Unified Brane Gravity: Cosmological Dark Matter from Scale Dependent Newton Constant

We analyze, within the framework of unified brane gravity, the weak-field perturbations caused by the presence of matter on a 3-brane. Although deviating from the Randall-Sundrum approach, the masslessness of the graviton is still preserved. In particular, the four-dimensional Newton force law is recovered, but serendipitously, the corresponding Newton constant is shown to be necessarily lower than the one which governs FRW cosmology. This has the potential to puzzle out cosmological dark matter. A subsequent conjecture concerning galactic dark matter follows.Comment: 6 pages, to be published in Phys. Rev.

Concentrating the Dark Matter in Galaxy Clusters through Tidal Stripping of Baryonically-Compressed Galactic Halos

Gravitational lensing observations of massive X-ray clusters imply a steep characteristic density profile marked by a central concentration of dark matter. The observed mass fraction within a projected radius of 150 kpc is twice that found in state-of-the-art dark matter simulations of the standard Lambda-CDM cosmology. A central baryon enhancement that could explain this discrepancy is not observed, leaving a major puzzle. We propose a solution based on the merger histories of clusters. A significant fraction of the final dark matter content of a cluster halo originates within galaxy-sized halos, in which gas can cool and compress the dark matter core to high densities. The subsequent tidal stripping of this compressed dark matter occurs in denser regions that are closer to the center of the cluster halo. Eventually, the originally cooled gas must be dispersed into the intracluster medium through feedback, for consistency with observations that do not find central baryon enhancements in clusters. Still, the early adiabatic compression of the galactic dark matter leaves a net effect on the cluster. Using a simple model for this process, we show that the central cluster profile is substantially modified, potentially explaining the observed discrepancy.Comment: 7 pages, 3 figures, submitted to MNRA

Singlet-Doublet Dirac Dark Matter and Neutrino Masses

We examine an extension of the Standard Model that addresses the dark matter puzzle and generates Dirac neutrino masses through the radiative seesaw mechanism. The new field content includes a scalar field that plays an important role in setting the relic abundance of dark matter. We analyze the phenomenology in the light of direct, indirect, and collider searches of dark matter. In this framework, the dark matter candidate is a Dirac particle that is a mixture of new singlet-doublet fields with mass mχ01≲1.1  TeV. We find that the allowed parameter space of this model is broader than the well-known Majorana dark matter scenario

Dark Matter Gravitational Interactions

We argue that the conjectured dark mater in the Universe may be endowed with a new kind of gravitational charge that couples to a short range gravitational interaction mediated by a massive vector field. A model is constructed that assimilates this concept into ideas of current inflationary cosmology. The model is also consistent with the observed behaviour of galactic rotation curves according to Newtonian dynamics. The essential idea is that stars composed of ordinary (as opposed to dark matter) experience Newtonian forces due to the presence of an all pervading background of massive gravitationally charged cold dark matter. The novel gravitational interactions are predicted to have a significant influence on pre-inflationary cosmology. The precise details depend on the nature of a gravitational Proca interaction and the description of matter. A gravitational Proca field configuration that gives rise to attractive forces between dark matter charges of like polarity exhibits homogeneous isotropic eternal cosmologies that are free of cosmological curvature singularities thus eliminating the horizon problem associated with the standard big-bang scenario. Such solutions do however admit dense hot pre-inflationary epochs each with a characteristic scale factor that may be correlated with the dark matter density in the current era of expansion. The model is based on a theory in which a modification of Einsteinian gravity at very short distances can be expressed in terms of the gradient of the Einstein metric and the torsion of a non-Riemannian connection on the bundle of linear frames over spacetime. Indeed we demonstrate that the genesis of the model resides in a remarkable simplification that occurs when one analyses the variational equations associated with a broad class of non-Riemannian actions.Comment: 40 pages, 4 Postscript figure

Asymmetric WIMP dark matter

In existing dark matter models with global symmetries the relic abundance of dark matter is either equal to that of anti-dark matter (thermal WIMP), or vastly larger, with essentially no remaining anti-dark matter (asymmetric dark matter). By exploring the consequences of a primordial asymmetry on the coupled dark matter and anti-dark matter Boltzmann equations we find large regions of parameter space that interpolate between these two extremes. Interestingly, this new asymmetric WIMP framework can accommodate a wide range of dark matter masses and annihilation cross sections. The present-day dark matter population is typically asymmetric, but only weakly so, such that indirect signals of dark matter annihilation are not completely suppressed. We apply our results to existing models, noting that upcoming direct detection experiments will constrain a large region of the relevant parameter space.Comment: 32 pages, 6 figures, updated references, updated XENON100 bounds, typo in figure caption correcte

Dark Matter from Split Seesaw

The seesaw mechanism in models with extra dimensions is shown to be generically consistent with a broad range of Majorana masses. The resulting democracy of scales implies that the seesaw mechanism can naturally explain the smallness of neutrino masses for an arbitrarily small right-handed neutrino mass. If the scales of the seesaw parameters are split, with two right-handed neutrinos at a high scale and one at a keV scale, one can explain the matter-antimatter asymmetry of the universe, as well as dark matter. The dark matter candidate, a sterile right-handed neutrino with mass of several keV, can account for the observed pulsar velocities and for the recent data from Chandra X-ray Observatory, which suggest the existence of a 5 keV sterile right-handed neutrino.Comment: 15 pages. v2: references added, a version accepted by PL