首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 656 毫秒
1.
It is surprising that we hardly know only 4% of the universe. Rest of the universe is made up of 73% of dark-energy and 23% of dark-matter. Dark-energy is responsible for acceleration of the expanding universe; whereas dark-matter is said to be necessary as extra-mass of bizarre-properties to explain the anomalous rotational-velocity of galaxy. Though the existence of dark-energy has gradually been accepted in scientific community, but the candidates for dark-matter have not been found as yet and are too crazy to be accepted. Thus, it is obvious to look for an alternative theory in place of dark-matter. Milgrom (Astrophys. J. 270:365, 1983a; 270:371, 1983b) has suggested a ‘Modified Newtonian Dynamics (MOND)’ which appears to be highly successful for explaining the anomalous rotational-velocity. But unfortunately MOND lacks theoretical support. The MOND, in-fact, is (empirical) modification of Newtonian-Dynamics through modification in the kinematical acceleration term ‘a’ (which is normally taken as a=\fracv2ra=\frac{v^{2}}{r}) as effective kinematic acceleration aeffective = a m(\fracaa0)a_{\mathit{effective}} = a \mu(\frac{a}{a_{0}}), wherein the μ-function is 1 for usual-values of accelerations but equals to \fracaa0 ( << 1)\frac{a}{a_{0}} (\ll1) if the acceleration ‘a’ is extremely-low lower than a critical value a 0(10−10 m/s2). In the present paper, a novel variant of MOND is proposed with theoretical backing; wherein with the consideration of universe’s acceleration a d due to dark-energy, a new type of μ-function on theoretical-basis emerges out leading to aeffective = a(1 -K \fraca0a)a_{\mathit{effective}} = a(1 -K \frac{a_{0}}{a}). The proposed theoretical-MOND model too is able to fairly explain ‘qualitatively’ the more-or-less ‘flat’ velocity-curve of galaxy-rotation, and is also able to predict a dip (minimum) on the curve.  相似文献   

2.
This letter presents expressions for: Newton’s law of universal gravitation, Milgrom’s expression of Modified Newtonian Dynamics [MOND] for the velocity of stars at the out-skirts of spiral-galaxies, and some new relations between mass and radius of: globular-clusters, spiral-galaxies, and the galactic-clusters without any need for the big G. These expressions suggest that the strength of gravity and velocities of stars at the out-skirts of spiral-galaxies would be different if the total-mass and radius of the universe were different.  相似文献   

3.
A new equivalence relation, named relation of ‘similarity’ is defined and applied in the restricted three-body problem. Using this relation, a new class of trajectories (named ‘similar’ trajectories) are obtained; they have the theoretical role to give us new details in the restricted three-body problem. The ‘similar’ coordinate systems allow us in addition to obtain a unitary and an elegant demonstration of some analytical relations in the Roche geometry. As an example, some analytical relations published by Seidov (in Astrophys. J. 603:283, 2004) are demonstrated.  相似文献   

4.
A new class of dark energy models in a Locally Rotationally Symmetric Bianchi type-II (LRS B-II) space-time with variable equation of state (EoS) parameter and constant deceleration parameter have been investigated in the present paper. The Einstein’s field equations have been solved by applying a variation law for generalized Hubble’s parameter given by Berman: Nuovo Cimento 74:182 (1983) which generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential-law form. Using these two forms, Einstein’s field equations are solved separately that correspond to expanding singular and non-singular models of the universe respectively. The dark energy EoS parameter ω is found to be time dependent and its existing range for both models is in good agreement with the three recent observations of (i) SNe Ia data (Knop et al.: Astrophys. J. 598:102 (2003)), (ii) SNe Ia data collaborated with CMBR anisotropy and galaxy clustering statistics (Tegmark et al.: Astrophys. J. 606:702 (2004)) and latest (iii) a combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift type Ia supernovae and galaxy clustering (Hinshaw et al.: Astrophys. J. Suppl. 180:225 (2009); Komatsu et al. Astrophys. J. Suppl. 180:330 (2009)). The cosmological constant Λ is found to be a positive decreasing function of time and it approaches a small positive value at late time (i.e. the present epoch) which is corroborated by results from recent supernovae Ia observations. The physical and geometric behaviour of the universe have also been discussed in detail.  相似文献   

