共查询到20条相似文献，搜索用时 31 毫秒
1.
G. F. R. Ellis W. R. Stoeger 《Monthly notices of the Royal Astronomical Society》2009,398(3):1527-1536
The causal limit usually considered in cosmology is the particle horizon, delimiting the possibilities of causal connection in the expanding Universe. However, it is not a realistic indicator of the effective local limits of important interactions in space–time. We consider here the matter horizon for the Solar system, i.e. the comoving region which has significantly contributed matter to our local physical environment. This lies inside the effective domain of dependence , which (assuming the universe is dominated by dark matter along with baryonic matter and vacuum-energy-like dark energy) consists of those regions that have had a significant active physical influence on this environment through effects such as matter accretion and acoustic waves. It is not determined by the velocity of light c , but by the flow of matter perturbations along their world lines and associated gravitational effects. We emphasize how small a region the perturbations which became our Galaxy occupied, relative to the observable universe – even relative to the smallest scale perturbations detectable in the cosmic microwave background radiation. Finally, looking to the future of our local cosmic domain, we suggest simple dynamical criteria for determining the present domain of influence and the future matter horizon . The former is the radial distance at which our local region is just now separating from the cosmic expansion. The latter represents the limits of growth of the matter horizon in the far future. 相似文献
2.
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
3.
Fulvio Melia † 《Monthly notices of the Royal Astronomical Society》2007,382(4):1917-1921
4.
J. A. Peacock 《Monthly notices of the Royal Astronomical Society》2007,379(3):1067-1074
It has been claimed that the observed magnitude of the vacuum energy density is consistent with the distribution predicted in anthropic models, in which an ensemble of universes is assumed. This calculation is revisited, without making the assumption that the cosmic microwave background (CMB) temperature is known, and considering in detail the possibility of a recollapsing universe. New accurate approximations for the growth of perturbations and the mass function of dark haloes are presented. Structure forms readily in the recollapsing phase of a model with negative Λ, so collapse fraction alone cannot forbid Λ from being large and negative. A negative Λ is disfavoured only if we assume that formation of observers can be neglected once the recollapsing universe has heated to T ≳ 8 K . For the case of positive Λ, however, the current universe does occupy an extremely typical position compared to the predicted distribution on the Λ− T plane. Contrasting conclusions can be reached if anthropic arguments are applied to the curvature of the universe, and we discuss the falsifiability of this mode of anthropic reasoning. 相似文献
5.
In this article we want to answer the cosmologically relevant question what, with some good semantic and physical reason, could be called the massM u of an infinitely extended, homogeneously matter‐filled and expanding universe. To answer this question we produce a space‐like sum of instantaneous cosmic energy depositions surrounding equally each spacepoint in the homogeneous universe. We calculate the added‐up instantaneous cosmic energy per volume around an arbitrary space point in the expanding universe. To carry out this sum we use as basic metrics an analogy to the inner Schwarzschild metric applied to stars, but this time applied to the spacepoint‐related universe. It is then shown that this leads to the added‐up proper energy within a sphere of a finite outer critical radius defining the point‐related infinity. As a surprise this radius turns out to be reciprocal to the square root of the prevailing average cosmic energy density. The equivalent mass of the universe can then also be calculated and, by the expression which is obtained here, shows a scaling with this critical radius of this universe, a virtue of the universe which was already often called for in earlier works by E. Mach, H. Thirring and F. Hoyle and others. This radius on the other hand can be shown to be nearly equal to the Schwarzschild radius of the so‐defined mass M u of the universe. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
6.
7.
