The first EPS (European Physical Society) Conference on Gravitation to be held at the Sapienza University (Rome, Italy) from February 19th to February 21th, 2019. The aim of the conference is to discuss aspects of Classical and Quantum Gravity, including General Relativity tests, measurements of the G constant, Geodesy, and obviously Gravitational Waves from the experimental, theoretical and data analysis point of views. Details about registration will be communicated in the next announcement. The conference is organised in days focused around key topics introduced by invited speakers and followed by contributed talks. There will also be a poster session, together with four Young Scientist Awards to the best poster contributions by skilled young researchers . The scientific program will be finalised in the upcoming days. The Local and Scientific Organising Committees are looking forward to welcoming you in Rome.
Please note that the conference desk will be open on Monday 18th, from 17:00 to 19:00 at the building Marconi of the Department of Physics of the Sapienza University.
Then, it will open again the morning of 19th at 8:30.
In this talk I will describe progresses in considering GR effects in the dynamics of structure formation. First I will present results of a nonlinear post-Friedman approximation, a kind of post-Newtonian formalism. Then I will focus on recent fully nonlinear numerical relativity simulations. Numerical relativity is a fundamental tool in the modelling of gravitational waves sources, but its application to cosmology is in its infancy. As more interdisciplinary work between the gravitational waves and the cosmology communities will develop, in the next few years numerical relativity may become a fundamental tool for understanding the extent to which we can trust standard newtonian N-body simulations on the largest scales. First results of simulations representing the full GR nonlinear evolution of initial perturbations in a Einstein de Sitter background are: 1) back-reaction effects on the overall expansion of the model are very small; 2) voids expansion rate is significantly higher than that of the background; 3) over-densities can reach turn-around much earlier than predicted by the standard top-hat model. To establish the significance of these results is the goal of future work.
I will talk about the GW signatures from the collisions of Oscillatons -- which are long-lived non-topological solutions of massive scalar fields. I will describe the technical challenges and present the GW wave forms of such collisions calculated using the numerical relativity code GRCHOMBO. I will show that for black hole end states, the total gravitational waves energy released in such collisions is more than that of BH-BH collisions of equal masses by a significant amount.
The upcoming sub-millimetre VLBI images of Sgr A* by the EHT Collaboration are expected to provide critical evidence for the existence of this supermassive black hole. In the near future, strong field images could be used to test both deviations from General Relativity as well as different models for the compact object at the Galactic Centre. In this work we assess our present and future ability to determine whether such images correspond to a Kerr black hole, a dilaton black hole from an alternative theory of gravity or a boson star. We perform GRMHD simulations and use GRRT calculations to generate synthetic images of the magnetised accretion flow onto each of these objects. We provide synthetic images based on the 2017 EHT and future VLBI observations and use image metrics to quantify our ability to distinguish between these objects.
I will review the status of TEOBResumS, that is a state of the art effective-one-body based waveform model for spin-aligned coalescing compact binaries, e.g. black hole binaries and neutron star binaries. I will focus on the impact of the numerical relativity information needed to complete the analytical model and focus on the performance of the model for gravitational wave data-analytsis purposes.
An extended accelerating body under ‘rigid motion’ by definition manifests unvarying separation between its constituents in all comoving inertial frames. Relationships be- tween constituents’ necessarily di↵ering yet constant accelerations—reflecting a nonuni- form, dynamically changing and moreover repulsive gravitational field—have been es- tablished by Woodhouse in 2003 using Minkowski spacetime, by Franklin in 2010 using Lorentz transformations, and by the present author in 2013 using unfamiliar yet simple inter-rocket radar period equations. A second ‘pseudo-rigor mortis’ attractive gravita- tional field scenario introduced in 2018 is now further considered. In both cases, radar trajectories are shown to exhibit unchanging inverse square root of two geodesic curvature on a corresponding real-metric spacetime surface of ubiquitously zero Gauss curvature.
Keywords: spacetime metric; own-surface; hemix; rigor mortis motion; radar paths; geodesics; Gauss curvature; gravitational fields.
Quantum mechanics is at the basis of our understanding of all matter and - since with the behavior of matter we explore space and time - also of our understanding of space-time. In the recent years, also quantum technologies became more and more important for practical purposes. This includes quantum sensors, quantum metrology, quantum information, quantum cryptography, quantum computing, etc. Of particular importance is the coupling of quantum matter to gravity. In this talk we collect the foundations of quantum mechanics, the foundations of relativistic gravity, and corresponding tests, in particular tests exploring the quantum-gravity interaction. Also the relevance of this research for practical purposes is described.
Archimedes is an experiment conceived to shed light on one of the most intriguing topics of the modern physics: the interaction between the gravitational field and the vacuum fluctuations. The experiment will measure the force exerted by the gravitational field on a Casimir cavity, whose vacuum energy is modulated with a superconductive transition, by using a balance as a small force detector. Archimedes is an INFN six-year project that will be installed in the SARGRAV laboratory placed in an old mine located the Sardinia italian region. This site is characterized by a very low seismic noise so it is the ideal environment for null force experiments and for third-generation gravitational waves interferometers like ET.
In a laboratory gravitational experiment one of the most critical elements is represented by the source of the gravitational field. For state-of-art measurements, a precise characterization of the field and the possibility to modulate the amplitude are needed. Usually one or more moving masses are used. We describe a new actuation technique for gravity experiments based on a liquid Field Mass. The basic idea is to modulate the gravity force acting on a test mass by controlling the level of a liquid in a suitable container. This allows us to produce a periodically varying gravity force without moving parts (apart the liquid level) close to the Test Mass. Italian INFN (National Institute of Nuclear Physics) has recently funded a R&D experiment, named LAG (Liquid Actuated Gravity) to test principle of operation and performance of the liquid gravity actuator. We will describe in detail the most relevant aspects of the experiment and discuss how it can be used in gravity measurements. In particular we analyse a proposed application for improving test of the inverse square law in the mm to cm region.
The Laser Ranged Satellites Experiment (LARASE) aims to test the gravitational interaction in the weak-field and slow-motion limit and compare, consequently, the predictions of Einstein’s theory of general relativity (GR) with those of other alternative theories of gravitation. In particular, a goal of LARASE is to improve the modelling of the non-gravitational perturbations (NGP) on the LAGEOS, LAGEOS II and LARES satellites in such a way to further improve their precise orbit determination in order to better extract, from their orbital residuals, the expected tiny relativistic effects. Indeed, the motion of these passive laser-ranged satellites along nearly geodesics of spacetime may be a posteriori reconstructed through a careful modelling of the main NGP that act on their surface and, in more general terms, of their overall dynamical models. We will focus upon two recent LARASE results: the development of a new model for the spin evolution of the satellites and of one to account for the very subtle effects on their orbits that are produced by the thermal thrust perturbations. Concerning the gravitational perturbations due to the deviation of the Earth's mass distribution from that of a perfect sphere, we will discuss our improvements in the modelling of the Earth's even zonal harmonics coefficients based on GRACE data, specifically in their time-dependency. Finally, we will show our new results for a refined measurement of the Lense-Thirring precession on the combined orbits of the LAGEOS, LAGEOS II and LARES satellites. This relativistic precession arises from the gravitomagnetic field of the Earth produced by its angular momentum. Gravitomagnetism describes, in Einstein’s GR, the curvature of spacetime produced by mass-currents, with important consequences in the astrophysics of high-energy phenomena as well as possible cosmological consequences related to Mach’s Principle.
The acceleration of free falling atoms as measured by light-pulse atom interferometry (AI) equals the exact value experienced by the atoms only if the acceleration is constant. Since the gravitational field is non-uniform a variable term is always present whose contribution is measured only approximately, the experimental value being systematically smaller than the theoretical one. The more the acceleration deviates from uniformity, the less good is the approximate value measured by the instrument. This systematic error limits the absolute measurement of the local gravitational acceleration $g$ as well as measurement of the universal constant of gravity $G$ or attempts to detect gravitational waves by means of gravity gradiometers based on AI. Testing the Universality of Free Fall (or the Weak Equivalence Principle) with atom interferometry requires a dual AI, whereby the free fall accelerations of different atoms in the field of the Earth are measured and subtracted, searching for tiny non-zero differences which would indicate a violation. Unless the two species are manipulated with the same laser, thus ensuring that the time interval between subsequent laser pulses is the same for both species, the systematic error reported here makes the test utterly impossible on ground, and sets severe limitations in orbit. With the same laser the systematic error cancels out, and less stringent limitations become dominant. However, such a requirement makes the choice of different atom species that can be subjected to testing very limited. In practice, most tests use $^{87}\rm Rb$ and $^{85}\rm Rb$, differing by two neutrons only.
