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GRTresna is a multigrid solver designed to solve the constraint equations for the initial data required in numerical relativity simulations. In particular it is focussed on scenarios with fundamental fields around black holes and inhomogeneous cosmological spacetimes. The following overview has been prepared as part of the submission of the code to the Journal of Open Source Software. The code is based on the formalism in Aurrekoetxea, Clough \& Lim arXiv:2207.03125 and can be found at https://github.com/GRTLCollaboration/GRTresna, Comment: To be submitted to the Journal of Open Source Software (JOSS). Code available at https://github.com/GRTLCollaboration/GRTresna

We explore the gravitational-wave emission from head-on collisions of equal-mass solitonic boson-star binaries from simulations spanning a two-dimensional parameter space, consisting of the central scalar-field amplitude of the stars and the solitonic potential parameter. We report the gravitational-wave energies emitted by boson-star binaries which, due to their combination of moderately high compactness with significant deformability, we often find to be louder by up to an order of magnitude than analogous black-hole collisions. The dependence of the radiated energy on the boson-star parameters exhibits striking needle-sharp features and discontinuous jumps to the value emitted by black-hole binaries. We explain these features in terms of the solitonic potential and the stability properties of the respective individual stars., Comment: 17 pages, 10 figures

We study EgalMAB, an egalitarian assignment problem in the context of stochastic multi-armed bandits. In EgalMAB, an agent is tasked with assigning a set of users to arms. At each time step, the agent must assign exactly one arm to each user such that no two users are assigned to the same arm. Subsequently, each user obtains a reward drawn from the unknown reward distribution associated with its assigned arm. The agent's objective is to maximize the minimum expected cumulative reward among all users over a fixed horizon. This problem has applications in areas such as fairness in job and resource allocations, among others. We design and analyze a UCB-based policy EgalUCB and establish upper bounds on the cumulative regret. In complement, we establish an almost-matching policy-independent impossibility result.

This review is an up-to-date account of the use of numerical relativity to study dynamical, strong-gravity environments in a cosmological context. First, we provide a gentle introduction into the use of numerical relativity in solving cosmological spacetimes, aimed at both cosmologists and numerical relativists. Second, we survey the present body of work, focusing on general relativistic simulations without approximations, organised according to the cosmological history -- from cosmogenesis, through the early hot Big Bang, to the late-time evolution of universe. In both cases, we discuss the present state-of-the-art, and suggest directions in which future work can be fruitfully pursued., Comment: 42 pages, 21 figures. To be submitted to Living Reviews in Relativity. Comments welcome!

In recent decades, the field of quantum computing has experienced remarkable progress. This progress is marked by the superior performance of many quantum algorithms compared to their classical counterparts, with Shor's algorithm serving as a prominent illustration. Quantum arithmetic circuits, which are the fundamental building blocks in numerous quantum algorithms, have attracted much attention. Despite extensive exploration of various designs in the existing literature, researchers remain keen on developing novel designs and improving existing ones. In this review article, we aim to provide a systematically organized and easily comprehensible overview of the current state-of-the-art in quantum arithmetic circuits. Specifically, this study covers fundamental operations such as addition, subtraction, multiplication, division and modular exponentiation. We delve into the detailed quantum implementations of these prominent designs and evaluate their efficiency considering various objectives. We also discuss potential applications of presented arithmetic circuits and suggest future research directions., Comment: Under review at the Royal Society's Philosophical Transactions A

We investigate the effects of large inhomogeneities in both the inflaton field and its momentum. We find that in general, large kinetic perturbations reduce the number of e-folds of inflation. In particular, we observe that inflationary models with sub-Planckian characteristic scales are not robust even to kinetic energy densities that are sub-dominant to the potential energy density, unless the initial field configuration is sufficiently far from the minimum. This strengthens the results of our previous work. In inflationary models with super-Planckian characteristic scales, despite a reduction in the number of e-folds, inflation is robust even when the potential energy density is initially sub-dominant. For the cases we study, the robustness of inflation strongly depends on whether the inflaton field is driven into the reheating phase by the inhomogeneous scalar dynamics., Comment: 8 pages, 5 figures, 1 appendix. Comments welcome!