A. M.Aliaga V.Quilis J. V.Arnau D.Sáez 《Monthly notices of the Royal Astronomical Society》2002,330(3):625-630
Non-linear evolution of cosmological energy density fluctuations triggers deviations from Gaussianity in the temperature distribution of the cosmic microwave background. A method to estimate these deviations is proposed. N -body simulations – in a Λ cold dark matter cosmology – are used to simulate the strongly non-linear evolution of cosmological structures. It is proved that these simulations can be combined with the potential approximation to calculate the statistical moments of the cosmic microwave background anisotropies produced by non-linear gravity. Some of these moments are computed and the resulting values are different from those corresponding to Gaussianity. 相似文献
8.
Jie Wang Gabriella De Lucia Manfred G. Kitzbichler Simon D. M. White 《Monthly notices of the Royal Astronomical Society》2008,384(4):1301-1315
We combine N -body simulations of structure growth with physical modelling of galaxy evolution to investigate whether the shift in cosmological parameters between the first- and third-year results from the Wilkinson Microwave Anisotropy Probe ( WMAP ) affects predictions for the galaxy population. Structure formation is significantly delayed in the WMAP3 cosmology, because the initial matter fluctuation amplitude is lower on the relevant scales. The decrease in dark matter clustering strength is, however, almost entirely offset by an increase in halo bias, so predictions for galaxy clustering are barely altered. In both cosmologies, several combinations of physical parameters can reproduce observed, low-redshift galaxy properties; the star formation, supernova feedback and active galactic nucleus feedback efficiencies can be played off against each other to give similar results. Models which fit observed luminosity functions predict projected two-point correlation functions which scatter by about 10–20 per cent on large scale and by larger factors on small scale, depending both on cosmology and on details of galaxy formation. Measurements of the pairwise velocity distribution prefer the WMAP1 cosmology, but careful treatment of the systematics is needed. Given present modelling uncertainties, it is not easy to distinguish between the WMAP1 and WMAP3 cosmologies on the basis of low-redshift galaxy properties. Model predictions diverge more dramatically at high redshift. Better observational data at z > 2 will better constrain galaxy formation and perhaps also cosmological parameters. 相似文献
9.
We investigate the background dynamics when dark energy is coupled to dark matter in the universe described by loop quantum
cosmology. We consider dark energy of the form modified Chaplygin gas. The dynamical system of equations is solved numerically
and a stable scaling solution is obtained. It henceforth resolves the famous cosmic coincidence problem in modern cosmology.
The statefinder parameters are also calculated to classify this dark energy model. 相似文献
10.
Ti-Pei Li Hao Liu Li-Ming Song Shao-Lin Xiong Jian-Yin Nie 《Monthly notices of the Royal Astronomical Society》2009,398(1):47-52
A remarkable similarity between the large-scale non-Gaussian pattern of cosmic microwave background (CMB) temperatures obtained by the Wilkinson Microwave Anisotropy Probe ( WMAP ) mission and the distribution features of observation numbers is noted. Motivated by such a similarity, in this work we check the WMAP data for the correlation between pixel temperature t and observation number N . Systematic effects of imbalance in the differential observations and significant t – N correlations in magnitude, distribution of non-Gaussianity and north–south asymmetry are found. Our results indicate that, for precision cosmology studies based on WMAP observations, the observation effect on released WMAP temperature maps requires further careful study. 相似文献
11.
Jalil Naji 《Astrophysics and Space Science》2014,350(1):385-392
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
12.
现代科学表明宇宙中99%以上的可观测物质都处于等离子体状态,从小尺度的微观粒子动力学集体过程与能量转换机制到大尺度的宇宙等离子天体结构状态与爆发活动现象,都是等离子天体物理学的研究课题.从宇宙演化历史、大尺度结构形成以及爆发活动现象等方面,系统地论述了等离子天体物理学在现代天文学发展以及现代等离子体宇宙观形成中的重要作用.同时,结合空间卫星科学探测研究及其对现代天文学的巨大影响,进一步阐述了地球磁层和日球层等空间等离子体实地探测研究在等离子天体物理学研究中所扮演的“天然实验室”的独特作用. 相似文献
13.
