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1.
《New Astronomy Reviews》2002,46(11):659-680
We present some phenomenological aspects of the pre-big-bang cosmological model inspired by the duality properties of string theory. In particular, assuming the spatial sections of the homogeneous background geometry to be isotropic, we discuss the quantum production of perturbations of the background fields (gravitons, dilatons, moduli fields), as well as the production of particles which do not contribute to the background, which we call “seeds”. As such we consider the cases of electromagnetic and axionic seeds. We also discuss their possible observational consequences, for example, we study whether they can provide the origin of primordial galactic magnetic fields, and whether they can generate the initial fluctuations leading to the formation of large-scale structure and the measured cosmic microwave background anisotropies. We finally analyze axion and photon production in four dimensional anisotropic pre-big-bang cosmological models.  相似文献   

2.
Limits on cosmic time scale variations of gravitational and cosmological `constants' are studied. The study is based on a function which can measure the temporal variation of the magnitude of the gradient of any scalar field defined inside a medium exposed to a gravitational field. The cosmic time dependent scalar fields are taken to be the gravitational and cosmological `constants'. The medium; in which those scalar fields are defined; is taken to be the spatially perturbed Friedman-Robertson-Walker (FRW) expanding universe. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

3.
We performed cosmological, magnetohydrodynamical simulations to follow the evolution of magnetic fields in galaxy clusters, exploring the possibility that the origin of the magnetic seed fields is galactic outflows during the starburst phase of galactic evolution. To do this, we coupled a semi-analytical model for magnetized galactic winds as suggested by Bertone, Vogt & Enßlin to our cosmological simulation. We find that the strength and structure of magnetic fields observed in galaxy clusters are well reproduced for a wide range of model parameters for the magnetized, galactic winds and do only weakly depend on the exact magnetic structure within the assumed galactic outflows. Although the evolution of a primordial magnetic seed field shows no significant differences to that of galaxy cluster fields from previous studies, we find that the magnetic field pollution in the diffuse medium within filaments is below the level predicted by scenarios with pure primordial magnetic seed field. We therefore conclude that magnetized galactic outflows and their subsequent evolution within the intracluster medium can fully account for the observed magnetic fields in galaxy clusters. Our findings also suggest that measuring cosmological magnetic fields in low-density environments such as filaments is much more useful than observing cluster magnetic fields to infer their possible origin.  相似文献   

4.
The neutrinos from the Big Bang or the Cosmic Neutrino Background (CNB) carry precious information from the early epoch when our universe was only 1 s old. Although not yet directly detected, CNB may be revealed indirectly through cosmological observations due to neutrino important cosmological influence.We review the cosmological role of neutrinos and the cosmological constraints on neutrino characteristics. Namely, we discuss the impact of neutrinos in the early universe: the cosmic expansion, neutrino decoupling, the role of neutrinos in the primordial production of light elements, leptogenesis, etc. We briefly discuss the role of neutrino at later stages of the universe.Due to the considerable cosmological influence of neutrinos, cosmological bounds on neutrino properties from observational data exist. We review the cosmological constraints on the effective number of neutrino species, neutrino mass and mixing parameters, lepton number of the universe, presence of sterile neutrino, etc.  相似文献   

5.
Bianchi Type III massive string cosmological model for perfect fluid distribution in the presence of magnetic field, is investigated. It is assumed that the universe is filled with barotropic perfect fluid. We have attempted to investigate Bianchi Type III string cosmological model incorporating perfect fluid with magnetic field. To get the deterministic model in terms of cosmic time, we have assumed that the expansion (θ) in the model is proportional to the shear. We have also assumed that F 12 is the only non-vanishing component of electromagnetic field tensor F ij . The behaviour of the model in presence and absence of magnetic field together with singularities in these models are also discussed.  相似文献   

6.
We obtain some cosmological models that are exact solutions of Einstein's field equations. The metric utilized is Marder's metric which is Bianchi Type I and the curvature source is a cloud of strings which are one dimensional objects. Bianchi type cosmological models play an important role in the study of the universe on a scale which anisotropy is not ignored. In this paper we have investigated the effect of cosmic strings on the cosmic microwave background anisotropy. Various physical and geometrical properties of the model are also discussed. The solutions have reported that the cosmic microwave background anisotropy may due to the cosmic strings.  相似文献   

