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We present a full general relativistic analytic solution for a radiation-pressure supported equilibrium fluid torus orbiting a rotating neutron star (NS). Previously developed analytical methods are thoroughly applied in the Hartle-Thorne geometry, including the effects of both the NS's angular momentum and quadrupole moment. The structure, size and shape of the torus are explored, focusing especially on the critically thick solution - the cusp tori. For the astrophysically relevant range of NS parameters, we examine how our findings differ from those obtained for the Schwarzschild spacetime. The solutions for rotating stars display signatures of the interplay between relativistic and Newtonian effects where the impact of NS angular momentum and quadrupole moment are almost counterbalanced at the given radius. Nevertheless, the spacetime parameters still strongly influence the size of tori, which can be shown in a coordinate-independent way. Finally, we discuss the importance of the size of the central neutron star, determining whether or not the surrounding torus may exist. We provide a set of tools in a Wolfram Mathematica code, which poses a basis allowing for a further investigation of the impact of the NSs' superdense matter equation of state on the spectral and temporal behaviour of accretion tori., Comment: 7 pages, 5 figures, submitted to A&A

The four characteristic oscillation frequencies of accretion flows are, in addition to the Keplerian orbital frequency, often discussed in the context of the time variability of the black hole and neutron star (NS) low-mass X-ray binaries (LMXBs). These are namely the frequencies of the axisymmetric radial and vertical epicyclic oscillations, and the frequencies of non-axisymmetric oscillations corresponding to the periastron (radial) and Lense-Thirring (vertical) precessions. In this context, we investigate the effect of the quadrupole moment of a slowly rotating NS and provide complete formulae for calculating these oscillation and precession frequencies, as well as their convenient approximations. Simple formulae corresponding to the geodesic limit of a slender torus (and test particle motion) and the limit of a marginally overflowing torus (torus exhibiting a critical cusp) are presented, and furthermore, more general approximate formulae are included to allow calculations for arbitrarily thick tori. We provide the Wolfram Mathematica code used for our calculations together with C++ and PYTHON codes for calculations of the frequencies. Our formulae can be used for various calculations describing the astrophysical signatures of the NSs' superdense matter equation of state. For instance, we demonstrate that, even for a given fixed number of free parameters, a model accounting for fluid flow precession better matches the frequencies of twin-peak quasiperiodic oscillations observed in NS LMXBs than a model using geodesic precession., Comment: 17 pages, 6 figures, submitted to A&A

The unification of quantum mechanics and general relativity has long been elusive. Only recently have empirical predictions of various possible theories of quantum gravity been put to test. The dawn of multi-messenger high-energy astrophysics has been tremendously beneficial, as it allows us to study particles with much higher energies and travelling much longer distances than possible in terrestrial experiments, but more progress is needed on several fronts. A thorough appraisal of current strategies and experimental frameworks, regarding quantum gravity phenomenology, is provided here. Our aim is twofold: a description of tentative multimessenger explorations, plus a focus on future detection experiments. As the outlook of the network of researchers that formed through the COST Action CA18108 "Quantum gravity phenomenology in the multi-messenger approach (QG-MM)", in this work we give an overview of the desiderata that future theoretical frameworks, observational facilities, and data-sharing policies should satisfy in order to advance the cause of quantum gravity phenomenology., Comment: Submitted to CQG for the Focus Issue on "Quantum Gravity Phenomenology in the Multi-Messenger Era: Challenges and Perspectives". Please contact us to express interesst of endorsement of this white paper

During the last century, two independent theories using the concept of dimensional reduction have been developed independently. The first, known as F\"oppl-von K\`arm\`an theory, uses Riemannian geometry and continuum mechanics to study the shaping of thin elastic structures which could become as complex as crumpled paper. The second one, known as Kaluza-Klein theory, uses Minkowskian geometry and general relativity to unify fundamental interactions and gravity under the same formalism. Here we draw a parallel between these two theories in an attempt to use concepts from elasticity theory of plates to recover the Einstein-Maxwell equations. We argue that Kaluza-Klein theory belongs to the same conceptual group of theories as three-dimensional elasticity, which upon dimensional reduction leads to the F\"oppl-von K\`arm\`an theory of two-dimensional elastic plates. We exploit this analogy to develop an alternative Kaluza-Klein formalism in the framework of elasticity theory in which the gravitational and electromagnetic fields are respectively associated with stretching-like and bending-like deformations. We show that our approach of dimensional reduction allows us to retrieve the Lagrangian densities of both gravitational, electromagnetic and Dirac spinors fields as well as the Lagrangian densities of mass and charge sources.

The Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the mid-2030s, is a gravitational wave observatory in space designed to detect sources emitting in the millihertz band. LISA is an ESA flagship mission, currently entering the Phase B development phase. It is expected to help us improve our understanding about our Universe by measuring gravitational wave sources of different types, with some of the sources being at very high redshifts $z\sim 20$. On the 23rd of February 2022 we organized the 1$^\mathrm{st}$ {\it LISA in Greece Workshop}. This workshop aimed to inform the Greek scientific and tech industry community about the possibilities of participating in LISA science and LISA mission, with the support of the Hellenic Space Center (HSC). In this white paper, we summarize the outcome of the workshop, the most important aspect of it being the inclusion of $15$ Greek researchers to the LISA Consortium, raising our total number to $22$. At the same time, we present a road-map with the future steps and actions of the Greek Gravitational Wave community with respect to the future LISA mission., Comment: 11 pages, 2 figures

Our limited understanding of the physical properties of matter at ultra-high density, high proton/neutron number asymmetry, and low temperature is presently one of the major outstanding problems in physics. As matter in this extreme state is known to only exist stably in the cores of neutron stars (NSs), complementary measurements from electromagnetic and gravitational wave astrophysical observations of NSs, combined with terrestrial laboratory constraints and further theoretical investigations, hold the promise to provide important insight into the properties of matter in a region of the quantum chromodynamics phase space that is otherwise inaccessible. This multidisciplinary endeavor imposes the following requirements for facilities and resources in the upcoming decade and beyond: * A next generation of gravitational wave detectors to uncover more double NS and neutron star-black hole mergers; * Sensitive radio telescopes to find the most massive and fastest spinning NSs; * Large-area, high-time-resolution and/or high angular resolution X-ray telescopes to constrain the NS mass-radius relation; * Suitable laboratory facilities for nuclear physics experiments to constrain the dense matter equation of state; * Funding resources for theoretical studies of matter in this regime; * The availability of modern large-scale high performance computing infrastructure. The same facilities and resources would also enable significant advances in other high-profile fields of inquiry in modern physics such as the nature of dark matter, alternative theories of gravity, nucleon superfluidity and superconductivity, as well as an array of astrophysics, including but not limited to stellar evolution, nucleosynthesis, and primordial black holes., Comment: Submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021) under Cosmic Frontier (CF07: Cosmic probes of fundamental physics); 30 pages, 8 figures

