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1.
Recently, the gravitational polarization of the quantum vacuum was proposed as alternative to the dark matter paradigm. In the present paper we consider four benchmark measurements: the universality of the central surface density of galaxy dark matter haloes, the cored dark matter haloes in dwarf spheroidal galaxies, the non-existence of dark disks in spiral galaxies and distribution of dark matter after collision of clusters of galaxies (the Bullet cluster is a famous example). Only some of these phenomena (but not all of them) can (in principle) be explained by the dark matter and the theories of modified gravity. However, we argue that the framework of the gravitational polarization of the quantum vacuum allows the understanding of the totality of these phenomena.  相似文献   

2.
Dwarf galaxy rotation curves and the core problem of dark matter haloes   总被引：1，自引：0，他引：1  
The standard cold dark matter (CDM) model has recently been challenged by the claim that dwarf galaxies have dark matter haloes with constant-density cores, whereas CDM predicts haloes with steeply cusped density distributions. Consequently, numerous alternative dark matter candidates have recently been proposed. In this paper we scrutinize the observational evidence for the incongruity between dwarf galaxies and the CDM model. To this end, we analyse the rotation curves of 20 late-type dwarf galaxies studied by Swaters. Taking the effects of beam smearing and adiabatic contraction into account, we fit mass models to these rotation curves with dark matter haloes with different cusp slopes, ranging from constant-density cores to r −2 cusps. Even though the effects of beam smearing are small for these data, the uncertainties in the stellar mass-to-light ratio and the limited spatial sampling of the halo's density distribution hamper a unique mass decomposition. Consequently, the rotation curves in our sample cannot be used to discriminate between dark haloes with constant-density cores and r −1 cusps. We show that the dwarf galaxies analysed here are consistent with CDM haloes in a ΛCDM cosmology, and that there is thus no need to abandon the idea that dark matter is cold and collisionless. However, the data are also consistent with any alternative dark matter model that produces dark matter haloes with central cusps less steep than r −1.5. In fact, we argue that based on existing H  i rotation curves alone, at best weak limits can be obtained on cosmological parameters and/or the nature of the dark matter. In order to make progress, rotation curves with higher spatial resolution and independent measurements of the mass-to-light ratio of the disc are required.  相似文献   

3.
Neutral hydrogen line observations of the extended rotation curves of spiral galaxies imply that there exist significant discrepancies between the luminous and dynamical mass beyond the bright optical discs. This means either that galaxies contain significant quantities of non-luminous matter (matter with a mass-to-light ratio very much higher than that of ordinary stars), or that the law of gravity on the scale of galaxies is not the usual Newtonian inverse square law. Attempts to account for the observed discrepancy in the context of these two explanations are reviewed here with emphasis given to the second and less conventional alternative. It is argued that the standard picture of spiral galaxy halo and disc formation in the context of cold dark matter cannot account for the observed systematics of the discrepancy — notably rotation curves which are seen to be flat and featureless from the bright inner regions where the visible matter dominates the dynamics (in some cases overwhelmingly) to the outer regions where the dark halo dominates. It is demonstrated that in those galaxies with well-observed rotation curves, the discrepancy apparently appears below a critical acceleration. Any dark matter explanation of the discrepancy must account for this fact. Moreover, this would also eliminate empirically motivated modifications of Newton's law in which the deviation from 1/r occurs beyond a fundamental length scale. The suggestion by Milgrom in which the force law becomes essentially 1/r below a critical acceleration (MOND) can account for most of the observed systematics of galaxy rotation curves and, significantly, leads to the observed luminosity-velocity relationship in spiral galaxies (the Tully-Fisher law). Generally covariant theories of gravity which predict this phenomenology in the weak-field limit are described. Although there is not yet a theory which obviously meets all of the requirements for a physically viable alternative to dark matter, a generalized scalar-tensor theory of the form suggested by Bekenstein (phase coupling gravitation) is the currently leading candidate and has the advantage of being testable locally.  相似文献   

