Itzhak Goldman

Afeka Tel-Aviv Academic College of Engineering | Afeka · Department of Physics

An analytic model for steady state turbulence is employed to obtain the inertial range power spectrum of compressible turbulence. We assume that for homogeneous turbulence, the timescales controlling the energy injected at a given wavenumber from all smaller wave-numbers, are equal for each spatial component. However, the longitudinal component ene...

The observational structure function of the scintillations of the radio pulsar PSR B0950+08, was fitted, a decade ago, with a power law with index 1 ± 0.05. This was interpreted as an appreciable deviation from the, commonly observed index of 5/3, expected for Kolmogorov turbulence. In this paper it is suggested that the observations are consistent...

The Fermi-LAT observational data of the diffuse $\gamma$ ray emission from the Large Magellanic Cloud (LMC), were examined to test for the existence of underlying long range correlations. A statistical test applied to the data indicated that the probability that data are random is $\sim 10^{-99}$. Thus we proceeded and have used the counts-number d...

The observational structure function of the scintillations of the radio pulsar PSR B0950+08, was fitted, a decade ago, with a power law with index $1 \pm 0.01$. This was interpreted as an {\em appreciable deviation} from the, commonly observed index of $5/3$, expected for Kolmogorov turbulence. In this paper it is suggested that the observations ar...

Mirror Dark Matter (MDM) has been considered as an elegant framework for a particle theory of Dark Matter (DM). It is supposed that there exists a dark sector which is mirror of the ordinary matter. Some MDM models allow particle interactions mirror and ordinary matter, in addition to the grav-itational interaction. The possibility of neutron to mi...

Mirror Dark Matter (MDM) has been considered as an elegant framework for a particle theory of Dark Matter (DM). It is supposed that there exists a dark sector which is mirror of the ordinary matter. Some MDM models allow particle interactions mirror and ordinary matter, in addition to the gravitational interaction. The possibility of neutron to mir...

Observational power spectra of the photospheric magnetic field turbulence, of the quiet-sun, were presented in a recent paper by Abramenko & Yurchyshyn. Here, I focus on the power spectrum derived from the observations of the Near InfraRed Imaging Spectrapolarimeter operating at the Goode Solar Telescope. The latter exhibits a transition from a pow...

Observational power spectra of the photospheric magnetic field turbulence, of the quiet-sun, were presented in a recent paper by Abramenko and Yurchyshyn. Here I focus on the power spectrum derived from the observations of the Near InfraRed Imaging Spectrapolarimeter (NIRIS) operating at the Goode Solar Telescope. The latter exhibits a transition f...

The possibility of neutron-mirror neutron transition has recently been discussed both from theoretical and experimental perspectives. We point out that such a transition would imply mass loss in neutron stars leading to a significant change of the orbital period of neutron star binary systems. Known limits on the period changes of such binaries the...

Gas accretion onto galaxies imports energy and mass (and sometimes net momentum or angular momentum). The flow resulting from the accretion can exhibit instabilities leading to turbulence. As an example we derive the power spectrum of the Kelvin-Helmholtz instability resulting from gas accretion onto a spiral galaxy. Fleck (1983) has attributed the...

Mergers of clusters of Galaxies are a central important implication of the standard LCDM cosmological model The dissipationless collision of the dark matter, of the merging galaxies, would induce a time dependent gravitational potential, that in turn would drive large scale flows. The shocks produced by the collision the merging baryons will also g...

This is an English translation of the Italian version of an encyclopedia chapter that appeared in the Italian Encyclopedia of the Physical Sciences, edited by Bruno Bertotti (1994). Following requests from colleagues we have decided to make it available to a more general readership. We present the motivation for constructing General Relativity, pro...

A self consistent model for an accretion disk is presented, in which the turbulent viscosity is due to turbulent convection and the vertical flux is the sum of the radiative and convective fluxes. The model allows for both gas pressure dominated region and radiation pressure dominated region also for highly super-critical accretion. The disk in bot...

We consider the implications of fermionic asymmetric dark matter for a "mixed neutron star" composed of ordinary baryons and dark fermions. We find examples, where for a certain range of dark fermion mass -- when it is less than that of ordinary baryons -- such systems can reach higher masses than the maximal values allowed for ordinary ("pure") ne...

We study the implications of asymmetric dark matter on neutron stars. we construct a "mixed neutron star" model composed of ordinary baryons and of asymmetric dark matter baryons. We derive the general relativistic structure equations for each specie, the equation for the mass within a given radius, and the redshift as function of radius. We presen...