7.
In spite of the large number of global three-dimensional (3-D) magnetohydrodynamic (MHD) simulations of accretion disks and astrophysical jets, which have been developed since 2000, the launching mechanisms of jets is somewhat controversial. Previous studies of jets have concentrated on the effect of the large-scale magnetic fields permeating accretion disks. However, the existence of such global magnetic fields is not evident in various astrophysical objects, and their origin is not well understood. Thus, we study the effect of small-scale magnetic fields confined within the accretion disk. We review our recent findings on the formation of jets in dynamo-active accretion disks by using 3-D MHD simulations. In our simulations, we found the emergence of accumulated azimuthal magnetic fields from the inner region of the disk (the so-called magnetic tower) and also the formation of a jet accelerated by the magnetic pressure of the tower. Our results indicate that the magnetic tower jet is one of the most promising mechanisms for launching jets from the magnetized accretion disk in various astrophysical objects. We will discuss the formation of cosmic jets in the context of the magnetic tower model.  相似文献   

8.
I review some basic properties of diffusive shock acceleration (DSA) in the context of young supernova remnants (SNRs). I also point out some key differences with cosmological, cluster-related shocks. DSA seems to be very efficient in strong, young SNR shocks. Provided the magnetic fields exceed some hundreds of μGauss (possibly amplified by CR related dynamics), these shocks can accelerate cosmic ray hadrons to PeV energies in the time available to them. Electron energies, limited by radiative losses, are likely limited to the TeV range. Injection of fresh particles at these shocks is poorly understood, but hadrons are much more easily injected than the more highly magnetized electrons. That seems supported by observational data, as well. So, while CR protons in young SNRs may play very major roles in the SNR evolution, the CR electron populations have minimal such impact, despite their observational importance.  相似文献   

9.
Confronted with microwave background observations by WMAP and with consternating supernova locations in the magnitude–redshift diagram modern cosmology feels enforced to call for cosmic vacuum energy as a necessary cosmological ingredient. Most often this vacuum energy is associated with Einstein’s cosmological constant Λ or with so-called “dark energy”. A positive value of Λ describes an inflationary action on cosmic dynamics which in view of recent cosmological data appears as an absolute need. In this article, however, we question the hypothesis of a constant vacuum energy density since not justifiable on physical grounds. Instead we show that gravitational binding energy of cosmic matter, connected with ongoing structure formation during cosmic expansion, acts similar to vacuum energy, since it reduces the effective gravitating proper mass density. Thus one may be encouraged to believe that actions of cosmic vacuum energy and gravitational binding energy concerning their cosmological effects are closely related to each other, perhaps in some respects even have identical phenomenologies.  相似文献   

10.
It is well known that Kasner-type cosmologies provide a useful framework for analyzing the three-dimensional anisotropic expansion because of the simplification of the anisotropic dynamics. In this paper relativistic multi-fluid Kasner-type scenarios are studied. We first consider the general case of a superposition of two ideal cosmic fluids, as well as the particular cases of non-interacting and interacting ones, by introducing a phenomenological coupling function q(t). For two-fluid cosmological scenarios there exist only cosmological scaling solutions, while for three-fluid configurations there exist not only cosmological scaling ones, but also more general solutions. In the case of triply interacting cosmic fluids we can have energy transfer from two fluids to a third one, or energy transfer from one cosmic fluid to the other two. It is shown that by requiring the positivity of energy densities there always is a matter component which violates the dominant energy condition in this kind of anisotropic cosmological scenarios.  相似文献   

11.
We describe a novel technique for probing the statistical properties of cosmic magnetic fields based on radio polarimetry data. Second-order magnetic field statistics like the power spectrum cannot always distinguish between magnetic fields with essentially different spatial structure. Synchrotron polarimetry naturally allows certain fourth-order magnetic field statistics to be inferred from observational data, which lifts this degeneracy and can thereby help us gain a better picture of the structure of the cosmic fields and test theoretical scenarios describing magnetic turbulence. In this work we show that a fourth-order correlator of specific physical interest, the tension force spectrum, can be recovered from the polarized synchrotron emission data. We develop an estimator for this quantity based on polarized emission observations in the Faraday rotation free frequency regime. We consider two cases: a statistically isotropic field distribution, and a statistically isotropic field superimposed on a weak mean field. In both cases the tension force power spectrum is measurable; in the latter case, the magnetic power spectrum may also be obtainable. The method is exact in the idealized case of a homogeneous relativistic electron distribution that has a power-law energy spectrum with a spectral index of   p = 3  , and assumes statistical isotropy of the turbulent field. We carry out numerical tests of our method using synthetic polarized emission data generated from numerically simulated magnetic fields. We show that the method is valid, that it is not prohibitively sensitive to the value of the electron spectral index p , and that the observed tension force spectrum allows one to distinguish between e.g. a randomly tangled magnetic field (a default assumption in many studies) and a field organized in folded flux sheets or filaments.  相似文献   

12.
A Bianchi type I string cosmological model in the presence of a magnetic flux is investigated. A few plausible assumptions regarding the parametrization of the cosmic string and magneto-fluid are introduced and some exact analytical solutions are presented.  相似文献   