5.
In the framework of ‘microscopic’ theory of black holes (J. Phys. Soc. Jpn. Suppl. B 70, 84, 2001; Astrophys. USSR 4, 659, 1996; 35, 335, 1991, 33, 143, 1990, 31, 345, 1989a; Astrophys. Space Sci. 1, 1992; Dokl. Akad. Nauk USSR 309, 97, 1989b), and references therein, we address the ‘pre-radiation time’ (PRT) of neutrinos from black holes, which implies the lapse of time from black hole’s birth till radiation of an extremely high energy neutrinos. For post-PRT lifetime, the black hole no longer holds as a region of spacetime that cannot communicate with the external universe. We study main features of spherical accretion onto central BH and infer a mass accretion rate onto it, and, further, calculate the resulting PRT versus bolometric luminosity due to accretion onto black hole. We estimate the PRTs of AGN black holes, with the well-determined masses and bolometric luminosities, collected from the literature by Woo Jong-Hak and Urry (Astrophys. J. 579, 530, 2002) on which this paper is partially based. The simulations for the black holes of masses M BH ≃(1.1⋅106 ÷4.2⋅109) M ⊙ give the values of PRTs varying in the range of about T BH ≃(4.3⋅105 ÷5.6⋅1011) yr. The derived PRTs for the 60 AGN black holes are longer than the age of the universe (∼13.7 Gyr) favored today. At present, some of remaining 174 BHs may radiate neutrinos. However, these results would be underestimated if the reservoir of gas for accretion in the galaxy center is quite modest, and no obvious way to feed the BHs with substantial accretion.  相似文献   

6.
The paper deals with a spatially homogeneous and isotropic FRW space-time filled with perfect fluid and dark energy components. The two sources are assumed to interact minimally, and therefore their energy momentum tensors are conserved separately. A special law of variation for the Hubble parameter proposed by Berman (Nuovo Cimento B 74:182, 1983) has been utilized to solve the field equations. The Berman’s law yields two explicit forms of the scale factor governing the FRW space-time and constant values of deceleration parameter. The role of dark energy with variable equation of state parameter has been studied in detail in the evolution of FRW universe. It has been found that dark energy dominates the universe at the present epoch, which is consistent with the observations. The physical behavior of the universe has been discussed in detail.  相似文献   

7.
The regularization of a new problem, namely the three-body problem, using ‘similar’ coordinate system is proposed. For this purpose we use the relation of ‘similarity’, which has been introduced as an equivalence relation in a previous paper (see Roman in Astrophys. Space Sci. doi:, 2011). First we write the Hamiltonian function, the equations of motion in canonical form, and then using a generating function, we obtain the transformed equations of motion. After the coordinates transformations, we introduce the fictitious time, to regularize the equations of motion. Explicit formulas are given for the regularization in the coordinate systems centered in the more massive and the less massive star of the binary system. The ‘similar’ polar angle’s definition is introduced, in order to analyze the regularization’s geometrical transformation. The effect of Levi-Civita’s transformation is described in a geometrical manner. Using the resulted regularized equations, we analyze and compare these canonical equations numerically, for the Earth-Moon binary system.  相似文献   

8.
An axially symmetric non-static space-time is considered in the presence of thick domain walls in the scalar–tensor theories formulated by Brans and Dicke (Phys. Rev. 124:925, 1961) and Saez and Ballester (Phys. Lett. A 113:467, 1985). Exact cosmological models, in both the theories, are presented with the help of special law of variation proposed by Berman (Nuovo Cim. B 74:182, 1983), for Hubble’s parameter. Some physical and kinematical properties of the models are discussed.   相似文献   

9.
It is argued that the iron nucleosynthesis rate in the universe due to SNI outbursts is dependent on the mass function of star formation. Since the mass function depends on the chemical composition and since the masses of SNI precursors have upper limits, the iron nucleosynthesis rate was low at an earlier evolutionary epoch of the universe when mainly massive stars were formed. The iron nucleosynthesis rate should reach a maximum near z ∼ 0.5. At such or similar value of z the well-known ‘step’ in the cosmic γ-ray background spectrum may be explained by the presence of γ-gray quanta accompanying the radioactive56Co →56Fe decay. An argument is presented against the identification of the hidden mass of the universe with black-hole remnants of ‘type III’ stars.  相似文献   

