Table of contents

The international conference ''The Modern Physics of Compact Stars and Relativistic Gravity'' took place in Yerevan, Armenia, from 18–21 September 2013. This was the second in a series of conferences which aim to bring together people working in astrophysics of compact stars, physics of dense matter, gravitation and cosmology, observations of pulsars and binary neutron stars and related fields. The conference was held on the occasion of 100th birthday of the founder of the Theoretical Physics Chair at the Department of Physics of Yerevan State University and prominent Armenian scientist Academician Gurgen S Sahakyan.

The field of compact stars has seen extraordinary development since the discovery of pulsars in 1967. Even before this discovery, pioneering work of a number of theoretical groups had laid the foundation for this development. A pioneer of this effort was Professor G S Sahakyan who, together with Professor Victor Ambartsumyan and a group of young scientists, started in the early sixties their fundamental work on the properties of superdense matter and on the relativistic structure of compact stellar objects. This conference explored the vast diversity of the manifestations of compact stars, including the modern aspects of the equation of state of superdense matter, its magnetic and thermal properties, rotational dynamics, superfluidity and superconductivity, phase transition from hadronic to quark matter, etc. The articles on these subjects collected in this volume are evidence of liveliness of the field and of the continuous feedback between theory and the experiment. A part of this volume is devoted to the cosmology and the theories of gravity — the subfields of astrophysics that are of fundamental importance to our understanding of the universe. The reader will find here articles touching on the most diverse aspects of these fields such as modern problems in Einstein's classical theory of gravity and its alternatives, string theory motivated cosmological models, theories of cosmic microwave background, quantum field theory in curved background, Casimir effect, etc. Thus, it is fair to say that the present volume covers a large number of actively pursued subjects of modern relativistic astrophysics.

We would like to thank all those individuals and organizations that helped us in organizing a successful conference. These include the members of the international advisory committee as well as the local organizing committee (whose names are listed separately above) and the sponsors of the conference — the Volkswagen Stiftung (Hannover, Germany), the HIC for FAIR Institute (Frankfurt am Main, Germany) and the State Committee for Sciences of Armenia.

Edvard Chubaryan, Aram Saharian and Armen SedrakianYerevan / Frankfurt-Main, 1 February, 2014

A list of participants is available in the PDF

The paper is dedicated to the Centenary of an Academician of NAS RA, Professor G S Sahakyan's birth, the Man that founded and headed the Chair of Theoretical Physics (CTP) of the Yerevan State University for almost half a century. The reference to school days of G S Sahakyan is made, information about his 7 years long service in the forces in the fields, about the establishment and administration by him of the Chair of Theoretical Physics in the Yerevan State University, about his collaboration with academician V A Ambartsumian, about the research associates of the G S Sahakyan's Chair, the students of CTP and the advancement of theoretical physics in Armenia is given. The personality characteristics of G S Sahakyan as a principal investigator and leader of CTP are analyzed.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

We study the abrupt changes in the characteristics of compact stars due to the quark deconfinement phase transition. The hadronic phase is described within the relativistic mean-field theory, including a scalar-isovector δ-meson effective field. To describe the quark phase, we use the MIT bag model, in which the interactions between u, d and s quarks inside the bag are taken into account in the one-gluon exchange approximation. We analyze catastrophic changes of the parameters of the near-critical configuration of compact star and compute the amount of the energy released by a corequake for the two extreme cases of the deconfinement phase transition scenarios. The first one corresponds to the ordinary first-order phase transition (Maxwell scenario) and the second one corresponds to the phase transition calculated using the bulk Gibbs equilibrium conditions and global charge neutrality (Glendenning scenario).

We suggest a new Bayesian analysis using disjunct M-R constraints for extracting probability measures for cold, dense matter equations of state. One of the key issues of such an analysis is the question of a deconfinement transition in compact stars and whether it proceeds as a crossover or rather as a first order transition. The latter question is relevant for the possible existence of a critical endpoint in the QCD phase diagram under scrutiny in present and upcoming heavy-ion collision experiments.

