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 265 毫秒
1.
There has been a renewed interest in the recent years in the possibility of deviations from the predictions of Newton’s “inverse-square law” of universal gravitation. One of the reasons for renewing this interest lies in various theoretical attempts to construct a unified elementary particle theory, in which there is a natural prediction of new forces over macroscopic distances. Therefore the existence of such a force would only coexist with gravity, and in principle could only be detected as a deviation from the inverse square law, or in the “universality of free fall” experiments. New experimental techniques such that of Sagnac interferometry can help explore the range of the Yukawa correction λ≥1014 m where such forces might be present. It may be, that future space missions might be operating in this range which has been unexplored for very long time. In this paper we derive the basic thermodynamic parameters of such a Yukawa stationary spherically symmetric black hole. First, the expression for the event horizon of such a black hole is derived, with the help of which the temperature, entropy and heat capacity of this particular black hole are obtained. We have also obtained analytical expressions for the change of mass of such black hole, and also its corresponding evaporation time.  相似文献   

2.
Quantum theory in Robertson – Walker spacetime suggests the existence of a minimal energy ε of the order of 10−45 erg. Reasonable forms for ε give the expansion factor R=R(t)(t= the cosmic time) with no need of gravitational field equations.Einstein's theory should be modified in gravitational fields of strength less than ε c/ħ ∼ 10−8 cm/s2 where c is the speed of light and ħ is the reduced Planck constant. The cosmological term λ is expected to decrease as the universe expands.In the Appendix, ε is derived from a big bang – big crunch Newtonian cosmology.  相似文献   

3.
In this paper we examine the recently introduced Dvali-Gabadadze-Porrati (DGP) gravity model. We use a space-time metric in which the local gravitation source dominates the metric over the contributions from the cosmological flow. Anticipating ideal possible solar system effects, we derive expressions for the signal time delays in the vicinity of the Sun. and for various ranges of the angle θ of the signal approach, The time contribution due to DGP correction to the metric is found to be proportional to b 3/2/c 2 r 0. For r 0 equal to 5 Mpc and θ in the range [−π/3,π/3], Δt is equal to 0.0001233 ps. This delay is extremely small to be measured by today’s technology but it could be probably measurable by future experiments.  相似文献   

4.
We analyze the evolution of the flare/postflare-loop system in the two-ribbon flare of November 3, 2003, utilizing multi-wavelength observations that cover the temperature range from several tens of MK down to 104 K. A non-uniform growth of the loop system enables us to identify analogous patterns in the height–time, h(t), curves measured at different temperatures. The “knees,” “plateaus,” and “bends” in a higher-temperature curve appear after a certain time delay at lower heights in a lower-temperature curve. We interpret such a shifted replication as a track of a given set of loops (reconnected field lines) while shrinking and cooling after being released from the reconnection site. Measurements of the height/time shifts between h(t) curves of different temperatures provide a simultaneous estimate of the shrinkage speed and cooling rate in a given temperature domain, for a period of almost ten hours after the flare impulsive phase. From the analysis we find the following: (a) Loop shrinkage is faster at higher temperatures – in the first hour of the loop-system growth, the shrinkage velocity at 5 MK is 20 – 30 km s−1, whereas at 1 MK it amounts to 5 km s−1; (b) Shrinking becomes slower as the flare decays – ten hours after the impulsive phase, the shrinkage velocity at 5 MK becomes 5 km s−1; (c) The cooling rate decreases as the flare decays – in the 5 MK range it is 1 MK min−1 in the first hour of the loop-system growth, whereas ten hours later it decreases to 0.2 MK min−1; (d) During the initial phase of the loop-system growth, the cooling rate is larger at higher temperatures, whereas in the late phases the cooling rate apparently does not depend on the temperature; (e) A more detailed analysis of shrinking/cooling around one hour after the impulsive phase reveals a deceleration of the loop shrinkage, amounting to ā ≈ 10 m s−2 in the T < 5 MK range; (f) In the same interval, conductive cooling dominates down to T ≈ 3 MK, whereas radiation becomes dominant below T ≈ 2 MK; (g) A few hours after the impulsive phase, radiation becomes dominant across the whole T < 5 MK range. These findings are compared with results of previous studies and discussed in the framework of relevant models.  相似文献   