10.
The present study deals with a spatially homogeneous and anisotropic Bianchi-I cosmological models representing massive strings. The energy-momentum tensor, as formulated by Letelier (1983), has been used to construct massive string cosmological models for which we assume the expansion scalar in the models is proportional to one of the components of shear tensor. The Einstein’s field equations have been solved by applying a variation law for generalized Hubble’s parameter in Bianchi-I space-time. We have analysed a comparative study of accelerating and decelerating models in the presence of string scenario. The study reveals that massive strings dominate in the decelerating universe whereas strings dominate in the accelerating universe. The strings eventually disappear from the universe for sufficiently large times, which is in agreement with current astronomical observations.  相似文献   

11.
Magnetic field extrapolation is an alternative method to study chromospheric and coronal magnetic fields. In this paper, two semi-analytical solutions of force-free fields (Low and Lou in Astrophys. J. 352:343, 1990) have been used to study the errors of nonlinear force-free (NLFF) fields based on force-free factor α. Three NLFF fields are extrapolated by approximate vertical integration (AVI) Song et al. (Astrophys. J. 649:1084, 2006), boundary integral equation (BIE) Yan and Sakurai (Sol. Phys. 195:89, 2000) and optimization (Opt.) Wiegelmann (Sol. Phys. 219:87, 2004) methods. Compared with the first semi-analytical field, it is found that the mean values of absolute relative standard deviations (RSD) of α along field lines are about 0.96–1.19, 0.63–1.07 and 0.43–0.72 for AVI, BIE and Opt. fields, respectively. While for the second semi-analytical field, they are about 0.80–1.02, 0.67–1.34 and 0.33–0.55 for AVI, BIE and Opt. fields, respectively. As for the analytical field, the calculation error of 〈|RSD|〉 is about 0.1∼0.2. It is also found that RSD does not apparently depend on the length of field line. These provide the basic estimation on the deviation of extrapolated field obtained by proposed methods from the real force-free field.  相似文献   

12.
The evidence for unseen mass (which is briefly reviewed) suggests that the cosmological density parameter Ω is at least 0.1–0.2. An Einstein-de-Sitter ‘flat’ universe with Ω = 1 — which is appealing for theoretical reasons — can only be reconciled with the data if the galaxies are more ‘clumped’ than the overall mass distribution, and are poor tracers of the unseen mass even on scales of several Mpc. Possible forms for the unseen mass are discussed; and feedback processes are outlined whereby galaxy formation can be suppressed in underdense regions.  相似文献   

13.
We revisit a set of symplectic variables introduced by Andre Deprit (Celest Mech 30, 181–195, 1983), which allows for a complete symplectic reduction in rotation invariant Hamiltonian systems, generalizing to arbitrary dimension Jacobi’s reduction of the nodes. In particular, we introduce an action-angle version of Deprit’s variables, connected to the Delaunay variables, and give a new hierarchical proof of the symplectic character of Deprit’s variables.  相似文献   

14.
The main goal of this work is investigation of NADE in the cyclic universe scenario. Since, cyclic universe is explained by a phantom phase (ω<−1), it is shown when there is no interaction between matter and dark energy, ADE and NADE do not produce a phantom phase, then can not describe cyclic universe. Therefore, we study interacting models of ADE and NADE in the modified Friedmann equation. We find out that, in the high energy regime, which it is a necessary part of cyclic universe evolution, only NADE can describe this phantom phase era for cyclic universe. Considering deceleration parameter tells us that the universe has a deceleration phase after an acceleration phase, and NADE is able to produce a cyclic universe. Also it is found valuable to study generalized second law of thermodynamics. Since the loop quantum correction is taken account in high energy regime, it may not be suitable to use standard treatment of thermodynamics, so we turn our attention to the result of Li et al. (Adv. High Energy Phys. 2009: 905705, 2009), which the authors have studied thermodynamics in loop quantum gravity, and we show that which condition can satisfy generalized second law of thermodynamics.  相似文献   

