共查询到20条相似文献，搜索用时 333 毫秒
1.
《New Astronomy》2024
We propose hydrostatic polytropic spheres governed by the Lane-Emden equation (LEE) of index as a novel set of physical models for axially averaged gravitational lenses anywhere in the Universe, alternative to the familiar singular isothermal sphere (SIS) and the Navarro–Frenk–White (NFW) profile, as such general polytropic spheres are conceptually simple, versatile in representing a series of equations of state, and able to address both the inner core and cusp features. As LEE is nonlinear, there exist several distinct classes of LEE solutions to serve as physical lens models. With a few scaling parameters, the complete problem can be readily reconstructed with full physical dimensions. A given mass density profile satisfying LEE produces lensing effects that are solely determined by a dimensionless parameter which contains geometric and kinematic information about the source-lens-observer system. The lens mapping and tangential shear or distortion profile are derived, first analytically for special cases and then asymptotically at the outskirts or near the edge of the lens. Numerical procedures for calculating full lensing profiles of a general lens are developed. Our results include the analytical “singular polytropic sphere” (SPS) profile which generalizes the SIS model and may outperform the latter in modeling dark matter halos among others. We further point out that dynamic models of general polytropic spheres in self-similar evolution can serve as several broad classes of gravitational lenses and produce time-dependent lensing effects slow or fast depending on the pertinent time scales. Astrophysical sources that can be lensed include electromagnetic wave sources in the entire frequency band, gravitational wave sources in the entire frequency band, gravitons even possibly with finite masses, neutrino sources of three different types, neutron sources, and ultra high energy cosmic rays (UHECRs) of electrically charged particles which can also interact with magnetic fields. We discuss and elabrate applications to dark matter halos, hypermassive black holes and supermassive black holes in the entire Universe including the early Universe, magnetized supermassive stars, static and dynamically evolving spherical and cylindrical lenses in contexts of astrophysics and cosmology. 相似文献
2.
We examine the possibility that a substantial fraction of the total energy density in a spatially flat Universe is composed
of a time-dependent and spatially inhomogeneous component whose equation-of-state differs from that of baryons, neutrinos,
dark matter, or radiation. In this lecture, we report on our investigations of the case in which the additional energy component,
dubbed "quintessence", is due to a dynamical scalar field evolving in a potential. We have computed the effects on the background
cosmological evolution, the cosmic microwave background (CMB) and mass power spectrum, finding a broad range of cosmologically
viable models. We stress three important features of the quintessence or Q-component: the time evolution of the equation-of-state;
the length-scale dependence of the speed of propagation of the fluctuations in the Q-component; and, the contribution of quintessence
fluctuations to the CMB anisotropy spectrum.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
Marek Biesiada 《Monthly notices of the Royal Astronomical Society》2001,325(3):1075-1080
It is hoped that the future generation of interferometric gravitational wave detectors will provide accurate measurements of the final stages of binary in-spirals. The sources probed by such experiments are of extragalactic origin and the observed chirp mass is the intrinsic chirp mass multiplied by (1+ z ) where z is the redshift of the source. Moreover the luminosity distance is a direct observable in such experiments. This creates the possibility to establish a new kind of cosmological test, supplementary to more standard ones.
Recent observations of distant type Ia supernovae light curves suggest that the expansion of the Universe has recently begun to accelerate. A popular explanation of the present accelerating expansion of the Universe is to assume that some part ΩQ of the matter–energy density is in the form of a dark component called 'the quintessence' with the equation of state p Q = wρ Q with w ≥−1 . In this paper we consider the predictions concerning observations of binary in-spirals in future LIGO-type interferometric experiments assuming a 'quintessence cosmology'. In particular we compute the expected redshift distributions of observed events in the a priori admissible range of parameters describing the equation of state for the quintessence. We find that this distribution has a robust dependence on the cosmic equation of state. 相似文献
Recent observations of distant type Ia supernovae light curves suggest that the expansion of the Universe has recently begun to accelerate. A popular explanation of the present accelerating expansion of the Universe is to assume that some part Ω
4.
We calculate analytically and numerically the distance–redshift equation in perfect fluid quintessence models and give an accurate fit to the numerical solutions for all the values of the density parameter and the quintessence equation of state. Then we apply our solutions to the estimation of H 0 from multiple image time delays and find that the inclusion of quintessence modifies significantly the likelihood distribution of H 0 , generally reducing the best estimate with respect to a pure cosmological constant. Marginalizing over the other parameters (Ω m and the quintessence equation of state) we obtain H 0 =71±6 km s−1 Mpc−1 for an empty beam and H 0 =64±4 km s−1 Mpc−1 for a filled beam. These errors, however, do not take into account the uncertainty on the modelling of the lens. We also discuss the future prospects for distinguishing quintessence from a cosmological constant with time delays. 相似文献
5.
