Hasil untuk "gr-qc"

L. Lanosa, O. Santillan

In the present work, the scattering between a light scalar particle $φ$ and a heavy scalar $σ$ in the eikonal limit is considered, for gravity scenarios containing higher order derivatives, such as the ones studied in \cite{stelle1}-\cite{modesto4}. It is suggested that if one of the new gravity scales introduced in the higher order action is smaller than the Planck mass, for instance of the order of $M_{GUT}\sim 10^{15}$ GeV, the functional form of the GR eikonal formulas appears changed by a factor. However, in this situation, the conditions for the eikonal approximation to hold has to be revised, this issue is analyzed in the text. The statements of the present work should be taken with a grain of salt, as the Schwarzschild radius for these polynomials theories is not yet established. The results presented here, in our opinion, are in agreement with the suppression of corrections to GR pointed out in \cite{brandhuber}, \cite{fradkin} and \cite{deser} for the Stelle gravity. The next to leading order approximation and part of the seagull diagrams are estimated. Different to the GR case, this order generically is non vanishing. An explicit regularization scheme is presented, based on Riesz and Hadamard procedures. The need of a regularization is partially expected, as the inclusion of small energy fluctuations may spoil the eikonal approximation.

Shubham Kejriwal, Francisco Duque, Alvin J. K. Chua et al.

The upcoming gravitational wave (GW) observatory LISA will measure the parameters of sources like extreme-mass-ratio inspirals (EMRIs) to exquisite precision. These measurements will also be sensitive to perturbations to the vacuum, GR-consistent evolution of sources, which might be caused by astrophysical environments or deviations from general relativity (GR). Previous studies have shown such ``beyond-vacuum-GR'' perturbations to potentially induce severe biases ($\gtrsim 10σ$) on recovered parameters under the ``null'' vacuum-GR hypothesis. While Bayesian inference can be performed under the null hypothesis using Markov Chain Monte Carlo (MCMC) samplers, it is computationally infeasible to repeat for more than a modest subset of all possible beyond-vacuum-GR hypotheses. We introduce bias-corrected importance sampling, a generic inference technique for nested models that is informed by the null hypothesis posteriors and the linear signal approximation to correct any induced inference biases. For a typical EMRI source that is significantly influenced by its environment but has been inferred only under the null hypothesis, the proposed method efficiently recovers the injected (unbiased) source parameters and the true posterior at a fraction of the expense of redoing MCMC inference under the full hypothesis. In future GW data analysis using the output of the proposed LISA global-fit pipeline, such methods may be necessary for the feasible and systematic inference of beyond-vacuum-GR effects.

M. Silva, F. Ferguson, Q. Cai et al.

Tauopathies are neurodegenerative diseases characterized by aberrant forms of tau protein accumulation leading to neuronal death in focal brain areas. Positron emission tomography (PET) tracers that bind to pathological tau are used in diagnosis, but there are no current therapies to eliminate these tau species. We employed targeted protein degradation technology to convert a tau PET-probe into a functional degrader of pathogenic tau. The hetero-bifunctional molecule QC-01–175 was designed to engage both tau and Cereblon (CRBN), a substrate-receptor for the E3-ubiquitin ligase CRL4CRBN, to trigger tau ubiquitination and proteasomal degradation. QC-01–175 effected clearance of tau in frontotemporal dementia (FTD) patient-derived neuronal cell models, with minimal effect on tau from neurons of healthy controls, indicating specificity for disease-relevant forms. QC-01–175 also rescued stress vulnerability in FTD neurons, phenocopying CRISPR-mediated MAPT-knockout. This work demonstrates that aberrant tau in FTD patient-derived neurons is amenable to targeted degradation, representing an important advance for therapeutics.

N. Mattsson, U. Andreasson, Staffan Persson et al.

Daniel Carney, Akira Matsumura

We analyze the framework recently proposed by Oppenheim et al (2023 Nat. Commun. 14; 2023 Phys. Rev. X 13 041040; arXiv:2302.07283 [gr-qc]; 2023 J. High Energy Phys. JHEP08(2023)163) to model relativistic quantum fields coupled to relativistic, classical, stochastic fields (in particular, as a model of quantum matter coupled to ‘classical gravity’). Perhaps surprisingly, we find that we can define and calculate scattering probabilities which are Lorentz-covariant and conserve total probability, at least at tree level. As a concrete example, we analyze 2→2 scattering of quantum matter mediated by a classical Yukawa field. Mapping this to a gravitational coupling in the non-relativistic limit, and assuming that we can treat large objects as point masses, we find that the simplest possible ‘classical-quantum’ gravity theory constructed this way gives predictions for 2→2 gravitational scattering which are inconsistent with simple observations of, e.g. spacecraft undergoing slingshot maneuvers. We comment on lessons learned for attempts to couple quantum matter to ‘non-quantum’ gravity, or more generally, for attempts to couple relativistic quantum and classical systems.