This is a solicited whitepaper for the Snowmass 2021 community planning exercise. The paper focuses on measurements and science with the Cosmic Microwave Background (CMB). The CMB is foundational to our understanding of modern physics and continues to be a powerful tool driving our understanding of cosmology and particle physics. In this paper, we outline the broad and unique impact of CMB science for the High Energy Cosmic Frontier in the upcoming decade. We also describe the progression of ground-based CMB experiments, which shows that the community is prepared to develop the key capabilities and facilities needed to achieve these transformative CMB measurements., Comment: contribution to Snowmass 2021

This short review is intended as a colloquium-level summary, for the Snowmass 2021 process, on recent theoretical results on infrared observables in quantum gravity. We rely on simple physical arguments, most notably a random walk intuition, to show how effects of quantum gravity in the ultraviolet (at the Planck length $\ell_p \approx 10^{-35} \mbox{ m}$) may integrate into the infrared when the large measurement length scale $L$ enters into the observable. A quantum uncertainty at lightsheet horizons would give rise to an accumulated effect of size $\delta L^2 \simeq \ell_p L/4 \pi$. We discuss how the random walk intuition falls out from more formal calculations, such as from AdS/CFT, from the dimensional reduction of the Einstein-Hilbert action to dilaton gravity, from multiple gravitational shockwaves generated by vacuum energy fluctuations, as well as from an effective description of gravity as a fluid. We overview experimental prospects for measuring this effect with a simple Michelson interferometer utilizing many of the tools developed for gravitational wave observatories., Comment: 16 pages, 3 figures

The double copy is, in essence, a map between scattering amplitudes in a broad variety of familiar field and string theories. In addition to the mathematically rich intrinsic structure, it underlies a multitude of active research directions and has a range of interesting applications in quantum, classical and effective field theories, including broad topics such as string theory, particle physics, astrophysics, and cosmology. This Snowmass white paper provides a brief introduction to the double copy, its applications, current research and future challenges., Comment: 53 pages + refs, 4 figures, contribution to Snowmass 2021; feedback welcome, contact: JJ Carrasco

High-energy physics is primarily concerned with uncovering the laws and principles that govern nature at the fundamental level. Research in this field usually relies on probing the boundaries of established physics, an undertaking typically associated with extreme energy and distance scales. It is therefore unsurprising that particle physics has traditionally been dominated by large-scale experimental methods often involving high energies, such as colliders and storage rings, cosmological and astrophysical observations, large-volume detector systems, etc. However, high-sensitivity measurements in smaller experiments, often performed at lower energies, are presently experiencing a surge in importance for particle physics for at least two reasons. First, they exploit synergies to adjacent areas of physics with recent advances in experimental techniques and technology. Together with intensified phenomenological explorations, these advances have led to the realization that challenges associated with weak couplings or the expected suppression factors for new physics can be overcome with such methods while maintaining a large degree of experimental control. Second, many of these measurements broaden the range of particle-physics phenomena and observables relative to the above set of more conventional methodologies. Combining such measurements with the conventional efforts above therefore casts both a wider and tighter net for possible effects originating from physics beyond the Standard Model (BSM). This paper argues that this assessment points at a growing impact of such methods and measurements on high-energy physics, and therefore warrants direct support as particle-physics research. Leveraging the recent rapid progress and bright outlook associated with such studies for high-energy physics, could yield high returns, but requires substantial and sustained efforts by funding agencies., Comment: Contribution to Snowmass 2021, 55 pages, 5 figures

This white paper summarizes recent progress in the cosmological bootstrap, an approach to the study of the statistics of primordial fluctuations from consistency with unitarity, locality and symmetry assumptions. We review the key ideas of the bootstrap method, with an eye towards future directions and ambitions of the program. Focusing on recent progress involving de Sitter and quasi-de Sitter backgrounds, we highlight the role of singularities and unitarity in constraining the form of the correlators. We also discuss nonperturbative formulations of the bootstrap, connections to anti-de Sitter space, and potential implications for holography., Comment: 52 pages, 7 figures; Contribution to Snowmass 2021

This Snowmass 2021 White Paper describes the Cosmic Microwave Background Stage 4 project CMB-S4, which is designed to cross critical thresholds in our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. We provide an overview of the science case, the technical design, and project plan., Comment: Contribution to Snowmass 2021. arXiv admin note: substantial text overlap with arXiv:1908.01062, arXiv:1907.04473

Gravitational wave detectors are formidable tools to explore strong-field gravity, especially black holes and neutron stars. These compact objects are extraordinarily efficient at producing electromagnetic and gravitational radiation. As such, they are ideal laboratories for fundamental physics and have an immense discovery potential. The detection of black hole binaries by third-generation Earth-based detectors, space-based detectors and pulsar timing arrays will provide exquisite tests of general relativity. Loud "golden" events and extreme mass-ratio inspirals can strengthen the observational evidence for horizons by mapping the exterior spacetime geometry, inform us on possible near-horizon modifications, and perhaps reveal a breakdown of Einstein's gravity. Measurements of the black-hole spin distribution and continuous gravitational-wave searches can turn black holes into efficient detectors of ultralight bosons across ten or more orders of magnitude in mass. A precise monitoring of the phase of inspiralling binaries can constrain the existence of additional propagating fields and characterize the environment in which the binaries live, bounding the local dark matter density and properties. Gravitational waves from compact binaries will probe general relativity and fundamental physics in previously inaccessible regimes, and allow us to address fundamental issues in our current understanding of the cosmos., Comment: Contribution to Snowmass 2021 - Cosmic Frontier (CF07: Cosmic probes of fundamental physics). 24 pages, 4 figures. v2: added list of endorsers and references to related White Papers, minor corrections

