed? These unique observations suggest that some of the matter may be making a great escape. No one would want to be so close to a black hole. Just a few hundred kilometres away from its deadly surface, space is a maelstrom of particles and radiation. Vast storms of particles are falling to their doom at close to the speed of light, raising the temperature to millions of degrees. Ordinarily, it takes just a millisecond for the particles to cross this final distance but hope may be at hand for a small fraction of them. Thanks to the new Integral observations, astronomers now know that this chaotic region is threaded by magnetic fields. Read more

Title: Black Strings, Low Viscosity Fluids, and Violation of Cosmic Censorship Authors: Luis Lehner, Frans Pretorius (Version v3)

We describe the behaviour of 5-dimensional black strings, subject to the Gregory-Laflamme instability. Beyond the linear level, the evolving strings exhibit a rich dynamics, where at intermediate stages the horizon can be described as a sequence of 3-dimensional spherical black holes joined by black string segments. These segments are themselves subject to a Gregory-Laflamme instability, resulting in a self-similar cascade, where ever-smaller satellite black holes form connected by ever-thinner string segments. This behaviour is akin to satellite formation in low-viscosity fluid streams subject to the Rayleigh-Plateau instability. The simulation results imply that the string segments will reach zero radius in finite asymptotic time, whence the classical space-time terminates in a naked singularity. Since no fine-tuning is required to excite the instability, this constitutes a generic violation of cosmic censorship.