G. Efstathiou 《Monthly notices of the Royal Astronomical Society》2003,346(2):L26-L30
The Wilkinson Microwave Anisotropy Probe ( WMAP ) has measured lower amplitudes for the temperature quadrupole and octopole anisotropies than expected in the best fitting (concordance) Λ-dominated cold dark matter (ΛCDM) cosmology. Some authors have argued that this discrepancy may require new physics. However, the statistical significance of this result is not clear. Some authors have applied frequentist arguments and claim that the discrepancy would occur by chance about 1 time in 700, if the concordance model is correct. Other authors have used Bayesian arguments to claim that the data show marginal evidence for new physics. I investigate these confusing and apparently conflicting claims in this Letter using a frequentist analysis and a simplified Bayesian analysis. On either analysis, I conclude that the WMAP results are consistent with the concordance ΛCDM model. 相似文献
14.
In this work we investigate the background dynamics when dark energy is coupled to dark matter with a suitable interaction
in the universe described by brane cosmology. Here DGP and the RSII brane models have been considered separately. Dark energy
in the form of modified Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is considered
in order to at least alleviate (if not solve) the cosmic coincidence problem. The dynamical system of equations is solved
numerically and a stable scaling solution is obtained. A significant attempt towards the solution of the cosmic coincidence
problem is taken. The statefinder parameters are also calculated to classify the dark energy models. Graphs and phase diagrams
are drawn to study the variations of these parameters. It is also seen that the background dynamics of modified Chaplygin
gas is completely consistent with the notion of an accelerated expansion in the late universe. Finally, it has been shown
that the universe in both the models follows the power law form of expansion around the critical point, which is consistent
with the known results. 相似文献
15.
Hongsheng Zhao David J. Bacon y N. Taylor Keith Horne 《Monthly notices of the Royal Astronomical Society》2006,368(1):171-186
We propose to use multiple-imaged gravitational lenses to set limits on gravity theories without dark matter, specifically tensor–vector–scalar (TeVeS) theory, a theory which is consistent with fundamental relativistic principles and the phenomenology of Modified Newtonian Dynamics (MOND) theory. After setting the framework for lensing and cosmology, we analytically derive the deflection angle for the point lens and the Hernquist galaxy profile, and study their patterns in convergence, shear and amplification. Applying our analytical lensing models, we fit galaxy-quasar lenses in the CfA-Arizona Space Telescope Lens Survey (CASTLES) sample. We do this with three methods, fitting the observed Einstein ring sizes, the image positions, or the flux ratios. In all the cases, we consistently find that stars in galaxies in MOND/TeVeS provide adequate lensing. Bekenstein's toy μ function provides more efficient lensing than the standard MOND μ function. But for a handful of lenses, a good fit would require a lens mass orders of magnitude larger/smaller than the stellar mass derived from luminosity unless the modification function μ and modification scale a 0 for the universal gravity were allowed to be very different from what spiral galaxy rotation curves normally imply. We discuss the limitation of present data and summarize constraints on the MOND μ function. We also show that the simplest TeVeS 'minimal-matter' cosmology, a baryonic universe with a cosmological constant, can fit the distance–redshift relation from the supernova data, but underpredicts the sound horizon size at the last scattering. We conclude that lensing is a promising approach to differentiate laws of gravity. 相似文献
16.