We review recent progress and future prospects for harnessing powerful tools from theoretical high-energy physics, such as scattering amplitudes and effective field theory, to develop a precise and systematically improvable framework for calculating gravitational-wave signals from binary systems composed of black holes and/or neutron stars. This effort aims to provide state-of-the-art predictions that will enable high-precision measurements at future gravitational-wave detectors. In turn, applying the tools of quantum field theory in this new arena will uncover theoretical structures that can transform our understanding of basic phenomena and lead to new tools that will further the cycle of innovation. While still in a nascent stage, this research direction has already derived new analytic results in general relativity, and promises to advance the development of highly accurate waveform models for ever more sensitive detectors., Comment: contribution to Snowmass 2021; 28+28 pages, 5 figures

The current understanding of dark matter comes largely from measurements of the total matter content in the universe, from the distribution of gravitating matter on very large scales, and from rotation curves and velocity dispersions on sub-galactic scales. However, small-scale structure may well hold the key to unlocking the particle physics of the still-mysterious 85% of matter in the universe. Novel small-scale astrophysical probes of new particles and dark matter will become possible with a large data volume of high-precision measurements enabled by next-generation gravitational-wave detectors and advanced astrometry instruments. Cosmic microwave background and large-scale structure surveys will provide complementary constraints on dark matter models with unique small-scale signatures. We lay out the studies of small-scale structures and compact objects as dark matter probes, and summarize the requirements to achieve the goals., Comment: 43 pages, 7 figures

Progress in the quantum readout and control of mechanical devices from single atoms to large masses may enable a first generation of experiments probing the gravitational interaction in the quantum regime, conceivably within the next decade. In this Snowmass whitepaper, we briefly outline the possibilities and challenges facing the realization of these experiments. In particular, we emphasize the need for detailed theories of modifications to the usual effective QFT of gravitons in the infrared regime $E/L^3 \ll m_{\rm Pl}/\ell_{\rm Pl}^3$ in which these experiments operate, and relations to possible UV completions., Comment: Short contribution to Snowmass 2021. 5 pages, 1 figure. v2: typo fixed

Small fluctuations around homogeneous and isotropic expanding backgrounds are the main object of study in cosmology. Their origin and evolution is sensitive to the physical processes that happen during inflation and in the late Universe. As such, they hold the key to answering many of the major open questions in cosmology. Given a large separation of relevant scales in many examples of interest, the most natural description of these fluctuations is formulated in terms of effective field theories. This was the main avenue for many of the important modern developments in theoretical cosmology, which provided a unifying framework for a plethora of cosmological models and made a clear connection between the fundamental cosmological parameters and observables. In this review we summarize these results in the context of effective field theories of inflation, large-scale structure, and dark energy., Comment: Contribution to Snowmass 2021

The next generation of gravitational-wave observatories can explore a wide range of fundamental physics phenomena throughout the history of the universe. These phenomena include access to the universe's binary black hole population throughout cosmic time, to the universe's expansion history independent of the cosmic distance ladders, to stochastic gravitational-waves from early-universe phase transitions, to warped space-time in the strong-field and high-velocity limit, to the equation of state of nuclear matter at neutron star and post-merger densities, and to dark matter candidates through their interaction in extreme astrophysical environments or their interaction with the detector itself. We present the gravitational-wave detector concepts than can drive the future of gravitational-wave astrophysics. We summarize the status of the necessary technology, and the research needed to be able to build these observatories in the 2030s., Comment: 31 pages, 5 figures, contribution to Snowmass 2021

The next generation of gravitational-wave detectors, conceived to begin operations in the 2030s, will probe fundamental physics with exquisite sensitivity. These observations will measure the equation of state of dense nuclear matter in the most extreme environments in the universe, reveal with exquisite fidelity the nonlinear dynamics of warped spacetime, put general relativity to the strictest test, and perhaps use black holes as cosmic particle detectors. Achieving each of these goals will require a new generation of numerical relativity simulations that will run at scale on the supercomputers of the 2030s to achieve the necessary accuracy, which far exceeds the capabilities of numerical relativity and high-performance computing infrastructures available today., Comment: contribution to Snowmass 2021

Quantum gravity places important consistency conditions on low-energy effective field theory, such as the absence of global symmetries. These may have important consequences in the search for particle physics beyond the Standard Model. We review some of these conditions and their phenomenological implications for the strong CP problem, the weak scale, new gauge interactions, and cosmology. We also offer some general comments on how these ideas can guide model building., Comment: Contribution to Snowmass 2021; 11 pages + references

Black holes provide a window into the microscopic structure of spacetime in quantum gravity. Recently the quantum information contained in Hawking radiation has been calculated, verifying a key aspect of the consistency of black hole evaporation with quantum mechanical unitarity. This calculation relied crucially on recent progress in understanding the emergence of bulk spacetime from a boundary holographic description. Spacetime wormholes have played an important role in understanding the underpinnings of this result, and the precision study of such wormholes, in this and other contexts, has been enabled by the development of low-dimensional models of holography. In this white paper we review these developments and describe some of the deep open questions in this subject. These include the nature of the black hole interior, potential applications to quantum cosmology, the gravitational explanation of the fine structure of black holes, and the development of further connections to quantum information and laboratory quantum simulation., Comment: 16 + 17 pages. v2: references added

In this paper, we explore the possibility of the formation of a traversable wormhole geometry in General Relativity sustained by particle creation mechanism. Gravitational particle creation mechanism is a thermodynamic approach to generate repulsive back-reaction gravitational effects through the thermodynamic creation of particles. This repulsive back-reaction pressure can be thought of as a source of sustaining a traversable wormhole. In the first part of this paper, we systematically model a wormhole geometry through the process of particle creation by assuming an inverse square variation law of particle creation pressure within the wormhole geometry, and the shape function for the wormhole is obtained, assuming the redshift function as constant and finite. The obtained shape function is found to adhere to the feasibility conditions (flaring out condition and asymptotic flatness condition) of a viable wormhole geometry. In the second part of the paper, again by assuming a constant and finite redshift function, we studied the radial evolutionary behaviour of the particle creation pressures for three commonly utilized toy shape-functions. The examination of the energy conditions namely the Null Energy Condition (NEC) and the Dominant Energy Condition (DEC) for all these models reveal that the NEC is violated for the shape-function derived from the first principles of particle creation mechanism, as well as for the toy shape functions. The DEC is however found to be satisfied for all the aforementioned cases. In essence, NEC violation occurs through the particle creation phenomena inside the wormhole structure in spite of the existence of non-exotic matter., Comment: Suggestions and Comments are welcome