13.
We find a solution for exponential inflation in Brans–Dicke cosmology endowed with a cosmological term, which includes time-varying shear and vorticity. We find that the scalar field and the scale factor increase exponentialy while shear, vorticity, energy density, cosmic pressure and the cosmological term decay exponentialy for negative beta, where beta is defined in the text.  相似文献   

14.
We propose and test new statistical tools to study the distribution of cosmic rays based on the use of the minimal spanning tree. The method described is particularly sensitive to filamentary structures, as those expected to arise from strong sources of charged cosmic rays which get deflected by intervening magnetic fields. We also test the method with data available from the AGASA and SUGAR surface detector arrays.  相似文献   

15.
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16.
We investigate numerically the contribution to the cosmic gamma-ray background from cosmic-ray ions and electrons accelerated at intergalactic shocks associated with cosmological structure formation. We show that the kinetic energy of accretion flows in the low-redshift intergalactic medium is thermalized primarily through moderately strong shocks, which allow for an efficient conversion of shock ram pressure into cosmic-ray pressure. Cosmic rays accelerated at these shocks produce a diffuse gamma-ray flux which is dominated by inverse Compton emission from electrons scattering off cosmic microwave background photons. Decay of neutral π mesons generated in p–p inelastic collisions of the ionic cosmic-ray component with the thermal gas contribute about 30 per cent of the computed emission. Based on experimental upper limits on the photon flux above 100 MeV from nearby clusters we constrain the efficiency of conversion of shock ram pressure into relativistic CR electrons to  ≲1 per cent  . Thus, we find that cosmic rays of cosmological origin can generate an overall significant fraction of order 20 per cent and no more than 30 per cent of the measured gamma-ray background.  相似文献   

17.
Various radio observations have shown that the hot atmospheres of galaxy clusters are magnetized. However, our understanding of the origin of these magnetic fields, their implications on structure formation and their interplay with the dynamics of the cluster atmosphere, especially in the centres of galaxy clusters, is still very limited. In preparation for the upcoming new generation of radio telescopes (like Expanded Very Large Array, Low Wavelength Array, Low Frequency Array and Square Kilometer Array), a huge effort is being made to learn more about cosmological magnetic fields from the observational perspective. Here we present the implementation of magnetohydrodynamics (MHD) in the cosmological smoothed particle hydrodynamics (SPH) code gadget . We discuss the details of the implementation and various schemes to suppress numerical instabilities as well as regularization schemes, in the context of cosmological simulations. The performance of the SPH–MHD code is demonstrated in various one- and two-dimensional test problems, which we performed with a fully, three-dimensional set-up to test the code under realistic circumstances. Comparing solutions obtained using athena , we find excellent agreement with our SPH–MHD implementation. Finally, we apply our SPH–MHD implementation to galaxy cluster formation within a large, cosmological box. Performing a resolution study we demonstrate the robustness of the predicted shape of the magnetic field profiles in galaxy clusters, which is in good agreement with previous studies.  相似文献   

18.
This paper presents anisotropic, homogeneous two-fluid cosmological models in a Bianchi type I space–time with a variable gravitational constant G and cosmological constant Λ. In the two-fluid model, one fluid represents the matter content of the universe and another fluid is chosen to model the CMB radiation. We find a variety of solutions in which the cosmological parameter varies inversely with time t. We also discuss in detail the behavior of associated fluid parameters and kinematical parameters. This paper pictures cosmic history when the radiation and matter content of the universe are in an interactive phase. Here, Ω is closing to 1 throughout the cosmic evolution.   相似文献   

19.
In this paper we consider a correspondence between the holographic dark energy density and interacting generalized cosmic Chaplygin gas energy density in flat FRW universe. Then, we reconstruct the potential of the scalar field which describe the generalized cosmic Chaplygin cosmology. In the special case we obtain time-dependent energy density and study cosmological parameters. We find stability condition of this model which is depend on cosmic parameter.  相似文献   

20.
One dimensional numerical results of the non-linear interaction between cosmic rays and a magnetic field are presented. These show that cosmic ray streaming drives large amplitude Alfvénic waves. The cosmic ray streaming energy is very efficiently transfered to the perturbed magnetic field of the Alfvén waves. Thus a magnetic field of interstellar values, assumed in models of supernova remnant blast wave acceleration, would not be appropriate in the region of the shock. The increased magnetic field reduces the acceleration time and so increases the maximum cosmic ray energy, which may provide a simple and elegant resolution to the highest energy galactic cosmic ray problem were the cosmic rays themselves provide the fields necessary for their acceleration. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

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