Black hole binaries in cubic Horndeski theories

We study black hole binary mergers in certain cubic Horndeski theories of gravity, treating them fully nonlinearly. In the regime of validity of effective field theory, the mismatch of the gravitational wave strain between Horndeski and general relativity (coupled to a scalar field) can be as large as 10%-13% in the Advanced LIGO mass range. The initial data and coupling constants are chosen such the theory remains in the weakly coupled regime throughout the evolution. In all cases that we have explored, we observe that the waveform in the Horndeski theory is shifted by an amount much larger than the smallness parameter that controls the initial data. This effect is generic and may be present in other theories of gravity involving higher derivatives

Stable non-uniform black strings below the critical dimension

The higher-dimensional vacuum Einstein equation admits translationally non-uniform black string solutions. It has been argued that infinitesimally non-uniform black strings should be unstable in 13 or fewer dimensions and otherwise stable. We construct numerically non-uniform black string solutions in 11, 12, 13, 14 and 15 dimensions. Their stability is investigated using local Penrose inequalities. Weakly non-uniform solutions behave as expected. However, in 12 and 13 dimensions, strongly non-uniform solutions appear to be stable and can have greater horizon area than a uniform string of the same mass. In 14 and 15 dimensions all non-uniform black strings appear to be stable.Comment: 26 pages, 11 figures. V2: reference added, matches published versio

Trust and trustworthiness under information asymmetry and ambiguity

We introduce uncertainty and ambiguity in the standard investment game. In the uncertainty treatment, investors are informed that the return of the investment is drawn from a publicly known distribution function. In the ambiguity treatment, investors are not informed about the distribution function. We find that both trust and trustworthiness are robust to the introduction of these changes

Information asymmetry and deception

Situations such as an entrepreneur overstating a project’s value, or a superior choosing to under or overstate the gains from a project to a subordinate are common and may result in acts of deception. In this paper we modify the standard investment game in the economics literature to study the nature of deception. In this game a trustor (investor) can send a given amount of money to a trustee (or investee). The amount received is multiplied by a certain amount, k, and the investee then decides on how to divide the total amount received. In our modified game the information on the multiplier, k, is known only to the investee and she can send a nonbinding message to the investor regarding its value. We find that 66% of the investees send false messages with both under and over, statement being observed. Investors are naive and almost half of them believe the message received. We find greater lying when the distribution of the multiplier is unknown by the investors than when they know the distribution. Further, messages make beliefs about the multiplier more pessimistic when the investors know the distribution of the multiplier, while the opposite is true when they do not know the distribution

Riding the kinematic waves in the Milky Way disk with Gaia

Gaia DR2 has delivered full-sky 6-D measurements for millions of stars, and the quest to understand the dynamics of our Galaxy has entered a new phase. Our aim is to reveal and characterize the kinematic sub-structure of the different Galactic neighbourhoods, to form a picture of their spatial evolution that can be used to infer the Galactic potential, its evolution and its components. We take ~5 million stars in the Galactic disk from the Gaia DR2 catalogue and build the velocity distribution of many different Galactic Neighbourhoods distributed along 5 kpc in Galactic radius and azimuth. We decompose their distribution of stars in the V_R-V_phi plane with the wavelet transformation and asses the statistical significance of the structures found. We detect many kinematic sub-structures (arches and more rounded groups) that diminish their azimuthal velocity as a function of Galactic radius in a continuous way, connecting volumes up to 3 kpc apart in some cases. The decrease rate is, on average, of ~23 km/s/kpc. In azimuth, the kinematic sub-structures present much smaller variations. We also observe a duality in their behaviour: some conserve their vertical angular momentum with radius (e.g., Hercules), while some seem to have nearly constant kinetic energy (e.g., Sirius). These two trends are consistent with the approximate predictions of resonances and of phase mixing, respectively. Besides, the overall spatial evolution of Hercules is consistent with being related to the Outer Lindblad Resonance of the Bar. We also detect structures without apparent counterpart in the vicinity of the Sun. The various trends observed and their continuity with radius and azimuth allows for future work to deeply explore the parameter space of the models. Also, the characterization of extrasolar moving groups opens the opportunity to expand our understanding of the Galaxy beyond the Solar Neighbourhood.Comment: 16 pages. Submitted to Astronomy and Astrophysics on 24th of May, 2018. Related on-line material available (see Appendix B

Puncture gauge formulation for Einstein-Gauss-Bonnet gravity and four-derivative scalar-tensor theories in d+1 spacetime dimensions

We develop a modified CCZ4 formulation of the Einstein equations in d + 1 spacetime dimensions for general relativity plus a Gauss-Bonnet term, as well as for the most general parity-invariant scalar-tensor theory of gravity up to four derivatives. We demonstrate well-posedness for both theories and provide full expressions for their implementation in numerical relativity codes. As a proof of concept, we study the so-called “stealth scalarization” induced by the spin of the remnant black hole after the merger. As in previous studies using alternative gauges, we find that the scalarization occurs too late after the merger to impact the tensor waveform, unless the parameters are finely tuned. Naively increasing the coupling to accelerate the growth of the scalar field risks a breakdown of the effective field theory, and therefore well-posedness, as the evolution is pushed into the strongly coupled regime. Observation of such an effect would therefore rely on the detection of the scalar radiation that is produced during scalarization. This work provides a basis on which further studies can be undertaken using codes that employ a moving-punctures approach to managing singularities in the numerical domain. It is therefore an important step forward in our ability to analyze modifications of general relativity in gravitational wave observations

Real-Time Dynamics of Plasma Balls from Holography

Plasma balls are droplets of deconfined plasma surrounded by a confining vacuum. We present the first holographic simulation of their real-time evolution via the dynamics of localized, finite-energy black holes in the five-dimensional anti–de Sitter (AdS) soliton background. The dual gauge theory is four-dimensional N = 4 super Yang-Mills theory compactified on a circle with supersymmetry-breaking boundary conditions. We consider horizonless initial data sourced by a massless scalar field. Prompt scalar field collapse produces an excited black hole at the bottom of the geometry together with gravitational and scalar radiation. The radiation disperses to infinity in the noncompact directions and corresponds to particle production in the dual gauge theory. The black hole evolves toward the dual of an equilibrium plasma ball on a time scale longer than naively expected. This feature is a direct consequence of confinement and is caused by long-lived, periodic disturbances bouncing between the bottom of the AdS soliton and the AdS boundary