The center of the Milky Way galaxy (MW) is an extreme environment which contains a massive black hole surrounded by a very dense star cluster, two other adjacent star clusters, and a giant molecular cloud which would serve as an incubator to a new generation of stars. The gas and dust in its vicinity are denser by 2-3 orders of magnitude than in ot...

According to the $\Lambda CDM$ cosmological framework, galaxies underwent multiple mergers in their history. In this paper we propose to use the power spectrum of the residual fluctuations of the rotation curve velocity as a probe of past mergers. The proposition relies on the assertion that mergers are expected to induce large scale flows and in c...

The spectra emitted from clouds near the Galactic Centre are investigated calculating the UV-optical-IR lines using the physical parameters and the element abundances constrained by the fit of mid-IR observations. The characteristic line ratios are compared with those observed in active galaxies. We have found that the physical conditions in the ne...

The timing data of the binary pulsar PSR1913+16, are used to establish an upper limit on the rate of continuous neutrino emission from neutron stars. Neutrino emission from each of the neutron stars of the binary system, increases the star binding energy and thus translates to a decrease in their masses. This in turn implies an increase with time o...

If Micro Black Holes (MBHs) can be produced at the LHC, they will decay very fast. We study hypothetical MBHs that do not decay; in particular, QCD effects on accretion by MBHs that are produced at rest. We explain why accretion of a nucleon by such MBHs is associated with pion emission. This pion emission results in a kick to the MBHs, such that t...

A recent paper by Stanimirovic etal (2008) presents quit interesting results from HI observations of the Magellanic Stream (MS) tip. The high spatial resolution of the data reveals rich and complex morphological and kinematic structures; notably four coherent HI substreams extending over angular size of about 20 deg were found. We use the position-...

A major problem confronting the understanding of tidal evolution of close solar-type binaries is the inefficiency of the turbulent convection. The value of the effective viscosity estimated, in the framework of the mixing length theory (MLT), implies circularization timescales which are almost two orders of magnitude longer than observed. Moreover,...

The spatial power spectrum of the HI 21 cm intensity in the Small Magellanic Cloud (Stanimirovic et al 1999) is a power law over scales as large as those of the SMC itself. It was interpreted as due to turbulence by Goldman (2000) and by Stanimirovic & Lazarian (2001). The question is whether the power spectrum is indeed the result of a dynamical t...

The online tutoring system CAPA was implemented at Afeka College in the academic year 2000-2001 in calculus based physics courses. It was used also in the academic year 2001-2002 and was very successful in improving understanding and achievements of the students. The paper describes the system features and the case study of its implementation. Less...

The online tutoring system CAPA was implemented at Afeka College in the academic year 2000-2001 in calculus based physics courses. It was used also in the academic year 2001-2002 and was very successful in improving understanding and achievements of the students. The paper describes the system features and the case study of its implementation. Less...

Models in which the "dark" halo particles have mutual and potentially also appreciable nuclear interactions have been considered by various authors. In this note we briefly point out strategies for a most sensitive search for these particles. We show that a particular matter/anti-matter symmetric variant due to Farrar et al. is excluded by combinin...

Recent 21 cm radio observations of H$_I$ regions in the Small Magellanic Cloud, have revealed spatial power spectra of the intensity, which are quite similar in shape to those previously deduced for the Galaxy. The similarity, in spite the differences in the physical parameters between the Galaxy and the SMC, suggests that the shape of the power sp...

We show, that SN1987A can serve as an astrophysical laboratory for testing the viability of the assertion that a new massive neutral fermion is implied by the KARMEN data. We show that a wide range of the parameters characterizing the proposed particle is ruled out by the above constraints making this interpretation very unlikely.

Observations in X-Ray, optical and radio suggest quit convincingly that mergers ofclusters with clusters are quit common. Such a merger will lead to violent relaxation of the dissipationless dark matter resulting in a time dependent gravitational potential which in turn will generate large scale flows. The intracluster gas will develop shocks. The...

There is an abstract at the text beginning.

Observations in X-ray and optical suggest that mergers of sub-clusters with galaxy clusters are quite common (for Coma see e.g., White et al. 1993; Colless & Dunn 1996; Ishizaka & Mineshige 1996). A merger leads to violent relaxation of the dissipationless dark matter resulting in a time-dependent gravitational potential. This in turn generates lar...

We present a selfconsistent model for stellar turbulent convection which is similar in spirit to the CM model (Canuto \& Mazzitelli 1991) since it accounts for the full spectrum of the turbulent eddies rather than only one eddy, as done in the mixing length theory (MLT). The model differs from the CM model in the treatment of the rate of energy inp...