4.
中子星可以通过重子物质和暗物质的相互作用吸积暗物质,且在一定条件下, 中子星吸积的暗物质粒子可以引发自引力塌缩形成小型黑洞, 生成的黑洞可能会进一步吞噬中子星.依据文献已有模型, 基于以上物理过程给出了在暗物质粒子不同质量下对暗物质粒子--中子的散射截面的限制.使用弱相互作用大质量粒子(Weakly Interacting Massive Particle, WIMP)模型, 并考虑暗物质粒子是玻色子的情形, 讨论了暗物质粒子有无自相互作用以及有无湮灭等条件下对限制暗物质参数的影响.既考虑了已发现的两个中子星系统来给出对暗物质参数空间的限制,也考虑了两个可能存在的年老中子星来预测未来观测可能对暗物质参数空间的限制.对于考虑玻色--爱因斯坦凝聚(Bose-Einstein Condensate, BEC)的玻色子暗物质, 在无自相互作用或有弱自相互作用, 无湮灭或有很小湮灭截面的条件下,中子星给出的间接观测对暗物质粒子-中子散射截面的限制的强度比XENON1T直接探测实验来得更强.未来, 如果在银心附近能观测到年老中子星, 其观测结果可以提升模型给出的对暗物质粒子--中子散射截面的限制, 从而帮助人们进一步理解暗物质.  相似文献   

5.
Some consequences of MOND (Modification of Newtonian dynamics) proposed as an alternative hypothesis to dark matter for various astrophysical situations is discussed. The ubiquitous occurrence of the fundamental acceleration invoked is pointed out.  相似文献   

6.
We study the distribution of dark matter in dwarf spheroidal galaxies by modelling the moments of their line-of-sight velocity distributions. We discuss different dark matter density profiles, both cuspy and possessing flat density cores. The predictions are made in the framework of standard dynamical theory of two-component (stars and dark matter) spherical systems with different velocity distributions. We compare the predicted velocity dispersion profiles to observations in the case of Fornax and Draco dwarfs. For isotropic models the dark haloes with cores are found to fit the data better than those with cusps. Anisotropic models are studied by fitting two parameters, dark mass and velocity anisotropy, to the data. In this case all profiles yield good fits, but the steeper the cusp of the profile, the more tangential is the velocity distribution required to fit the data. To resolve this well-known degeneracy of density profile versus velocity anisotropy, we obtain predictions for the kurtosis of the line-of-sight velocity distribution for models found to provide best fits to the velocity dispersion profiles. It turns out that profiles with cores typically yield higher values of kurtosis which decrease more steeply with distance than the cuspy profiles, which will allow us to discriminate between the profiles once the kurtosis measurements become available. We also show that with present quality of the data the alternative explanation of velocity dispersions in terms of Modified Newtonian Dynamics cannot yet be ruled out.  相似文献   

7.
Making robust predictions for the phase-space distribution of dark matter at the solar neighbourhood is vital for dark matter direct-detection experiments. To date, almost all such predictions have been based on simulations that model the dark matter alone. Here, we use three cosmological hydrodynamic simulations of bright, disc-dominated galaxies to include the effects of baryonic matter self-consistently for the first time. We find that the addition of baryonic physics drastically alters the dark matter profile in the vicinity of the solar neighbourhood. A stellar/gas disc, already in place at high redshift, causes merging satellites to be dragged preferentially towards the disc plane where they are torn apart by tides. This results in an accreted dark matter disc that contributes ∼0.25–1.5 times the non-rotating halo density at the solar position. The dark disc, unlike dark matter streams, is an equilibrium structure that must exist in disc galaxies that form in a hierarchical cosmology. Its low rotation lag with respect to the Earth significantly boosts Weakly Interacting Massive Particle (WIMP) capture in the Earth and Sun, boosts the annual modulation signal and leads to distinct variations in the flux as a function of recoil energy that allow the WIMP mass to be determined.  相似文献   

8.
《New Astronomy Reviews》2002,46(12):755-766
An overview is presented of the main properties of dark matter haloes, as we know them from observations, essentially from rotation curves around spiral and dwarf galaxies. Detailed rotation curves are now known for more than a thousand galaxies, revealing that they are not so flat in the outer parts, but rising for late-types, and falling for early-types. A well-established result now is that most bright galaxies are not dominated by dark matter inside their optical disks. Only for dwarfs and LSB (Low Surface Brightness galaxies) dark matter plays a dominant role in the visible regions. The 3D-shape of haloes are investigated through several methods that will be discussed: polar rings, flaring of HI planes, X-ray isophotes. It is not yet possible with rotation curves to know how far haloes extend, but tentatives have been made. It will be shown that the dark matter appears to be coupled to the gas in spirals and dwarfs, suggesting that dark baryons could play a major role in rotation curves. Theories proposing to replace the non-baryonic dark matter by a different dynamical or gravity law, such as MOND, have to take into account the dark baryons, especially since their spatial distribution is likely to be quite different from the visible matter.  相似文献   