Sagnac Gyroscopes attached to the Earth surface are sensitive to gravito-magnetism, and in principle the confrontation with independent measurements of the Earth rotation rate gives the possibility to measure the Lense Thirring effect without the necessity to map the gravitational field, independently from the space measurements. This is feasible or not depending on how ideal is the Sagnac gyroscopes we have at our disposal. Moreover the same kind of apparatus built on small scale are in principle ideal for the many kind of applications necessary to develop inertial platforms. Ring laser gyros are at present the most sensitive Sagnac gyros, the data of the prototype GINGERINO are used to discuss the feasibility of large scale apparatus and short size apparatus.
It has been realized that nonlocality might be a key ingredient for the formulation of a quantum renormalizable theory of gravitation. In facts, nonlocal gravitational models are earning growing interest in the scientific community, since they are super-renormalizable or even finite at quantum level. In this seminar I will introduce nonlocal field theories and discuss their general features. In particular, I will discuss power counting renormalization and finiteness. Moreover, I will show how Cutkosky rules are generalized to the case of nonlocal theories, so that the perturbative unitarity is easily established. Finally, I will discuss the problem of causality in nonlocal theories.
Gravitational-wave astronomy can give us access to structure of black holes, potentially probing microscopic corrections at the horizon scale. Some quantum-gravity models of exotic compact objects replace the event horizon by a reflective surface. Spinning horizonless compact objects with these properties may be unstable against an ergoregion instability.
In this talk we investigate a model consisting of a Kerr geometry with a reflective surface near the horizon and we analyse its instability under scalar, electromagnetic and gravitational perturbations. We derive analytically the quasi-normal mode frequencies and the instability time scale of unstable modes in the black-hole limit. We show that the instability for electromagnetic and gravitational perturbations is generically stronger than in the scalar case and it requires larger absorption at the surface to be quenched. This result has important consequences for the viability of exotic compact objects as alternatives to black holes.
Effective field theory methods suggest that some rather-general extensions of General Relativity include higher-order curvature corrections, with small coupling constants. In this talk, we discuss black hole solutions in such a framework. First, we construct spherically symmetric black hole solutions and study gravitational perturbation around them. Despite the higher-order operators of the theory, we show that linearized field equations obey second-order differential equations. We also study slowly rotating solutions around spherically symmetric black hole solutions and show that the spacetimes do not have $Z_2$ symmetry due to the parity violating term.
Corpuscular gravity has originated from the observation that a black hole can be viewed as a Bose-Einstein condensate at the critical point, with a large occupation number of (soft off-shell) gravitons and no central singularity. This innovative approach moves away from the semi-classical picture of quantum field theory on curved backgrounds and considers self-gravitating systems as truly quantum. We shall introduce the idea that the gravitational state of the whole Universe can be described as a Cosmological Dark Energy Condensate behaving like a quasi-de Sitter Universe and then discuss the crucial role of regular baryonic matter and the way it interacts with the cosmological condensate.
Despite the good agreement between theoretical predictions and observational results, the cosmological constant is not a satisfactory explanation for the accelerated expansion of the universe. Hence an intense theoretical effort has been devoted to the study of models beyond General Relativity plus a cosmological constant.
In this talk, I will present ongoing work on the study of the properties of the dark sector to describe Horndeski models in terms of a non-trivial fluid within the Equation of State formalism. I will apply the framework to derive approximate expressions which give a simple physical intuition of the problem at hand and to understand theoretically modified gravity parameters and effects on observables. This will be linked to results from other perturbative approaches used in the literature.
In the new era of gravitational-wave astronomy, one of the most exciting targets for future observations is the stochastic gravitational-wave background (SGWB). While we have yet to detect the SGWB, we expect that by studying the angular power spectrum of its anisotropies, we may learn about the large-scale structure of the Universe (analogous to studies of the CMB). With this in mind, we develop detailed models of the SGWB anisotropies from two important sources of gravitational waves: unresolved compact binary coalescences, and cosmic strings.
Gravitational Waves discovery has recently be announced by the LIGO and the Virgo collaborations.
Due to their weak amplitude, Gravitational Waves are expected to produce a very small effect on free- falling masses, which undergo a displacement of the order of 10^(-18) m for a km-scale mutual distance. This discovery showed that interferometric detectors are suitable to reveal such a feeble effect, and therefore represent a new tool for astronomy, astrophysics and cosmology in the understanding of the Universe.
To better reconstruct the position of the Gravitational Wave source and increase the signal-to-noise ratio of the events by means of multiple coincidence, a network of detectors is
necessary. In the USA, a couple of twin 4 kilometer-long arms detectors, which are placed in Washington State and Louisiana, constitute the LIGO project. Advanced Virgo (AdV) is a 3 kilometer-long arms second generation interferometer situated in Cascina, near Pisa in Italy.
On August 1st 2017, Virgo has joined LIGO in the second observation run (O2) for about one month, during which Virgo has detected for the first time a Gravitational Wave produced by binary black holes and binary neutron stars systems. In view of the next science run which will start in Spring
2019, hardware upgrades have been performed, such as monolithic suspension installation and injection of squeezed light, which have improved the detector performances in the mid-low and high frequency range, respectively. Furthermore, thanks to unceasing noise hunting and commissioning
activities addressed towards the noise reduction and fine tuning of the interferometer working point, a broad improvement of the detector sensitivity over the whole frequency range was achieved.
Thanks to all of these actions, Advanced Virgo will be able to join the third observation run with a sensitivity two times
higher than it was during O2.
Continuous waves (CW) are still undetected gravitational wave signals emitted by rotating neutron stars, isolated or in binary systems. The estimated number of isolated neutron stars in our Galaxy is 10^8-10^9. Information provided by electromagnetic observations is crucial to constrain the signal parameter space, lower the computational cost of a CW search and increase the number of potential targets. Accordingly to the information available about the source, different searches can be set up.
In this work we present prospects for the directed search of CW signals in advanced LIGO-Virgo data using the BSD-directed search method. A list of potentially interesting sources, which are present in the main astronomical catalogues, along with some young supernova remnants, is investigated and theoretical indirect upper limits are computed when possible. Estimate of the computational power needed to perform a directed search for the selected sources is also provided.
With the detection of GW170817 we have observed the first multi messenger signal from two merging neutron stars.
This signal carried a multitude of information about the underlying equation of state(EOS) of nuclear matter, which so far is not known for densities above nuclear saturation.
In particular it is not known if exotic states or even a phase transition to quark matter can occur at densities so extreme that they can't be probed by any current experiment.
I will show how the information carried in the gravitational wave signal of GW170817 can be used to constrain the EOS at densities above saturation and what we can learn about the possible existence of phase transitions.
I will also comment on how we can use future gravitational wave detections in order to set limits on the existence of neutron stars having a quark matter core.
Finally, I will discuss the detectability of a quark-hadron phase transition taking place in a neutron star merger event.
Precision tests of the strong gravity regime through direct observations of gravitational wave events, will provide new crucial insights on the nature of gravity. In this regard, a long lasting questions that has survived one century of investigation, is wether the graviton is massive or not.
In this talk we present new results obtained by studying gravitational perturbations of non-spinning black holes, when the underlying theory of gravity features gravitons with a non-vanishing mass term. We provide a detailed study of the gravitational signals produced when a small particle plunges or inspirals into a large black hole. Our results should also describe the gravitational collapse to black holes and explosive events such as supernovae. We show how merging objects up to 1Gpc away or collapsing stars in the nearby galaxy can be used to constrain the mass of the graviton to be smaller than ∼ 10^(−23) eV, with low-frequency detectors. We also present a detailed investigation of new modes, that suggest how the absence of dipolar gravitational waves from black hole binaries may be used to rule out entirely such theories. These results are particularly relevant for next generation of space interferometers like LISA, which has extreme-mass-ratio mergers among its primary targets.
Now that gravitational waves from compact binary coalescences have been successfully detected by the global LIGO-Virgo network, a key challenge is to improve the detector sensitivity in order to detect more transient sources — weaker or located further away. The detectors' sensitivity can be enhanced by increasing the laser power travelling within the arm cavities, for it reduces the effect of the laser quantum phase noise, which is the fundamental noise that dominates the sensitivity in the high-frequency range (above a few hundreds of Hz). However, a nonlinear optomechanical phenomenon that has long been studied, and which is called parametric instabilities (PI), may limit the amount of energy stored in the Fabry-Perot resonator, and thus the laser power.
PI comes from the coupling of three modes: a mirror mechanical mode (MM) that sets the mirror surface in motion, the fundamental optical mode of an optical cavity (TEM00), and a higher order optical mode (HOM). Photons scattering from the TEM00 to a HOM can generate an optical beat note if the difference in frequencies of the two optical modes is equal to the MM resonance frequency. This beat note, in turn, can either damp or increase the mechanical motion via radiation pressure. The latter effect could lead to an excitation, that is, first exponentially growing, and then reaches a plateau after some time. The signal associated with this mirror excitation would be aliasing in the detection band, thus saturating the electronics.