David M. Coward Ronald R. Burman David G. Blair 《Monthly notices of the Royal Astronomical Society》2001,324(4):1015-1022
We investigate the stochastic gravitational wave background that results from neutron star birth throughout the Universe. The neutron star birth rate, as a function of redshift, is calculated using an observation-based model for the evolving star formation rate, together with an estimate of the rate of core-collapse supernovae in the nearby Universe and an estimate of the neutron star/black hole branching ratio. Using three sample waveforms, based on numerical models of stellar core collapse by Zwerger & Müller, the spectral flux density, spectral strain, spectral energy density and duty cycle of the background have been computed. Our results show, contrary to recent claims, that the spectrum of the stochastic background clearly reflects the different physics in the core-collapse models. For a star formation model that is corrected for dust extinction, the neutron star formation rate throughout the Universe is high enough to result in a nearly continuous background of gravitational waves, with spectral features that can be related to emission mechanisms. 相似文献
6.
P. F. L. Maxted L. Ferrario T. R. Marsh D. T. Wickramasinghe 《Monthly notices of the Royal Astronomical Society》2000,315(3):L41-L44
We present a new interpretation of recent observations suggesting that the expansion of the Universe has recently started to accelerate. A cosmological model with a quintessence field driven by a potential motivated by M-theory is used to study the energy density and equation of state for the Universe. We find that late-time acceleration does not have to lead to the usual predictions of perpetual acceleration. The model allows another broad class of scenarios in which today's acceleration is a transient phenomenon, which is succeeded by a return to matter domination and decelerating expansion. 相似文献
7.
D. I. Podolsky 《Astronomy Letters》2002,28(7):434-437
The recently detected accelerated expansion of the Universe is related to the existence of a new type of matter called the Λ field or quintessence. Constraints were obtained on its equation of state from the absence of clustering of this matter on scales much smaller than the cosmological horizon. The question of how these constraints affect the possibility of fitting the accelerated expansion by such cosmological models as the Chaplygin gas model is discussed. 相似文献
8.
Dark energy has a dramatic effect on the dynamics of the Universe, causing the recently discovered acceleration of the expansion. The dynamics are also central to the behaviour of the growth of large-scale structure, offering the possibility that observations of structure formation provide a sensitive probe of the cosmology and dark energy characteristics. In particular, dark energy with a time-varying equation of state can have an influence on structure formation stretching back well into the matter-dominated epoch. We analyse this impact, first calculating the linear perturbation results, including those for weak gravitational lensing. These dynamical models possess definite observable differences from constant equation of state models. Then we present a large-scale numerical simulation of structure formation, including the largest volume to date involving a time-varying equation of state. We find the halo mass function is well described by the Jenkins et al. mass function formula. We also show how to interpret modifications of the Friedmann equation in terms of a time-variable equation of state. The results presented here provide steps toward realistic computation of the effect of dark energy in cosmological probes involving large-scale structure, such as cluster counts, the Sunyaev–Zel'dovich effect or weak gravitational lensing. 相似文献
9.
S. D. Katore K. S. Adhav A. Y. Shaikh M. M. Sancheti 《Astrophysics and Space Science》2011,333(1):333-341
We consider a self-consistent system of Plane symmetric cosmology and binary mixture of perfect fluid and dark energy. The
perfect fluid is taken to be one obeying the usual equation of state p=γρ with γ∈[0,1]. The dark energy is considered to be either the quintessence or Chaplygin gas. Exact solutions to the corresponding
Einstein’s field equations are obtained as a quadrature. The cases of Zeldovich Universe, Dust Universe and Radiation Universe
and models with power-law and exponential expansion have discussed in detail. For large t, the models tend to be isotropic. 相似文献
10.
A. M. Swinbank R. G. Bower Graham P. Smith Ian Smail J.-P. Kneib R. S. Ellis D. P. Stark A. J. Bunker 《Monthly notices of the Royal Astronomical Society》2006,368(4):1631-1645
We exploit the gravitational potential of massive cluster lenses to probe the emission-line properties of six z = 1 galaxies which appear as highly magnified luminous arcs. Using the Gemini Multi-Object Spectrograph (GMOS) integral field spectrograph together with detailed cluster lens models, we reconstruct the intrinsic morphologies and two-dimensional velocity fields in these galaxies on scales corresponds to ∼0.5 kpc (unlensed) at z = 1 . Four of the galaxies have stable disc-like kinematics, whilst the other two resemble interacting or starburst galaxies. These galaxies lie close to the mean rest-frame I -band Tully–Fisher relation for nearby spirals suggesting a clear preference for hierarchical growth of structure. In the rest-frame B band, the observations suggest 0.5 ± 0.3 mag of brightening, consistent with increased star-formation activity at z = 1 . However, the galaxies with stable disc kinematics have more slowly rising rotation curves than expected from galaxies with similar surface brightness in the local Universe. We suggest that this may arise because the distant galaxies have lower bulge masses than their local counterparts. Whilst this study is based on only six galaxies, the gain in flux and in spatial resolution achieved via gravitational magnification provides a much more detailed view of the high-redshift Universe than that possible with conventional surveys. 相似文献
11.