It has been shown that entropy differences between certain states of perturbative quantum gravity can be computed without specifying an ultraviolet completion. This is analogous to the situation in classical statistical mechanics, where entropy differences are defined but absolute entropy is not. Unlike in classical statistical mechanics, however, the entropy differences computed in perturbative quantum gravity do not have a clear physical interpretation. Here we construct a family of perturbative black hole states for which the entropy difference can be interpreted as a relative counting of states. Conceptually, this paper begins with the algebra of mass fluctuations around a fixed black hole background, and points out that while this is a type I algebra, it is not a factor and therefore has no canonical definition of entropy. As in previous work, coupling the mass fluctuations to quantum matter embeds the mass algebra within a type II factor, in which entropy differences (but not absolute entropies) are well defined. It is then shown that for microcanonical wavefunctions of mass fluctuation, the type II entropy difference equals the logarithm of the dimension of the extra Hilbert space that is needed to map one microcanonical window to another using gauge-invariant unitaries. The paper closes with comments on type II entropy difference in a more general class of states, where the von Neumann entropy difference does not have a physical interpretation, but "one-shot" entropy differences do., Comment: 5 pages

Ever since its discovery by Roy Kerr in 1963, the geometry around rotating, electrostatically-neutral black holes, otherwise known as Kerr black holes, has significantly contributed to theoretical developments in the fields of general relativity, thermodynamics and beyond. Extremal Kerr black holes, in which the spin parameter of the rotating black hole is at a maximum without breaching certain postulates, has especially been of interest to the research community due to its sensitivity to higher order corrections to Einstein's theory of relativity. In this paper, we review some unresolved open questions regarding the discontinuity of certain properties of the Kerr black hole as it approaches the extreme limit. These include the form of the innermost stable circular orbit (ISCO), the nature of vanishing entropy, and the disappearance of the trapped surface as a Kerr black hole approaches extremality. We conclude that current theories are likely not suitable for studies of extremal black holes, and we posit that further development in theory may likely require a quantum approach to gravity. G=c=1 and Einstein summation convention are assumed throughout this paper., Comment: 2 figures, 9 pages. Literature review

This paper investigates Buchdahl transformations within the framework of Einstein and Einstein-Scalar theories. Specifically, we establish that the recently proposed Schwarzschild-Levi-Civita spacetime can be obtained by means of a Buchdahl transformation of the Schwarschild metric along the spacelike Killing vector. The study extends Buchdahl's original theorem by combining it with the Kerr-Schild representation. In doing so, we construct new vacuum-rotating black holes in higher dimensions which can be viewed as the Levi-Civita extensions of the Myers-Perry geometries. Furthermore, it demonstrates that the double copy scheme within these new generated geometries still holds, providing an example of an algebraically general double copy framework. In the context of the Einstein-Scalar system, the paper extends the corresponding Buchdahl theorem to scenarios where a static vacuum seed configuration, transformed with respect to a spacelike Killing vector, generates a hairy black hole spacetime. We analyze the geometrical features of these spacetimes and investigate how a change of frame, via conformal transformations, leads to a new family of black hole spacetimes within the Einstein-Conformal-Scalar system.

This paper combines the post-Minkowskian expansion of general relativity with the language of intersection theory. Due to the nature of the soft limit inherent to the post-Minkowskian expansion, the intersection-based approach is of enhanced utility in that theory compared to a generic QFT. In the language of intersection theory, Feynman integrals are rephrased in terms of twisted cocycles. The intersection number is a pairing between two such cocycles and its existence allows for the direct projection onto a basis of master integrals. In this paper we use this approach to compute the 2PM contribution to the scattering of two compact astronomical objects, getting results in agreement with previous findings., Comment: Ten pages, two figures

From Event Horizon Telescope (EHT) and other observations on M87* and Sagittarius A* (Sgr A*), which are supermassive objects in M87 and our galaxy, respectively, we can put a constrain on the electrical or alternative charges of M87* and Sgr A*. The constraints on the charges by the EHT collaboration are based on the change rate of the radius of a photon sphere while their general relativistic magnetohydrodynamic simulations of black holes implies that the observed rings are formed by the gravitational lensing of synchrotron radiations from a hot plasma near supermassive black holes. On this paper, we investigate the constraints on the charges of M87* and Sgr A* by using the change rate of size of not the photon sphere but the lensing ring, which is the image of a disk edge at the innermost stable circular orbit of a particle, lensed nearly outside of the photon sphere as the simplest model. We show that the bounds of the charge by using the change rate of size of the lensing ring is more relaxed than the ones by the photon sphere obtained by the EHT collaboration. We concentrate on the Reissner-Nordstr\"{o}m black hole spacetime in this paper, but our result implies the relaxation of the bound of the charge parameters on other black hole spacetimes., Comment: 12 pages, 9 figures

The nature of equation of state for the matter in the neutron star plays an important role in determining its maximal mass. In addition, it must comply with the condition of causality. Noting that the central density of a maximally massive neutron star is well above the nuclear saturation density, a deconfined quark core in the central region is motivated in this paper. We analyze this scenario by employing the MIT bag model to represent the core region and one of the unified equations of state for the region outside the core. Such combination is found to solve the problem of causality violation. In each case of the combined equations of state, the radial profile of $\rho r^2$ displays a peak and dominant contribution to the total mass of the star comes from the region around the peak value of $\rho r^2$, whereas the contribution is small from the regions near the center and the surface. This peak occurs in the region of hadronic matter for the combinations considered in this paper. Importantly, we find that the position of the peak in $\rho r^2$ is well-correlated with the maximal mass -- the highest value of $1.98\ M_\odot$ obtains for the case with the peak occurring farthest from the center. This gravitational threshold being obtained for a non-rotating neutron star, we expect the threshold to lie well above 2 $ M_\odot$ for a rapidly rotating neutron star, that may explain the existance of massive pulsars from recent astronomical observations.