15.
An exact Bianchi type-V perfect fluid cosmological model is obtained in a scalar tensor theory proposed by Sen (Z. Phys. 149:311, 1957) based on Lyra Manifold in case of β is a constant and it is shown that this cosmological model exists only in the case of Radiation Universe (ρ=3p) if β is a function of ‘t’ using negative constant deceleration parameter. Some physical and geometrical properties of these models are discussed.  相似文献   

16.
Due to the Hubble redshift, photon energy, chiefly in the form of CMBR photons, is currently disappearing from the universe at the rate of nearly 1055 erg s−1. An ongoing problem in cosmology concerns the fate of this energy. In one interpretation it is irretrievably lost, i.e., energy is not conserved on the cosmic scale. Here we consider a different possibility which retains universal energy conservation. Treating gravitational potential energy conventionally as ‘negative’, it has earlier been proposed that the Hubble shift flips positive energy (photons) to negative energy (gravitons) and vice versa. The lost photon energy would thus be directed towards gravitation, making gravitational energy wells more negative. Conversely, within astrophysical bodies, the flipping of gravitons to photons would give rise to a ‘Hubble luminosity’ of magnitude −UH 0, where U is the internal gravitational potential energy of the object. Preliminary evidence of such an energy release is presented in bodies ranging from planets, white dwarfs and neutron stars to supermassive black holes and the visible universe.  相似文献   

17.
Exact solution of Einstein’s field equations is obtained for massive string cosmological model of Bianchi III space-time using the technique given by Letelier (Phys. Rev. D 20:2414, 1983) in presence of perfect fluid and decaying vacuum energy density Λ. To get the deterministic solution of the field equations the expansion θ in the model is considered as proportional to the eigen value s2 2\sigma^{2}_{~2} of the shear tensor sj i\sigma^{j}_{~i} and also the fluid obeys the barotropic equation of state. The vacuum energy density Λ is found to be positive and a decreasing function of time which is supported by the results from recent supernovae Ia observations. It is also observed that in early stage of the evolution of the universe string dominates over the particle whereas the universe is dominated by massive string at the late time. Some physical and geometric properties of the model are also discussed.  相似文献   

18.
A search for microflare activity in the extreme ultraviolet (EUV) quiet Sun using the Coronal Diagnostic Spectrometer (CDS) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft has not resulted in the identification of microflare activity, but has resulted in the identification of a hitherto unknown phenomenon: enhancements of a factor of 2–3 in the flux of transition region lines at network junctions. A total of some 6 hours of observation of 5 different target areas showed this ‘blinker’ activity at each area, with durations ranging from 1 to 30 min and averaging 13 min, and thermal energy content of order 10-6 that of a ‘standard’ flare. Assuming that the observations are of typical quiet Sun, and projecting these data to predict a distribution of these events over the entire Sun, the total thermal energy content of these ‘blinkers’ is insignificant when compared to the energy required to heat the corona. The nature of these events and their significance are discussed in this paper.  相似文献   

19.
In this paper it is suggested that inclusion of mutual gravitational interactions among the particles in the early dense universe can lead to a ‘pre-big bang’ scenario, with particle masses greater than the Planck mass implying an accelerating phase of the universe, which then goes into the radiation phase when the masses fall below the Planck mass. The existence of towers of states of such massive particles (i.e. multiples of Planck mass) as implied in various unified theories, provides rapid acceleration in the early universe, similar to the usual inflation scenario, but here the expansion rate goes over ‘smoothly’ to the radiation dominated universe when temperature becomes lower than the Planck temperature.  相似文献   

20.
A series of numerical experiments is carried out to simulate the dynamical evolution of groups of galaxies in different models of gravitation. The simulation results are compared with observations of the Local Group and of the CenA/M83 and M81/M82 galactic groups. It is found that the LCDM model and the MOND (Modified Newtonian Dynamics) model are in substantially better agreement with the observations than the other two models examined here. The dispersion in the velocities relative to Hubble’s law in these models is small, but the MOND model yields slightly elevated values. For initial conditions close to virial equilibrium, the mean square deviations in the velocities of the galaxies are ~30 km/s, in agreement with observational estimates.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号