Axion dark matter can form stable, self-gravitating, and coherent configurations known as axion stars, which are rendered unstable above a critical mass by the Chern-Simons coupling to electromagnetism. We study, using numerical relativity, the merger and subsequent decay of compact axion stars. We show that two sub-critical stars can merge, and form a more massive, excited and critical star, which survives for a finite period before rapidly decaying via electromagnetic radiation. We find a rich multimessenger signal, composed of gravitational waves, electromagnetic radiation, and axion radiation. The gravitational wave signal is broken into two parts: a weak and broad signal from the merger, followed by a much stronger signal of almost fixed frequency from the decay. The electromagnetic radiation follows only the gravitational waves from the decay, while the axion signal is continuous throughout the process. We briefly discuss the detectability of such a signal., Comment: 14 pages, 11 figures. Movie: https://www.youtube.com/watch?v=SItsbjsr_tg

Rapid progress in the design of scalable, robust quantum computing necessitates efficient quantum circuit implementation for algorithms with practical relevance. For several algorithms, arithmetic kernels, in particular, division plays an important role. In this manuscript, we focus on enhancing the performance of quantum slow dividers by exploring the design choices of its sub-blocks, such as, adders. Through comprehensive design space exploration of state-of-the-art quantum addition building blocks, our work have resulted in an impressive achievement: a reduction in Toffoli Depth of up to 94.06%, accompanied by substantial reductions in both Toffoli and Qubit Count of up to 91.98% and 99.37%, respectively. This paper offers crucial perspectives on efficient design of quantum dividers, and emphasizes the importance of adopting a systematic design space exploration approach., Comment: This is accepted for publication in ISCAS 2024

Imitation learning empowers artificial agents to mimic behavior by learning from demonstrations. Recently, diffusion models, which have the ability to model high-dimensional and multimodal distributions, have shown impressive performance on imitation learning tasks. These models learn to shape a policy by diffusing actions (or states) from standard Gaussian noise. However, the target policy to be learned is often significantly different from Gaussian and this mismatch can result in poor performance when using a small number of diffusion steps (to improve inference speed) and under limited data. The key idea in this work is that initiating from a more informative source than Gaussian enables diffusion methods to mitigate the above limitations. We contribute both theoretical results, a new method, and empirical findings that show the benefits of using an informative source policy. Our method, which we call BRIDGER, leverages the stochastic interpolants framework to bridge arbitrary policies, thus enabling a flexible approach towards imitation learning. It generalizes prior work in that standard Gaussians can still be applied, but other source policies can be used if available. In experiments on challenging simulation benchmarks and on real robots, BRIDGER outperforms state-of-the-art diffusion policies. We provide further analysis on design considerations when applying BRIDGER. Code for BRIDGER is available at https://github.com/clear-nus/bridger.

LISA, the Laser Interferometer Space Antenna, will usher in a new era in gravitational-wave astronomy. As the first anticipated space-based gravitational-wave detector, it will expand our view to the millihertz gravitational-wave sky, where a spectacular variety of interesting new sources abound: from millions of ultra-compact binaries in our Galaxy, to mergers of massive black holes at cosmological distances; from the beginnings of inspirals that will venture into the ground-based detectors' view to the death spiral of compact objects into massive black holes, and many sources in between. Central to realising LISA's discovery potential are waveform models, the theoretical and phenomenological predictions of the pattern of gravitational waves that these sources emit. This white paper is presented on behalf of the Waveform Working Group for the LISA Consortium. It provides a review of the current state of waveform models for LISA sources, and describes the significant challenges that must yet be overcome., Comment: 239 pages, 11 figures, white paper from the LISA Consortium Waveform Working Group, invited for submission to Living Reviews in Relativity, updated with comments from community

Light carries intrinsic spin angular momentum (SAM) when the electric or magnetic field vector rotates over time. A familiar vector equation calculates the direction of light's SAM density using the right hand rule with reference to the electric and magnetic polarisation ellipses. Using Maxwell's equations, this vector equation can be decomposed into a sum of two distinct terms, akin to the well-known Poynting vector decomposition into orbital and spin currents. We present the first general study of this spin decomposition, showing that the two terms, which we call canonical and Poynting spin, are chiral analogies to the canonical and spin momenta of light in its interaction with matter. Both canonical and Poynting spin incorporate spatial variation of the electric and magnetic fields and are influenced by optical orbital angular momentum (OAM). The decomposition allows us to show that the OAM of a linearly polarised vortex beam can impart a first-order preferential force to chiral matter in the absence of spin., Comment: 17 pages, 2 figures. SI included