In 2015, during the Observing Run 1 (O1), LIGO observed PIs when a mirror mechanical mode at 15 kHz became unstable, for an intracavity power of 50 kW. That is why we study the effects of PIs for the Virgo configuration with various parameters in order to scan PIs around theoretical and computed values so that we could take hypothetical errors into account. We, as well, compare results using perfect spherical maps and measured mirror maps. Alongside with which we study the effects of optical losses that can counterintuitively increase the parametric gain. Finally, we show that the O3 nominal intracavity power of 272 kW, could bring from zero to a few tens of unstable modes, depending on the radii of curvatures of the mirrors, if all the mechanical modes quality factors are assumed to be equal to 10^7.
I will present few initial steps towards a new general relativistic magneto hydrodynamic (GRMHD) code devoted to the study of compact binary mergers with finite temperature equations of state and neutrino emission. Numerical modeling of neutron stars binaries (NS-NS), black holes binaries (BH-BH) and neutron star black hole binaries (NS-BH) has now become one of the most important fields of study in theoretical astrophysics, because it allows to extract physical information from gravitational wave (GW) and electromagnetic signals by comparing simulated data with observations. Focusing on the NS-NS and NS-BH cases in particular, it has been shown many times that only a fully general relativistic treatment taking also into account magnetic fields may give a complete picture of this scenario and this requires to solve the equations of GRMHD. From a numerical point of view, one of the most demanding conditions imposed by the system of equations to be solved is the so-called divergence-free condition for the magnetic field. In my work I chose to consider the magnetic field coming out from a vector potential, in order to let the aforementioned conditions be automatically satisfied. In addition, I also consider a general treatment for the NS Equation Of State (EOS) allowing for the use of finite temperature tabulated EOS. This new code will also implement neutrino cooling in order to provide a more accurate study of the post merger phase.
One fruitful approach to quantum gravity has been the framework of Causal Dynamical Triangulations (CDT). This path integral approach describes space-time in a nonperturbative and background-independent manner. The Planckian regime is characterized by wild and highly non-classical geometries and general relativity is recovered as an effective theory in the IR limit.
An important question is how to describe operators that are meaningful at the Planck length and flow towards classical equivalents in the IR limit. In this talk I will discuss the construction of "Quantum Ricci Curvature". This novel definition of a quantum implementation of the classical Ricci tensor relies on the average distance between spheres. It contains a natural scale in terms of the radius of these spheres and is applicable in a large variety of situations. I will show how quantum Ricci curvature is constructed and measured and if time permits I will discuss the results obtained for dynamical triangulations. This is based on arXiv 1712.08847 and 1802.10524
Constraining the equation of state (EoS) of cold dense matter in neutron stars (NS) is a major science goal for observing programmes conducted with X-ray,
radio, and gravitational wave telescopes. In this talk we demonstrate how to combine gravitational-wave and electromagnetic observations in order to solve
the relativistic inverse stellar problem, i.e. to reconstruct the main features of the EoS. Using a full Bayesian analysis, we apply this approach to the
data obtained by the LIGO/Virgo collaboration for the binary neutron star detection GW170817, setting new constraints on the parameters that characterise
the behaviour of matter at supra-nuclear densities.
The second LIGO-Virgo observational witnessed the birth of gravitational-wave multi-messenger astronomy. The first ever gravitational-wave (GW) detection from the coalescence of two neutron stars, GW170817, associated to its gamma-ray counterpart, GRB 170817A, as well as its optical, X-ray and radio counterparts (AT 2017gfo).
In this talk, we will describe the O2 low-latency program of the LIGO/Virgo collaboration, enabling multi-messenger discovery. We will focus on the online candidate alerts shared with observing partners during O2. First, we will describe the distribution of gravitational-wave alerts : we will highlight the validation process, especially from the detector characterization effort to separate transient noise from real GW signal. We will also show the gravitational-wave observables which were sent in the alerts to enable searches for their counterparts.
The next observation campaign O3 (starting coming Spring) promises multiple GW detections: the expected rate of binary black hole (BBH) triggers will be around one every few days whereas binary neutron stars rate is estimated to be one every two weeks at most. The LIGO-Virgo Collaboration (LVC) has developed a robust alert system for the open public alerts (OPA) era in order to enable a prompt electromagnetic follow-up. This requires some automations as: selection of the best GW trigger candidate among the different online searches and the validation process to reject transients originated from the noise. This talk will present as a second part an overview of the O3 LIGO-Virgo low-latency multi-messenger program.
Recently, two simple criteria were proposed to assess if vacua emerging from an effective scalar field theory are part of the string "landscape" or "swampland". The former are the vacua that emerge from string compactifications; the latter are not obtained by any such compactification and hence may not survive in a UV completed theory of gravity. So far, these criteria have been applied to inflationary and dark energy models. Here we consider them in the context of solitonic compact objects made up of scalar fields: boson stars. Analysing several models (static, rotating, with and without self-interactions), we find that, in this context, the criteria are not independent. Furthermore, we find the universal behaviour that in the region wherein the boson stars are expected to be perturbatively stable, the compact objects may be part of the landscape. By contrast, in the region where they may be faithful black hole mimickers, in the sense they possess a light ring, the criteria fail (are obeyed) for static (rotating) ultracompact boson stars, which should thus be part of the swampland (landscape). We also consider hairy black holes interpolating between these boson stars and the Kerr solution and establish the part of the domain of existence where the swampland criteria are violated. In interpreting these results one should bear in mind, however, that the swampland criteria are not quantitatively strict.
The most accurate semi-analytical models for gravitational wave models for compact binaries are based on the effective-one-body (EOB) approach and calibrated to numerical relativity (NR) simulations. These models are however too slow to be used in parameter estimation runs, when $\sim 10^7$ waveforms have to be generated. This has prompted the construction of surrogate models, based on the reduced-order modeling (ROM) technique, in order to shorten the waveform evaluation time.
The post-adiabatic approximation is used in state of the art EOB-NR models to generate initial conditions with low eccentricity. In arxiv:1805.03891, this approximation has been extended and used to determine the complete dynamics of the binary system. This avoids the numerical solution of two of the four Hamilton's equations (for non-precessing systems), which constitutes the main contribution to the waveform computation cost.
In practice, one can analytically compute the momenta of the system at any given radius, under the approximation that the gravitational wave flux is small. The time and orbital phase are then recovered by means of numerical quadratures on a very sparse grid. In the regime where this approximation is no longer valid (tipically the last few orbits), it can be used to generate the initial conditions of the ordinary dynamics as done usually.
Using this approach, it is possible to evaluate a binary neutron star waveform from 10 Hz in 50 milliseconds, with respect to the $\sim 1$ second that we obtain numerically solving the four ordinary differential equations (these times refer to the TEOBResumS model). Even when this time-domain waveform is interpolated (using the standard GLS interpolator) on a uniform-in-time grid, the evaluation times become 0.37 and 1.7 seconds respectively.
The use of the post-adiabatic approximation hence drastically reduces the computational cost of EOB-NR models while retaining their full flexibility. This makes it possible to directly use the best available semi-analytical models in parameter estimation runs and, above all, tests of general relativity.
This approach should be adaptable to the case of eccentric binaries and would be of certain use for extreme mass ratio inspirals that will be detected by LISA.
Wave-function collapse following a measurement process is a longstanding controversial issue of quantum physics. It introduces an element of strong non-linearity and irreversibility in an otherwise unitary and reversible dynamics. Several proposals of modification of Quantum Mechanics have been put forward in the past few decades in order to solve such a dichotomy. Among them, some approaches considered the possible role of gravity in the wave-function collapse as a result of the incompatibility of general relativity and unitary time evolution of Quantum Mechanics. In this contribution we present a nonunitary model of Newtonian Gravity (NNG) [1-3], which shows several appealing features: while reproducing at a macroscopic level the ordinary Newtonian interaction, it presents a mass threshold for gravitational localization. In particular, it provides a mechanism for the evolution of macroscopic coherent superpositions of states into ensembles of pure states.
In particular, we explicitly show how a one-parameter generalization of our NNG model is free from any causality-violation problem for any finite value of parameter [4]. The basic idea is to look at the single particle Newton-Schroedinger equation as the mean-field approximation of an equation of N identical copies of the particle, interacting via usual gravitational interaction, when N goes to infinity. The general N-copy model is a fully consistent quantum theory, in which superluminal communications are automatically avoided. This feature is shown to be a consequence of the intrinsic mechanism of spontaneous state reduction, built in in our NNG model and completely suppressed in the Newton-Schroedinger limit. We discuss in detail a specific (ideal) EPR-like experiment involving the superposition of two distinct Center of Mass position states of a massive body and show that the absence of causality violations leads to the appearance of unusual communications among Everett branches of the wave function. Our results agree with previous findings by Polchinski [5], obtained for a general class of nonlinear models characterized by nonlinear observables which depend only on the density matrix.
References
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[3] G. Scelza, F. Maimone, A. Naddeo, J. Phys. Comm. 2, 015014 (2018).
[4] F. Maimone, G. Scelza, A. Naddeo, arXiv:1809.08653.