9.
In this paper we explore the differences between a warm dark matter (WDM) model and a cold dark matter (CDM) model where the power on a certain scale is reduced by introducing a narrow negative feature ('dip'). This dip is placed in a way so as to mimic the loss of power in the WDM model: both models have the same integrated power out to the scale where the power of the dip model rises to the level of the unperturbed CDM spectrum again.
Using N -body simulations we show that some of the large-scale clustering patterns of this new model follow more closely the usual CDM scenario while simultaneously suppressing small-scale structures (within galactic haloes) even more efficiently than WDM. The analysis in the paper shows that the new Dip model appears to be a viable alternative to WDM, but it is based on different physics. Where WDM requires the introduction of a new particle species, the Dip model is based on a non-standard inflationary period. If we are looking for an alternative to the currently challenged standard ΛCDM structure formation scenario, neither the ΛWDM nor the new Dip model can be ruled out based on the analysis presented in this paper. They both make very similar predictions and the degeneracy between them can only be broken with observations yet to come.  相似文献   

10.
Various compact dark matter objects (CDOs) were discussed in the literature. Typically parameters of CDOs, such as the mass and the distance, were evaluated by using the gravitational microlensing effect. However, this method has limitations. We propose an alternative method for detecting and measuring parameters of CDOs. It is based on the scenario where there is a star having one planet, such that the orbital plane of the planet does not contain the star. This indicates the presence of a gravitating object located far away at the axis directed from the star to the planetary orbital plane. If in this direction there is no visible star, this could mean that the distant gravitating object is a CDO. We derived analytical expressions for determining the unknown mass of the CDO and its unknown distance from the star based on the parameters of the planetary orbit. We believe that this method could help obtaining additional observational data on the CDOs in particular and therefore on dark matter in general.  相似文献   

11.
Early reionization of the intergalactic medium (IGM), which is favoured from the WMAP temperature–polarization cross-correlations, contests the validity of the standard scenario of structure formation in the cold dark matter (CDM) cosmogony. It is difficult to achieve early enough star formation without rather extreme assumptions such as a very high escape fraction of ionizing photons from protogalaxies or a top-heavy initial mass function (IMF). Here, we propose an alternative scenario that additional fluctuations on small scales induced by primordial magnetic fields trigger early structure formation. We found that ionizing photons from Population III stars formed in dark haloes can easily reionize the Universe by   z ≃ 15  if the strength of primordial magnetic fields is between 0.7 and  1.5 × 10−9 G  .  相似文献   

12.
  相似文献   

13.
Starting from the standard Newtonian theory, we derive in the propagation equation of perturbations an effectively Yukawalike potential invoked by several authors as an alternative to dark matter. This approach provides with a simple explanation of the segregation observed for dynamically ‘hot’ systems in the plane defined by their effective parameters.  相似文献   

14.
The effect of background dynamics of the universe on formation of large scale structures in the framework of Modified Newtonian Dynamics (MOND) is investigated. A spherical collapse model is used for modeling the formation of the structures. This study is done in two extreme cases: (i) assuming a universe with a low-density baryonic matter without any cold dark matter and dark energy; (ii) a dark energy dominated universe with baryonic matter, without cold dark matter. We show that for the case (ii) the structures virialize at lower redshifts with larger radii compared to the low-density background universe. The dark energy slow downs the collapse of the structures. We show that our results are compatible with recent simulations of the structure formation in MOND.  相似文献   

15.
A two-fluid dark matter model, in which dark matter is represented as a two-component fluid thermodynamic system, without interaction between the constituent particles of different species, and with each distinct component having a different four-velocity, was recently proposed in Harko and Lobo [T. Harko, F.S.N. Lobo, Phys. Rev. D 83 (2011) 124051]. In the present paper we further investigate the two-fluid dark matter model, by assuming that the two dark matter components are pressureless, non-comoving fluids. For this particular choice of equations of state the dark matter distribution can be described as a single anisotropic fluid, with vanishing tangential pressure, and non-zero radial pressure. We investigate the properties of this model in the region of constant velocity galactic rotation curves, where the dynamics of the test particles is essentially determined by the dark matter only. By solving the general relativistic equations of mass continuity and hydrostatic equilibrium we obtain the geometric and physical parameters of the dark matter halos in the constant velocity region in an exact analytical form. The general, radial coordinate dependent, functional relationship between the energy density and the radial pressure is also determined, and it differs from a simple barotropic equation of state.  相似文献   