Sanders 《Monthly notices of the Royal Astronomical Society》1998,296(4):1009-1018
It is well known that the application of Newtonian dynamics to an expanding spherical region leads to the correct relativistic expression (the Friedmann equation) for the evolution of the cosmic scalefactor. Here, the cosmological implications of Milgrom's modified Newtonian dynamics (MOND) are considered by means of a similar procedure. Earlier work by Felten demonstrated that in a region dominated by modified dynamics the expansion cannot be uniform (separations cannot be expressed in terms of a scalefactor) and that any such region will eventually recollapse regardless of the initial expansion velocity and mean density. Here I show that, because of the acceleration threshold for the MOND phenomenology, a region dominated by MOND will have a finite size which, in the earlier Universe ( z >3), is smaller than the horizon scale. Therefore, uniform expansion and homogeneity on the horizon scale are consistent with MOND-dominated non-uniform expansion and the development of inhomogeneities on smaller scales. In the radiation-dominated era, the amplitude of MOND-induced inhomogeneities is much smaller than that implied by observations of the cosmic background radiation, and the thermal and dynamical history of the Universe is identical to that of the standard big bang model. In particular, the standard results for primordial nucleosynthesis are retained. When matter first dominates the energy density of the Universe, the cosmology diverges from that of the standard model. Objects of galaxy mass are the first virialized objects to form (by z =10), and larger structure develops rapidly. At present, the Universe would be inhomogeneous out to a substantial fraction of the Hubble radius. 相似文献
17.
The fluctuations of the cosmic microwave background (CMB) are investigated for a hyperbolic universe with finite volume. Four-component models with radiation, matter, vacuum energy and an extra spatially constant dark energy X -component are considered. The general solution of the Friedmann equation for the cosmic scalefactor a ( η ) is given for the four-component models in terms of the Weierstrass ℘-function. The lower parts of the angular power spectra C l of the CMB anisotropy are computed for nearly flat models with Ωtot ≤0.95. It is shown that the particular compact fundamental cell that is considered in this paper leads to a suppression in C l for l ≲10 and Ωtot ≲0.9. 相似文献
18.
Both dark energy and the thermodynamics on apparent horizon in cosmology have been broadly investigated in recent several
years. In order to maintain the continuity equation of the total matter in the universe, a new interacting dark energy in
the framework of Brans-Dicke theory is proposed. Considering this new interacting dark energy, an equilibrium thermodynamics
in Brans-Dicke theory is constructed successfully. Moreover, this new interacting dark energy can be regarded as arising from
the “Holographic Dark Energy” models. 相似文献
19.
Y. Ascasibar R. Sevilla G. Yepes V. Müller S. Gottlöber 《Monthly notices of the Royal Astronomical Society》2006,371(1):193-203
The aim of this work is to show that, contrary to popular belief, galaxy clusters are not expected to be self-similar, even when the only energy sources available are gravity and shock-wave heating. In particular, we investigate the scaling relations between mass, luminosity and temperature of galaxy groups and clusters in the absence of radiative processes. Theoretical expectations are derived from a polytropic model of the intracluster medium and compared with the results of high-resolution adiabatic gasdynamical simulations. It is shown that, in addition to the well-known relation between the mass and concentration of the dark matter halo, the effective polytropic index of the gas also varies systematically with cluster mass, and therefore neither the dark matter nor the gas profiles are exactly self-similar. It is remarkable, though, that the effects of concentration and polytropic index tend to cancel each other, leading to scaling relations whose logarithmic slopes roughly match the predictions of the most-basic self-similar models. We provide a phenomenological fit to the relation between polytropic index and concentration, as well as a self-consistent scheme to derive the non-linear scaling relations expected for any cosmology and the best-fitting normalizations of the M – T , L – T and F – T relations appropriate for a Λ cold dark matter universe. The predicted scaling relations reproduce observational data reasonably well for massive clusters, where the effects of cooling and star formation are expected to play a minor role. 相似文献
20.
Manoj Kaplinghat 《New Astronomy Reviews》2003,47(11-12):893
With WMAP putting the phenomenological standard model of cosmology on a strong footing, one can look forward to mining the cosmic microwave background (CMB) for fundamental physics with higher sensitivity and on smaller scales. Future CMB observations have the potential to measure absolute neutrino masses, test for cosmic acceleration independent of supernova Ia observations, probe for the presence of dark energy at z2, illuminate the end of the dark ages, measure the scale-dependence of the primordial power spectrum and detect gravitational waves generated by inflation. 相似文献