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In this paper, our prime objective is to investigate the thermal nature of the de- Sitter space generated due to the entanglement between a pair of Unruh-De-Witt detectors in the paradigm of open quantum systems. The Master-equation of pair of a two-level atomic system within a framework of weakly interacting limit in the de-Sitter space is solved. One of the most important phenomena occurring due to the vacuum-fluctuations of a conformally coupled scalar field is the Casimir effect is studied in the framework of open quantum systems with a weakly-interacting environment degrees of freedom. Such vacuum fluctuations enhance the entanglement between the two atoms thereby resulting in the thermalization of the space-time and encodes the Unruh Effect into it. We study how the Resonance Casimir- Polder interaction between the two atoms manifests the curvature of space-time. Another main objective of this paper is to investigate the asymptotic entanglement between the two atoms giving rise to an ensemble of thermal states encoded within the vacuum state of the scalar field. These thermal phenomena is obtained by the Gibbons-Hawking temperature is widely studied with respect to various toy model Hamiltonian for an entangled pair of atoms weakly conformally coupled to a scalar field.
We show that compressible, Riemann-S type ellipsoids can emit gravitational waves (GWs) with a chirp-like structure (chirping ellipsoids, CELs). The potential detection of these type of sources with eLISA and other future space-based GW observatories can reveal previously undetected astrophysical processes in system of compact objects (e.g. the postmerger object of white dwarf binary mergers). We study the intrinsic phase-time evolution and the fourier transform, in order to demonstrate that the waveform of CELs (mass $\sim1$~M$_\odot$, radius $\sim10^3$~km, polytropic equation of state with index $n\approx 3$) is almost indistinguishable from that emitted by extreme mass-ratio inspirals (EMRIs) composed of an intermediate-mass (e.g.~$10^3~M_\odot$) black hole and a planet-like (e.g.~$10^{-4}~M_\odot$) companion. In addition, the miss-match between the two waveforms is computed and we show that for one year of observation, they are almost indistinguishable. The detection of these EMRIs is relevant for the understanding of planetary formation and dynamics in crowded stellar systems. From reasonable astrophysical assumptions, the rate in the local Universe of CEL and EMRIs in the mass range considered here, are very similar.
Kaluza-Klein [1,2] mechanism was rejected for inconsistencies due to the Planck length scale.
Nelson [3] derivation of Quantum Mechanics was rejected for having assumed, without justification, a periodic multivalued action and the presence of universal fluctuations. Both the mechanisms can be reinterpreted without problems at the light of the guiding center (for Kaluza-Klein) and gyrocenter (for Nelson) transformations. It is demonstrated that with the use of guiding center coordinates the fields follow an Einstein’s equation on the phase-space extended to time and Gravitation includes electromagnetism if considered on such extended phase space. Once electromagnetic (now also gravitational) fluctuations are considered it is shown that the gyrocenter follows the Schroedinger equation. It is proposed to explaining the discrepancies between GR and QM in the simplest way: they are describing different objects (guiding center and gyrocenter, respectively) that are both representative of charges and/or masses. In this way a unified theory is accessible and it can be depicted without inconsistencies [4].
[1] Kaluza, Th. (1921) Sitzungsberichte der Preussischen Akademie der Wissenschaftenzu Berlin Math. Phys, K1, 374.
[2] Klein, O. (1926) Zeitschrift für Physik, 37, 895-906.
https://doi.org/10.1007/BF01397481
[3] Nelson, E. (1966) Physical Review, 150, 1079.
https://doi.org/10.1103/PhysRev.150.1079
[4] Di Troia, C (2018) Journal of Modern Physics, 9, 701
I review the approach of precanonical quantization based on the structures of the De Donder-Weyl Hamiltonian formulation of field theories. The approach requires no splitting to space and time and it leads to a new understanding of quantum fields as a hypercomplex generalization of quantum theory rather than an infinite-dimensional one. A relationship with the standard QFT in the functional Schroedinger representation, which emerges from precanonical quantization as a singular limiting case, is explained using the example of scalar field theory in flat and curved space-time. We apply the framework to the Palatini formulation of GR in vielbein and spin-connection variables and derive the precanonical analogue of the Schroedinger equation for quantum gravity, which is a PDE on the total space of the bundle of spin-connection coefficients over space-time. We discuss the Hilbert space in this formulation, the quantum gravitational avoidance of curvature singularities and the emergence of the Einstein equations in the classical limit. Using a simplified cosmological model we also show that the approach leads to a very specific prediction of the Levy-type of statistics of quantum fluctuations of spin-connection that may point to a fractal nature of quantum space-time.
We analyze the propagation of gravitational
waves in a medium containing bounded
subsystems ("molecules''), able to induce significant Macroscopic Gravity effects. We establish a precise constitutive relation between the
average quadrupole and the amplitudes of a vacuum gravitational wave, via the geodesic deviation equation. Then we determine
the modified equation for the wave inside the medium and the associated dispersion relation. A phenomenological analysis shows that
anomalous polarizations of the wave emerge
with an appreciable experimental detectability
if the medium is identified with a typical galaxy. Both the modified dispersion relation
(wave velocity less than the speed of light)
and anomalous oscillations modes could be detectable by the incoming LISA or pulsar timing arrays experiments, having the appropriate size to see the concerned wavelengths
(larger than the molecular size) and
the appropriate sensitivity to detect the
expected deviation from vacuum General Relativity.
The simplicity of black holes, as characterized by no-hair theorems, is one of the most important mathematical results in the framework of general relativity. Are these theorems unique to black hole spacetimes, or do they also constrain the geometry around regions of spacetime with arbitrarily large (although finite) redshift? In this talk I will discuss the answer to this question, showing how to extend Israel's theorem to static spacetimes without event horizons that contain small deviations from spherical symmetry. This provides a first extension of no-hair theorems to ultracompact stars, wormholes, and other exotic objects, and paves the way for the construction of similar results for stationary spacetimes describing rotating objects.
Gravitational-wave standard sirens present a novel approach for the determination of the Hubble constant. After the recent spectacular confirmation of the method thanks to GW170817 and its optical counterpart, additional standard siren measurements from future gravitational-wave sources are expected to constrain the Hubble constant to high accuracy. At the same time, improved constraints are expected from observations of cosmic microwave background (CMB) polarization and from baryon acoustic oscillations (BAO) surveys. We explore the role of future standard siren constraints on $H_0$ in light of expected CMB+BAO data.
Considering a $10$-parameters cosmological model, in which curvature, the dark energy equation of state, and the Hubble constant are unbounded by CMB observations, we find that a combination of future CMB+BAO data will constrain the Hubble parameter to $\sim 1.5 \%$. Further extending the parameter space to a time-varying dark energy equation of state, we find that future CMB+BAO constraints on $H_0$ are relaxed to $\sim 3.0 \%$. These accuracies are within reach of future standard siren measurements from the Hanford-Livingston-Virgo and the Hanford-Livingston-Virgo-Japan-India networks of interferometers, showing the cosmological relevance of these sources. If future gravitational-wave standard siren measurements reach $1\%$ on $H_0$, as expected, they would significantly improve future CMB+BAO constraints on curvature and on the dark energy equation of state by up to a factor $\sim 3$. We also show that the inclusion of $H_0$ constraints from gravitational-wave standard sirens could result in a reduction of the dark energy figure-of-merit (i.e., the cosmological parameter volume) by up to a factor of $\sim 400$.
The postmerger-ringdown waveform of coalescing, non-precessing, spinning
binary black holes in the \texttt{TEOBresumS} model is given by a closed form, analytic,
time-domain family of template waveforms, informed by a large set of
Numerical Relativity waveforms from different codes such as the BAM code, the SpEC code and
state-of-the-art test-particle waveforms. The NR waveforms cover the parameter space
from the equal mass case till the test-particle limit. This includes 5 waveforms generated by the BAM
code with mass ratio $m_1/m_2=18$ and with the heavier BH spinning with spins up to $\pm 0.8$.
The peak is fitted with an error of at most $2\%$ in both amplitude and frequency. The phase (amplitude)
is fitted with an accuracy of at least $0.1rad$ ($10\%$) over the first 10-15M
after the peak, with the exception of 3 outliers.
Stand alone the model can be used for several studies independently as well.
The postmerger and ringdown of very heavy black hole binary systems,
such as GW170729, will still be in the observable band of 2nd generation
gravitational wave detectors such as advanced LIGO and advanced Virgo.
Analyzing the signals directly in the time-domain, using the analytical
postmerger-ringdown waveform model, gives a fully independent
measurement of source parameters. Further, it was demonstrated in arXiv:1811.08744 that
fits of key waveform characteristics, such as peak amplitude and frequency,
could be used for consistency tests of general relativity in the strong-field
regime.
After a pedagogical introduction to the analytic setup I will discuss the
set of Numerical Relativity waveforms used to inform the model and highlight
several technical details of the fitting procedure. I will discuss the fitting of the
peak structures for the complex multipolar waveforms. I will conclude by showing
results of the application of the model to the analysis of real gravitational wave data.