Beatrice Costeri, C. Dappiaggi, Michele Goi

We consider a self-interacting, massive, real scalar field on a four-dimensional globally hyperbolic spacetime and the associated stress-energy tensor. Using techniques proper of the algebraic approach to perturbative quantum field theory, we study the associated, Wick-ordered, quantum observable. In particular, we generalize a construction, first developed in the free field theory scenario by Moretti in (Commun Math Phys 232: 189–221, 2003. arXiv:gr-qc/0109048 [gr-qc]), aimed at exploiting the existing freedoms in the definition of the classical stress-energy tensor, in order to define a quantum counterpart which is divergence free. We focus on Minkowski spacetime proving that this procedure can be adapted also to cubic or quartic self-interactions, at least up to order $${\mathcal {O}}(\lambda ^3)$$ O ( λ 3 ) in perturbation theory. We remark that this result can be extended to arbitrary globally hyperbolic spacetimes, although in this case one needs to exploit the existing regularization freedom in the construction of the Wick ordered stress-energy tensor.

P. Channuie

In this work, we extend the investigation of the consequences of thermal fluctuations on the Casimir effect within the context of a traversable wormhole, recently proposed by Garattini \&Faizal, arXiv:2403.15174 [gr-qc], subject to charge contributions. Specifically, we focus on scenarios where the plates exhibit both constant and radial variations. In our analysis, we initially concentrate on the high temperature approximation, considering solely the influence of charge on the thermal Casimir wormholes. Additionally, upon incorporating Generalized Uncertainty Principle (GUP) corrections to the Casimir energy, we obtain a new class of wormhole solutions. Notably, we establish that the flare-out condition remains consistently satisfied. Intriguingly, our findings reveal that both the charge and GUP contributions serve to further enlarge the throat's size in the radial variation.

M. A. M. Smith, V. Paschalidis, G. Bozzola

In a previous paper, arXiv:2411.11960 [gr-qc], we initiated a study of high-energy interactions of charged binary black holes near the scattering threshold, focusing on zoom-whirl orbits. In this second paper in our series, we focus on merger remnant properties and energetics with new simulations of equal-mass, equal-charge, nonspinning binary black holes with variable impact parameter. We find near-extremal merger remnants with Kerr-Newman parameter reaching $\Upsilon_f = 0.97$, and observe that the maximum $\Upsilon_f$ increases monotonically with $\lambda$ for a fixed initial Lorentz factor. We find that binaries with larger $\lambda$ radiate less total energy despite having stronger electromagnetic emission. The maximum energy radiated by a binary in our study is $31\%$ of its gravitational mass. Increasing $\lambda$ has little effect on the maximum angular momentum radiated, which was $\approx 72\%$ of the spacetime total angular momentum for each $\lambda$ explored here. Lastly, we provide additional evidence for the universality with the irreducible mass that we discovered in arXiv:2411.11960 [gr-qc]. The black hole horizon areal radius determines a fundamental, gauge-invariant length scale governing BH interactions near the scattering threshold.

Eleftherios Tselentis, Amin Baumeler

We present a tight inequality to test the dynamical nature of spacetime. A general-relativistic violation of that inequality certifies change of curvature, in the same sense as a quantum-mechanical violation of a Bell inequality certifies a source of entanglement. The inequality arises from a minimal generalization of the Bell setup. It represents a limit on the winning chance of a collaborative multi-agent game played on the M\"obius graph. A long version of this Letter including other games and how these games certify the dynamical character of the celebrated quantum switch is accessible as arXiv:2309.15752 [gr-qc].

