arXiv subjects

Vitor Cardoso

Publications and source records attributed to Vitor Cardoso.

At least 181 records · Page 10Linked to original sources

arXiv2018

Remarks on the maximum luminosity

The quest for fundamental limitations on physical processes is old and venerable. Here, we investigate the maximum possible power, or luminosity, that any event can produce. We show, via full nonlinear simulations of Einstein's equations, that there exist initial conditions which give rise to arbitrarily large luminosities. However, the requirement that there is no past horizon in the spacetime seems to limit the luminosity to below the Planck value, ${{\cal L}_\textrm{P}\!=\!c^5/G}$. Numerical relativity simulations of critical collapse yield the largest luminosities observed to date, ${\approx \! 0.2 {\cal L}_\textrm{P}}$. We also present an analytic solution to the Einstein equations which seems to give an unboundedly large luminosity; this will guide future numerical efforts to investigate super-Planckian luminosities.

gr-qc↗

arXiv2018

Characterization of echoes: A Dyson-series representation of individual pulses

The ability to detect and scrutinize gravitational waves from the merger and coalescence of compact binaries opens up the possibility to perform tests of fundamental physics. One such test concerns the dark, nature of compact objects: are they really black holes? It was recently pointed out that the absence of horizons -- while keeping the external geometry very close to that of General Relativity -- would manifest itself in a series of echoes in gravitational wave signals. The observation of echoes by LIGO/Virgo or upcoming facilities would likely inform us on quantum gravity effects or unseen types of matter. Detection of such signals is in principle feasible with relatively simple tools, but would benefit enormously from accurate templates. Here we analytically individualize each echo waveform and show that it can be written as a Dyson series, for arbitrary effective potential and boundary conditions. We further apply the formalism to explicitly determine the echoes of a simple toy model: the Dirac delta potential. Our results allow to read off a few known features of echoes and may find application in the modelling for data analysis.

gr-qc↗

arXiv2018

The Penrose process, superradiance and ergoregion instabilities

Superradiant scattering is a radiation enhancement process that takes place in many contexts, and which has recently found exciting applications in astro and particle physics. In the framework of curved spacetime physics, it has been associated with the classical Penrose process for particles. Superradiance is usually also associated with bosonic fields around geometries with ergoregions and horizons. These notions are in clear tension however: the Penrose process occurs for horizonless geometries, and particles are composed of fermions. Here, we resolve the tension in its different aspects, by showing that (i) superradiance occurs for self-interacting fermions on flat spacetime; (ii) superradiance occurs also for horizonless geometries, where it leads to an ergoregion instability. Ultracompact, horizonless geometries will usually respond with echoes of growing amplitude, until rotational (or electrostatic) energy is extracted from the object; (iii) the Fourier-domain analysis leads to absence of superradiance when horizons are not present. We elucidate why this analysis fails to give meaningful results; (iv) finally, we show that superradiant, ergoregion instabilities have a particle analog of similar growth timescales and which can power the formation of a structure outside a compact, rotating star.

gr-qc↗

arXiv2018

Probing Planckian corrections at the horizon scale with LISA binaries

Several quantum-gravity models of compact objects predict microscopic or even Planckian corrections at the horizon scale. We explore the possibility of measuring two model-independent, smoking-gun effects of these corrections in the gravitational waveform of a compact binary, namely the absence of tidal heating and the presence of tidal deformability. For events detectable by the future space-based interferometer LISA, we show that the effect of tidal heating dominates and allows one to constrain putative corrections down to the Planck scale. The measurement of the tidal Love numbers with LISA is more challenging but, in optimistic scenarios, it allows to constrain the compactness of a supermassive exotic compact object down to the Planck scale. Our analysis suggests that highly-spinning, supermassive binaries at 1-20 Gpc provide unparalleled tests of quantum-gravity effects at the horizon scale.

gr-qc↗

arXiv2018

Quasinormal modes and Strong Cosmic Censorship

The fate of Cauchy horizons, such as those found inside charged black holes, is intrinsically connected to the decay of small perturbations exterior to the event horizon. As such, the validity of the strong cosmic censorship (SCC) conjecture is tied to how effectively the exterior damps fluctuations. Here, we study massless scalar fields in the exterior of Reissner--Nordstrom--de Sitter black holes. Their decay rates are governed by quasinormal modes of the black hole. We identify three families of modes in these spacetimes: one directly linked to the photon sphere, well described by standard WKB-type tools; another family whose existence and timescale is closely related to the de Sitter horizon. Finally, a third family which dominates for near-extremally-charged black holes and which is also present in asymptotically flat spacetimes. The last two families of modes seem to have gone unnoticed in the literature. We give a detailed description of linear scalar perturbations of such black holes, and conjecture that SCC is violated in the near extremal regime.