We improve and generalize to all multipoles the factorization and resummation approach of Nagar and Shah,designed to improve the strong-field behavior of the post-Newtonian (PN) residual waveform amplitudes $f_{\ell m}$'s entering the effective-one-body, circularized, gravitational waveform for spinning coalescing binaries. For a test-particle orbiting a Kerr black hole, each multipolar amplitude is truncate at relative 6~post Newtonian (PN) order, both for the orbital and spin factors. By taking a certain Pad\'e approximant of the orbital factor in conjuction with the inverse Taylor (iResum) representation of the spin factor, it is possible to push the analytical/numerical agreement of the energy flux at the level of $5\%$ at the last-stable-orbit for a spinning black hole with dimensionless spin parameter $+0.99$.
When the procedure is generalized to comparable-mass binaries, each orbital factor is
kept at relative $3^{+3}$PN order, i.e. the globally 3PN-accurate comparable-mass terms
are hybridized with higher PN test-particle terms up to 6PN relative order in each mode.
The same Pad\'e resummation is used for continuity. By contrast, the spin factor is only
kept at the highest comparable-mass PN-order currently available. We illustrate that the
consistency between different truncations in the spin content of the waveform amplitudes
is more marked in the resummed case than when using the standard Taylor-expanded form.
We finally introduce a method to consistently hybridize comparable-mass and test-particle
information {\it also} in the presence of spin.
The improved, factorized and resummed, multipolar waveform amplitudes presented
here are expected to set a new standard for effective-one-body-based
gravitational waveform models.
Premetric teleparallel gravity is a generalization of GR based on two field equations similar to Maxwell’s equations of electrodynamics. In gravitational model, the scalar-valued 2-forms of electrodynamics are replaced by two vector-valued 2-forms. The energy-momentum 3-form of matter and gravity field serves as a source. A general linear constitutive relation between two basic fields provides a rich family of gravitational models. When this gravitational constitutive tensor is restricted to a metric-dependent expression with a special choice of dimensionless parameters, the standard GR in its teleparallel equivalent version is recovered. The energy-momentum current and the Lorentz-type force density expressions are defined for the whole family of models. They loss their invariant meaning only when the construction is restricted to GR itself. It is because of an additional local Lorentz transformation of frame field. We show that this gravitational Lorentz force yields the proper geodesic equation for the whole family of models (even including GR) when an additional mechanical linear constitutive relation between the velocity and the momentum of a point-wise particle is assumed. This fact solves the known dichotomy between geodesic and autoparallel trajectories in gravity models with a general connection.
References:
1) Itin, Y., Hehl, F. W., & Obukhov, Y. N. (2017). Premetric equivalent of general relativity: Teleparallelism. Physical Review D, 95(8), 084020.
2) Itin, Y., Obukhov, Y. N., Boos, J., & Hehl, F. W. (2018). Premetric teleparallel theory of gravity and its local and linear constitutive law. The European Physical Journal C, 78(11), 907.
3) Itin, Y. (2018). Premetric representation of mechanics, electromagnetism and gravity. International Journal of Geometric Methods in Modern Physics, 1840002.
I introduce the concept of fake particle and study how it is used to formulate a consistent theory of quantum gravity. Fakeons arise from a new quantization prescription, alternative to the Feynman one, for the poles of higher-derivative theories, which avoids the problem of ghosts. The fake particles mediate interactions and simulate true particles in many situations. Nevertheless, they are not asymptotic states and cannot be detected directly. The Wick rotation and the S matrix are regionwise analytic and the amplitudes can be calculated in all regions starting from the Euclidean one by means of an unambiguous, but nonanalytic operation. By reconciling renormalizability and unitarity in higher-derivative theories, the models containing both true and fake particles are good candidates to explain quantum gravity. In pole position is the unique theory that is strictly renormalizable. One of the major physical predictions due to the fakeons is the violation of microcausality. I discuss the classical limit of the theory and the acausal corrections to the Einstein equations.
We shall present a SO(2) duality-symmetric form of the
linearized ADM action principle when the linearization is performed on
anti de Sitter and Kasner backgrounds. The analysis is based on the
two-potential formalism, obtained upon resolution of the Hamiltonian
constraints, and generalizes previous works that focused on Minkowski
and de Sitter backgrounds. The implications of our results for
holography and cosmology will be discussed.
It is widely accepted that the general theory of relativity ceases to explain phenomena in the vicinity and inside a black hole. Wheeler holds the idea that on the surface of a Black hole the space parameters turn into quantum foam. He even contemplates on the geometry of space parameters as one zooms down on a point particle. In this work we aim to solve challenges by employing a pair of spacetime parameters of a quantum character. We call these of quantum character because they are not experimentally measurable but provide a very useful relations between different physical quantities. This pair of spacetime parameters are linked together through the limiting quantity of space: the Planck length. The realization of the idea of Wheeler helps to achieve a beautiful connection between gravitation and electromagnetism, an effort on which Einstein worked for around 40 years. These steps respect the hierarchy of the physical quantities of energy, momentum and force. This approach respects the idea of Einstein who searched for a theory that had the concepts of the energy – or the field – at its center. The space parameters are derived from a simple relation and are valid for the whole range of masses, from the mass of the electron to the mass of the Universe. From this relation the gravitational constant, and the electrostatic force are derived.
TEOBResumS and SEOBNR are the two mainstream analytical waveform models (informed by numerical-relativity information) that accurately describe the dynamics of two coalescing compact objects of masses m1 and m2 and spins S1 and S2 up to merger and ringdown. Both use the effective-one-body (EOB) approach, which maps the relative dynamics of two objects into the dynamics of a (spinning) particle of mass mu=m1m2/(m1+m2) and spin moving in a deformed Kerr metric. In doing so, the post-Newtonian expanded Hamiltonian is resummed in special ways so to improve its behavior and its predictability in the strong-field, fast-velocity regime up to merger. We compare in detail the analytical choices made in the two models, focusing in particular on the treatment of spin-orbit and spin-spin effects.
We start from the state-of-the-art effective one-body (EOB) model and suitably expand it up to 5.5PN order in the orbital (nonspinning) case.
By comparing the so-obtained high-order PN phasing with the corresponding EOB one, we conclude that the 5.5PN approximation delivers a reliable phasing description up to $M\omega = 0.05$ for comparable mass binaries. Although such cutoff frequency is reduced when the mass ratio is increased,the EOB/PN agreement is better with the 5.5PN approximant with respect to the standardly used 3.5PN one, expecially at low frequencies. Beyond that, an injection/recovery study is done. The injected waveforms are {\tt TEOBResumS} ones and we recover them using the TaylorF2 with a certain tidal part, that can be either the 6PN one (leading order), or the NRTidal one. We illustrate that the purely analytical point mass information is relevant in reducing the PE biases on the tidal parameters, analyzing our result for the Sly and h4 equations of state.
Although the existence of dark matter (DM) has been shown through various observations, nobody knows the identity of the DM. Axions were firstly introduced to solve the strong CP problem, and one of the possible candidate of DM.
Due to the super-radiance instability, axion cloud is formed around Kerr BHs.
Recently, it was suggested that once coupling to photons is considered, the laser of photons is emitted from the cloud through the coupling.
In this talk, we solve numerically the Axion-Maxwell system, and confirm the existence of laser-like emission from clouds.
In this work, we study the effect of a magnetic field on the growth of cosmological perturbations. We develop a mathematical consistent treatment in which a perfect fluid and a uniform magnetic field evolve together in a Bianchi I universe. We then study the energy density perturbations on this background with particular emphasis on the effect of the background magnetic field. We develop a solution in the Newtonian treatment with adiabatic sound speed in the isotropic limit. We also find the full relativistic anisotropic solution for perturbations in the directions parallel and perpendicular to the background magnetic field when the sound speed is zero. This represents a new result in GR since all the present studies deal with isotropic systems: our solutions show a clear anisotropic behaviour and are far more complicated than the FRW ones. We also write an equation for the general solution of the problem which could be numerically integrated.
In this talk we will present a new method to construct homogenous AdS black strings/p-branes in General Relativity and Lovelock gravity. These configurations are obtained by means of a specific "scalar dressing" of the extended coordinates. We will show how to use this method to construct the Schwarzschild AdS black string, the black string extension of the Boulware-Deser black hole and some black strings configurations in the presence of matter fields.
We show that (Eur Phys J C, 2018), the gravity can be considered as defined not by one but two fundamental constants which enables us to explain quantitatively both dark energy (the cosmological constant) in GR equations and dark matter in weak-field limit simultaneously. Then, in order to throw more light on the nature of the constants appearing here, we generalize the Newton theorem on the 'sphere-point mass' equivalency to arbitrary dimensions. We also turn into gravitational lensing, where this additional term predict a critical value for the involved weak-field parameter. If this value will be established at future observations, this will mark the first discrepancy with GR of the conventional weak-field limit, directly linked to the nature of the dark sector of the Universe.