We study the formation of spinning primordial black holes during an early matter-dominated era. Using non-linear 3+1D general relativistic simulations, we compute the efficiency of mass and angular momentum transfer in the process -- which we find to be $\mathcal{O}(10\%)$ and $\mathcal{O}(5\%)$, respectively. We show that subsequent evolution is important due to the seed PBH accreting non-rotating matter from the background, which decreases the dimensionless spin. Unless the matter era is short, we argue that the final dimensionless spin will be negligible., Comment: 12 pages, 5 figures. 1 YouTube video $\href{https://youtu.be/CC4xBLol4aE}{here}$

If the dark matter is composed of axions, then axion stars are expected to be abundant in the Universe. We demonstrate in fully non-linear (3+1) numerical relativity the instability of compact axion stars due to the electromagnetic Chern-Simons term. We show that above the critical coupling constant $g_{a\gamma}^\mathrm{crit} \propto M_s^{-1.35}$, compact axion stars of mass $M_s$ are unstable. The instability is caused by parametric resonance between the axion and the electromagnetic field. The existence of stable compact axion stars requires approximately Planck-suppressed couplings to photons. If the coupling exceeds the critical value, then all stable axion stars are necessarily non-compact. Unstable axion stars decay leaving behind a less massive, less compact, remnant. The emitted radiation peaks at frequency $\omega \sim 1/R_s$, where $R_s$ is the axion star radius., Comment: 7 pages, 5 figures

We present a generalization of the curative initial data construction derived for equal-mass compact binaries in Helfer {\it et al} (2019 Phys. Rev. D 99 044046; 2022 Class. Quantum Grav. 39 074001) to arbitrary mass ratios. We demonstrate how these improved initial data avoid substantial spurious artifacts in the collision dynamics of unequal-mass boson-star binaries in the same way as has previously been achieved with the simpler method restricted to the equal-mass case. We employ the improved initial data to explore in detail the impact of phase offsets in the coalescence of equal- and unequal-mass boson star binaries., Comment: 37 pages, 12 figures, to match published version in CQG

In this work, we point out the problem of observed adversaries for deep policies. Specifically, recent work has shown that deep reinforcement learning is susceptible to adversarial attacks where an observed adversary acts under environmental constraints to invoke natural but adversarial observations. This setting is particularly relevant for HRI since HRI-related robots are expected to perform their tasks around and with other agents. In this work, we demonstrate that this effect persists even with low-dimensional observations. We further show that these adversarial attacks transfer across victims, which potentially allows malicious attackers to train an adversary without access to the target victim.

In this work we study the long-lived post-merger gravitational wave signature of a boson-star binary coalescence. We use full numerical relativity to simulate the post-merger and track the gravitational afterglow over an extended period of time. We implement recent innovations for the binary initial data, which significantly reduce spurious initial excitations of the scalar field profiles, as well as a measure for the angular momentum that allows us to track the total momentum of the spatial volume, including the curvature contribution. Crucially, we find the afterglow to last much longer than the spin-down timescale. This prolonged gravitational wave afterglow provides a characteristic signal that may distinguish it from other astrophysical sources., Comment: Movie: https://youtu.be/JE5FRG7kgvU Data: https://github.com/ThomasHelfer/BosonStarAfterglow

In numerical relativity simulations with non-trivial matter configurations, one must solve the Hamiltonian and momentum constraints of the ADM formulation for the metric variables in the initial data. We introduce a new scheme based on the standard Conformal Transverse-Traceless (CTT) decomposition, in which instead of solving the Hamiltonian constraint as a 2nd order elliptic equation for a choice of mean curvature $K$, we solve an algebraic equation for $K$ for a choice of conformal factor. By doing so, we evade the existence and uniqueness problem of solutions of the Hamiltonian constraint without using the usual conformal rescaling of the source terms. This is particularly important when the sources are fundamental fields, as reconstructing the fields' configurations from the rescaled quantities is potentially problematic. Using an iterative multigrid solver, we show that this method provides rapid convergent solutions for several initial conditions that have not yet been studied in numerical relativity; namely (i) periodic inhomogeneous spacetimes with large random Gaussian scalar field perturbations and (ii) asymptotically flat black hole spacetimes with rotating scalar clouds., Comment: 14 pages, 4 figures, 1 appendix. Matches version published in CQG