We discuss to which extent the modifications of the hyperon-scalar-meson coupling constants affect the equation of state (EoS) hypernuclear matter. The study is carried out within a relativistic density functional theory. The nucleonic matter is described in terms of a density-dependent parametrization of nucleon-meson couplings, whereas the hyperon-meson couplings are deduced from the octet model. We identify the parameter space of hyperon-meson couplings for which massive stellar configurations with M ≤ 2.25M_⊙ exist. We also discuss the EoS at finite temperatures with and without of a trapped neutrino component and show that neutrinos stiffen the EoS and change qualitatively the composition of stellar matter.

In this contribution nuclear constraints on the equation of state for a neutron star are discussed. A combined fit to nuclear masses and charge radii leads to improved values for the symmetry energy and its derivative at nuclear saturation density, S_v = 31 MeV and L = 68 ± 8 MeV. As an application the sensitivity of some properties of rotating supramassive neutron stars on the EoS is discussed.

The equilibrium states of hot strange stars are studied. The equation of state of hot strange quark matter is determined on the basis of the MIT bag model. It is shown that for hot strange quark stars the mass-central density and mass-radius relations do not depend on the central temperature if quarks are ultrarelativistic. For these relations the weight of the thermal energy is of fundamental significance. That explains the nature of change in the radius of a strange star when it cools down.

New possibilities of hypernuclear studies at modern electron accelerators based on recently developed radio frequency photomultiplier tubes are discussed.

We discuss the possible implication of the recent predictions of two new properties of high momentum distribution of nucleons in asymmetric nuclei for neutron star dynamics. The first property is about the approximate scaling relation between proton and neutron high momentum distributions weighted by their relative fractions (x_p and x_n) in the nucleus. The second is the existence of inverse proportionality of the high momentum distribution strength of protons and neutrons to x_p/n. Based on these predictions we model the high momentum distribution functions for asymmetric nuclei and demonstrate that it describes reasonably well the high momentum characteristics of light nuclei. We also extrapolate our results to heavy nuclei as well as infinite nuclear matter and calculate the relative fractions of protons and neutrons with momenta above k_F. Our results indicate that for neutron stars starting at three nuclear saturation densities the protons with x_p = 1/9 will populate mostly the high momentum tail of the momentum distribution while only 2% of the neutrons will do so. Such a situation may have many implications for different observations of neutron stars which we discuss.

We report on recent progress in understanding pairing phenomena in low-density nuclear matter at small and moderate isospin asymmetry. A rich phase diagram has been found comprising various superfluid phases that include a homogeneous and phase-separated BEC phase of deuterons at low density and a homogeneous BCS phase, an inhomogeneous LOFF phase, and a phase-separated BCS phase at higher densities. The transition from the BEC phases to the BCS phases is characterized in terms of the evolution, from strong to weak coupling, of the condensate wavefunction and the second moment of its density distribution in r-space. We briefly discuss approaches to higher-order clustering in low-density nuclear matter.

We study the stability of strange dwarfs, superdense stars with a small self-confining core (M_core < 0.02M_⊙) containing strange quark matter and an extended crust consisting of atomic nuclei and degenerate electron gas. The mass and the radius of these stars are of the same order of magnitude as those of ordinary white dwarfs. It is shown that any study of their stability must examine the dependence of the mass on two variables, which can, for convenience, be taken to be the rest mass (total baryon number) of the quark core and the energy density ρ_tr of the crust at the surface of the quark core. The range of variation of these quantities over which strange dwarfs are stable is determined. This region is referred to as the stability valley for strange dwarfs. The mass and radius obtained from theoretical models of strange dwarfs are compared with observational data obtained through the HIPPARCOS program and the most probable candidate strange dwarfs are identified.

It is well known that neutron star crust in a wide range of mass densities and temperatures is in a crystal state. At a given density, the crystal is made of fully ionized atomic nuclei of a single species immersed in a nearly incompressible (i.e., constant and uniform) charge compensating background of electrons. This model is known as the Coulomb crystal model. In this talk we analyze thermodynamic and elastic properties of the Coulomb crystals and discuss various deviations from the ideal model. In particular, we study the Coulomb crystal behavior in the presence of a strong magnetic field, consider the effect of the electron gas polarizability, outline the main properties of binary Coulomb crystals, and touch the subject of quasi-free neutrons permeating the Coulomb crystal of ions in deeper layers of neutron star crust.