I try to revive, and possibly reconcile, a debate started a few years ago, about the relative roles of a bare cosmological constant and of a vacuum energy, by taking the attitude to try to get the most from the physics now available as established. I take as starting point the proposals on how to regularize the particles vacuum energy without violating Lorentz invariance. I notice that the bare cosmological constant of the Einstein equations, which is there ever since GR emerged, is actually constrained (if not measured) indirectly from the effective cosmological constant observed now, as given by CDM Precision Cosmology and from the cumulative vacuum contribution of Standard Model, SM, particles, when this is evaluated using the well-established physics of Quantum Field Theory, QFT. Therefore the fine tuning, implied by the compensation to a small positive value of the two large contributions, could be seen as offered by Nature, which provides one more fundamental constant, the bare Lambda. The possibility is then discussed of constraining (measuring) directly such a bare cosmological constant by the features of primordial gravitational wave signals coming from hypothetical epoch’s precedent to the creation of particles. A hint is briefly discussed for a possible bare Lambda inflation process.
The detection of GW170817, the first neutron star-neutron star merger observed by advanced LIGO and Virgo, and its following analyses represent the first contributions of gravitational wave data to understanding dense matter. Imposing a lower limit on the maximum mass value and parametrizing the high density section of the equation of state of both neutron stars through spectral decomposition led to an estimate of the stars' radii of $R_1 = 11.9_{- 1.4}^{+ 1.4}$ km and $R_2 = 11.9_{- 1.4}^{+ 1.4}$ km. These values do not, however, take into account the uncertainty owed to the arbitrary choice of the crust low-density equation of state, which was fixed to reproduce the SLy equation of state model. We here re-analyze GW170817 data and establish that different crust models do not strongly impact the mass or tidal deformability of a neutron star, making it impossible to distinguish between low density models through GW analysis. However, the crust does have an effect on radius. We predict the systematic error due to this effect using neutron star structure equations, and compare the prediction to results from full parameter estimation runs. For GW170817, this systematic error affects the radius estimate by 0.3 km, approximately $3\%$ of the neutron stars' radii.
Basic aspects of the Hamiltonian structure of the parity-violating Poincar\'e gauge theory are studied. We found all possible primary constraints, identified the corresponding critical parameters, and constructed the generic form of the canonical Hamiltonian. In addition to being important in their own right, these results offer dynamical information that is essential for a proper understanding of the particle spectrum of the theory, calculated in the weak field approximation around the Minkowski background.
Time in physics is always viewed from the point of view of duration, but this interpretation causes many problems and paradoxes. I would like to explore a change of interpretation of time. By thinking time as a cut, many problems could be easily solved. Particularly, with such interpretation, quantum gravity theories based on 3+1 spacetime (e.g. Kuchar or Ellis' evolving block universe) may open unexpected and fruitful views.
References: L. Foschini, Fisica del Tempo (Bonomo, Bologna, 2018).
The objective, statistical nature of SDSS astrophysical datasets, which were not driven by any theoretical agenda, reveal false and misleading prior measurements (e.g., redshift-distance) driven by confirmation bias in the context of such agendas. SDSS theta-z, redshift-magnitude (both spectroscopic and photometric pipelines), and galaxy population-density data are shown to conflict with the ΛCDM standard cosmological model. However, all four of these distinct and independent data sets are similarly consistent with a new cosmological model. That new model, which is consistent with Willem de Sitter’s exact solution to the Einstein field equations, and which derives from simple considerations of symmetry and local proper time modeled as a geometric object, motivated by Minkowski, represents a major paradigm shift in cosmology. The canonical idea of a non-relativistic universal time coordinate (i.e., ‘Cosmic time’ from initial singularity) is supplanted by a relativistic, strictly-local time coordinate involving no such unphysical singularity. The confrontation of all new predictive equations, having no free parameters, with corresponding SDSS data sets definitively resolves the modern quandary of ‘dark energy,’ purported to cause the improbable phenomenon of accelerating cosmic expansion.
A very general reconstruction and estimation of the gravitational wave features, i.e. not based on prior knowledge of the waveform models, is useful to catch unexpected characteristics of the signal. In addition, it can complement the analyses based on parametrized models of the detected emissions from compact binary coalescences. In fact, we know that parametrized models may not always accurately cover all the known possible variety of the emissions, such as orbital eccentricity, misaligned spins and post-merger signals from neutron star remnants. We will overview the un-modeled methods developed to characterize the waveforms and the comparison with modeled analyses in the case of the LIGO-Virgo signal catalog GWTC-1.
The second observing run of Advanced LIGO and Virgo ended on August 2017 with the detection of several coalescences. We report the results of a general search for gravitational waves of short duration using minimal assumption on the waveforms and sensitive to a wide range of sources. The analysis includes the results of three different algorithms over the frequency from 32 to 4096 Hz targeting signals with duration up to seconds. We will also mention the results on the detected gravitational waves from coalescences of compact object.
Gravitational waves (GWs) have been detected from mergers of binary black holes and binary neutron stars.
Core collapse supernovae (CCSNe) are recognized as the most energetic explosions in the modern Universe
The main reason GWs from CCSN have not yet been detected is the low event rate, about one per century,
observable within the Milky Way.
We report on the construction of a Convolutional Neural Network to focus on gravitational waves produced in one of the most dramatic events in the cosmos, supernova explosions.
We use only whitened time series of measured gravitational-wave strain as an input, and we train and test on simulated core-collapse supernovae signals in synthetic Gaussian noise representative of Advanced LIGO sensitivity. We show that our network can classify signal from noise.
In the context of the unmodelled search for gravitational waves associated to gamma-ray bursts (GRB), we present a sensitivity study conducted using X-Pipeline, a software which combines the data from LIGO and Virgo in correlation with the GRB direction in the sky. The goal is to understand how the addition of Virgo to the network of interferometers impacts the sensitivity of the search. The overall sensitivity, limited by non-stationary noise, is estimated through the efficiency in recovering simulated gravitational waves signals injected in the data, and then it is compared to the sensitivity obtained without including Virgo. For 4 out of 9 GRBs detected in August 2017, adding Virgo results in lower upper limits on the amplitudes of the injected waveforms in the [20, 500] Hz band, improving the sensitivity up to a factor of ~60%. For the 5 GRBs left, the addition of Virgo reduced the sensitivity up to ~25%. We find that the crucial factor is the ratio between the detector angular response and its noise power spectrum: when this quantity computed for Virgo is smaller than for LIGO, the Virgo inclusion results in a better sensitivity. This gives us a metric for the Virgo inclusion in this search for the next observing run.
We compute the Zero Point Energy (ZPE) induced by quantum fluctuations around a fixed background with the help of a reformulation of the Wheeler-DeWitt equation. A variational approach is used for the calculation with Gaussian Trial Wave Functionals. The one loop contribution of the graviton to the ZPE is extracted keeping under control the UltraViolet divergences by means of a distorted gravitational field knows as Gravity's Rainbow. The finite ZPE is here interpreted as an induced Cosmological Constant. A comparison with other methods of keeping under control ultraviolet divergences is also discussed.
Anti-de Sitter spacetime is a solution of Einstein’s equations with a negative cosmological constant. This fact allows for unusual black hole solutions with non-spherical horizon topology. We calculate the renormalised vacuum polarisation for black holes with spherical, flat and hyperbolic event horizons, following the “extended coordinates” method, which uses a mode-sum representation for the Hadamard parametrix. Renormalisation counter terms are subtracted from the Green’s function mode-by-mode, leaving each individual term manifestly finite.
Driven by the the fact that a wide family of Ricci-based metric-affine theories of gravity can be reduced to a metric compatible framework, a formal correspondence between the space of solutions of these generalized gravity theories and the space of solutions of general relativity will be presented. The correspondence will be detailed in the cases where modified gravity is coupled to scalar and charged matter. These results allows to use well-established methods and results from General relativity to explore new gravitational physics beyond it.
The presentation will be based on the following recent publications : arXiv:1810.04239, arXiv:1807.06385, arXiv:1705.03806.
Using a geocentric ecliptical coordinate system to analyze the data of a proposed new Earth's satellite, provisionally named ELXIS, in a circular orbit perpendicular to both the equator and the reference direction of the Vernal Equinox should allow, in principle, to measure the general relativistic Lense-Thirring and De Sitter effects on the satellite's inclination $I$ and node $\Omega$ to a relative accuracy of $\simeq 10^{-2},~10^{-5}$, respectively. Indeed, the long-term perturbations on $I,~\Omega$, referred to the ecliptic, due to the zonal harmonic coefficients $J_\ell,~\ell=2,3,4,5,\ldots$ of the geopotential vanish for $e=0,~I = \Omega = 90\deg$. Departures $\Delta I=\Delta\Omega\simeq 0.01-0.1\deg$ from such an ideal orbital configuration would not compromise the stated accuracy goals. The most insidious competing perturbations are due to the ocean component of the $K_1$ tide of degree $\ell=2$ and order $m=1$: they do not vanish for $I = \Omega = 90\deg$, and our knowledge of its tidal height $C_{2,1,K_1}^{+}$ is relatively inaccurate. A suitable linear combination of the rates of change of $I,~\Omega$ allows to cancel out them and enforce the De Sitter effect. By assuming a relative uncertainty of the order of $\simeq 10^{-3}$ in $C_{2,1,K_1}^{+}$ from a comparison of some rather recent global ocean tide models, the resulting systematic bias on each of the Lense-Thirring precessions would be at the percent level. Other sources of potential systematic uncertainties like the 3rd-body perturbations due to the Moon and the non-gravitational accelerations allow to meet the desired accuracy levels.
