共查询到20条相似文献,搜索用时 15 毫秒
1.
In this paper we have considered the Universe to be filled with Modified Gas and the Cosmological Constant Λ to be time-dependent
with or without the Gravitational Constant G to be time-dependent. We have considered various phenomenological models for Λ, viz.,
and
. Using these models it is possible to show the accelerated expansion of the Universe at the present epoch. Also we have shown
the natures of G and Λ over the total age of the Universe. Using the statefinder parameters we have shown the diagrammatical representation
of the evolution of the Universe starting from radiation era to ΛCDM model. 相似文献
2.
S. I. Blinnikov 《Astrophysics and Space Science》1978,59(1):13-17
Recently some authors have questioned whether Newton's law of gravitation is actually true on scales less than 1 km. The available constraints on the gravitational constant show that is laboratory valueG
0 may differ from the value at infinityG
by 40%. Long (1976) reported experimental evidence for departures from Newton's law. In this note it is shown that the difference betweenG
0 andG
modifies the mass-radius relation of degenerate stars. The observations of white dwarfs are consistent with the theory of stellar evolution only ifG
0 differs fromG
by not more than 10%. This estimate may be improved by a higher accuracy of observations. 相似文献
3.
4.
Using the method of apparent motion parameters, we have studied the relative motion of the components in 561 pairs of wide (ρ > 2″) and relatively nearby (Hipparcos parallaxes > 0.01″) visual double stars based on data from the WDS catalog. The minimum masses of the double stars have been calculated at given parallaxes. We have identified 358 optical pairs. For 11 stellar pairs, we have found the minimum mass to exceed the estimate corresponding to their spectral types and luminosities. This excess is 5–7 M ⊙ for two stars, ADS 7446 and 9701. 相似文献
5.
Corrado Massa 《Astrophysics and Space Science》1993,209(2):307-308
The time variation of the gravitational constantG is discussed in the light of the gravity modified form of quantum electrodynamics. From the experimental upper limit |a/| < 5 × 10–15 yr–1 on the time variation of the electromagnetic fine structure constant one finds |/G| < 5 × 10–13 yr–1. 相似文献
6.
Sanjay Oli 《Astrophysics and Space Science》2008,314(1-3):89-94
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.
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7.
8.
9.
E. A. Semenko M. E. Sachkov T. A. Ryabchikova D. O. Kudryavtsev N. E. Piskunov 《Astronomy Letters》2008,34(6):413-422
We present the results of our abundance analysis for the magnetic chemically peculiar star HD 115708 based on high-resolution spectra. The atmospheric chemical composition of HD 115708 (T eff = 7550 K) is shown to be typical of cool Ap stars with a significant ionization disequilibrium for the first and second rare earth ions, which is commonly observed in the atmospheres of pulsating Ap (roAp) stars. Our study of the vertical distribution of elements has shown that Mg, Ca, Cr, and Fe concentrate in deeper atmospheric layers, with their abundances decreasing sharply in the upper layers. The jumps in abundance are 1.5–3 orders of magnitude. Silicon is distributed in depth almost uniformly in the atmosphere of HD 115708. The derived empirical Cr and Fe distributions agree qualitatively with the results of diffusion calculations. Since the atmospheric chemical peculiarities in HD 115708 correspond to roAp stars, we have performed a spectroscopic monitoring to find nonradial pulsations. We have been able to determine only an upper limit for the amplitude of the possible radial velocity pulsations, ~100 m s?1, due to the insufficient temporal resolution and instability of the main stellar spectrograph (MSS) of the 6-m telescope. 相似文献
10.
A. A. Saharian 《Astrophysics》2005,48(1):122-133
The vacuum expectations of the surface energy-momentum tensor generated on a brane in AdS space-time by quantum fluctuations of a scalar field with an arbitrary coupling parameter are studied. It is assumed that the field satisfies mixed boundary conditions on the brane. A generalized zeta function method is used as a regularization procedure. Two regions, located to the left (L-region) and right (R-region) of the brane, are considered. It is shown that the surface energies for both these regions contain pole and finite contributions. Analytic expressions are derived for both parts. When calculating the total surface energy including the contributions from the L- and R-regions, the pole terms add out in odd spatial dimensions. The surface energy-momentum tensor induced by vacuum quantum effects corresponds to the generation of a cosmological constant on the brane. These results are applied to the second Randall-Sundrum model.Translated from Astrofizika, Vol. 48, No. 1, pp. 151–164 (February 2005). 相似文献
11.