In a recent paper, the NANOGrav collaboration studied new physics explanations of the observed pulsar timing residuals consistent with a stochastic gravitational wave background (SGWB), including cosmic strings in the Nambu-Goto (NG) approximation. Analysing one of current models for the loop distribution, it was found that the cosmic string model is disfavored compared to other sources, for example, super massive black hole binaries (SMBHBs). When both SMBHB and cosmic string models are included in the analysis, an upper bound on a string tension $G\mu \lesssim 10^{-10}$ was derived. However, the analysis did not accommodate results from cosmic string simulations in an underlying field theory, which indicate that at most a small fraction of string loops survive long enough to emit GW. Following and extending our previous study, we suppose that a fraction $f_{\rm NG}$ of string loops follow NG dynamics and emit only GWs, and study the three different models of the loop distribution discussed in the LIGO-Virgo-KAGRA (LVK) collaboration analyses. We re-analyze the NANOGrav 15yrs data with our signal models by using the NANOGrav $\texttt{ENTERPRISE}$ analysis code via the wrapper $\texttt{PTArcade}$. We find that loop distributions similar to LVK Model B and C yield higher Bayes factor than Model A analyzed in the NANOGrav paper, as they can more easily accommodate a blue-tilted spectrum of the observed amplitude. Furthermore, because of the degeneracy of $G\mu$ and $f_{\rm NG}$ in determining the signal amplitude, our posterior distribution extends to higher values of $G\mu$, and in some cases the uppermost value of credible intervals is close to the Cosmic Microwave Background limit $G\mu \lesssim 10^{-7}$. Hence, in addition to the pulsar timing array data, further information about the fraction of long-lived loops in a cosmic string network is required to constrain the string tension., Comment: 22 pages, 9 figures; v2: references are added

In order to solve problem of spacetime singularity in theoretical physics, researchers proposed the regular black holes (BH). The generalized black-bounce (GBB) spacetime, as a unified treatment of distinct kinds of geometries in the framework of general relativity (e.g. regular BH and wormholes), has been extensively studied. Firstly, we derive to give the explicit forms of Lagrangian for a nonlinear electromagnetic field and potential for a non-canonical phantom field in the action of gravitational system corresponding to GBB solution. Secondly, this paper computes the radius of the innermost stable circular orbit (ISCO) and the stable circular orbit region for different types of celestial bodies in GBB spacetime. The research suggests that traversable wormholes may have two ISCOs or one ISCO depending on the throat's scale, whereas regular BH and extremal BH possess only one ISCO. Thirdly, quasi-periodic oscillations (QPOs) have been found to be a reliable tool for testing gravitational theories. Therefore, we compute the radial and azimuthal epicyclic angular frequencies of particles oscillating on stable circular orbits around various celestial bodies and compare them with the oscillation frequency properties of schwarzschild BH. Moreover, due to the limited amount of research on the high-frequency quasi-periodic oscillations (HFQPOs) phenomenon and its generation mechanisms around particles near wormholes using observational data, this paper aims to study theoretical models that can simultaneously describe both BH and wormholes by fitting observational data. Using resonance models and associated frequency ratios, we are able to locate the resonances of different celestial bodies within the GBB spacetime., Comment: 20 pages, 8 figures

In this paper, we explore the effects of General Relativity modification on the Mass-Radius relations of Neutron Stars induced by the presence of the Quintessence field. We consider, in particular, the Kiselev model, according to which the Quintessence field, being present in the entire Universe, might also be present around massive objects. Considering the Equation of State (EoS) for Baryonic matter BSk22 derived by A. Y. Potekhin et al., we infer the upper limit for NS masses in the presence of Quintessence. The presence of Quintessence generates a peculiar effect for which the Mass-Radius relation is unvaried and therefore the presence of Quintessence is indistinguishable from ordinary matter, at least for the Kiselev model studied in this paper., Comment: 5 pages, 2 figures

We investigate dynamical effects of particles moving around rotating SV regular BH and traversable wormholes, and the deviation from Kerr BH. We found that the rotating SV traversable wormhole spacetime has a closer innermost stable circular orbit to the central object than Kerr and rotating regular BHs. Furthermore, the paper provides the general form of the epicycle frequency in this spacetime, presenting the radial profile of the angular frequency of particles undergoing oscillatory motion. Unlike the commonly used $\chi$-square analysis method, with considering a generalized data analysis approach in this paper we hypothesize that the observational data of three microguasars can be explained through an axisymmetric SV spacetime, fitting the data to find the range of spin values for three types of SV objects, and analyzing the possible mechanisms of HFQPOs (high-frequency quasi-periodic oscillations) generation for different types of axisymmetric SV objects. We found that for a Kerr BH with $l^*=0$, the observational data of three microquasars can be explained by the HFQPOs model used in this paper (excluding ER4 model); for a regular BH with $l^*=1$, the HFQPO phenomenon observed through the three microquasars can be explained by the ER0, ER1, ER2, ER3, RP0, RP1, RP2, TD and WD models. For a traversable wormhole with $l^*=3$, the mechanism of HFQPO can be interpreted by the ER1, ER3, RP0, RP1, RP2, TD and WD models. Moreover, we observed that for the same HFQPOs model, the limiting range of the spin parameter $\alpha^*$ increases with the increase of the parameter $l^*$., Comment: 18 pages, 6 figures

In most analytical studies of light ray propagation in curved spacetimes around a gravitating object surrounded by a medium, it is assumed that the medium is a cold nonmagnetized plasma. The distinctive feature of this environment is that the equations of motion of the rays are independent of the plasma velocity, which, however, is not the case in other media. In this paper, we consider the deflection of light rays propagating near a spherically symmetric gravitating object in a moving dispersive medium given by a general refractive index. The deflection is studied when the motion of the medium is defined either as a radially falling onto a gravitating object (e.g., black hole), or rotating in the equatorial plane. For both cases the deflection angles are obtained. These examples demonstrate that fully analytic expressions can be obtained if the Hamiltonian for the rays takes a rather general form as a polynomial in a given momentum component. The general expressions are further applied to three specific choices of refractive indices and these cases are compared. Furthermore, the light rays propagating around a gravitating object surrounded by a generally moving medium are further studied as a small perturbation of the cold plasma model. The deflection angle formula is hence expressed as a sum of zeroth and first order components, where the zeroth order term corresponds to the known cold plasma case and the first order correction can be interpreted as caused by small difference in the refractive index compared to the cold plasma. The results presented in this paper allow to describe the effects caused by the motion of a medium and thus go beyond cold nonmagnetized plasma model.