5.
We consider a system of nonlinear spinor and a Bianchi type I gravitational fields in presence of viscous fluid. The nonlinear term in the spinor field Lagrangian is chosen to be λ F, with λ being a self-coupling constant and F being a function of the invariants I an J constructed from bilinear spinor forms S and P. Self-consistent solutions to the spinor and BI gravitational field equations are obtained in terms of τ, where τ is the volume scale of BI universe. System of equations for τ and ε, where ε is the energy of the viscous fluid, is deduced. This system is solved numerically for some special cases.   相似文献   

6.
  相似文献   

7.
If cooled-down neutron stars have a thin atomic crystalline–iron crust, they must diffract X-rays of appropriate wavelength. If the diffracted beam is to be visible from Earth (an extremely rare but possible situation), the illuminating source must be very intense and near the reflecting star. An example is a binary system composed of two neutron stars in close orbit, one of them inert, the other an X-ray pulsar. (Perhaps an “anomalous” X-ray pulsar or magnetar, not powered by gas absorption from the companion or surrounding space, would be the cleanest example.) The observable to be searched for is a secondary peak added (quasi-) periodically to the main X-ray pulse. The distinguishing feature of this secondary peak is that it appears at wavelengths related by simple integer numbers, λ,λ/2,λ/3,…,λ/n because of Bragg’s diffraction law.  相似文献   

8.
We present a simple method of forecasting the geomagnetic storms caused by high-speed streams (HSSs) in the solar wind. The method is based on the empirical correlation between the coronal hole area/position and the value of the Dst index, which is established in a period of low interplanetary coronal mass ejection (ICME) activity. On average, the highest geomagnetic activity, i.e., the minimum in Dst, occurs four days after a low-latitude coronal hole (CH) crosses the central meridian. The amplitude of the Dst dip is correlated with the CH area and depends on the magnetic polarity of the CH due to the Russell – McPherron effect. The Dst variation may be predicted by employing the expression Dst(t)=(−65±25×cos λ)[A(t *)]0.5, where A(t *) is the fractional CH area measured in the central-meridian slice [−10°,10°] of the solar disc, λ is the ecliptic longitude of the Earth, ± stands for positive/negative CH polarity, and t−t *=4 days. In periods of low ICME activity, the proposed expression provides forecasting of the amplitude of the HSS-associated Dst dip to an accuracy of ≈30%. However, the time of occurrence of the Dst minimum cannot be predicted to better than ±2 days, and consequently, the overall mean relative difference between the observed and calculated daily values of Dst ranges around 50%.  相似文献   

9.
The results of morphological and spectral study of the galaxies Kaz 5, Kaz 92, and Kaz 390 are presented. The observations were made on the 2.6-m telescope at the Byurakan Observatory with the VAGR microlenses spectrograph. Isophotes of the images of the galaxies are constructed in the Hα, [NII] λλ6584 , 6548, and [SII] λλ6731, 6717 emission lines and in the continuum. The masses of Kaz 5 and Kaz 92 are determined to be 8.6 × 108 M⊙ and 6.1 × 109 M⊙ , respectively. The mass of the gaseous component in the centers of regions I and IV of Kaz 390, which encompass a 1 pixel area, are also determined. The morphological structure of the central region of Kaz 5 in the observed spectral range, λλ6400–6800?, differs completely from the structure of the same part of the galaxy observed with the 6-m and 2.5-m telescopes. It is shown that these differences in the structure in images of Kaz 5 are mostly explained by the comparatively low resolution of the telescope in combination with the VAGR spectrograph. Absorbing matter also contributes to this effect. It is also shown that a “deficit” of nitrogen has been observed in the region of Kaz 390 studied here (a circle of diameter 40″). Translated from Astrofizika, Vol. 52, No. 1, pp. 63–74 (February 2009).  相似文献   

10.
Assuming the time-dependent equation of state p=λ(t)ρ, five dimensional cosmological models with viscous fluid for an open universe (k=−1) and flat universe (k=0) are presented. Exact solutions in the context of the rest mass varying theory of gravity proposed by Wesson (Astron. Astrophys. 119, 145, 1983) are obtained. It is found that the phenomenon of isotropisation takes place in this theory, i.e. the mass scale factor A(t) which characterizes the rest mass of a typical particle is evolving with cosmic time just as the spatial scale factor R(t). It is further found that rest mass is approximately constant in the present universe.  相似文献   