It is well known that a spinning body moving in a fluid suffers a force orthogonal to its velocity and rotation axis --- it is called the Magnus effect. Recent simulations of spinning black holes and (indirect) theoretical arguments suggest that a somewhat analogous effect may occur for purely gravitational phenomena. The magnitude and precise direction of this "gravitational Magnus effect" is however still the subject of debate. Starting from the rigorous equations of motion for spinning bodies in General Relativity (Mathisson-Papapetrou equations), we show that indeed such an effect takes place. We compute it explicitly for some astrophysical systems of interest: galactic dark matter haloes, black hole accretion disks, and the cosmological FLRW background. It is seen to lead to secular orbital precessions potentially observable by future astrometric experiments and gravitational-wave detectors.
In this talk we will consider the possibility to enlarge the class of symmetries under which a local (polynomial) action is invariant by introducing nonlocal (non-polynomial) operators. In particular, we will show how to construct nonlocal actions, consisting of infinite order derivatives, which are invariant under a wider class of symmetries, containing the Galilean shift symmetry as a subclass. Motivated by this, we will consider the case of a scalar field and discuss the pole structure of the propagator which has infinitely many complex conjugate poles, but satisfies the tree-level unitarity. We will also consider the possibility to construct UV complete Galilean theories by showing how the ultraviolet behavior of loop integrals can be ameliorated. Moreover, we will consider kinetic operators respecting the same symmetries in the context of linearized gravity. In such a scenario, the graviton propagator turns out to be ghost-free and the spacetime metric generated by a point-like source is nonsingular. These new models might open a new branch of nonlocal theories.
We consider time-independent scattering of gravitational waves by a compact horizon-less body such as a neutron star. The neutron star is modelled with a polytropic equation of state. The metric perturbation in the exterior can be solved for using the gauge invariant master functions and formalism presented by Martel and Poisson [2005]. For the interior we work in Regge-Wheeler gauge and solve the perturbed Einstein field equations for odd and even parity. We will show that in this scenario, there can be rainbow scattering, similar to that seen in nuclear experiments. An associated caustic (a focusing of null geodesics) forms near the body’s surface. This feature is imitated in the wave picture. We will compare our results with black hole scattering studies.
Scenarios that strive to describe quantum theory as an emergent, non-primitive concept typically run into difficulties when trying to address a relativistic generalization. In this talk we discuss a possible way out of this situation by showing that the observed relativistic behavior in the quantum world might well be just a statistically emergent phenomenon out of deeper no-relativistic level of quantum dynamics. We start by discussing complex dynamical systems whose statistical behavior can be explained in terms of a superposition of simpler underlying dynamics — the so-called superstatistics paradigm. Then we go on by showing that the combination of two cornerstones of contemporary physics — namely Einstein’s special relativity and quantum-mechanical dynamics is mathematically identical to a complex dynamical system described by two interlocked processes operating at different energy scales. The combined dynamic obeys Einstein’s special relativity even though neither of the two underlying dynamics does. This implies that Einstein’s special relativity might well be an emergent concept in the quantum realm.
To model the double process in question, we consider quantum mechanical dynamics in a background space consisting of a number of small crystal-like domains varying in size and composition, known as polycrystalline space. There, particles exhibit a Brownian motion. The observed relativistic dynamics then comes solely from a particular grain distribution in the polycrystalline space. In the cosmological context such distribution might form during the early universe’s formation.
Ensuing implications for quantum field theory and cosmology (leptogenesis) will be also briefly discussed.
Related articles:
[1] P. Jizba and F. Scardigli, Special Relativity Induced by Granular Space, Eur. Phys. J. C (2013) 73: 2491
[2] P. Jizba and F. Scardigli, The emergence of Special and Doubly Special Relativity, Phys. Rev. D (2012) 86: 025029
[3] P.Jizba and H. Kleinert, Superstatistics approach to path integral for a relativistic particle, Phys. Rev. D (2010) 82: 085016
We analyze the semiclassical stability of the Schwarzschild AdS black hole
in the Euclidean partition function approach. We perform this computation
in the large D limit and focus on scalar perturbations. We obtain the equa-
tions for non-spherically symmetric scalar perturbations in a simple form.
For a class of perturbations stability is demonstrated by the S-deformation
method. For some other classes we rule out unstable modes of O(D^2). We
also analyze the spherically symmetric perturbations and demonstrate the
appearance of an unstable mode for small black holes in the large D limit.
We obtain an expression for the eigenvalue corresponding to the unstable
mode to next to leading order in a 1/D expansion. This result agrees with
a previously obtained numerical bound on this eigenvalue. For cosmological
constant zero, our answer matches a previous result obtained for the corre-
sponding eigenvalue for the D dimensional Schwarzschild-Tangherlini black
hole to next to leading order in a 1/D expansion.
In this talk, I explore the Casimir effect for a massless scalar field in the context of a generic curved background. In this perspective, the mean vacuum energy density and the pressure between the binding plates are the relevant physical objects to evaluate. After the above general discussion, a systematic procedure to derive interesting pieces of information on the free parameters of extended theories of gravity is presented. In particular, I focus the attention on some recent results regarding the Standard Model Extension and several quadratic models of gravity.
In this work, we study the impact of quantum entanglement on the two-point correlation function and the associated primordial power spectrum of mean square vacuum fluctuation in a bipartite quantum field theoretic system. The field theory that we consider is the effective theory of axion field arising from Type IIB string theory compactified to four dimensions. We compute the expression for the power spectrum of vacuum fluctuation in three different approaches, namely (1) field operator expansion (FOE) technique with the quantum entangled state, (2) reduced density matrix (RDM) formalism with mixed quantum state and (3) the method of non-entangled state (NES). For massless axion field, in all these three formalism, we reproduce, at the leading order, the exact scale-invariant power spectrum which is well known in the literature. We observe that due to quantum entanglement, the sub-leading terms for these thee formalisms are different. Thus, such correction terms break the degeneracy among the analysis of the FOE, RDM and NES formalisms in the super-horizon limit. On the other hand, for massive axion field, we get a slight deviation from scale invariance and exactly quantify the spectral tilt of the power spectrum in small scales. Apart from that, for massless and massive axion field, we find distinguishable features of the power spectrum for the FOE, RDM, and NES on the large scales, which is the result of quantum entanglement. We also find that such large-scale effects are comparable to or greater than the curvature radius of the de Sitter space. Most importantly, in the near future, if experiments probe for early universe phenomena, one can detect such small quantum effects. In such a scenario, it is possible to test the implications of quantum entanglement in primordial cosmology.
Following Whittaker(Proc.Roy.Soc.A116,720(1927)) we derive the partial differential equation whose solution is the electrostatic potential in curved spaces with the metric given :
a.in 2 + 1 dimensions by i)S.Deser,R.Jackiw and G.'tHooft,Ann.Phys.152,220(1984) and G.Clement,Int.J.Theor.Phys.24,267(1985)and by ii)M.Banados,C.Teitelboim and J.Zanelli,Phys.Rev.Lett.69,1849(1992) and
b.in 3 + 1 dimensions by C.S.Trendafilova and S.Fulling,Eur.J.Phys.32,1663(2011)
With an exact solution to the partial differential equation as the objective,the telling interest here is the contrast between the resulting partial differential equations in 3 + 1 dimensions from the use of the Schwarzschild metric -and this was dealt with in detail by Whittaker(see eq.(25) et seq. in the reference cited) -versus the cylindrically symmetric metric given by Trendafilova and Fulling in item b. above
Finally, in the planar case an exact form for the electrostatic potential seems less accessible for the Banados-Teitelboim-Zanelli metric relative to the Deser-Jackiw-'tHooft and Clement solution.
We show that in the post-merger phase of binary neutron star (BNS) merger are present convective instabilities that excite inertial mode oscillations. These oscillations emit gravitational waves in the frequency band where ground-based detectors are within reach of planned third-generation detectors and could be used also as sensitive probes of the rotational and thermal state of matter in the neutron star remnant. Within the limits of the input physics of our simulations (which neglect magnetic fields and neutrino transport) their presence appear to be quite general for remnants that live more than 50 ms and show up for four different equations of state (EOS), parametrized as piecewise polytropics plus a thermal component with $\Gamma = 1.8$. We also analyze the gravitational wave signal emitted by the remnant after the merger for each EOS in order to characterize the rate of change of the frequencies and the damping of the amplitudes for both the main and excited mode.