Compact stars having strong magnetic fields (magnetars) have been observationally determined to have surface magnetic fields of order of 10¹⁴ – 10¹⁵ G, the implied internal field strength being several orders larger. We study the equation of state and composition of hypernuclear matter and quark matter - two forms of dense matter in strong magnetic fields. We find that the magnetic field has substantial influence on the properties of hypernuclear matter and quark matter for magnetic field B ≥ 10¹⁷ G and B ≥ 10¹⁸ G respectively. In particular the matter properties become anisotropic. Moreover, above a critical field B_cr, both hypernuclear and quark matter show instability, although the values of B_cr are different for two kinds of matter.

The influence of small-scale magnetic field on the polar cap heating by returning positrons is considered. The returning positron current is calculated in the framework of two models of rapid and gradual screening. To calculate the electron-positron pair production rate we take into account only the curvature radiation of primary electrons and its absorption in magnetic field. We use the polar cap model with steady space charge limited electron flow. It is shown that the rapid screening model is in the better agreement with observations of old (age > 10⁶ years) radio pulsars. The gradual screening model usually leads to too strong heating and too large X-ray luminosities.

We formulate a model of pulsar spin evolution (braking, inclination angle evolution and radiative precession) taking into account the non-rigidity of neutron star rotation. We discuss two simple limiting cases of this model and show that the evolution of the inclination angle substantially depends on the model of crust-core interaction. The non-rigidity of core rotation accelerates the inclination angle evolution and makes all pulsars evolve to the orthogonal state. The size of the effect depends on the amount of differentially rotating matter and mechanism of its interaction with the rest of the star. Since the rapid evolution of the inclination angle apparently contradicts the observational data, our results may be used as an additional test for the theories of the cores of neutron stars.

We show that all reliably known temperature data of neutron stars including those belonging to Cassiopea A can be comfortably explained in our "nuclear medium cooling" scenario of neutron stars. The cooling rates account for medium-modified one-pion exchange in dense matter, polarization effects in the pair-breaking-formation processes operating on superfluid neutrons and protons paired in the 1S₀ state, and other relevant processes. The emissivity of the pair-breaking-formation process in the 3P₂ state is a tiny quantity within our scenario. Crucial for a successful description of the Cassiopeia A cooling proves to be the thermal conductivity from both, the electrons and nucleons, being reduced by medium effects. Moreover, we exploit an EoS which stiffens at high densities due to an excluded volume effect and is capable of describing a maximum mass of 2.1 M_⊙, thus including the recent measurements of PSR J1614-2230 and PSR J0348+0432.

We report on our observational attempt to constrain the compactness of the isolated neutron stars via X-ray spin phase-resolved spectroscopy. There are seven thermally emitting neutron stars known from X-ray and optical observations, which are young (up to few Myrs), nearby (hundreds of pc), and radio-quiet with blackbody-like X-ray spectra. A model with a condensed iron surface and partially ionized hydrogen-thin atmosphere allows us to fit simultaneously the observed general spectral shape and the broad absorption feature (observed at 0.3 keV) in different spin phases. We constrain a number of physical properties of the X-ray emitting areas, including their temperatures, magnetic field strengths at the poles, and their distribution parameters. In addition, we place some constraints on the geometry of the emerging X-ray emission and the gravitational redshift of three isolated neutron stars.

The innermost stable circular orbit and the splitting of epicyclic and orbital frequencies are among strong-field signatures of general relativity searched for in astronomical objects, particularly in the X-ray data accumulated in observations of neutron stars and black holes. It may come as a surprise that these effects are present in the Newtonian physics of rapidly rotating gravitating bodies, such as the classic Maclaurin spheroids.