We study the generation and propagation of gravitational waves in scalar-tensor gravity using numerical relativity simulations of scalar field collapses beyond spherical symmetry. This allows us to compare the tensor and additional massive scalar waves that are excited. As shown in previous work in spherical symmetry, massive propagating scalar waves decay faster than 1/r and disperse, resulting in an inverse chirp. These effects obscure the ringdown in any extracted signal by mixing it with the transient responses of the collapse during propagation. In this paper we present a simple method to rewind the extracted signals to horizon formation, which allows us to clearly identify the ringdown phase and extract the amplitudes of the scalar quasinormal modes, quantifying their excitation in strong gravity events and verifying the frequencies to perturbative calculations. The effects studied are relevant to any theories in which the propagating waves have a dispersion relation, including the tensor case., Comment: 10 pages, 8 figures. Movies: https://youtu.be/YYBwyAbH5Fk. Matches version published in PRD

GRChombo is an open-source code for performing Numerical Relativity time evolutions, built on top of the publicly available Chombo software for the solution of PDEs. Whilst GRChombo uses standard techniques in NR, it focusses on applications in theoretical physics where adaptability, both in terms of grid structure, and in terms of code modification, are key drivers., Comment: JOSS submission for first official release and review of the GRChombo GitHub repository: https://github.com/GRChombo/GRChombo, https://doi.org/10.21105/joss.03703

We demonstrate the flexibility and utility of the Berger-Rigoutsos Adaptive Mesh Refinement (AMR) algorithm used in the open-source numerical relativity code GRChombo for generating gravitational waveforms from binary black-hole inspirals, and for studying other problems involving non-trivial matter configurations. We show that GRChombo can produce high quality binary black-hole waveforms through a code comparison with the established numerical relativity code Lean. We also discuss some of the technical challenges involved in making use of full AMR (as opposed to, e.g. moving box mesh refinement), including the numerical effects caused by using various refinement criteria when regridding. We suggest several "rules of thumb" for when to use different tagging criteria for simulating a variety of physical phenomena. We demonstrate the use of these different criteria through example evolutions of a scalar field theory. Finally, we also review the current status and general capabilities of GRChombo., Comment: 46 pages, 10 figures, to match version published in CQG

We study the formation of black holes from subhorizon and superhorizon perturbations in a matter dominated universe with 3+1D numerical relativity simulations. We find that there are two primary mechanisms of formation depending on the initial perturbation's mass and geometry -- via $\textit{direct collapse}$ of the initial overdensity and via $\textit{post-collapse accretion}$ of the ambient dark matter. In particular, for the latter case, the initial perturbation does not have to satisfy the hoop conjecture for a black hole to form. In both cases, the duration of the formation the process is around a Hubble time, and the initial mass of the black hole is $M_\mathrm{BH} \sim 10^{-2} H^{-1} M_\mathrm{Pl}^2$. Post formation, we find that the PBH undergoes rapid mass growth beyond the self-similar limit $M_\mathrm{BH}\propto H^{-1}$, at least initially. We argue that this implies that most of the final mass of the PBH is accreted from its ambient surroundings post formation., Comment: 12 pages, 8 figures. 2 YouTube videos here: https://youtube.com/playlist?list=PLSkfizpQDrcZuKZPQydJA9l_pxxWpqIMz

We show that the so-called "post-Born" effects of weak lensing at 4th order are equivalent to lens-lens couplings in the Born Approximation. We demonstrate this by explicitly showing the equivalence of the canonical weak lensing approach at 4th order using the anisotropy remapping method, to that of the 4th order calculation of the lens-lens coupling effects using the Boltzmann equation approach that was first developed in [Phys. Rev. D89, 123006]. Furthermore, we argue that to incorporate true "post-Born" effects, i.e. taking into account non-straight photon paths, require the addition of a photon deflection term which has not been taken into account in the canonical formalism nor the Boltzmann method., Comment: 21 pages

Through numerical simulations of boson-star head-on collisions, we explore the quality of binary initial data obtained from the superposition of single-star spacetimes. Our results demonstrate that evolutions starting from a plain superposition of individual boosted boson-star spacetimes are vulnerable to significant unphysical artefacts. These difficulties can be overcome with a simple modification of the initial data suggested in [PRD 99 (2018) 044046] for collisions of oscillatons. While we specifically consider massive complex scalar field boson star models up to a 6th-order-polynomial potential, we argue that this vulnerability is universal and present in other kinds of exotic compact systems and hence needs to be addressed., Comment: 28 pages, 9 figures, submitted to Classical Quantum Gravity