16.
Although the existence of dark matter is generally accepted by the mainstream scientific community, there is no generally agreed direct detection of it. Also, observations show that some Bok globules are prolate in some regions without suitable explanation for its cause. In this paper, we investigate the effect of dark matter sub-halo in transformation of the Bok globules from spherical to the prolate shape. We limit the investigation to a particular case that the magnetic field and turbulent effects are negligible through the Bok globule. We consider the gravitational effect of dark matter sub-halo on the isothermal Bok globule that is exposed to suitable distance of it. The results show that the dark matter sub-halo can justify the transformation of Bok globules in some regions. In this paper, we introduce a new method for proving the existence of dark matter sub halo.  相似文献   

17.
We calculate the gravitational lensing probabilities by cold dark matter (CDM) halos with different density profiles, and compare them with current observations from the Cosmic Lens All-Sky Survey (CLASS) and the Jodrell-Bank VLA Astrometric Survey (JVAS). We find that the lensing probability is dramatically sensitive to the clumping of the dark matter, or quantitatively, the concentration parameter. We also find that our predicted lensing probabilities in most cases show inconsistency with the observations. It is argued that high lensing probability may not be an effective tool for probing the statistical properties of inner structures of dark matter halos.  相似文献   

18.
On the nature of superoutbursts in dwarf novae   总被引：1，自引：0，他引：1  
We determine a crucial feature of the dark halo density distribution from the fact that the luminous matter dominates the gravitational potential at about one disc scalelength R d, but at the optical edge     the dark matter has already become the main component of the galaxy density. From the kinematics of 137 spirals we find that the dark matter halo density profiles are self-similar at least out to R opt and show core radii much larger than the corresponding disc scalelengths. The luminous regions of spirals consist of stellar discs embedded in dark haloes with roughly constant density. This invariant dark matter profile is very difficult to reconcile with the fundamental properties of the density distribution of cold dark matter haloes. With respect to previous work, the present evidence is obtained by means of a robust method and for a large and complete sample of normal spirals.  相似文献   

19.
There are a number of theoretical and observational hints that large numbers of low-mass galaxies composed entirely of dark matter exist in the field. The theoretical considerations follow from the prediction of cold dark matter theory that there exist many low-mass galaxies for every massive one. The observational considerations follow from the observed paucity of these low-mass galaxies in the field but not in dense clusters of galaxies; this suggests that the lack of small galaxies in the field is due to the inhibition of star formation in the galaxies as opposed to the fact that their small dark matter haloes do not exist. In this work we outline the likely properties of low-mass dark galaxies, and describe observational strategies for finding them, and where in the sky to search. The results are presented as a function of the global properties of dark matter, in particular the presence or absence of a substantial baryonic dark matter component. If the dark matter is purely cold and has a Navarro, Frenk & White density profile, directly detecting dark galaxies will only be feasible with present technology if the galaxy has a maximum velocity dispersion in excess of 70 km s−1, in which case the dark galaxies could strongly lens background objects. This is much higher than the maximum velocity dispersions in most dwarf galaxies. If the dark matter in galaxy haloes has a baryonic component close to the cosmic ratio, the possibility of directly detecting dark galaxies is much more realistic; the optimal method of detection will depend on the nature of the dark matter. A number of more indirect methods are also discussed.  相似文献   

20.
In a cold dark matter (CDM) framework of structure formation, the dark matter haloes around galaxies assemble through successive mergers with smaller haloes. This merging process is not completely efficient, and hundreds of surviving halo cores, or subhaloes, are expected to remain in orbit within the halo of a galaxy like the Milky Way. While the dozen visible satellites of the Milky Way may trace some of these subhaloes, the majority are currently undetected. A large number of high-velocity clouds (HVCs) of neutral hydrogen are observed around the Milky Way, and it is plausible that some of the HVCs may trace subhaloes undetected in the optical. Confirming the existence of concentrations of dark matter associated with even a few of the HVCs would represent a dramatic step forward in our attempts to understand the nature of dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle physics currently suggest neutralinos as a natural well-motivated candidate for the non-baryonic dark matter of the universe. If this is indeed the case, then it may be possible to detect dark matter indirectly as it annihilates into neutrinos, photons or positrons. In particular, the centres of subhaloes might show up as point sources in gamma-ray observations. In this work, we consider the possibility that some of the unidentified EGRET γ-ray sources trace annihilating neutralino dark matter in the dark substructure of the Local Group. We compare the observed positions and fluxes of both the unidentified EGRET sources and the HVCs with the positions and fluxes predicted by a model of halo substructure, to determine up to what extent any of these three populations could be associated.  相似文献   

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