We explore the prospects for using future supernova observations to probe the dark energy. We focus on quintessence, an evolving scalar field that has been suggested as a candidate for the dark energy. After simulating the observations that would be expected from the proposed SuperNova / Acceleration Probe satellite ( SNAP ), we investigate two methods for extracting information concerning quintessence from such data. First, by expanding the quintessence equation of state as w Q ( z ) = w Q (0) −α ln(1 + z ) , to fit the data, it is possible to reconstruct the quintessence potential for a wide range of smoothly varying potentials. Secondly, it will be possible to test the basic properties of the dark energy by constraining the parameters Ω Q , w Q and α. We show that it may be possible, for example, to distinguish between quintessence and the cosmological constant in this way. Furthermore, when supernova data are combined with other planned cosmological observations, the precision of reconstructions and parameter constraints is significantly improved, allowing a wider range of dark energy models to be distinguished. 相似文献
12.
Robert J. Nemiroff 《Astrophysics and Space Science》1988,145(1):53-59
It is shown that gravitational lensing could produce images of comparable brightness which are separated by an angular distance larger than the angular size of the Einstein ring of the lens distribution. Hence, specific lens configurations allow given image separations to be derived with significantly less mass than a standard single lens model. A specific example is supplied, a simple case of two properly positioned lenses acting instead of one. Observational consequences are discussed that would result if such a model was used to explain the candidate gravitational lens systems Hazard 1146+111 B, C and PKS 1145–071. Such a lens distribution might, in many cases, be verifiable with VLBI techniques. 相似文献
13.
We investigate the effect of microlensing on parameters of the images of distant sources seen near the critical curves of complex gravitational lenses, which are represented as a sum of compact structures—microlenses (stars, star-like or planet-like bodies) and diffusely distributed matter (dust and gas clouds etc.). The observation of merging, cross-shaped, annular, or arc-shaped source images is an indication that the images are close to the critical curves of gravitational lenses. Our analysis and numerical solution have allowed us to determine the structures of the critical curves and caustics formed by macro-and microlenses, as well as to estimate the characteristic perturbations introduced by microlenses at their various positions relative to the critical curve of a regular gravitational lens. We show that, the closer are the microlenses to the critical curve, the larger is the discrepancy between our results and those obtained previously with standard (linearized) allowance for the effect of a regular gravitational lens. 相似文献
14.
Shortly the vacuum component of the Universe from the geometry point of view and from the point of view of the standard model of physics of elementary particles is discussed. Some arguments are given to the calculated value of the cosmological constant (Zel’dovich’s approximation). A new component of space vacuum (the gravitational vacuum condensate) is involved the production of which has fixed time in our Universe. Also the phenomenon of vacuum selforganization must be included in physical consideration of the Universe evolution. 相似文献
15.
R. G.Vishwakarma 《Monthly notices of the Royal Astronomical Society》2002,331(3):776-784
We examine the status of various dark energy models in light of the recently observed SN 1997ff at z ≈1.7 . The modified data still fit a pure cosmological constant Λ or a quintessence with an equation of state similar to that of Λ. The kinematical Λ models, Λ∼ S -2 and Λ∼ H 2 , also fit the data reasonably well and require less dark energy density (hence more matter energy density) than is required by the constant Λ model. However, the model Λ∼ S -2 with low energy density becomes unphysical as it cannot accommodate higher redshift objects.
We also examine an alternative explanation of the data, namely the absorption by the intervening whisker-like dust, and find that the quasi-steady state (QSS) model and the Friedmann–Robertson–Walker (FRW) model Ωm0 =0.33 without any dark energy also fit the data reasonably well.
We notice that the addition of SN 1997ff to the old data has worsened the fit to most of the models, except a closed FRW model with a constant Λ and a closed quintessence model with ω φ =-0.82 , and the models have started departing from each other as we go above z =1 . However, to make a clear discrimination possible, a few more supernovae with z >1 are required.