Altimetric measurements of the GEOSAT satellite were used for the determination of geopotential scale factorR
0. The geopotential valueW
0 on the geoid surface was then computed (W
0 =GM/R
0).The GEOSAT Geophysical Data Records (GDR's) covering an initial period of the Exact Repeat Mission (ERM) were filtered and processed. The necessary corrections were made in order to allow a precise detection of the sea surface. Gravitational geopotential, rotation and permanent tides were taken into account and the equipotential surface which is the best approximation of the sea surface was found.The determination of the potential valueW
0 on the mean geoid surface in this way is very promising. An associated value withW
0 - the geopotential scale factorR
0 - seems to be a very good Earth dimension defining quantity. Moreover, there are many possible applications ofW
0 (R
0) in modern geophysics.The incorporation of one of these parameters - we now recommendR
0 - into the set of the Primary Geodetic Parameters (PGP) is discussed and suggested. 相似文献
12.
S. Kailas 《Journal of Astrophysics and Astronomy》1986,7(2):53-57
From the experimentally measured (p,n) cross-sections for92,94Zr,93Nb,95,96,98Mo,103Rh,107,109Ag,110Pd,115In,117,122Sn nuclides, for proton energies below 7 MeV, thermonuclear reaction rates in the temperature range 1≤T
9≤5(T9=109 K) have been extracted. These reaction rates have been fitted to a three parameter empirical expression proposed by Fowler. 相似文献
13.
Aster Gebrekirstos Martin Worbes Demel Teketay Masresha Fetene Ralph Mitlhner 《Global and Planetary Change》2009,66(3-4):253-260
Although several proxies have been proposed to trace the course of environmental and climatological fluctuations, precise paleoclimate records from the tropics, notably from Africa are still sorely lacking today. Stable carbon isotopes (δ13C) in tree rings are an attractive record of climate. In this study, the patterns and climatic signals of δ13C ratios were determined on tree rings of deciduous (Acacia senegal, Acacia tortilis, Acacia seyal) and an evergreen (Balanites aegyptiaca) species, from a semi-arid Acacia Woodland in Ethiopia. δ13C inter-annual patterns are synchronous among the co-occurring species. A declining trend with time was observed in δ13C, notably for B. aegyptiaca, which could be due to anthropogenic increases in atmospheric CO2 concentration and decrease in atmospheric δ13C. Tree ring δ13C values of all the species revealed significant negative correlation with precipitation amount but not with temperature and relative humidity. The δ13C series of the deciduous species shows a higher correlation (r = − 0.70 to − 0.78) with precipitation than the evergreen species (r = − 0.55). A master δ13C series, composed of the average of the three Acacia species, displayed stronger significant correlation (r = − 0.82) than any of the individual species δ13C series. The weak relationship between temperature and δ13C in this study indicates that photosynthetic rate is not a significant factor. Moisture stress, however, may have a direct impact on the stomatal conductance and explain the strong negative relationship between δ13C and precipitation. The results demonstrate the potential of δ13C in tree rings to reflect physiological responses to environmental changes as a vehicle for paleoclimatic reconstruction, which is important to understand tree response to past and future climate change. 相似文献
14.
We present a new technique to extract the cosmological information from high-redshift supernova data in the presence of calibration errors and extinction due to dust. While in the traditional technique the distance modulus of each supernova is determined separately, in our approach we determine all distance moduli at once, in a process that achieves a significant degree of self-calibration. The result is a much reduced sensitivity of the cosmological parameters to the calibration uncertainties. As an example, for a strawman mission similar to that outlined in the SNAP satellite proposal, the increased precision obtained with the new approach is roughly equivalent to a factor of five decrease in the calibration uncertainty. 相似文献
15.
