Narrow Results

It is well-known that traversable wormholes are valid solutions of the Einstein field equations, but these structures can only be maintained by violating the null energy condition. In this paper, we have obtained such wormhole solutions in an isothermal galactic halo, as well as in a void. We have shown that the null energy condition is violated, with the help of a suitable redshift function obtained from flat galactic rotation curves.

The concept of dark matter has been imported to explain the observed velocity profile of the spiral galaxies, the flat rotational velocity. Taking the flatness of rotation curves as an input and assuming that the galactic halo is filled with charged perfect fluid having known mass density function, we obtain a space time metric in the galactic halo region. The acquired solution indicates to a (nearly) flat universe, consistent with the present day cosmological observations. Various other aspects of the solution such as attractive gravity in the halo region, stability of the circular orbit, etc., are also analyzed.

The concept of dark matter (DM) hypothesis comes out as a result from the input of the observed flat rotational velocity. With the assumption that the galactic halo is pseudo-spheroidal and filled with charged perfect fluid, we have obtained a solution which has inkling to a (nearly) flat universe, compatible with the modern day cosmological observations. Various other important aspects of the solution such as attractive gravity in the halo region and the stability of the circular orbit are also explored. Also, the matter in the halo region satisfies the known equation of state which indicates its non-exotic nature.

Using the observed flat galactic rotation curve feature and taking into account the presence of anisotropic dark matter with a Hernquist density profile, the space-time geometry of the halo region in galaxies is derived. The gravitational field inside the halo is attractive in nature, and the resultant space-time is flat. We find that our solution affirms the existence of stable circular orbits in the dark halo region. The expression for the equation of state parameter indicates that the matter within the dark halo is non-exotic in nature. An analysis is also conducted on several other aspects of the solution.

Based on the widely recognized Karmarkar condition, or embedding class 1 technique, this work searches for charged wormhole solutions with spherical symmetry for the Einstein field equations. With charged traversable wormhole geometry, we use the Karmarkar condition to derive a wormhole shape function. The obtained shape function satisfies the necessary traversability conditions. In addition, we discuss the embedding diagram in Euclidean space, in both two and three dimensions, and verify the proper radial distance to display the wormhole configurations. Next, we examine a model with a quintessence field and a second field that represents regular matter and has an anisotropic pressure, providing energy for developing wormhole spacetime. Then, we obtain the Einstein field equations to verify the energy conditions. After that, we take three density profiles, which are the pseudo-isothermal density profile, the Navarro–Frenk–White density profile, and the density profile of Einasto Dark Matter of the galactic halo, and observe that for all of these density profiles, the NEC is violated. Hence, wormhole structures are firmly maintained by the composition of exotic matter within them. Furthermore, we use the volume integral quantifier to determine the necessary quantity of exotic matter near the wormhole throat.

In this paper, we investigate static spherically symmetric wormhole solutions with galactic halo region in the background of $F(T,T_G)$ gravity. Here, $T$ represents torsion scalar and $T_G$ is teleparallel equivalent Gauss–Bonnet term. For this purpose, we consider a diagonal tetrad and two specific $F(T,T_G)$ models. We analyze the wormhole structure through shape function graphically for both models. We also investigate the behavior of null/weak energy conditions. Finally, we evaluate the equilibrium condition to check stability of the wormhole solutions. It is concluded that there exists physically viable wormhole solution only for the first model that turns out to be stable.

This paper explores static spherically symmetric wormhole solutions in the galatic halo region for $f({𝒢},T)$ gravity (${𝒢}$ and $T$ represent the Gauss–Bonnet invariant and trace of the energy–momentum tensor, respectively). We formulate the explicit expressions for matter variables and evaluate wormhole solutions either specifying $f({𝒢},T)$ model to construct shape function or taking specific form of the shape function to determine $f({𝒢},T)$ model. It is found that null energy condition for the effective energy–momentum tensor is violated throughout the evolution in both cases while physically acceptable wormhole solutions exist only for a considered $f({𝒢},T)$ model.

In this work, the spacetime geometry of the halo region in spiral galaxies is obtained considering the observed flat galactic rotation curve feature, invoking the Tully–Fisher relation and assuming the presence of cold dark matter in the galaxy. The gravitational lensing analysis is performed treating the so-obtained spacetime as a gravitational lens. It is found that the aforementioned spacetime as the gravitational lens can consistently explain the galaxy–galaxy weak gravitational lensing observations and the lensing observations of the well-known Abell 370 and Abell 2390 galaxy clusters.

Gravitational microlensing is a powerful method to constrain the abundance of massive dark objects in the Milky Way halo. We calculate the optical depth and the microlensing rate for events caused by Primordial Black Holes (PBHs) eventually distributed in the Milky Way halo, towards some selected directions of observation, as the Galactic bulge, the Large and the Small Magellanic Clouds and the M31 galaxy. The capability of the Euclid space telescope to constraint the abundance of PBHs with mass ≥ 10⁻⁷M_⊙ in observation towards the Galactic bulge is also discussed.

We present here the proposal to use the LISA interferometer for detecting the gravitomagnetic field due to the rotation of the Milky Way, including the contribution given by the dark matter halo. The galactic signal would be superposed to the gravitomagnetic field of the Sun. The technique to be used is based on the asymmetric propagation of light along the closed contour of the space interferometer (Sagnac-like approach). Both principle and practical aspects of the proposed experiment are discussed. The strategy for disentangling the sought for signal from the kinematic terms due to proper rotation and orbital motion is based on the time modulation of the time of flight asymmetry. Such modulation will be originated by the annual oscillation of the plane of the interferometer with respect to the galactic plane. Also the effect of the gravitomagnetic field on the polarization of the electromagnetic signals is presented as an in principle detectable phenomenon.

It is described the distribution of small-scale dark matter (DM) clumps in the Galactic halo. These clumps are efficiently destructed by tidal forces at early stages of structure formation starting from time of clump detachment from the expansion of the Universe. Only a small fraction of these clumps survives the stage of hierarchical clustering. The survived clumps can be further destructed in the Galaxy by tidal interactions with stars in the Galactic bulge, disk and halo. It is shown that collective gravitational field of the Galactic disc provides the dominant contribution to the destruction of small-scale clumps. The resulting enhancement (boost factor) of annihilation signal due to the halo clumpiness strongly depends on the primordial perturbation spectrum and varies in the range 10-100.

The EROS-2 collaboration has monitored during 6.7 years millions of stars in the Magellanic Clouds, in order to search for microlensing phenomena due to hypothetical compact dark objects in the Milky Way Halo ('machos'), constituting a component of Galactic dark matter. I will present the limit obtained by the EROS experiment on the macho content of the galactic halo, using only the brightest, well resolved stars (~ 7 million). The number of microlensing effects found is by far not numerous enough to account for the galactic dark matter. Our robust limit (f < 0.07 at 95% CL for 0.4 M_⊙) is now clearly in conflict with the MACHO experiment positive result (0.05 < f < 0.42 for 0.4 M_⊙). More generally, machos in the mass range 10^-7M_⊙ < M < 5M_⊙ are ruled out as the primary occupants of the Milky Way Halo. Furthermore, a new constraint for higher machos masses (up to 150 solar masses) has been performed using the complete star sample. A preliminary limit is presented here.

An exact model in Einstein-Maxwell gravity describing a magnetized galactic disk-halo system is presented. The description of properties of the stationary metric and its source are discussed. All the expressions are presented in terms of an Harmonic function. A “generalization” of the Kuzmin potential is used as a particular example. The solution obtained is asymptotically Minkowskian in general and turns out to be singularity free. All the relevant quantities show a reasonable physical behavior.