Humans display the remarkable ability to sense the world through tools and other held objects. For example, we are able to pinpoint impact locations on a held rod and tell apart different textures using a rigid probe. In this work, we consider how we can enable robots to have a similar capacity, i.e., to embody tools and extend perception using standard grasped objects. We propose that vibro-tactile sensing using dynamic tactile sensors on the robot fingers, along with machine learning models, enables robots to decipher contact information that is transmitted as vibrations along rigid objects. This paper reports on extensive experiments using the BioTac micro-vibration sensor and a new event dynamic sensor, the NUSkin, capable of multi-taxel sensing at 4~kHz. We demonstrate that fine localization on a held rod is possible using our approach (with errors less than 1 cm on a 20 cm rod). Next, we show that vibro-tactile perception can lead to reasonable grasp stability prediction during object handover, and accurate food identification using a standard fork. We find that multi-taxel vibro-tactile sensing at sufficiently high sampling rate led to the best performance across the various tasks and objects. Taken together, our results provides both evidence and guidelines for using vibro-tactile perception to extend tactile perception, which we believe will lead to enhanced competency with tools and better physical human-robot-interaction., Comment: 9 pages, 7 figures. This version adds additional related work and updated results

Characterizing the physical properties of the stochastic gravitational waves background (SGWB) is a key step towards identifying the nature of its possible origin. We focus our analysis on SGWB anisotropies. The existence of a non-trivial primordial scalar-tensor-tensor (STT) correlation in the squeezed configuration may be inferred from the effect that a long wavelength scalar mode has on the gravitational wave power spectrum: an anisotropic contribution. Crucially, such contribution is correlated with temperature anisotropies in the cosmic microwave background (CMB). We show that, for inflationary models that generate suitably large STT non-Gaussianities, cross-correlating the CMB with the stochastic background of gravitational waves is a very effective probe of early universe physics. The resulting signal can be a smoking-gun for primordial SGWB anisotropies., Comment: 6 pages, 2 figures. Version matching the published manuscript

We construct, for the first time, the time-domain gravitational wave strain waveform from the collapse of a strongly gravitating Abelian Higgs cosmic string loop in full general relativity. We show that the strain exhibits a large memory effect during merger, ending with a burst and the characteristic ringdown as a black hole is formed. Furthermore, we investigate the waveform and energy emitted as a function of string width, loop radius and string tension $G\mu$. We find that the mass normalized gravitational wave energy displays a strong dependence on the inverse of the string tension $E_{\mathrm{GW}}/M_0\propto 1/G\mu$, with $E_{\mathrm{GW}}/M_0 \sim {\cal O}(1)\%$ at the percent level, for the regime where $G\mu\gtrsim10^{-3}$. Conversely, we show that the efficiency is only weakly dependent on the initial string width and initial loop radii. Using these results, we argue that gravitational wave production is dominated by kinematical instead of geometrical considerations., Comment: 15 pages, 16 figures, 2 YouTube movies: https://youtu.be/-dhYA2788LA https://youtu.be/0sSH54gXu4U

We study the robustness of single-field inflation against inhomogeneities. We derive a simple analytic criterion on the shape of the potential for successful inflation in the presence of inhomogeneities, and demonstrate its validity using full 3+1 dimensional numerical relativity simulations on several classes of popular models of single-field inflation. We find that models with convex potentials are more robust to inhomogeneities than those with concave potentials, and that concave potentials that vary on super-Planckian scales are significantly more robust than those that vary on sub-Planckian scales., Comment: 16 pages, 12 figures. Summary video: https://youtu.be/yk9sGuG8hdI

We investigate the physics of black hole formation from the head-on collisions of boosted equal mass Oscillatons (OS) in full numerical relativity, for both the cases where the OS have equal phases or are maximally off-phase (anti-phase). While unboosted OS collisions will form a BH as long as their initial compactness $\mathcal{C}\equiv GM/R$ is above a numerically determined critical value $\mathcal{C}>0.035$, we find that imparting a small initial boost counter-intuitively \emph{prevents} the formation of black holes even if $\mathcal{C}> 0.035$. If the boost is further increased, at very high boosts $\gamma>1/12\mathcal{C}$, BH formation occurs as predicted by the hoop conjecture. These two limits combine to form a "stability band" where collisions result in either the OS "passing through" (equal phase) or "bouncing back" (anti-phase), with a critical point occurring around ${\cal C}\approx 0.07$. We argue that the existence of this stability band can be explained by the competition between the free fall and the interaction timescales of the collision.