gr-qc↗

arXiv2018

Constraining the mass of dark photons and axion-like particles through black-hole superradiance

Ultralight bosons and axion-like particles appear naturally in different scenarios and could solve some long-standing puzzles. Their detection is challenging, and all direct methods hinge on unknown couplings to the Standard Model of particle physics. However, the universal coupling to gravity provides model-independent signatures for these fields. We explore here the superradiant instability of spinning black holes triggered in the presence of such fields. The instability taps angular momentum from and limits the maximum spin of astrophysical black holes. We compute, for the first time, the spectrum of the most unstable modes of a massive vector (Proca) field for generic black-hole spin and Proca mass. The observed stability of the inner disk of stellar-mass black holes can be used to derive \emph{direct} constraints on the mass of dark photons in the mass range $ 10^{-13}\,{\rm eV}\lesssim m_V \lesssim 3\times 10^{-12}\,{\rm eV}$. By including also higher azimuthal modes, similar constraints apply to axion-like particles in the mass range $6\times10^{-13}\,{\rm eV}\lesssim m_{\rm ALP} \lesssim 10^{-11}\, {\rm eV}$. Likewise, mass and spin distributions of supermassive BHs --~as measured through continuum fitting, K$α$ iron line, or with the future space-based gravitational-wave detector LISA~-- imply indirect bounds in the mass range approximately $10^{-19}\,{\rm eV}\lesssim m_V, m_{\rm ALP} \lesssim 10^{-13}\, {\rm eV}$, for both axion-like particles and dark photons. Overall, superradiance allows to explore a region of approximately $8$ orders of magnitude in the mass of ultralight bosons.

gr-qc↗

arXiv2017

Black-hole head-on collisions in higher dimensions

The collision of black holes and the emission of gravitational radiation in higher-dimensional spacetimes are of interest in various research areas, including the gauge-gravity duality, the TeV gravity scenarios evoked for the explanation of the hierarchy problem, and the large-dimensionality limit of general relativity. We present numerical simulations of head-on collisions of nonspinning, unequal-mass black holes starting from rest in general relativity with $4 \leq D\leq 10$ spacetime dimensions. We compare the energy and linear momentum radiated in gravitational waves with perturbative predictions in the extreme mass ratio limit, demonstrating the strength and limitations of black-hole perturbation theory in this context.

gr-qc↗

arXiv2017

Gravitational Wave Signatures of Highly Compact Boson Star Binaries

Solitonic boson stars are stable objects made of a complex scalar field with a compactness that can reach values comparable to that of neutron stars. A recent study of the collision of identical boson stars produced only non-rotating boson stars or black holes, suggesting that rotating boson stars may not form from binary mergers. Here we extend this study to include an analysis of the gravitational waves radiated during the coalescence of such a binary, which is crucial to distinguish these events from other binaries with LIGO and Virgo observations. Our studies reveal that the remnant's gravitational wave signature is mainly governed by its fundamental frequency as it settles down to a non-rotating boson star, emitting significant gravitational radiation during this post-merger state. We calculate how the waveforms and their post-merger frequencies depend on the compactness of the initial boson stars and estimate analytically the amount of energy radiated after the merger.

gr-qc↗

arXiv2017

General first-order mass ladder operators for Klein-Gordon fields

We study the ladder operator on scalar fields, mapping a solution of the Klein-Gordon equation onto another solution with a different mass, when the operator is at most first order in derivatives. Imposing the commutation relation between the d'Alembertian, we obtain the general condition for the ladder operator, which contains a non-trivial case which was not discussed in the previous work [V. Cardoso, T. Houri and M. Kimura, Phys.Rev.D 96, 024044 (2017), arXiv:1706.07339]. We also discuss the relation with supersymmetric quantum mechanics.

hep-th↗

arXiv2017

Black Hole Spectroscopy: Systematic Errors and Ringdown Energy Estimates

The relaxation of a distorted black hole to its final state provides important tests of general relativity within the reach of current and upcoming gravitational wave facilities. In black hole perturbation theory, this phase consists of a simple linear superposition of exponentially damped sinusoids (the quasinormal modes) and of a power-law tail. How many quasinormal modes are necessary to describe waveforms with a prescribed precision? What error do we incur by only including quasinormal modes, and not tails? What other systematic effects are present in current state-of-the-art numerical waveforms? These issues, which are basic to testing fundamental physics with distorted black holes, have hardly been addressed in the literature. We use numerical relativity waveforms and accurate evolutions within black hole perturbation theory to provide some answers. We show that (i) a determination of the fundamental $l=m=2$ quasinormal mode to within $1\%$ or better requires the inclusion of at least the first overtone, and preferably of the first two or three overtones; (ii) a determination of the black hole mass and spin with precision better than $1\%$ requires the inclusion of at least two quasinormal modes for any given angular harmonic mode $(\ell,\,m)$. We also improve on previous estimates and fits for the ringdown energy radiated in the various multipoles. These results are important to quantify theoretical (as opposed to instrumental) limits in parameter estimation accuracy and tests of general relativity allowed by ringdown measurements with high signal-to-noise ratio gravitational wave detectors.

