Most of the properties of black holes can be mimicked by horizonless compact objects such as gravastars and boson stars. We show that these ultracompact objects develop a strong ergoregion instability when rapidly spinning. Instability time scales can be of the order of 0.1 seconds to 1 week for objects with mass $M=1-{10}^6{M}_{\odot }$ and angular momentum $J>0.4M^2$. This provides a strong indication that ultracompact objects with large rotation are black holes. Explosive events due to ergoregion instability have a well-defined gravitational-wave signature. These events could be detected by next-generation gravitational-wave detectors such as Advanced LIGO or LISA.

Liquid argon is a bright scintillator with potent particle identification properties, making it an attractive target for direct-detection dark matter searches. The DarkSide-50 dark matter search here reports the first WIMP search results obtained using a target of low-radioactivity argon. DarkSide-50 is a dark matter detector, using a two-phase liquid argon time projection chamber, located at the Laboratori Nazionali del Gran Sasso. The underground argon is shown to contain ${}^{39}\mathrm{Ar}$ at a level reduced by a factor $(1.4\pm 0.2)\times {10}^3$ relative to atmospheric argon. We report a background-free null result from $(2616\pm 43)\mathrm{kg}\mathrm{d}$ of data, accumulated over 70.9 live days. When combined with our previous search using an atmospheric argon, the 90% C.L. upper limit on the WIMP-nucleon spin-independent cross section, based on zero events found in the WIMP search regions, is $2.0\times {10}^{-44}{\mathrm{cm}}^2$ ($8.6\times {10}^{-44}{\mathrm{cm}}^2$, $8.0\times {10}^{-43}{\mathrm{cm}}^2$) for a WIMP mass of $100\mathrm{GeV}/c^2$ ($1\mathrm{TeV}/c^2$, $10\mathrm{TeV}/c^2$).

We present a microscopical derivation of the entropy of the black hole solutions of the Jackiw-Teitelboim theory. We show that the asymptotic symmetry of two-dimensional (2D) anti–de Sitter space is generated by a central extension of the Virasoro algebra. Using a canonical realization of this symmetry and Cardy’s formula we calculate the statistical entropy of 2D black holes, which turns out to agree, up to a factor $\sqrt{2}$, with the thermodynamical result.