Neutron stars, notably binary radio pulsars, are used to set limits on long-range scalar and vector fields. Such fields suggested by various particle physics models give rise to long-range forces in addition to gravity: a scalar attractive force and a vector repulsive force. It has been argued that if the coupling strengths of the two forces are eq...

A self-consistent solution for a thin accretion disk with turbulent convection is presented. The disk viscosity and the convective flux are derived from a physical model for turbulence, and expressed in terms of the local physical conditions of the disk which, in turn, are controlled by the former two. In the gas pressure region, two distinct solut...

We call attention to the surprising similarity between the newly discovered planetary system around PSR B1257+12 and the inner solar system. The similarity is in the ratios of the orbital radii and the masses of the three planets. Comment: 4 pages including 1 figure. uuencoded compressed postscript file

The tidal evolution of close binaries in the limit of e approaching 1 is studied in this work. We use Hut equations to obtain the time derivatives and timescales for the evolution of the eccentricity, semimajor axis, and stellar rotation rate in the high-eccentricity binaries. We find that in some of the highly eccentric binaries the tidal shear ch...

Inflation is found to naturally provide mechanisms for the direct generation of cosmic turbulence on the same scales as those on which density perturbations are formed. Since the observational constraints on turbulence could translate into constraints on inflation itself, these results call for a reexamination of the issue of cosmic turbulence. It...

Neutrino emission from the newly formed neutron star reduces the star's gravitational mass and thus reduces the depth of the gravitational potential well, out of which the mantle and envelope layers have to be pushed. We estimated that this effect can contribute about 10 percent of the energy required for a Type II supernova explosion. It can thus...

The author discusses the use of observed spin-down of radio pulsars for setting limits on a possible variation of the gravitational constant with cosmic time. The method applies for pulsars with small observed spin-down rate, and negligible contributions to period derivative from matter accretion or from acceleration relative to the solar system. A...

It is shown that the binary radio pulsar PSR 0655 + 64 is an effective astrophysical laboratory for testing gravity theories that violate the strong equivalence principle (SEP). It is suggested that PSR 0655 + 64 has the characteristics required for testing an aspect of SEP violation: the emission of dipole gravitational radiation. A rate of change...

A modified approach to the tidal circularization of short-period binaries with convective envelopes is presented. It accounts for the reduction of the stellar viscosity expected when the tidal shear varies on a timescale which is comparable to or shorter than the typical convection turnover timescale. The above reduction depends on the value of the...

The typical scale and velocity of the dominant turbulent eddies excited by the motion of galaxies in clusters are determined from basic considerations which are valid if a steady state is attained. Hydrodynamics turbulence in the intracluster plasma presumably generates magnetic fields; the present estimates of the typical scale and mean strength o...

A modified approach to the tidal circularization of short-period binaries with convective envelopes is suggested. The proposed model is based on Zahn's (1966) theory and is motivated by his recent comment about the reduction of stellar viscosity due to the fact that the tidal variation time scale is comparable to, or shorter than, the typical conve...

A new expression for the baryon number of a uniformly rotating cold star is derived within the framework of general relativity. It gives the baryon number, in terms of integrals involving the energy density and the pressure, rather than the baryon-number density. The expression is used to obtain a relation between the star mass, its baryon number,...

The use of the observed spin-down rates of radio pulsars to set upper limits on a possible variation of the gravitational constant with cosmic time is discussed. The method applies for pulsars with rates of change of the period, due to matter accretion and to acceleration, which are small compared to the observed spin-down rate. The present method...

Neutron stars are used to set constraints on the characteristics of weakly interacting massive particles (WIMP's) suggested as dark-matter candidates. Some special classes of WIMP's are ruled out because they would be trapped in neutron stars, concentrate towards the star center, and become self-gravitating. This results in the formation of a mini...

Atomic and gravitational clocks are governed by the laws of electrodynamics and gravity, respectively. While the strong equivalence principle (SEP) assumes that the two clocks have been synchronous at all times, recent planetary data seem to suggest a possible violation of the SEP. Our past analysis of the implications of an SEP violation on differ...

Results are presented from the analysis of solar system astrometric data, notably the range data to the Viking landers on Mars. A least-squares fit of the parameters of the solar system model to these data limits a simple time variation in the effective Newtonian gravitational constant to (0.2 0.4) 10–11 year–1 and a rate of drift of atomic clocks...

