Sownak Bose, Baojiu Li, Durham), Alexandre Barreira, Garching), Jian-hua He, Durham), Wojciech A. Hellwing, Kazuya Koyama, Portsmouth), Claudio Llinares, Durham), Gong-Bo Zhao, Beijing, ICG, Portsmouth)

We describe and demonstrate the potential of a new and very efficient method for simulating certain classes of modified gravity theories, such as the widely studied $f(R)$ gravity models. High resolution simulations for such models are currently very slow due to the highly nonlinear partial differential equation that needs to be solved exactly to predict the modified gravitational force. This nonlinearity is partly inherent, but is also exacerbated by the specific numerical algorithm used, which employs a variable redefinition to prevent numerical instabilities. The standard Newton-Gauss-Seidel iterative method used to tackle this problem has a poor convergence rate. Our new method not only avoids this, but also allows the discretised equation to be written in a form that is analytically solvable. We show that this new method greatly improves the performance and efficiency of $f(R)$ simulations. For example, a test simulation with $512^3$ particles in a box of size $512 \, \mathrm{Mpc}/h$ is now 5 times faster than before, while a Millennium-resolution simulation for $f(R)$ gravity is estimated to be more than 20 times faster than with the old method. Our new implementation will be particularly useful for running very high resolution, large-sized simulations which, to date, are only possible for the standard model, and also makes it feasible to run large numbers of lower resolution simulations for covariance analyses. We hope that the method will bring us to a new era for precision cosmological tests of gravity.

Jahed Abedi, Hannah Dykaar, Niayesh Afshordi

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of $8 M \log M$ (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the "look elsewhere" effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at $2.9\sigma$ significance level (corresponding to false detection probability of 1 in 270). Future data releases from LIGO collaboration, along with more physical echo templates, will definitively confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in ${\it firewall}$ or ${\it fuzzball}$ paradigms.

Remi Geiger

We present the perspective of using atom interferometry for gravitational wave (GW) detection in the mHz to about 10 Hz frequency band. We focus on light-pulse atom interferometers which have been subject to intense developments in the last 25 years. We calculate the effect of the GW on the atom interferometer and present in details the atomic gradiometer configuration which has retained more attention recently. The principle of such a detector is to use free falling atoms to measure the phase of a laser, which is modified by the GW. We highlight the potential benefits of using atom interferometry compared to optical interferometry as well as the challenges which remain for the realization of an atom interferometry based GW detector. We present some of the important noise sources which are expected in such detectors and strategies to cirucumvent them. Experimental techniques related to cold atom interferometers are briefly explained. We finally present the current progress and projects in this rapidly evolving field.

Sachin Pandey, Sridip Pal, Narayan Banerjee

The present work sheds light on the nature of mathematical equivalence of Jordan frame and its conformally transformed version, the Einstein frame, as far as Brans-Dicke theory is concerned. It is shown that question of equivalence surviving a quantization of cosmological models in the theory is intricately related to whether the canonical structure breaks down while going from one frame to the other. It is found that that the consistent operator ordering to make two frames equivalent requires a non-canonical transformation that mixes gravity sector with scalar sector to undo the non minimal coupling, present in Jordan frame. The question of equivalence thus depends on the details of quantization prescriptions, i.e. whether we are allowed to make any such non-canonical transformation at classical level before quantizing the theory. We work with the Wheeler-deWitt quantization scheme and take up quite a few anisotropic cosmological models as examples.

Haipeng An, Jiayin Gu, Lian-Tao Wang

A light stop with mass almost degenerate with the lightest neutralino has important connections with both naturalness and dark matter relic abundance. This region is also very hard to probe at colliders. In this paper, we demonstrate the potential of searching for such stop particles at the LHC from sbottom decays, focusing on two channels with final states $2\ell+E^{\rm miss}_{\rm T}$ and $1b1\ell+E^{\rm miss}_{\rm T}$. We found that, if the lightest sbottom has mass around or below 1 TeV and has a significant branching ratio to decay to stop and $W$ ($\tilde{b} \to \tilde{t}\,W$), a stop almost degenerate with neutralino can be excluded up to about 500-600 GeV at the 13 TeV LHC with $300\,{\rm fb}^{-1}$ data. The searches we propose are complementary to other SUSY searches at the LHC and could have the best sensitivity to the stop-bino coannihilation region. Since they involve final states which have already been used in LHC searches, a reinterpretation of the search results already has sensitivity. Further optimization could deliver the full potential of these channels.

Martin Bauer, Anja Butter, Nishita Desai, Juan Gonzalez-Fraile, Tilman Plehn

An effective theory of dark matter offers an attractive framework for global analyses of dark matter. In the light of global fits we test the validity of the link between the non-relativistic dark matter annihilation, or the predicted relic density, and LHC signatures. Specifically, we study how well the effective theory describes the main features of simple models with s-channel and t-channel mediators coupling to the Standard Model at tree level or through one-loop diagrams. Our results indicate that global dark matter analyses in terms of effective Lagrangians are highly non-trivial to interpret in term of actual models.

Stephen W. Hawking, Malcolm J. Perry, Andrew Strominger

It is shown that black hole spacetimes in classical Einstein gravity are characterized by, in addition to their ADM mass M , momentum P ~ , angular momentum J ~ and ~ an infinite head of supertranslation hair. The distinct black holes boost charge K, are distinguished by classical superrotation charges measured at infinity. Solutions with supertranslation hair are diffeomorphic to the Schwarzschild spacetime, but the diffeomorphisms are part of the BMS subgroup and act nontrivially on the physical phase space. It is shown that a black hole can be supertranslated by throwing in an asymmetric shock wave. A leading-order Bondi-gauge expression is derived for the linearized horizon supertranslation charge and shown to generate, via the Dirac bracket, supertranslations on the linearized phase space of gravitational excitations of the horizon. The considerations of this paper are largely classical augmented by comments on their implications for the quantum theory.