S. Schippling, L. Balk, F. Costello et al.

A. F. Zakharov

Recently Alexeyev et al. published paper (J. Theor. Exper. Phys. v. 165, N 4, p. 508 in Russian; arXiv:2404.16079 [gr-qc], the reference is given also in [1]). In the paper the authors discussed an opportunity of estimating spins from the analysis of the shadow reconstruction of black holes, theoretically considered using the nonlocal gravity model proposed earlier for the description of"quantum"black holes. However, in essence, this paper considered circular photon orbits, and the fact that the corresponding motion parameters determine the shape and size of shadows, similarly to Kerr black holes, remained unproven. It is also remained unproven the statement that for an equatorial observer the shadow size in the direction of rotation of quantum black holes remains independent of spin. A long time ago the shadow property was established for the Kerr black hole case.

Muhamad Ashraf Azman, N. Yusof, H. A. Kassim et al.

The dark energy star is a hypothetical model proposed as an alternative to address problems related to the structure of black holes (BHs), such as the presence of singularities. While dark energy star models have been previously explored, their application to BHs remains unexplored. This paper aims to investigate the concept of dark energy stars and compare their properties to BHs. The primary objective is to explore the physical profiles of dark energy stars and evaluate their similarity to the physical properties of BHs described by the Schwarzschild solution. To achieve this, specific properties of the dark energy star models need to be satisfied in the context of BHs, and their physical profiles are studied. The metric function [Formula: see text] proposed by M. R. Finch and J. E. F. Skea [Class. Quantum Grav. 6, 467 (1989)], as adopted by A. Banerjee, M. K. Jasim and A. Pradhan [Mod. Phys. Lett. A 35, 2050071 (2020), arXiv:1911.09546 [gr-qc]], is used and parametrized, making it close to BH spacetimes. The findings show that the model exhibits properties similar to BHs in terms of the stellar radius, compactness, surface redshift, and nature of gravity. Specifically, the dark energy star model behaves like BHs with a dark energy parameter [Formula: see text], satisfying all energy conditions. However, it should be noted that the investigation is limited to static spherically symmetric cases and further studies are required to explore the model in rotating cases. Overall, this study sheds light on the potential of dark energy star models in explaining BH properties and presents promising avenues for further research in understanding the nature of BHs and dark energy.

Luís F. Dias da Silva, F. Lobo, G. Olmo et al.

The imaging by the Event Horizon Telescope (EHT) of the supermassive central objects at the heart of the M87 and Milky Way (Sgr A$^\star$) galaxies, has marked the first step into peering at the photon rings and central brightness depression that characterize the optical appearance of black holes surrounded by an accretion disk. Recently, Vagnozzi et. al. [S.~Vagnozzi, \textit{et al.} arXiv:2205.07787 [gr-qc]] used the claim by the EHT that the size of the {\it shadow} of Sgr A$^\star$ can be inferred by calibrated measurements of the bright ring enclosing it, to constrain a large number of spherically symmetric space-time geometries. In this work we use this result to study some features of the first and second photon rings of a restricted pool of such geometries in thin accretion disk settings. The emission profile of the latter is described by calling upon three analytic samples belonging to the family introduced by Gralla, Lupsasca and Marrone, in order to characterize such photon rings using the Lyapunov exponent of nearly bound orbits and discuss its correlation with the luminosity extinction rate between the first and second photon rings. We finally elaborate on the chances of using such photon rings as observational discriminators of alternative black hole geometries using very long baseline interferometry.

Yvonne Stahl, S. Grabowski, A. Bleckmann et al.

A. Alho, Jos'e Nat'ario, P. Pani et al.

The purpose of this review it to present a renewed perspective of the problem of self-gravitating elastic bodies under spherical symmetry. It is also a companion to the papers (2022 Phys. Rev. D 105 044025, 2022 Phys. Rev. D 106 L041502) and (arXiv:2306.16584 [gr-qc]), where we introduced a new definition of spherically symmetric elastic bodies in general relativity, and applied it to investigate the existence and physical viability, including radial stability, of static self-gravitating elastic balls. We focus on elastic materials that generalize fluids with polytropic, linear, and affine equations of state, and discuss the symmetries of the energy density function, including homogeneity and the resulting scale invariance of the TOV equations. By introducing invariant characterizations of physically admissible initial data, we numerically construct mass-radius-compactness diagrams, and conjecture about the maximum compactness of stable physically admissible elastic balls.

