Narrow Results

The nature of the cosmological dark matter (DM) remains elusive. Recent studies have advocated the possibility that DM could be composed of ultra-light, self-interacting bosons, forming a Bose–Einstein condensate (BEC) in the very early Universe. We consider models which are charged under a global U(1)-symmetry such that the DM number is conserved. It can then be described as a classical complex scalar field which evolves in an expanding Universe. We present a brief review on the bounds on the model parameters from cosmological and galactic observations, along with the properties of galactic halos which result from such a DM candidate.

The contribution of the thermal dust component in galactic halo rotation is explored based on the microwave data of Planck satellite. The temperature asymmetry of Doppler nature revealed for several edge-on galaxies at several microwave frequencies is analyzed regarding the contribution of the thermal dust emission. We derive the dust contribution to the galactic halo rotation using the data in three bands, 353, 545, and 857 GHz for two nearby galaxies M81 and M82. The relevance of the revealed properties on the halo rotation is then discussed in the context of the modified gravity theories proposed to describe the dark matter configurations.

We develop a new model for a spherically symmetric dark matter fluid sphere containing two regions: (i) Isotropic inner region with constant density and (ii) Anisotropic outer region. We solve the system of field equation by assuming a particular density profile along with a linear equation of state. The obtained solutions are well-behaved and physically acceptable which represent equilibrium and stable matter configuration by satisfying the Tolman–Oppenheimer–Volkoff (TOV) equation and causality condition, condition on adiabatic index, Harrison–Zeldovich–Novikov criterion, respectively. We consider the compact star EXO 1785-248 (Mass $M=1.3M_{\odot }$ and radius R$=$8.8 km) to analyze our solutions by graphical demonstrations.

The sparsity parameter for clusters of galaxies is obtained in the context of $\mathrm{\Lambda }$-gravity. It is shown that the theoretical estimated values are within the reported error limits of the measured data. Thus, in the future the sparsity parameter can serve as an informative new test to detect the discrepancy between General Relativity and $\mathrm{\Lambda }$-gravity.

In this paper, we involve the galactic halo observational data to test the weak field General Relativity involving the cosmological constant. By using the data for 15 hydrogen (Hi) VLA super spirals and the Tully–Fisher relation, we obtain constraints for each galaxy. The results are consistent with previous results for spiral galaxies, as well as with the scaling relations for the halos, thus confirming the efficiency of the use of dark halo data and Tully–Fisher relation, including the baryonic Tully–Fisher index (BTFR), for testing modified gravity models.

The rotation of galactic halos is a particularly difficult subject to be dealt with. It has been shown that CMB data toward nearby galaxies can be used to probe the galactic halo rotation and can be ascribed to cold molecular clouds populating the halos. We present some methods to study the physical properties and distribution of such molecular gas clouds in the M31 galaxy halo.

The study of Planck microwave temperature maps toward several nearby spiral edge-on galaxies had revealed frequency-independent temperature asymmetry detection of Doppler origin in their halos. The contribution of the dust component to that effect is studied in this paper, particularly for the case of the M31 galaxy, using the models of dust emission and the phenomenological profiles of the dark matter configurations. The obtained results are in accordance with those inferred from the microwave temperature asymmetry data, thus indicating the possible contribution of dust, among other radiation mechanisms, in revealing the dark halo parameters.

In this work, we study and compare the features of gravitational entropy near the throat of transversable wormholes formed by exotic matter and wormholes in galactic halos. We have verified that gravitational entropy and entropy density of these wormholes in regions near their throats are indistinguishable for objects of same throat, despite the fact that they are described by different metrics and by distinct energy–momentum tensors. We have found that the gravitational entropy density diverges near the throat for both cases, probably due to a nontrivial topology at this point, however, allowing the interesting interpretation that a maximum flux of information can be carried through the throat of these wormholes. In addition, we have found that both are endowed with an entropic behaviour similar to Hawking–Bekenstein’s entropy of nonrotating and null charge black holes.

The classification of Einstein clusters based on the analysis of the stability of circular orbits according to the effective potential theory is compared with that resulting from the application of the maximum binding energy criterion. The stability properties are investigated for different choices of the energy density profile. The cases of clusters with constant energy density, those characterized by arbitrarily large values of the central gravitational redshift and clusters with Burkert-type and Navarro-Frenk-White-type energy density profiles used in the literature to model galactic halos are discussed.

The possibility to describe in a consistent way both the supermassive compact object at the center and the halo of galaxies is explored by assuming the existence of a self-gravitating semi-degenerate system of fermions in thermodynamical equilibrium.

The asymmetry in the cosmic microwave background (CMB) towards several nearby galaxies detected by Planck data is probably due to the rotation of “cold gas” clouds present in the galactic halos. In 1995 it had been proposed that galactic halos are populated by pure molecular hydrogen clouds which are in equillibrium with the CMB. More recently, it was shown that the equillibrium could be stable. Nevertheless, the cloud chemical composition is still a matter to be studied. To investigate this issue we need to trace the evolution of these virial cloud from the time of their formation to the present, and to confront the model with the observational data. The present paper is a short summary of a paper. Here we only concentrate on the evolution of these clouds from the last scattering surface (LSS) up to the formation of first generation of stars (population-III stars).