This paper analyzes the effective field theory perspective on modern physics through the lens of the quantum theory of gravitational interaction. The historical part argues that the search for a theory of quantum gravity stimulated the change in outlook that characterizes the modern approach to the Standard Model of particle physics and General Relativity. We present some landmarks covering a long period, i.e., from the beginning of the 1930s until 1994, when, according to Steven Weinberg, the modern bottom-up approach to General Relativity began. Starting from the first attempt to apply the quantum field theory techniques to perturbatively quantize Einstein's theory, we explore its developments and interaction with the top-down approach encoded by String Theory. In the last part of the paper, we focus on this last approach to describe the relationship between our modern understanding of String Theory and Effective Field Theory in today's panorama. To this end, the non-historical part briefly explains the modern concepts of moduli stabilization and Swampland to understand another change in focus that explains the present framework where some string theorists move., Comment: Accepted for publication by European Physics Journal H for the Special Issue: History for Physics: Contextualizing Modern Developments in the Foundations of Quantum Theory

Earth-based Very Long Baseline Interferometry (VLBI) has made rapid advances in imaging black holes. However, due to the limitations imposed on terrestrial VLBI by the Earth's finite size and turbulent atmosphere, it is imperative to have a space-based component in future VLBI missions. Herein, this paper investigates the effect of Earth's oblateness, also known as the $J_{2}$ effect, on orbiters in Earth-Space and Space-Space VLBI. The paper provides an extensive discussion on how the $J_{2}$ effect can directly impact orbit selection for black hole observations and how through informed choices of orbital parameters, the effect can be used to the mission's advantage, a fact that has not been addressed in existing space-VLBI investigations. We provide a comprehensive study of how the orbital parameters of several current space VLBI proposals will vary specifically due to the $J_{2}$ effect. For black hole accretion flow targets of interest, we have demonstrated how the $J_{2}$ effect leads to modest increase in shorter baseline coverage, filling gaps in the $(u,v)$ plane. Subsequently, we construct a simple analytical formalism that allows isolation of the impact of the $J_{2}$ effect on the $(u,v)$ plane without requiring computationally intensive orbit propagation simulations. By directly constructing $(u,v)$ coverage using the $J_{2}$ affected and invariant equations of motion, we obtain distinct coverage patterns for M87* and SgrA* that show extremely dense coverage on short baselines as well as long term orbital stability on longer baselines., Comment: Accepted in ApJ

This research paper presents a black hole solution with a rational non-linear electrodynamics source in the Rastall gravity framework. The paper analyzes the thermodynamic properties of the solution in normal phase space and explores its critical behavior. The phase structure is examined using the extended first law of thermodynamics, with the cosmological constant $\Lambda$ serving as pressure $P$. The isotherms exhibit van der Waals behavior at small values of horizon $r_+$. The paper also investigates the Gibbs free energy behavior and finds two critical points with two pressures where the reentrant phase transition occurs and disappears. On the other hand, we explore the prevalent microstructure of black holes in Ruppeiner geometry, uncovering significant deviations in the nature of particle interactions from conventional practice. Moreover, the thermodynamic geometry is analyzed using the Ruppeiner formalism, with the normalized Ricci scalar indicating possible point-phase transitions of the heat capacity, and the normalized extrinsic curvature having the same sign as the normalized Ricci scalar. The three-phase transitions of the heat capacity are those that we find for the normalized Ruppeiner curvatures. Thus, there is an absolute correspondence., Comment: 23 pages, 7 figures

In recent years, there has been an increase in the number of papers regarding general-relativistic explanations for the dark matter phenomena in disc galaxies. The main focus of this scientific discussion is whether a previously unexamined relativistic dragging vortex could support flat rotation curves, with various research groups taking different stances on its feasibility. In this paper, we discuss the different points of view by placing the various arguments within a general theoretical context. We explicitly state the conceptual assumptions, and indicate what we believe to be the correct interpretation for the physical quantities of interest. We show how the dragging conjecture fails under certain hypotheses, and discuss the flaws of the most common dragging models: the linearised gravitomagnetic description and the one of Balasin&Grumiller. On the other hand, we illustrate how to avoid failure scenarios for the conjecture, emphasizing the features for a physically reasonable disc galaxy dragging model. In particular, we stress that the non-linearities of the Einstein Equations must play an essential role in generating what we define as ``pseudo-solitonic" solutions -- the only non-trivial physically viable solutions for the class of models considered. Furthermore, the dragging vortex is proven to show important contributions to the gravitational lensing in these models, thus providing an ulterior measure of its relevance. Moreover, by qualitatively exploring these pseudo-solitonic solutions, we find that a dragging speed of just a few kilometers per second would be enough to explain a non-negligible fraction of the galactic dark matter. Finally, we propose and analyse the feasibility of three independent measurements which could be carried out to detect the presence of dragging vortices in disc galaxies., Comment: 28 pages, 4 figures

In this paper, we propose a new Analogue Gravity example - a spinning (or Kerr) Black Hole in an extended fluid model. The fluid model receives Berry curvature contributions and applies to electron dynamics in Condensed Matter lattice systems in the hydrodynamic limit. We construct the acoustic metric for sonic fluctuations that obey a structurally relativistic wave equation in an effective curved background. In a novel approach of dimensional analysis, we have derived explicit expressions for effective mass and angular momentum per unit mass in the acoustic metric (in terms of fluid parameters), to identify with corresponding parameters of the Kerr metric. The spin is a manifestation of the Berry curvature-induced effective noncommutative structure in the fluid. Finally we put the Kerr Black Hole analogy in a robust setting by revealing explicitly the presence of horizon and ergo-region for a specific background fluid velocity profile. We also show that near horizon behavior of the phase-space trajectory of a probe particle agrees with Kerr Black Hole analogy. In fluid dynamics perspective, presence of a horizon signifies the wave blocking phenomenon., Comment: Accepted in Physical Review D and this paper has overlap with our earlier paper arXiv:2306.12201 [gr-qc]