We investigate the gravitational radiation from binary systems in conformal gravity (CG) and conformal Einstein-Weyl gravity (CEWG). CG might explain observed galaxy rotation curves without dark matter, and both models are of interest in the context of quantum gravity. Gravitational radiation emitted by compact binaries allows us to strongly constrain both models.
We derive the linearized fourth-order equation of motion for the metric, which describes massless and massive modes of propagation and we show that the gravitational radiation is due to the time-dependent quadrupole moment of a nonrelativistic source. Further, we derive the gravitational energy-momentum tensor for both models and apply our findings to the case of close binaries on circular orbits.
Our results are that in CG one cannot explain the orbital decay of binary systems via gravitational radiation, and replace dark matter simultaneously. In CEWG with small masses of the graviton, again one cannot reproduce the orbit of binaries by the emission of gravitational waves. On the other hand, for large graviton masses, the orbital period of compact binaries is in agreement with the data, as CEWG reduces to GR in this limit.
GW170817 with its coincident optical counterpart has led to a first "standard siren" measurement of the Hubble constant independent of the cosmological distance ladder. The Schutz "statistical" method, which is expected to work in the absence of uniquely identified hosts, has also started bringing in its first estimates. In this talk we report the current results of the gravitational-wave measurement of the Hubble constant and discuss the prospects with observations during the upcoming runs of the Advanced LIGO-Virgo detector network.
We derive the form of the partition function in conformal gravity using an extended form of the Faddeev-Popov method. The method uses conformal gauge fixing and special (third) conformal ghosts. In this way, at one-loop, the theory is proven to be conformally invariant also on the quantum level without performing an additional final conformal transformation. The partition function is discussed on a general background as well as on Ricci-flat and maximally symmetric.
Nowadays, our modeling of the Universe depends critically on our understanding of gravity; despite the fact General Relativity (GR) is the standard theory of gravity, deviations from GR could profoundly impact our conclusions on the best theory suitable to explain the "dark" ingredients that make up the Universe. On the other hand, experimental verifications of the GR weak effects are difficult, but could be as fundamental and complementary as any other observations that test manifestly the validity of Einstein's field equations, which underpin strong gravity.
Gaia-like missions are offering the unique possibility of being a multi laboratory for extensively testing weak gravitational fields at local (Solar System) and more distant (MIlky Way) scales.
In particular, the potential of Gaia is to probe the validity of GR by testing the structure of our Galaxy as a product of the cosmological evolution shaped by gravity (Local Cosmology), namely the relations among baryonic structures (and their evolution) and the dark components of the Universe.
In particular, we present the first attempt to apply the relativistic kinematics delivered by Gaia to trace the MW rotation curves within a general relativistic scenario.
We consider the dynamics of the Mixmaster Universe, focusing on the Bianchi IX cosmological model (this model have a closed Universe with a spherical topology). We use f (R) gravity, which is the simplest modification of the general relativity, considering the Palatini formalism, and compare the results with those of the quadratic correction. To describe the Mixmaster model, we use the Misner Chitrè – like variables, in the scalar-tensor framework. The form of the potential well depends on the self interacting scalar field potential. We start our work introducing f (R) gravity theories and their most important aspects covering the largest portion of the literature. Furthermore, we demonstrate the non-chaotic nature of the Mixmaster model, if it is described by the scalar-tensor version of the f (R) gravity. This work, derived in the homogeneous cosmological setting, can be easily extended to the inhomogeneous case.
After a brief description of what is a traversable wormhole we describe the connection between traversability and the Casimir effect. With the help of an equation of state we also discuss different form of solutions generated by the Casimir source. A connection with the Quantum Weak Energy Condition is also presented.
Regular models of black holes replace the central singularity with a
nonsingular spacetime region, in which an effective classical geometric
description is available. It has been argued that these models provide
an effective, but complete, description of the evaporation of black
holes at all times up to their eventual disappearance. However, I will
show that known models fail to be self-consistent: the regular core is
exponentially unstable against perturbations with a finite timescale,
while the evaporation time is infinite, therefore making the instability
impossible to prevent.
MICROSCOPE is a CNES-ESA-DLR-ONERA-CNRS-OCA-ZARM space mission that aimed to test the Weak Equivalence Principle (WEP) at the $10^{-15}$ level, i.e. two orders of magnitude better than the best "pre-MICROSCOPE" on-ground tests. The WEP is the cornerstone of General Relativity, the postulate that led Einstein to establish his theory: it states that all bodies fall at the same rate, independently of their mass and composition. Alternative theories of gravity, like those developed to overcome such conundrums as dark energy or the unification of gravity with the forces of the standard model of particle physics, generically predict a small violation of the WEP. As a consequence, not only does MICROSCOPE test the very foundation of General Relativity, but it also provides new constraints on theories beyond Einstein's.
Launched on April 25, 2016, the MICROSCOPE satellite and its scientific instruments provided high-quality data during the entire mission, which came to its end on October 16, 2018. The first results, based on 7% of the total data, ruled out a violation of the WEP greater than $2\times10^{-14}$. Since then, more data have been taken and additional analyses have been conducted to better constrain the level of systematic errors. While the remaining data is still being analysed, the final MICROSCOPE results should come closer to the $10^{-15}$ precision; they should be published at the end of 2019.
In this talk, I will first introduce the MICROSCOPE mission, in particular its scientific goals and measurement principles. Then, I will discuss its first results and mention what constraints MICROSCOPE can bring on some modified gravity models.
Black holes comprise a remarkably elegant set of solutions of the Einstein field equations. Aside from their rich mathematical structure, they are nowadays accepted as legitimate astrophysical objects and are routinely used in order to explain astrophysical observations. Nonetheless, General Relativity black holes were for a long time regarded with skepticism by many, even Einstein himself, as they imply infinite curvatures at their core (singularities) as well as infinite redshift at the horizon. What if nature tames somehow such infinities? How could we distinguish such “regularised black holes” from the standard ones? In this talk I shall try to present an overview of possibilities and their possible phenomenology with a specific attention at what could be observationally done in the near and far future.
General Relativity tells us that that a spinning source of gravity produces, in weak field approximation, both an attractive Newton-like force and a gravito-magnetic interaction. This is of course true for the whole Milky Way and in particular for its dark halo, if it exists. Here I discuss the opportunity of putting upper limits to the intensity of a possible galactic gravito-magnetic field, by terrestrial experiments. When a gravito-magnetic field concatenates with a loop closed in the space of a given observer, it causes a difference in the time of flight of right- and left-handed electromagnetic signals along that loop: this is the generalized version of the Sagnac effect, combining kinematical rotations and general relativity. Terrestrial devices exploiting this effect are for instance ring lasers. A galactic gravito-magnetic field would practically be constant in the whole internal solar system, but a ring fixed on the surface of the earth at a given latitude would daily oscillate its normal with respect to the axis of the Milky Way by an angular amplitude as big as the latitude. This diurnal (stellar day) modulation would be a possible footprint of the galactic gravito-magnetic interaction.
To date, the most precise tests of General Relativity have been achieved
through pulsar timing, albeit in the weak-field regime. Since pulsars are
some of the most precise and stable "clocks" in the Universe, present
observational efforts are focused on detecting pulsars in the vicinity of
supermassive black holes (most notably in the Galactic Centre), enabling
pulsar timing to be used as an extremely precise probe of strong-gravity
regime.
In this work, test-particle dynamics is described in general black-hole
spacetimes and used to study binary systems comprising a pulsar orbiting
a black hole. It is shown that, by adopting a fully general-relativistic
description of test-particle motion, independent of any particular theory
of gravity, observations of pulsars give reliable constraints on
alternative theories of gravity.
The supermassive black hole in the center of our galaxy is the closest of its kind and the largest in the sky. It is surrounded by a small cluster of high velocity stars called S-stars. Their trajectories are governed by the gravitational field of the black hole. We used the Very Large Telescope (VLT) instruments GRAVITY and SINFONI to follow the star S2/S-02 during its pericenter passage, collecting astrometric and spectroscopic data, respectively. These joint data allow a now robust detection of the combined gravitational redshift and transverse Doppler effect for S2/S-02. During high emission states (bright flares), GRAVITY also recorded continuous changes in position and polarisation of the IR source Sgr A* itself. These are attributed to a compact source of synchrotron emission (hot spot) from the innermost stable circular orbit around the black hole. I will discuss how we obtained our recent result and what it means in the context of gravity theories.
Abstract: I will present a simple and generic class of scalar-tensor theories that successfully realize dynamical damping of the effective cosmological constant, therefore providing a viable dynamical resolution of the fine-tuning cosmological constant problem. In contrast to early versions of this approach, the models considered do not suffer from unacceptable variations of Newton's constant, as one aims at a small but strictly positive late-time curvature. I will then show that the original fine-tuning issue is traded for a hierarchy of couplings, and further suggest a way to naturally generate this hierarchy based on fermion condensation and softly broken field shift symmetry. This talk is based on Phys. Rev. D 98, 124031 (2018), arXiv:1810.12336.