We describe the properties of evanescent gravitational waves (EGWs)---wave solutions of Einstein equations which decay exponentially in some direction while propagating in another. Evanescent waves are well-known in acoustics and optics and have recently received much attention due to their extraordinary properties such as their transverse spin and spin-momentum locking. We show that EGWs possess similarly remarkable properties, carrying transverse spin angular momenta and driving freely falling test masses along in elliptical trajectories. Hence, test masses on a plane transverse to the direction of propagation exhibit correlated vector and scalar-like deformation---correlations which can be used to distinguish it from modified gravity. We demonstrate that EGWs are present and dominant in the vicinity of sub-wavelength sources such as orbiting binaries., Comment: 13 pages, movie link https://youtu.be/DB7mHGqsrLk

The next generation of instruments designed to measure the polarization of the cosmic microwave background (CMB) will provide a historic opportunity to open the gravitational wave window to the primordial Universe. Through high sensitivity searches for primordial gravitational waves, and tighter limits on the energy released in processes like phase transitions, the CMB polarization data of the next decade has the potential to transform our understanding of the laws of physics underlying the formation of the Universe., Comment: 5 pages + references; Submitted to the Astro2020 call for science white papers

We present the first fully general relativistic dynamical simulations of Abelian Higgs cosmic strings using 3+1D numerical relativity. Focusing on cosmic string loops, we show that they collapse due to their tension and can either (i) unwind and disperse or (ii) form a black hole, depending on their tension $G\mu$ and initial radius. We show that these results can be predicted using an approximate formula derived using the hoop conjecture, and argue that it is independent of field interactions. We extract the gravitational waveform produced in the black hole formation case and show that it is dominated by the $l=2$ and $m=0$ mode. We also compute the total gravitational wave energy emitted during such a collapse, being $0.5\pm 0.2~ \%$ of the initial total cosmic string loop mass, for a string tension of $G\mu=1.6\times 10^{-2}$ and radius $R=100~M_{pl}^{-1}$. We use our results to put a bound on the production rate of planar cosmic strings loops as $N \lesssim 10^{-2}~\mathrm{Gpc}^{-3}~\mathrm{yr}^{-1}$., Comment: Movies: https://www.youtube.com/playlist?list=PLSkfizpQDrcaAxkuQ3BtjILn_tJu-jXxY

Axions and axion-like particles are compelling candidates for the missing dark matter of the universe. As they undergo gravitational collapse, they can form compact objects such as axion stars or even black holes. In this paper, we study the formation and distribution of such objects. First, we simulate the formation of compact axion stars using numerical relativity with aspherical initial conditions that could represent the final stages of axion dark matter structure formation. We show that the final states of such collapse closely follow the known relationship of initial mass and axion decay constant $f_a$. Second, we demonstrate with a toy model how this information can be used to scan a model density field to predict the number densities and masses of such compact objects. In addition to being detectable by the LIGO/VIRGO gravitational wave interferometer network for axion mass of $10^{-9} < m_a < 10^{-11}$ eV, we show using peak statistics that for $f_a < 0.2M_{pl}$, there exists a "mass gap" between the masses of axion stars and black holes formed from collapse.

We numerically investigate the gravitational waves generated by the head-on collision of equal-mass, self-gravitating, real scalar field solitons (oscillatons) as a function of their compactness $\mathcal{C}$. We show that there exist three different possible outcomes for such collisions: (1) an excited stable oscillaton for low $\mathcal{C}$, (2) a merger and formation of a black-hole for intermediate $\mathcal{C}$, and (3) a pre-merger collapse of both oscillatons into individual black-holes for large $\mathcal{C}$. For (1), the excited, aspherical oscillaton continues to emit gravitational waves. For (2), the total energy in gravitational waves emitted increases with compactness, and possesses a maximum which is greater than that from the merger of a pair of equivalent mass black-holes. The initial amplitudes of the quasi-normal modes in the post-merger ring-down in this case are larger than that of corresponding mass black-holes -- potentially a key observable to distinguish black-hole mergers with their scalar mimics. For (3), the gravitational wave output is indistinguishable from a similar mass, black-hole--black-hole merger., Comment: 8 Pages, 8 figures, movies : https://www.youtube.com/playlist?list=PLSkfizpQDrcahgvc5TvBk5qtXAzkSyHPe