gr-qc↗

arXiv2017

Gravitational wave searches for ultralight bosons with LIGO and LISA

Ultralight bosons can induce superradiant instabilities in spinning black holes, tapping their rotational energy to trigger the growth of a bosonic condensate. Possible observational imprints of these boson clouds include (i) direct detection of the nearly monochromatic (resolvable or stochastic) gravitational waves emitted by the condensate, and (ii) statistically significant evidence for the formation of "holes" at large spins in the spin versus mass plane (sometimes also referred to as "Regge plane") of astrophysical black holes. In this work, we focus on the prospects of LISA and LIGO detecting or constraining scalars with mass in the range $m_s\in [10^{-19},\,10^{-15}]$ eV and $m_s\in [10^{-14},\,10^{-11}]$ eV, respectively. Using astrophysical models of black-hole populations calibrated to observations and black-hole perturbation theory calculations of the gravitational emission, we find that, in optimistic scenarios, LIGO could observe a stochastic background of gravitational radiation in the range $m_s\in [2\times 10^{-13}, 10^{-12}]$ eV, and up to $10^4$ resolvable events in a $4$-year search if $m_s\sim 3\times 10^{-13}\,{\rm eV}$. LISA could observe a stochastic background for boson masses in the range $m_s\in [5\times 10^{-19}, 5\times 10^{-16}]$, and up to $\sim 10^3$ resolvable events in a $4$-year search if $m_s\sim 10^{-17}\,{\rm eV}$. LISA could further measure spins for black-hole binaries with component masses in the range $[10^3, 10^7]~M_\odot$, which is not probed by traditional spin-measurement techniques. A statistical analysis of the spin distribution of these binaries could either rule out scalar fields in the mass range $\sim [4 \times 10^{-18}, 10^{-14}]$ eV, or measure $m_s$ with ten percent accuracy if light scalars in the mass range $\sim [10^{-17}, 10^{-13}]$ eV exist.

gr-qc↗

arXiv2017

Stochastic and resolvable gravitational waves from ultralight bosons

Ultralight scalar fields around spinning black holes can trigger superradiant instabilities, forming a long-lived bosonic condensate outside the horizon. We use numerical solutions of the perturbed field equations and astrophysical models of massive and stellar-mass black hole populations to compute, for the first time, the stochastic gravitational-wave background from these sources. In optimistic scenarios the background is observable by Advanced LIGO and LISA for field masses $m_s$ in the range $\sim [2\times 10^{-13}, 10^{-12}]\,{\rm eV}$ and $\sim 5\times[ 10^{-19}, 10^{-16}]\,{\rm eV}$, respectively, and it can affect the detectability of resolvable sources. Our estimates suggest that an analysis of the stochastic background limits from LIGO O1 might already be used to marginally exclude axions with mass $\sim 10^{-12.5}{\rm eV}$. Semicoherent searches with Advanced LIGO (LISA) should detect $\sim 15~(5)$ to $200~(40)$ resolvable sources for scalar field masses $3\times 10^{-13}$ ($10^{-17}$) eV. LISA measurements of massive BH spins could either rule out bosons in the range $\sim [10^{-18}, 2\times 10^{-13}]$ eV, or measure $m_s$ with ten percent accuracy in the range $\sim[10^{-17}, 10^{-13}]$ eV.

gr-qc↗

arXiv2017

Orbital fingerprints of ultralight scalar fields around black holes

Ultralight scalars have been predicted in a variety of scenarios, and advocated as a possible component of dark matter. These fields can form compact regular structures known as boson stars, or---in the presence of horizons---give rise to nontrivial time-dependent scalar hair and a stationary geometry. Because these fields can be coherent over large spatial extents, their interaction with "regular" matter can lead to very peculiar effects, most notably resonances. Here we study the motion of stars in a background describing black holes surrounded by non-axially symmetric scalar field profiles. By analyzing the system in a weak-field approach, we find that the presence of a scalar field gives rise to secular effects akin to ones existing in planetary and accretion disks. Particularly, the existence of resonances between the orbiting stars and the scalar field may enable angular momentum exchange between them, providing mechanisms similar to planetary migration. Additionally, these mechanisms may allow \textit{floating orbits}, which are stable radiating orbits. We also show, in the full relativistic case, that these effects also appear when there is a direct coupling between the scalar field and the stellar matter, which can arise due to the presence of a scalar core in the star or in alternative theories of gravity.