Future gravitational wave observatories can probe dark matter by detecting the dephasing in the waveform of binary black hole mergers induced by dark matter overdensities. Such a detection hinges on the accurate modelling of the dynamical friction, induced by dark matter on the secondary compact object in intermediate and extreme mass ratio inspirals. In this paper, we introduce NbodyIMRI, a new publicly available code designed for simulating binary systems within cold dark matter `spikes'. Leveraging higher particle counts and finer timesteps, we validate the applicability of the standard dynamical friction formalism and provide an accurate determination of the maximum impact parameter of particles which can effectively scatter with a compact object, across various mass ratios. We also show that in addition to feedback due to dynamical friction, the dark matter also evolves through a `stirring' effect driven by the time-dependent potential of the binary. We introduce a simple semi-analytical scheme to account for this effect and demonstrate that including stirring tends to slow the rate of dark matter depletion and therefore enhances the impact of dark matter on the dynamics of the binary., Comment: 11 pages, 9 figures + appendices. Companion paper to "Sharpening the dark matter signature in gravitational waveforms I: Accretion and eccentricity evolution" (arXiv:2402.13053). NbodyIMRI code available here: https://github.com/bradkav/NbodyIMRI

This paper provides an analytical examination of non-radial geodesics within the context of the spatially flat Friedmann Lema\^itre Robertson Walker (FLRW) spacetime. Using the symmetry properties of the system, two constants of motion related to this dynamical system are derived. This treatment enables us to explicitly compute the radial and angular peculiar velocities, as well as the evolution of the comoving radial distance and angle over time. The study introduces three distinct methods to characterize geodesic solutions, and additionally examines the radial geodesic limit, the null geodesic limit, and the comoving geodesic limit. Additionally, the paper investigates both non-radial and radial free geodesics within the Lambda Cold Dark Matter ({\Lambda}CDM) model, presenting detailed results and discussions. Lastly, we explore the total angle swept by a free particle from the initial singularity to infinity as measured by a comoving observer, offering insights into the dynamics of free particles in FLRW spacetime., Comment: 39 pages, 11 figures

We investigate the status of the gravitational arrow of time in the case of a spherical collapse of a fluid that conducts heat and radiates energy. In particular, we examine the results obtained by W. B. Bonnor in his 1985 paper where he found that the gravitational arrow of time was opposite to the thermodynamic arrow of time. The measure of gravitational epoch function $P$ used by Bonnor was given by the ratio of the Weyl square to the Ricci square. In this paper, we have assumed the measure of gravitational entropy $P_{1}$ to be given by the ratio of the Weyl scalar to the Kretschmann scalar. Our analysis indicates that Bonnor's result seems to be validated, i.e., the gravitational arrow and the thermodynamic arrow of time point in opposite directions. This strengthens the opinion that the Weyl proposal of gravitational entropy applies only to the universe as a whole (provided that we exclude the white wholes)., Comment: 10 pages

Cosmological correlators hold the key to high-energy physics as they probe the earliest moments of our Universe, and conceal hidden mathematical structures. However, even at tree-level, perturbative calculations are limited by technical difficulties absent in flatspace Feynman diagrammatics. In this paper, we introduce CosmoFlow: a new accurate open source Python code that computes tree-level cosmological correlators by tracing their time flow. This code is specifically designed to offer a simple, intuitive and flexible coding environment to theorists, primordial and late-time cosmologists. It can typically serve to complement analytical computations, to provide physical intuition when studying various inflationary theories, and to obtain exact results in regimes that are analytically out of reach. This paper presents the basic structure of CosmoFlow, leads the reader through an in-depth user-guide, and illustrates how it can be used with a series of worked examples. Our hope is that this first building block sets the stage for a bank of theoretical data, which can be nurtured and enhanced collaboratively by the community. CosmoFlow is publicly available on GitHub., Comment: 43 pages, 15 figures

In this paper, we explore the Joule-Thomson expansion (JTE) process for the Einstein-Power-Young-Mills (EPYM) and the AdS Kerr Sen (AKS) black holes. We study the effect of free parameters on the Joule-Thomson coefficient (JTC), the inversion curve, and the $T_i^{min}/T_{c}$. The isenthalpic curves of the AKS black hole show cooling or heating behavior depending on the inversion curve, which is affected by the mass and the parameters $b$ and $a$ of the black hole. If we assume the parameter $b$ to be zero, the results reduce to the Kerr-AdS black holes[1]. In [2,3], for the Einstein-Power-Yang-Mills AdS black hole with $q>1$ and $n=2$, the $T_i^{min}/T_{c}$ is $1/2$. But in this paper, for the AdS-Maxwell-power-Yang-Mills black hole, when $q>1$, the $T_i^{min}/T_{c}$ is almost equal to $1/2$ for the increase of Maxwell's charge $C$, and when $q=1/2$, the $T_i^{min}/T_{c}$ is equal to $1/2$ for all values of $C$. Also, when $1/2

This paper presents an analytical study of the behavior of radial free-geodesics in the Friedmann-Lema\^itre-Robertson-Walker (FLRW) spacetime within the Lambda Cold Dark Matter ({\Lambda}CDM) model. Using the radial free motion solutions, we provide two methods for characterizing the geodesics and defines a general formula that encapsulates all possible solutions, determined by two initial conditions. We show that the past light cone, event horizon, and particle horizon, can be considered as special cases of this overarching formula. Furthermore, the paper explores the free geodesics within the currently accepted cosmological model based on the recent Planck results, thoroughly examining the various possible geodesic scenarios., Comment: 28 pages, 6 figures, and 1 table

As members of the Virgo Collaboration -- one of the large scientific collaborations that explore the universe of gravitational waves together with the LIGO Scientific Collaboration and the KAGRA Collaboration -- we became aware of biased citation practices that exclude Virgo, as well as KAGRA, from achievements that collectively belong to the wider LIGO/Virgo/KAGRA Collaboration. Here, we frame these practices in the context of Merton's Matthew effect, extending the reach of this well studied cognitive bias to include large international scientific collaborations. We provide qualitative evidence of its occurrence, displaying the network of links among published papers in the scientific literature related to Gravitational Wave science. We note how the keyword LIGO is linked to a much larger number of papers and variety of subjects than the keyword Virgo. We support these qualitative observations with a quantitative study based on a year-long monitoring of the relevant literature, where we scan all new preprints appearing in the arXiv electronic preprint database. Over the course of one year we identified all preprints failing to assign due credits to Virgo. As a further step, we undertook positive actions by asking the authors of problematic papers to correct them. We also report on a more in-depth investigation which we performed on problematic preprints that appeared in the first three months of the period under consideration, checking how frequently their authors reacted positively to our request and corrected their papers. Finally, we measure the global impact of papers classified as problematic and observe that, thanks to the changes implemented in response to our requests, the global impact (measured as the number of citations of papers which still contain Virgo visibility issues) was halved. We conclude the paper with general considerations for future work in a wider perspective., Comment: 17 pages, 9 figures, supplementary material (data and software) in linked public repository