Extending our previous work on the robustness of inflation to perturbations in the scalar field, we investigate the effects of perturbations in the transverse traceless part of the extrinsic curvature on the evolution of an inhomogeneous inflaton field. Focusing on small field models, we show that these additional metric inhomogeneities initially reduce the total number of e-folds as the amplitude increases, but that the reduction saturates and even reverses above a certain amplitude. We present an argument that this is due to the presence of a large initial Hubble friction when metric perturbations are large., Comment: 16 pages, 15 figures

The classical equations of motion for an axion with potential $V(\phi)=m_a^2f_a^2 [1-\cos (\phi/f_a)]$ possess quasi-stable, localized, oscillating solutions, which we refer to as "axion stars". We study, for the first time, collapse of axion stars numerically using the full non-linear Einstein equations of general relativity and the full non-perturbative cosine potential. We map regions on an "axion star stability diagram", parameterized by the initial ADM mass, $M_{\rm ADM}$, and axion decay constant, $f_a$. We identify three regions of the parameter space: i) long-lived oscillating axion star solutions, with a base frequency, $m_a$, modulated by self-interactions, ii) collapse to a BH and iii) complete dispersal due to gravitational cooling and interactions. We locate the boundaries of these three regions and an approximate "triple point" $(M_{\rm TP},f_{\rm TP})\sim (2.4 M_{pl}^2/m_a,0.3 M_{pl})$. For $f_a$ below the triple point BH formation proceeds during winding (in the complex $U(1)$ picture) of the axion field near the dispersal phase. This could prevent astrophysical BH formation from axion stars with $f_a\ll M_{pl}$. For larger $f_a\gtrsim f_{\rm TP}$, BH formation occurs through the stable branch and we estimate the mass ratio of the BH to the stable state at the phase boundary to be $\mathcal{O}(1)$ within numerical uncertainty. We discuss the observational relevance of our findings for axion stars as BH seeds, which are supermassive in the case of ultralight axions. For the QCD axion, the typical BH mass formed from axion star collapse is $M_{\rm BH}\sim 3.4 (f_a/0.6 M_{pl})^{1.2} M_\odot$., Comment: 19 pages, 11 figures, link to movies: http://www.grchombo.org/publications

We consider the effects of inhomogeneous initial conditions in both the scalar field profile and the extrinsic curvature on different inflationary models. In particular, we compare the robustness of small field inflation to that of large field inflation, using numerical simulations with Einstein gravity in 3+1 dimensions. We find that small field inflation can fail in the presence of subdominant gradient energies, suggesting that it is much less robust to inhomogeneities than large field inflation, which withstands dominant gradient energies. However, we also show that small field inflation can be successful even if some regions of spacetime start out in the region of the potential that does not support inflation. In the large field case, we confirm previous results that inflation is robust if the inflaton occupies the inflationary part of the potential. Furthermore, we show that increasing initial scalar gradients will not form sufficiently massive inflation-ending black holes if the initial hypersurface is approximately flat. Finally, we consider the large field case with a varying extrinsic curvature K, such that some regions are initially collapsing. We find that this may again lead to local black holes, but overall the spacetime remains inflationary if the spacetime is open, which confirms previous theoretical studies., Comment: 14 pages, 14 figures, updated for journal version and amended sign convention for chi

We investigate numerically the critical behaviour which occurs in the collapse of both spherically symmetric and asymmetric scalar field bubbles with full general relativity. We use a minimally coupled scalar field subject to a "double well" interaction potential, with the bubble wall spanning the barrier between two degenerate minima. We find that the spherically symmetric case exhibits Type 2 critical behaviour with the critical index consistent with a value of gamma = 0.37 as expected. In the asymmetric case, we find that again our results are consistent with a value of gamma = 0.37 for the dominant unstable mode. We do not see strong evidence of echoing in the solutions, which could be due to the fact that the coordinates are not well adapted to the echoing behaviour, or due to being still too far from the critical point to properly observe the critical solution., Comment: 22 pages, 12 Figures