The equations of magnetohydrodynamics are used to show that the energy released at the inner surface of the crust of a neutron star generates magnetosonic wave beams that propagate to the star's surface. These equations can be linearized under the conditions appropriate for the matter in the crust of neutron stars and for the frequency range 10⁷ – 10¹¹ Hz. The solutions describe a beam of standing wave with approximately constant transverse cross-section (radius). The outer base of this beam on the star's surface is a source of radio emission. Electrical currents are excited in this source and it becomes an antenna that emits radio waves into the circumstellar space. The intensity of the radio emission decreases at higher frequencies, so that the spectrum of emitted radiation by pulsars is limited from above (ω ≤ 10¹¹ Hz).

We discuss a time-dependent generalization of the stationary Ginzburg-Landau theory for two-flavor color superconducting quark matter and its modification in the presence of rotation. General expressions are obtained for the relaxation time-scales of the order parameter and color-magnetic fields and for the dissipative function, which obtains contributions from the relaxation of the order parameter and Ohmic dissipation. We also obtain a stationary equation that governs the penetration of the color-electric field in the color superconductor.

The spin-up of magnetized plasma and its role in the post-glitch response of neutron stars was first studied over thirty years ago, where it was demonstrated that co-rotation between crust and plasma is established rapidly (within a few seconds) by a process analogous to Ekman pumping in a viscous fluid. However, if the magnetized plasma is considered to be ideal, conservation of energy implies that the final state cannot be co-rotation. Using an exact analytical solution for the coupled motion of the crust and plasma, we demonstrate that the system oscillates persistently and explore the consequences for neutron star observations.

This paper is devoted to the investigation of conformally-related variants of the modified tensor-scalar Jordan theory on the example of determination of the comparative characteristics of the model Universe in "Einstein" and "proper" frames. Within the framework of this model, we consider the possibility for the accelerated expansion of the Universe at the recent epoch.

To explain the accelerated expansion of the universe, models with interacting dark components (dark energy and dark matter) have been considered recently in the literature. Generally, the dark energy component is physically interpreted as the vacuum energy of the all fields that fill the universe. As the other side of the same coin, the influence of the vacuum energy on the gravitational collapse is of great interest. We study such collapse adopting different parameterizations for the evolution of the vacuum energy. We discuss the homogeneous collapsing star fluid, that interacts with a vacuum energy component, using the stiff matter case as example. We conclude this work with a discussion of the Cahill-McVittie mass for the collapsed object.

In this contribution to the proceedings of the Conference on Modern Physics of Compact Stars and Relativistic Gravity in Yerevan, Armenia (September 18-21, 2013), I review recent work attempting to give a fundamental definition to string evolution in a dynamical, fully compact universe, and present a sketch of how the resulting formalism can be used for addressing questions of phenomenological significance in the field of string gas cosmology.

The most prominent successes of weakly interacting massive particle (WIMP) models of dark matter are at comoving scales of megaparsecs. At kiloparsec scales they face challenges in explaining the density profiles, abundances and phase space distributions of satellite galaxies in our local group. An alternate dark matter candidate, the giant 't Hooft-Polyakov monopole, is proposed which may share the large distance successes of WIMPs while evading their short distance problems. These are classical field theory solutions of a dark sector including a nonabelian gauge field, an adjoint scalar field and fundamental fermions. In such models each halo consists of a single monopole characterized by a conserved integer charge.

We attempt to address the well known problem of quantum gravity - the information paradox, through the study of transplanckian collisions, that may proceed with the production of intermediate black hole states. We apply the S-matrix approach to study 2 → 2 scattering of particles with the center of mass energy much greater than the Planck mass, and investigate the dependence of the scattering amplitude on the impact parameter. It is expected that the main properties of the S-matrix (unitarity, analyticity and crossing) may provide sufficiently strong constrain for the scattering amplitude to unravel non-trivial information about the underlying properties of quantum gravity. However, we argue that our present understanding is far from complete and additional novel ideas are required to resolve this puzzle.

A large scale B-mode signal in the CMB polarization would constitute a smoking gun of Inflation and is the main target of several ongoing and upcoming experiments. In this contribution, I consider distinguishing features of another potential source of primordial B-modes - magnetic fields. In particular, the Faraday Rotation of CMB polarization provides a distinctive signature of cosmic magnetic fields through the characteristic frequency dependence and the mode-coupling correlations of the CMB variables. I discuss constraints on primordial magnetism that can be expected from future CMB experiments, taking into account the obstruction caused by the magnetic field of the Milky Way.