gr-qc↗

arXiv2017

Self-gravitating oscillons and new critical behavior

The dynamical evolution of self-interacting scalars is of paramount importance in cosmological settings, and can teach us about the content of Einstein's equations. In flat space, nonlinear scalar field theories can give rise to localized, non-singular, time-dependent, long-lived solutions called {\it oscillons}. Here, we discuss the effects of gravity on the properties and formation of these structures, described by a scalar field with a double well potential. We show that oscillons continue to exist even when gravity is turned on, and we conjecture that there exists a sequence of critical solutions with infinite lifetime. Our results suggest that a new type of critical behavior appears in this theory, characterized by modulations of the lifetime of the oscillon around the scaling law and the modulations of the amplitude of the critical solutions.

gr-qc↗

arXiv2017

Mass Ladder Operators from Spacetime Conformal Symmetry

Ladder operators can be useful constructs, allowing for unique insight and intuition. In fact, they have played a special role in the development of quantum mechanics and field theory. Here, we introduce a novel type of ladder operators, which map a scalar field onto another massive scalar field. We construct such operators, in arbitrary dimensions, from closed conformal Killing vector fields, eigenvectors of the Ricci tensor. As an example, we explicitly construct these objects in anti-de Sitter spacetime (AdS) and show that they exist for masses above the Breitenlohner-Freedman (BF) bound. Starting from a regular seed solution of the massive Klein-Gordon equation (KGE), mass ladder operators in AdS allow one to build a variety of regular solutions with varying boundary condition at spatial infinity. We also discuss mass ladder operator in the context of spherical harmonics, and the relation between supersymmetric quantum mechanics and so-called Aretakis constants in an extremal black hole.

hep-th↗

arXiv2017

Theia: Faint objects in motion or the new astrometry frontier

In the context of the ESA M5 (medium mission) call we proposed a new satellite mission, Theia, based on relative astrometry and extreme precision to study the motion of very faint objects in the Universe. Theia is primarily designed to study the local dark matter properties, the existence of Earth-like exoplanets in our nearest star systems and the physics of compact objects. Furthermore, about 15 $\%$ of the mission time was dedicated to an open observatory for the wider community to propose complementary science cases. With its unique metrology system and "point and stare" strategy, Theia's precision would have reached the sub micro-arcsecond level. This is about 1000 times better than ESA/Gaia's accuracy for the brightest objects and represents a factor 10-30 improvement for the faintest stars (depending on the exact observational program). In the version submitted to ESA, we proposed an optical (350-1000nm) on-axis TMA telescope. Due to ESA Technology readiness level, the camera's focal plane would have been made of CCD detectors but we anticipated an upgrade with CMOS detectors. Photometric measurements would have been performed during slew time and stabilisation phases needed for reaching the required astrometric precision.

astro-ph.IM↗

arXiv2017

Scattering of point particles by black holes: gravitational radiation

Gravitational waves can teach us not only about sources and the environment where they were generated, but also about the gravitational interaction itself. Here we study the features of gravitational radiation produced during the scattering of a point-like mass by a black hole. Our results are exact (to numerical error) at any order in a velocity expansion, and are compared against various approximations. At large impact parameter and relatively small velocities our results agree to within percent level with various post-Newtonian and weak-field results. Further, we find good agreement with scaling predictions in the weak-field/high-energy regime. Lastly, we achieve striking agreement with zero-frequency estimates.

gr-qc↗

arXiv2017

Superradiance in rotating stars and pulsar-timing constraints on dark photons

In the presence of massive bosonic degrees of freedom, rotational superradiance can trigger an instability that spins down black holes. This leads to peculiar gravitational-wave signatures and distribution in the spin-mass plane, which in turn can impose stringent constraints on ultralight fields. Here, we demonstrate that there is an analogous spindown effect for conducting stars. We show that rotating stars amplify low frequency electromagnetic waves, and that this effect is largest when the time scale for conduction within the star is of the order of a light crossing time. This has interesting consequences for dark photons, as massive dark photons would cause stars to spin down due to superradiant instabilities. The time scale of the spindown depends on the mass of the dark photon, and on the rotation rate, compactness, and conductivity of the star. Existing measurements of the spindown rate of pulsars place direct constraints on models of dark sectors. Our analysis suggests that dark photons of mass $m_V \sim 10^{-12}$ eV are excluded by pulsar-timing observations. These constraints also exclude superradiant instabilities triggered by dark photons as an explanation for the spin limit of observed pulsars.

gr-qc↗