Torsten Hartmann Michael H. Soffel Thomas Kioustelidis 《Celestial Mechanics and Dynamical Astronomy》1994,60(1):139-159
The purpose of this article is to emphasize the usefulness of STF-tensors in celestial mechanics. Using STF-mass multipole moments and Cartesian coordinates the derivations of equations of motion, the interaction- and tidal-potentials for an isolated system ofN arbitrarily shaped and composed, purely gravitationally interacting bodies are particularly simple. Using simple relations between STF-tensors and spherical harmonics it is shown how all Cartesian formulas can be converted easily into the usual spherical representations. Some computational aspects of STF-tensors and spherical harmonics are discussed. A list of useful formulas for STF-tensors is provided. 相似文献
16.
The SMF algorithms were recently developed by the authors as a multistep generalization of the ScheifeleG-functions one-step method. Like the last, the proposed codes integrate harmonic oscillations without truncation error and the perturbing parameter appears as a factor of that error when integrating perturbed oscillations. Therefore they seemed to be convenient for the accurate integration of orbital problems after the application of linearizing transformations, such as KS or BF. In this paper we present several numerical experiments concerning the propagation of Earth satellite orbits, that illustrate the performance of the the SMF method. In general, it provides greater accuracy than the usual standard algorithms for similar computational cost. 相似文献
17.
Diego F. Torres Shu Zhang Olaf Reimer Xavier Barcons Amalia Corral Valentí Bosch-Ramon Josep M. Paredes Gustavo E. Romero Jin Qu Werner Collmar Volker Schönfelder Yousaf Butt 《Astrophysics and Space Science》2007,309(1-4):17-21
The COMPTEL unidentified source GRO J 1411-64 was observed by INTEGRAL and XMM-Newton in 2005. The Circinus Galaxy is the
only source detected within the 4σ location error of GRO J1411-64, but in here excluded as the possible counterpart. At soft X-rays, 22 reliable and statistically
significant sources (likelihood >10) were extracted and analyzed from XMM-Newton data. Only one of these sources, XMMU J141255.6-635932,
is spectrally compatible with GRO J1411-64 although the fact the soft X-ray observations do not cover the full extent of the
COMPTEL source position uncertainty make an association hard to quantify and thus risky. At the best location of the source,
detections at hard X-rays show only upper limits, which, together with MeV results obtained by COMPTEL suggest the existence
of a peak in power output located somewhere between 300–700 keV for the so-called low state. Such a spectrum resembles those
in blazars or microquasars, and might suggest at work by the models accordingly. However, an analysis using a microquasar
model consisting on a magnetized conical jet filled with relativistic electrons, shows that it is hard to comply with all
observational constrains. This fact and the non-detection at hard X-rays introduce an a-posteriori question mark upon the
physical reality of this source, what is discussed here. 相似文献
18.
Lorenzo Iorio 《Monthly notices of the Royal Astronomical Society》2007,376(4):1727-1730
In this paper, we directly constrain possible spatial variations of the Newtonian gravitational constant G over the range ≈ 0.01–5 au in various extrasolar multiplanet systems. Using the third Kepler law, we determine the quantity ΓXY = G X / G Y for each couple of planets X and Y located at different distances from their parent star; deviations of the measured values of Γ from unity would signal variations of G . The obtained results for η= 1 −Γ are found to be very compatible with zero within the experimental errors (η/δη≈ 0.2–0.3) . We make a comparison with an analogous test previously performed in our Solar system. 相似文献
19.
R. A. Kandalyan 《Astrophysics》2005,48(3):355-362
The properties of OH megamaser galaxies in the x-ray and radio ranges are discussed. Based on a sample of OH megamaser galaxies it is shown that the x-ray emission and the width of the OH radio line are closely related. The line width is also related to the radio emission from megamasers, but it depends only weakly on the infrared emission and this relationship is statistically less significant. The OH line width and the radio and x-ray emissions depend on the central mass of megamasers. There is a weak correlation between the radio and x-ray emissions owing to multiple scattering of synchrotron photons and bursts from the galactic nucleus. These results indicate that an active nucleus can be dominant in OH megamaser galaxies.__________Translated from Astrofizika, Vol. 48, No. 3, pp. 421–430 (August 2005). 相似文献