A new version of relativistic theory of gravitation is proposed, in which the GR is supplemented with two postulates. Owing to these postulates it turns possible to introduce into consideration a covariant energy-momentum tensor of gravitational field. Additionally the influence of Universe on the gravitating system under consideration is taken into account. The equations describing the gravitation field in the framework of proposed tensor mono-metric theory of gravitation are derived. In its simplest version the proposed theory contains nine free parameters. At a particular choice of these parameters the theory is reduced to GR with a cosmological term.

Two-point functions for the electromagnetic field in background of (D + 1)-dimensional dS spacetime are evaluated assuming that the field is prepared in the Bunch-Davies vacuum state. By using these functions, the vacuum expectation values (VEVs) of the field squared and the energy-momentum tensor are investigated in the geometry of a conducting plate. The VEVs are explicitly decomposed into the boundary-free and plate-induced parts. For points outside of the plate the renormalization is needed for the first parts only. Because of the maximal symmetry of the background spacetime and of the Bunch-Davies vacuum state, the boundary-free parts do not depend on spacetime coordinates, whereas the plate-induced parts are functions of the proper distance of the observation point from the plate. The plate-induced part in the VEV of the energy-momentum tensor vanishes for D = 3 which is a direct consequence of a conformal invariance of the electromagnetic field for this spatial dimension. For D > 3, in addition to the diagonal components, the vacuum energy-momentum tensor has nonzero off-diagonal component which describes energy flux along the direction normal to the plate.

The existence of the many unanswered questions in fundamental physics, in particular, in astrophysics allows for a great variety of theories to remain viable candidates for becoming the correct theory at energies not accessible in current experiments. One special class of these type of theories is the class of extra-dimensional brane-world models. Besides answering many fundamental problems, for instance, the hierarchy problem, they may produce testable predictions. In this work, we find and investigate brane-world induced black string horizon corrections, when the black hole solution has a post-Newtonian parameter. For suitable choices of such a parameter, the Hawking radiation on the brane is precluded, and the Hawking radiation in the bulk causes the black hole to slightly recoil into the bulk, which modifies the black hole apparent horizon. It has an impact on quasars luminosity and, therefore, might be detected and measured.

We discuss the features of the Casimir effect for two parallel plates in background of de Sitter and anti-de Sitter spacetimes. A massive scalar field with general curvature coupling parameter obeying Robin boundary conditions on the plates is considered. The corresponding results are compared with those in Minkowski spacetime.

In the framework of an idea of separation of rotational and vibrational motions, we have examined the problem of reducing the general three-body problem. The class of differentiable functions allowing transformation of the 6D Euclidean space to the 6D conformal-Euclidean space is defined. Using this fact the general classical three-body problem is formulated as a problem of geodesic flows on the energy hypersurface of the bodies system. It is shown that when the total potential depends on relative distances between the bodies, three from six ordinary differential equations of second order describing the non-integrable hamiltonian system are integrated exactly, thus allowing reducing the initial system in the phase space to the autonomous system of the 6th order. In the result of reducing of the initial Newtonian problem the geometry of reduced problem becomes curved. The latter gives us new ideas related to the problem of geometrization of physics as well as new possibilities for study of different physical problems.

We suggest to realize the computer simulation and calculation by the algebraic structure built on the basis of the logic inherent to processes in physical systems (called physical computing). We suggest a principle for the construction of quantum algorithms of neuroinformatics of quantum neural networks. The role of academician Sahakyan is emphasized in the development of quantum physics in Armenia.

We consider differential operators acting on densities of arbitrary weights on manifold M identifying pencils of such operators with operators on algebra of densities of all weights. This algebra can be identified with the special subalgebra of functions on extended manifold . On one hand there is a canonical lift of projective structures on M to affine structures on extended manifold . On the other hand the restriction of algebra of all functions on extended manifold to this special subalgebra of functions implies the canonical scalar product. This leads in particular to classification of second order operators with use of Kaluza-Klein-like mechanisms.