A new expression for the baryon number of a cold star is derived within the framework of general relativity. The expression is analogous to the Tolman mass formula and when combined with it yields a relation between the mass, the baryon number, the surface red-shift, and the ratio between the volume averages of the pressure and the energy density....

A model for fully developed turbulence is proposed whose predictions compare favorably with those of the direct interaction approximation (DIA) model and whose main equations are easy to handle. Four different expressions for the turbulent viscosity are derived which contain no free parameters. Two of the expressions are given in terms of propertie...

The neutrino events from the supernova SN1987A observed by the Kamiokande II and Irvine-Michigan-Brookhaven detectors are used to suggest constraints on neutrino physics. In particular, bounds on the value of the electron neutrino magnetic moment μve≲(10-12-10-13)μB are suggested.

An eccentric neutron-star binary, formed in a fast rotating supernova collapse, whose orbit decays due to emission of gravitational radiation, is studied. Consideration is given to the conditions for an enhanced neutrino emission at periastron passages that could result in a detectable modulation of the neutrino light curve with the orbital period....

A model for stationary, fully developed turbulence is presented in which the turbulent spectral energy function is completely determined once the time scale for the energy fed into the eddy interaction is known. The form of the eddy correlation time scale determining the turbulent viscosity is suggested by the basic equation of the model itself, up...

A model for the companion of SN 1987 A, observed recently by optical speckle interferometry, is proposed. It is suggested that the observed relativistic velocity and the high luminosity can be naturally explained within a framework of a rotating collapse, producing two neutron stars in a binary system which decays due to emission of gravitational r...

The 11 + 8 neutrino events observed at Kamiokande1 and IMB 2 detectors have been analysed by several authors1-12. The inferred total neutrino luminosity Wv and its distribution among the various neutrino species, vary between the different analyses depending on the events included in the analysis, the parameterization of the neutrino spectra, the r...

Theoretical models of neutrino emission, neutron-star formation, and SN explosion are examined on the basis of IMB and Kamiokande observations of neutrinos from SN 1987A. Particular attention is given to neutrino masses, mixing, magnetic moment, and decay; the total energy emitted as neutrinos; and lepton number conservation. Assuming that a neutro...

Recent work on the evolution of the solar nebula and the subsequent formation of planets is reviewed, and the stages of star formation thought to lead to a protosun and an accompanying solar nebula are considered. Photochemical results suggest that concentrations of O2, O3, and H2CO, and the ratio of CO/CO2 in the prebiological paleoatmosphere are...

Terrestrial and solar system experiments are shown to severely constrain the strength of an antigravity field with range much greater than 1 AU. The small upper limits set on the absolute value of alpha by the Eotvos-Dicke experiments (less than about 10 to the -9th), the gravitational redshift experiment (less than about 7 x 10 to the -5th), and t...

The paper presents an analytical model for large-scale turbulence, based on a new closure which depends on the growth rate of the instability-generating turbulence. For convection in stars the results of the mixing-length theory are recovered, and for laboratory convection the N = R to the 1/3 power law is recovered. The present model can readily b...

A formula for the Shakura-Suniaev alpha parameter is proposed in terms of the growth rate of the unstable modes of the physical mechanisms that generates turbulence. Turbulent convection is discussed as a particular example. The effect of rotation on turbulent viscosity is considered, and some remarks are made on convective fluxes, disk stability,...

Planetary range data offer the most promising means to test the validity of the Strong Equivalence Principle (SEP). Analytical expressions for the perturbation in the 'range' expected from an SEP violation predicted by the 'variation-of-G' method and by the 'two-times' approach are derived and compared. The dominant term in both expressions is quad...

Results are presented from the analysis of solar-system astrometric data, notably the range data to the Viking landers on Mars. A least-squares fit of the parameters of the solar system model to these data limits a simple time variation in the effective Newtonian gravitational constant to (2 + or - 4) x 10 to the -12th/yr and a rate of drift of ato...

The existence of a possible influence of the large-scale structure of the universe on local physics is discussed. A particular realization of such an influence is discussed in terms of the behavior in time of atomic and gravitational clocks. Two natural categories of metric theories embodying a cosmic infuence exist. The first category has geodesic...

The consequences of a supposed violation of the strong equivalence principle (SEP) for photons are analyzed theoretically and investigated using published observational data on the age of stars and globular clusters, the past temperature of the earth, the 3-K black-body radiation, and big-bang nucleosynthesis. It is shown that the photon number is...

