Dylan L. Jow, Simon Foreman, Ue-Li Pen, Wei Zhu

Wave effects are often neglected in microlensing studies; however, for coherent point-like sources, such as pulsars and fast radio bursts (FRBs), wave effects will become important in their gravitational lensing. In this paper, we describe the wave optics formalism, its various limits, and the conditions for which these limits hold. Using the simple point lens as an example, we will show that the frequency dependence of wave effects breaks degeneracies that are present in the usual geometric optics limit, and constructive interference results in larger magnifications further from the lens. This latter fact leads to a generic increase in cross section for microlensing events in the wave-optics regime compared to the geometric optics regime. For realistic percent-level spectral sensitivities, this leads to a relative boost in lensing cross section of more than an order of magnitude. We apply the point-lens model to the lensing of FRBs and pulsars and find that these radio sources will be lensed in the full wave-optics regime by isolated masses in the range of $0.1-100\,M_\oplus$, which includes free-floating planets (FFPs), whose Einstein radius is smaller than the Fresnel scale. More generally, the interference pattern allows an instantaneous determination of lens masses, unlike traditional microlensing techniques which only yield a mass inference from the event timescale.

Francesca Lepori, Julian Adamek, Ruth Durrer, Chris Clarkson, Louis Coates

We investigate for the first time weak lensing in a high-resolution relativistic N-body simulation, which includes all relevant effects from general relativity. We integrate the photon geodesics backwards from the observer to the emitters. Our analysis is fully relativistic and non-perturbative for the scalar part of the gravitational potential and first-order in the vector part, frame dragging. We solve the Sachs optical equations and study in detail the weak-lensing convergence, ellipticity and rotation. We present the angular power spectra and one-point probability distribution functions for the weak-lensing variables, which we find are broadly in agreement with comparable Newtonian simulations. Our geometric approach, however, is more robust and flexible, and can therefore be applied consistently to non-standard cosmologies and modified theories of gravity.

F. Calore, A. Cuoco, T. Regimbau, S. Sachdev, P. D. Serpico

Unveiling the origin of the coalescing binaries detected via gravitational waves (GW) is challenging, notably if no multi-wavelength counterpart is detected. One important quantity for diagnostics is their distribution with respect to the large scale structures of the universe, as encoded, for instance, in their (linear) biases. We discuss the perspectives of inferring these quantities via the cross-correlation of galaxy catalogs with mock GW ones, using both existing and forthcoming galaxy catalogs and using realistic Monte Carlo simulations of GW events. We find that the bias should be marginally detectable for a 10-year data taking of current generation detectors at design sensitivity, at least for binary neutron star mergers. The forthcoming five detector network would allow for a firmer detection of this signal and, in combination with future cosmological surveys, would also permit the detection of the coalescing black hole bias. Such a measurement may also unveil, for instance, a primordial origin of coalescing black holes. To attain this goal, we find that it is crucial to adopt a tomographic approach and to reach a sufficiently accurate localization of GW events. The depth of forthcoming surveys will be fully exploited by third generation GW detectors, which will allow one to perform precision studies of the coalescing black holes bias and attain rather advanced model discrimination capabilities, at a few percent level.

David Alonso, Giulia Cusin, Pedro G. Ferreira, Cyril Pitrou

The spatial and temporal discreteness of gravitational wave sources leads to shot noise that may, in some regimes, swamp any attempts at measuring the anisotropy of the gravitational wave background. Cross-correlating a gravitational wave background map with a sufficiently dense galaxy survey can alleviate this issue, and potentially recover some of the underlying properties of the gravitational wave background. We quantify the shot noise level and we explicitly show that cross-correlating the gravitational wave background and a galaxy catalog improves the chances of a first detection of the background anisotropy with a gravitational wave observatory operating in the frequency range (10Hz,100Hz), given sufficient sensitivity.

Giuseppe Dibitetto, Nicolò Petri, Marjorie Schillo

We study non-perturbative instabilities of AdS spacetime in General Relativity with a cosmological constant in arbitrary dimensions. In this simple setup we explicitly construct a class of gravitational instantons generalizing Witten's bubble of nothing. We calculate the corresponding Euclidean action and show that its change is finite. The expansion of these bubbles is described by a lower-dimensional de Sitter geometry within a non-compact foliation of the background spacetime. Moreover we discuss the existence of covariantly constant spinors as a possible topological obstruction for such decays to occur. This mechanism is further connected to the stability of supersymmetric vacua in string theory.

Ezequiel Alvarez, Federico Lamagna, Cesar Miquel, Manuel Szewc

Current daily paper releases are becoming increasingly large and areas of research are growing in diversity. This makes it harder for scientists to keep up to date with current state of the art and identify relevant work within their lines of interest. The goal of this article is to address this problem using Machine Learning techniques. We model a scientific paper to be built as a combination of different scientific knowledge from diverse topics into a new problem. In light of this, we implement the unsupervised Machine Learning technique of Latent Dirichlet Allocation (LDA) on the corpus of papers in a given field to: i) define and extract underlying topics in the corpus; ii) get the topics weight vector for each paper in the corpus; and iii) get the topics weight vector for new papers. By registering papers preferred by a user, we build a user vector of weights using the information of the vectors of the selected papers. Hence, by performing an inner product between the user vector and each paper in the daily Arxiv release, we can sort the papers according to the user preference on the underlying topics. We have created the website IArxiv.org where users can read sorted daily Arxiv releases (and more) while the algorithm learns each users preference, yielding a more accurate sorting every day. Current IArxiv.org version runs on Arxiv categories astro-ph, gr-qc, hep-ph and hep-th and we plan to extend to others. We propose several new useful and relevant implementations to be additionally developed as well as new Machine Learning techniques beyond LDA to further improve the accuracy of this new tool.

Zvi Bern, Harald Ita, Julio Parra-Martinez, Michael S. Ruf

We demonstrate the universality of the gravitational classical deflection angle of massless particles through O(G^3) by studying the high-energy limit of full two-loop four-graviton scattering amplitudes in pure Einstein gravity as well as N >= 4 supergravity. As a by-product, our first-principles calculation provides a direct confirmation of the massless deflection angle in Einstein gravity proposed long ago by Amati, Ciafaloni and Veneziano, and is inconsistent with a recently proposed alternative.