11.
In this paper, we present the design of a high resolution Chirp Transform Spectrometer (CTS) which is part of the GREAT (German REceiver for Astronomy at Terahertz frequencies) instrument onboard SOFIA, the Stratospheric Observatory For Infrared Astronomy. The new spectrometer will provide unique spectral resolving power and linearity response, since the analog Fourier transform performed by the CTS spectrometer was improved through a new design, that we call “Adaptive Digital Chirp Processor (ADCP)”. The principle behind the ADCP consists on digitally generating the dispersive signal which adapts to the compressor dispersive properties, achieving maximum spectral resolution and higher dynamic range. Excellent test results have been obtained such as a white noise dynamic range of 30 dB, and a spectral resolution (FWHM) of 41.68 kHz which would mean if analyzing signals with the high frequency band receiver on the GREAT instrument (4.7 THz) a spectral resolving power (λ/Δ λ) higher than 108.  相似文献   

12.
Some Bianchi type-I viscous fluid string cosmological models with magnetic field are investigated. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density ξ(t)=ξ 0 ρ m , where ξ 0 and m are constants. To get a determinate model, we assume conditions ρ=(1+ω)λ, where ρ is rest energy density, ω a positive constant and λ the string tension density and expansion θ is proportional to eigen value σ 11 of the shear tensor σ j i . The behaviour of the models from physical and geometrical aspects in presence and absence of magnetic field is discussed.   相似文献   

13.
Based on our spectroscopic observations of the variable planetary nebula IC 4997 in 2003–2009, we have obtained the relative fluxes in optical emission lines. The interstellar extinction c = 0.35 has been found from the Balmer decrement by taking into account the effect of self-absorption in hydrogen lines in dense nebular regions. The variations in the Balmer decrement point to variability of the self-absorption and circumstellar extinction. We have investigated the variations in the relative intensities of some spectral lines and their ratios with time. The drop in the ratios F(λ4363)/F(Hγ) and F(λ363)/F(λ4959) that began back in 1990–1995 has continued, suggesting a decrease in the electron density and temperature in the central nebular region. The ratio F(λ6731)/F(λ6717) has remained constant. It gives an estimate for the electron density in the outer regions of IC 4997, N e ∼ 104 cm−3.  相似文献   

14.
In this paper we study the evolution of a LRS Bianchi I Universe, filled with a bulk viscous cosmological fluid in the presence of time varying constants “but” taking into account the effects of a c-variable into the curvature tensor. We find that the only physical models are those which “constants” G and c are growing functions on time t, while the cosmological constant Λ is a negative decreasing function. In such solutions the energy density obeys the ultrastiff matter equation of state i.e. ω = 1.  相似文献   

15.
We investigate five-dimensional Brans–Dicke cosmology with spacetime described by the homogeneous, anisotropic and flat spacetime with the topology M 1×R 3×S 1 where S 1 is taken in the form of a circle. We conjecture throughout this letter that the extra-dimension compactifies as the visible dimensions expand like b(t)≈a −1(t) and that the non-minimal coupling between the scalar field and the matter is of the form f(φ)∝ φ 2. The model gives rise to a transition from a decelerated epoch to an accelerated epoch for large values of the Brans–Dicke parameter ω. The model predicts crossing of the phantom divided barrier unless the universe is governed by a growing matter field.  相似文献   

16.
Observations of the inner radian of the Galactic disk at very high energy (VHE) gamma-rays have revealed at least 16 new sources. Besides shell type super-nova remnants, pulsar wind nebulae (PWN) appear to be a dominant source population in the catalogue of VHE gamma-ray sources. Except for the Crab nebula, the newly discovered PWN are resolved at VHE gamma-rays to be spatially extended (5–20 pc). Currently, at least 3 middle aged (t>10 kyrs) PWN (Vela X, G18.0-0.7, and G313.3+0.6 in the “Kookaburra” region) and 1 young PWN MSH 15-52 (t=1.55 kyrs) have been identified to be VHE emitting PWN (sometimes called “TeV Plerions”). Two more candidate “TeV Plerions” have been identified and have been reported at this conference (Carrigan, These proceedings, in preparation). In this contribution, the gamma-ray emission from Vela X is explained by a nucleonic component in the pulsar wind. The measured broad band spectral energy distribution is compared with the expected X-ray emission from primary and secondary electrons. The observed X-ray emission and TeV emission from the three middle aged PWN are compared with each other.  相似文献   