A possible explanation of why the advanced solutions of Maxwell's equations are not observed in nature is by way of absorption by an opaque universe. As Davies has shown, the ever expanding, general relativistic cosmological models fail to provide the needed absorption. The absorption mechanism calling for an interplay between local physics and cos...

Theoretical and observational aspects of an SEP violation are discussed. A two-times theory is presented as a possible framework to handle an SEP violation, and tests performed to check the compatibility of such violation with a host of data ranging from nucleosynthesis to geophysics are summarized. Also discussed are the dynamical equations needed...

A heuristic asymmetrical model for a quasar emission-line region, with material streaming out, is described. The physical parameters are essentially the same as those of static models; the geometry turns out to be the dominant factor in determining the emission line profiles. Calculations are carried out for a variety of parameterized flow models a...

Since a possible time variability ofG has received renewed attention (Wesson and Goodson, 1981), we think it is important to stress a conceptual aspect so far not sufficiently appreciated and which puts the variability ofG in a much wider context. VariableG is a popular but incomplete representation of a much deeper problem:Is the Strong Equivalenc...

Dirac’s large number hypothesis (LNH), in the formG/G 0=HH 0 −1 , is applied to the matter-dominated cosmological era, using the framework of the scale covariant theory (Canuto et al., 1977). We obtain explicit expressions forR andβ a as functions ofR E , whereR andR E are the scale factors of the cosmological Robertson-Walker metric, expressed...

Widely separated, eccentric pulsating X-ray binaries are shown to be appropriate systems for studying the basic properties of stellar wind accretion. They are free from the complications encountered in closed binaries, where the optical star nearly fills its critical lobe (Avni, 1977). A framework of four observational tests was set up to study the...

The expected X-ray emission from Cyg X-1, considered a massive neutron star (8-15 solar masses) according to some gravity theories, is studied within the framework of Rosen's bimetric gravity theory (1973, 1974). It is shown that in such massive neutron stars, the innermost stable orbit lies far outside the star surface, and therefore the X-ray spe...

The eccentric binary hypothesis for transient X-ray sources is examined in the framework of the gradual acceleration stellar wind model proposed by Barlow and Cohen (1977). It is found that a consideration of the ratio of maximum to minimum luminosities and of the ratio of the durations of the high and low states, for a typical transient X-ray sour...

An X-ray burster model is considered which involves thermonuclear flashes on an accreting neutron star. Previous studies of this model have assumed that the neutron star is in hydrostatic equilibrium during a burst and that the burst luminosity is isotropic. It is shown that when general relativity is taken into account, the assumption of hydrostat...

The stability of a cold neutron star against dissociation into small grains and the stability of equilibrium neutron-star configurations against small radial perturbations are investigated. The steady-state spherically symmetric case corresponding to a cold neutron star is considered in the framework of the bimetric theory of gravitation, and a pro...

Field equations for plane waves are set up and some solutions are obtained. Transverse, longitudinal, and mixed waves are possible. In the purely transverse case the energy density is positive definite. In the purely longitudinal case it may be negative. However, in one example investigated it is positive if the Riemannian metric tensor satisfies a...

In the bimetric theory of gravitation, the static spherically symmetric case involving matter characterized by density and pressure is considered. It is found that the condition that the mass be stationary under small variations of the field variables (including the density) for a fixed number of baryons leads to the field equations and to the equi...

In the framework of the bimetric theory of gravitation, a homogeneous isotropic cosmological model is set up having negative spatial curvature (k = -1) and containing matter having negligible pressure. The solutions of the field equations are obtained as expansions in powers of a small parameter, the present value of which can be determined, in pri...

The inertial mass of a static spherically symmetric body is evaluated in the framework of Rosen's bimetric theory of gravitation. It is shown that the inertial mass equals the gravitational mass independently of the specific equation of state chosen, and hence that the theory satisfies the equivalence principle.

It is shown that the field equations of the bimetric theory of gravitation have solutions corresponding to a class of homogeneous isotropic cosmological models with negative spatial curvature (k=–1). Some examples are given.

The field equations of a noncovariant theory of gravitation, based on the existence of a preferred frame of reference in the universe, are applied to the homogeneous isotropic cosmological model. One is naturally led to a particular value of the previously undetermined constant present in the equations. The second-order equation determining the rad...

Since a homogeneous isotropic universe can be embedded in a flat space of five dimensions, the question is considered under what conditions a more general universe can be embedded in a five-dimensional flat space. On the assumption that the deviation from homogeneity is small, it is found that real inhomogeneities can occur only in the case of a un...