O. Lunin, S. Mathur

Abstract Near-extremal black holes are obtained by exciting the Ramond sector of the D1-D5 CFT, where the ground state is highly degenerate. We find that the dual geometries for these ground states have throats that end in a way that is characterized by the CFT state. Below the black hole threshold we find a detailed agreement between propagation in the throat and excitations of the CFT. We study the breakdown of the semiclassical approximation and relate the results to the proposal of gr-qc/0007011 for resolving the information paradox: semiclassical evolution breaks down if hypersurfaces stretch too much during an evolution. We find that a volume V stretches to a maximum throat depth of V /2G .

V. Cardoso, R. Vicente, Z. Zhong

Energy exchange mechanisms have important applications in particle physics, gravity, fluid mechanics, and practically every field in physics. In this letter we show, both in frequency and time domain, that energy enhancement is possible for waves scattering off fundamental solitons (time-periodic localized structures of bosonic fields), without the need for rotation nor translational motion. We use two-dimensional Q-balls as a testbed, providing the correct criteria for energy amplification, as well as the respective amplification factors, and we discuss possible instability mechanisms. Our results lend support to the qualitative picture drawn in ( arXiv:2212.03269 [gr-qc] ); however we show that this enhancement mechanism is not of superradiant-type, but instead a"blueshift-like"energy exchange between scattering states induced by the background Q-ball, which should occur generically for any time-periodic fundamental soliton. This mechanism does not seem to lead to instabilities.

Eva Lope-Oter, Aneta Wojnar

We demonstrate how to construct GR-independent equations of state. We emphasize the importance of using theory-based principles instead of relying solely on astrophysical observables and General Relativity (GR). We build a set of equations of state based on first principles, including chiral perturbation theory and perturbation theory in quantum chromodynamics. Interpolation methods are employed to assume thermodynamic stability and causality in the intermediate region. These equations of state are then used to constrain quadratic Palatini $f(\mathcal R)$ gravity, indicating that the parameter lies within the range $-6.47 \lesssim β\lesssim 1.99$ km$^2$. Additionally, we briefly discuss the problem of phase transitions and twin stars.

Siao-Jing Li

The Witten spinor Hamiltonian formulation has previously been shown to be able to yield a quasilocalization for the GR energy-momentum which leads to a proof of the positive energy when the spinor satisfies the Witten equation. In this work we investigate whether this formulation can also describe the GR angular momentum. We conceive four candidates of the quadratic spinor Hamiltonian for the angular momentum based on Witten's scheme. The first one is acquired by substituting the spinor pseudovectorial parameterization for the spinor vectorial parameterization of the displacement in the Witten Hamiltonian. The other three each are composed of other quadratic spinor terms, all with the displacement consisting of the spinor parameterization of an antisymmetric 2-rank tensor and a position vector, one having 4 terms and the others each having twice distinct 2 of the 4 terms. With possible field perturbations around the at spacetime we find none of the four candidates can give the angular momentum quasilocalization. The importance and prospects for successfully including the angular momentum in Witten's formalism are discussed.

A. Napoly, Tom Grassmann, F. Meier et al.

In urban areas, dense atmospheric observational networks with high-quality data are still a challenge due to high costs to deploy and maintain them over time. Citizen weather stations (CWS) could be one answer to that issue. Since more and more owners of CWS share their measurement data publicly, crowdsourcing, i.e., the automated collection of large amounts of data from an undefined crowd of citizens, opens new pathways for atmospheric research. However, the most critical issue is found to be the quality of data from such networks. In this study, a statistically-based quality control (QC) is developed to identify suspicious air temperature (T) measurements from crowdsourced data sets. The newly developed QC exploits the combined knowledge of the dense network of CWS to statistically identify implausible measurements, independent of external reference data. The evaluation of the QC is performed using data from Netatmo CWS in Toulouse, France, and Berlin, Germany, over a one-year period (July 2016 to June 2017), comparing the quality-controlled data with data from a network of reference stations. The new QC efficiently identifies erroneous data due to solar exposition and siting issues, which are common error sources of CWS. Estimation of T is improved when averaging data from a group of stations within a restricted area rather than relying on data of individual CWS. However, a positive deviation in CWS data compared to reference data is identified, particularly for daily minimum T. To illustrate the transferability of the newly developed QC and the use of CWS data, a mapping of CWS data is performed over the city of Paris, France, where spatial density of CWS is especially high.