17.
We report solar flare plasma to be multi-thermal in nature based on the theoretical model and study of the energy-dependent timing of thermal emission in ten M-class flares. We employ high-resolution X-ray spectra observed by the Si detector of the “Solar X-ray Spectrometer” (SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003. Firstly we model the spectral evolution of the X-ray line and continuum emission flux F(ε) from the flare by integrating a series of isothermal plasma flux. We find that the multi-temperature integrated flux F(ε) is a power-law function of ε with a spectral index (γ)≈−4.65. Next, based on spectral-temporal evolution of the flares we find that the emission in the energy range E=4 – 15 keV is dominated by temperatures of T=12 – 50 MK, while the multi-thermal power-law DEM index (δ) varies in the range of −4.4 and −5.7. The temporal evolution of the X-ray flux F(ε,t) assuming a multi-temperature plasma governed by thermal conduction cooling reveals that the temperature-dependent cooling time varies between 296 and 4640 s and the electron density (n e) varies in the range of n e=(1.77 – 29.3)×1010 cm−3. Employing temporal evolution technique in the current study as an alternative method for separating thermal from nonthermal components in the energy spectra, we measure the break-energy point, ranging between 14 and 21±1.0 keV.  相似文献   

18.
We study a massive cosmic strings with BII symmetries cosmological models in two contexts. The first of them is the standard one with a barotropic equation of state. In the second one we explore the possibility of taking into account variable “constants” (G and Λ). Both models are studied under the self-similar hypothesis. We put special emphasis in calculating the numerical values for the equations of state. We find that for ω∈(0,1], G, is a growing time function while Λ, behaves as positive decreasing time function. If ω=0, both “constants”, G and Λ, behave as true constants.  相似文献   

19.
The longitudinal magnetic field measured using the Fe I λ 525 and Fe I λ 524.7 nm lines and global magnetic field of the sun differ depending on the observatory. To study the cause of these discrepancies, we calculate the H ‖(525)/H ‖(524.7) ratios for various combinations of magnetic elements and compare them with the corresponding observed values. We use the standard quiet model of the solar photosphere suggesting that there are magnetic fields of different polarities in the range between zero and several kilogauss. The magnetic element distribution is found as a function of magnetic field strength and the parameters of this distribution are determined for which the calculated H ‖(525)/H ‖(524.7) ratio agrees with the observed one. The sigma-components are found to be shifted differently for various points of the Fe I λ 525 nm profile calculated for the inhomogeneous magnetic field. The farther the point is from the line center, the larger the sigma-components shift. Such a peculiarity of the profiles may be responsible for the discrepancies in the measured values of the global magnetic field obtained at different observatories. The increase in modulus of the global magnetic field during the maxima of solar activity can be due to a larger fraction of magnetic elements with kilogauss magnetic fields.  相似文献   

20.
We present a catalog of 520 most isolated nearby galaxies with angular velocities V LG < 3500 km/s, covering the entire sky. This population of “space orphans” makes up 4.8% among 10 900 galaxies with measured radial velocities. We describe the isolation criterion used to select our sample, called the “Local Orphan Galaxies”(LOG), and discuss their basic optical and HI properties. A half of the LOG catalog is occupied by the Sdm, Im and Ir morphological type galaxies without a bulge. The median ratio M gas/M star in the LOG galaxies exceeds 1. The distribution of the catalog galaxies on the sky looks uniform with some signatures of a weak clustering on the scale of about 0.5 Mpc. The LOG galaxies are located in the regions where the mean local density of matter is approximately 50 times lower than the mean global density. We indicate a number of LOG galaxies with distorted structures, which may be the consequence of interaction of isolated galaxies with massive dark objects.  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