We have also calculated the age of the Universe in these models and find that, in the models with a constant Λ, the expansion age is uncomfortably close to the age of the globular clusters. Quintessence models show even lower age. The kinematical Λ models are, however, interesting in this connection (especially the model Λ∼ H2 ) , as they give a remarkably large age of the Universe. 相似文献
We also examine an alternative explanation of the data, namely the absorption by the intervening whisker-like dust, and find that the quasi-steady state (QSS) model and the Friedmann–Robertson–Walker (FRW) model Ω
We notice that the addition of SN 1997ff to the old data has worsened the fit to most of the models, except a closed FRW model with a constant Λ and a closed quintessence model with ω
We have also calculated the age of the Universe in these models and find that, in the models with a constant Λ, the expansion age is uncomfortably close to the age of the globular clusters. Quintessence models show even lower age. The kinematical Λ models are, however, interesting in this connection (especially the model Λ∼ H
16.
Cheongho Han 《Monthly notices of the Royal Astronomical Society》2002,335(1):189-194
If a source star is gravitationally microlensed by a multiple lens system, the resulting light curve can have significant deviations from the standard form of a single lens event. The chance of producing significant deviations becomes important when the separations between the component lenses are equivalent to the combined angular Einstein ring radius of the system. For multiple lens systems composed of more than two lenses, however, this condition is difficult to meet because the orbits of such systems are unstable. Even if events are caused by a multiple lens system with stable orbits where a pair of lenses are closely located and the other component (a third body) has a wide separation from the pair, identifying the lens multiplicity photometrically will be difficult because the event will be identified by either a binary lens event caused by the close pair of lenses or a single lens event caused by the third body. In this paper, we show that if a seemingly binary lens event is followed up astrometrically using future high-precision interferometers, the existence of an additional third body can be identified via a repeating event. We show that the signatures of third bodies can be unambiguously identified from the characteristic distortions they make in the centroid shift trajectories. We also show that owing to the long-range astrometric effect of third bodies, the detection efficiency will be considerable even for third bodies with large separations from their close lens pairs. 相似文献
17.
Dr. Bijan Saha PhD 《Astrophysics and Space Science》2006,302(1-4):83-91
We consider a self-consistent system of Bianchi type-I (BI) gravitational field and a binary mixture of perfect fluid and
dark energy given by a cosmological constant. The perfect fluid is chosen to be the one obeying either the usual equation
of state, i.e., p = ζ, with ζ ∊ [0, 1] or a van der Waals equation of state. Role of the Λ term in the evolution of the BI Universe has been
studied. 相似文献
18.
We consider small-scale spheroidal clusters of weakly interacting massive particles in our Galaxy as non-compact gravitational microlenses and predict the appearance of caustics in the plane of a lensed source. The crossing of these caustics by a lensed star can produce a large variety of light curves, including some observed in actual microlensing events that have been interpreted as manifestations of binary gravitational lenses. We consider also observable effects during the gravitational microlensing of stars of non-zero angular size with a given brightness distribution across their disks by such an exotic objects as natural wormholes and objects whose space-time environment is described with the NUT metric. We demonstrate that, under certain conditions, the microlensing light curves, chromatic and polarizational effects due to the properties of the lens and the star disk brightness distributions can differ considerably from those observed for a Schwarzschild gravitational lens, so that their analysis can facilitate the identification of such objects. 相似文献
19.
20.
Clusters of galaxies are the most recently assembled, massive, bound structures in the Universe. As predicted by General Relativity,
given their masses, clusters strongly deform space-time in their vicinity. Clusters act as some of the most powerful gravitational
lenses in the Universe. Light rays traversing through clusters from distant sources are hence deflected, and the resulting
images of these distant objects therefore appear distorted and magnified. Lensing by clusters occurs in two regimes, each
with unique observational signatures. The strong lensing regime is characterized by effects readily seen by eye, namely, the production of giant arcs, multiple images, and arclets.
The weak lensing regime is characterized by small deformations in the shapes of background galaxies only detectable statistically. Cluster
lenses have been exploited successfully to address several important current questions in cosmology: (i) the study of the lens(es)—understanding cluster mass distributions and issues pertaining to cluster formation and evolution, as well as constraining
the nature of dark matter; (ii) the study of the lensed objects—probing the properties of the background lensed galaxy population—which is statistically at higher redshifts and of lower
intrinsic luminosity thus enabling the probing of galaxy formation at the earliest times right up to the Dark Ages; and (iii)
the study of the geometry of the Universe—as the strength of lensing depends on the ratios of angular diameter distances between the lens, source and observer, lens
deflections are sensitive to the value of cosmological parameters and offer a powerful geometric tool to probe Dark Energy.
In this review, we present the basics of cluster lensing and provide a current status report of the field. 相似文献