This paper reviews the physics of stars, the type, structure, evolution and stability. Simple thermodynamics and statistical mechanics are used to show the inner working of white dwarf and neutron stars. The major concentration of the paper will be on white dwarf stars although in some places references will also be made to neutron stars where the relations can be extended easily. It can be shown that a maximum mass limit is attached to each type of star which can be derived rigorously. Maximum entropy can be used to show that the gravitational contraction is balanced by the degeneracy pressure created by the electrons in the case of white dwarfs, and neutrons and protons which constitute the matter of the neutron star. Finally the kinetic equations which describe the luminosity of the star and the radiative transfer are introduced., Comment: arXiv admin note: text overlap with arXiv:cond-mat/9909291 by other authors

This paper analyses the impact of various parameter changes on the estimation of parameters for massive black hole binary (MBHB) systems using a Bayesian inference technique. Several designed MBHB systems were chosen for comparison with a fiducial system to explore the influence of parameters such as sky location, inclination angle, anti-spin, large mass ratio and light mass. And the two reported MBHB candidates named OJ287 and Tick-Tock are also considered. The study found that the network of TianQin and LISA can break certain degeneracies among different parameters, improving the estimation of parameters, particularly for extrinsic parameters. Meanwhile, the degeneracies between different intrinsic parameters are highly sensitive to the value of the parameters. Additionally, the small inclination angles and limited detection of the inspiral phase can introduce significant bias in the estimation of parameters. The presence of instrument noise will also introduce bias and worsen the precision. The paper concludes that the network of TianQin and LISA can significantly improve the estimation of extrinsic parameters by about one order of magnitude while yielding slight improvements in the intrinsic parameters. Moreover, parameter estimation can still be subject to biases even with a sufficiently high signal-to-noise ratio if the detected signal does not encompass all stages of the inspiral, merger, and ringdown., Comment: 17 pages, 10 figures

This paper explores the evolution of the over-dense region of dark matter in the presence of a non-minimally coupled scalar field which is used to model quintessence and phantom-like dark energy. We focus on algebraic coupling, where the interaction Lagrangian is independent of the derivatives of the scalar field. To make our model more relativistic, like the minimal coupling scenario we studied earlier, we consider a spacetime structure that is internally closed Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime and externally the generalized Vaidya spacetime. This structure allows non-zero matter flux at the boundary of the over-dense region. Our investigation reveals that an increment of the coupling strength causes dark energy to cluster with dark matter at a certain cosmological scale where the influence of dark energy cannot be ignored. This phenomenon arises from the specific nature of the non-minimal coupling considered in this paper. While the evolution of matter's energy density remains unchanged, the scalar field's Klein-Gordon equation is modified, causing dark energy to deviate from its homogeneous state and cluster with dark matter. Similar to minimal coupling scenarios, closed spherical regions do not collapse within certain parameter ranges, exhibiting eternal expansion within the spatially flat FLRW spacetime acting as voids with decreasing matter density. The study extends our understanding of the cosmological scenarios where the virialization of the over-dense regions of dark matter is influenced by the non-minimally coupled dark energy., Comment: 18 pages, 8 figures

Matter loses its original characteristics after entering a black hole, thus becoming a new kind of (black hole) matter. The property of this new matter cannot be measured experimentally, but some of it can be deduced theoretically from the Einstein equations and the conservation laws which it must still satisfy. In a previous paper, this matter is modelled by an ideal fluid, with an equation of state $p(r)=-\xi\r(r)$ between the pressure $p(r)$ and the density $\rho(r)$. In order for this matter to fill the inside of a black hole so that its property can be teased out from the Einstein and conservation equations, it must possess a negative pressure ($\xi>0$) to counter the gravitation attraction which draws all matter to the center. In that case a solution of the Einstein and conservation equations exists if and only if the constant $\xi$ is confined within a narrow range, between 0.1429 and 0.1716. In the present paper, we try to find out its dynamical response by injecting additional matter into the black hole over a period of time. The resulting solutions of the six time-dependent Einstein equations and conservation laws are presented in perturbation theory, valid if the total amount of injection is small. Even in perturbation, the solutions can be obtained only with a special trick. The result shows that the equation of state $p(r,t)=-\xi\r(r,t)$ remains unchanged with the same $\xi$ when the injection rate is constant. When the rate changes with time, $\xi$ requires a correction, $\xi\to\xi+\xi_1(r,t)$, where $\xi_1(r,t)$ appears to be correlated with the acceleration of the injected matter in a way to be shown in the text.

Adiabatic binary inspiral in the small mass ratio limit treats the small body as moving along a geodesic of a large Kerr black hole, with the geodesic slowly evolving due to radiative backreaction. Up to initial conditions, geodesics are typically parameterized in two ways: using the integrals of motion energy $E$, axial angular momentum $L_z$, and Carter constant $Q$; or, using orbit geometry parameters semi-latus rectum $p$, eccentricity $e$, and (cosine of ) inclination $x_I \equiv \cos I$. The community has long known how to compute orbit integrals as functions of the orbit geometry parameters, i.e., as functions expressing $E(p, e, x_I)$, and likewise for $L_z$ and $Q$. Mappings in the other direction -- functions $p(E, L_z, Q)$, and likewise for $e$ and $x_I$ -- have not yet been developed in general. In this note, we develop generic mappings from ($E$, $L_z$, $Q$) to ($p$, $e$, $x_I$). The mappings are particularly simple for equatorial orbits ($Q = 0$, $x_I = \pm1$), and can be evaluated efficiently for generic cases. These results make it possible to more accurately compute adiabatic inspirals by eliminating the need to use a Jacobian which becomes singular as inspiral approaches the last stable orbit., Comment: 10 pages, 5 figures, version to appear in Physical Review D. Incorporates helpful feedback from the referee and a few other comments received since this paper was originally posted. Posting includes Mathematica notebook and C++ code which implements the methods developed in this paper