Standard Dark Matter Scenario Research Articles

Galaxy formation and dark matter: small scale problems and quantum effects on astrophysical scales

Although non-baryonic dark matter seems essential in the context of the currently favoured cosmological model, the standard dark matter scenario is facing problems: experimental searches have failed to find the relevant particles, closing the mass-corssection window of the ‘WIMP miracle’, and the model suffers from problems on (sub) galactic scales. The cold dark matter (CDM) invoked may turn out to be too cold and needs to be heated; so that its solution to the dearth of visible matter in the outer parts of galaxies is not accompanied by the problem of an excess of matter in their centres (along with other possibly related problems, such as the numerical excess of predicted satellites). After a heuristic introduction to some aspects of the rationales that lead to the CDM paradigm, I discuss the properties of self gravitating CDM structures (haloes) and the proposed reasons for their apparently ‘universal profiles’ (including new simulations attempting to explain aspects of their advent), the galactic-scale problems associated with them, and proposed solutions, focussing on baryonic solutions and the recently topical ultra-light axion particles as replacement for the standard weakly interacting massive particles (WIMPs). It is hoped that at least parts of this review would be helpful to a general physics audience interested in the problem of dark matter in an astrophysical context.

Redshift-space distortions from vector perturbations. II. Anisotropic signal

We study the impact on the galaxy correlation function of the presence of a vector component in the tracers' peculiar velocities, in the case in which statistical isotropy is violated. We present a general framework - based on the bipolar spherical harmonics expansion - to study this effect in a model independent way, without any hypothesis on the origin or the properties of these vector modes. We construct six new observables, that can be directly measured in galaxy catalogs in addition to the standard monopole, quadrupole and hexadecapole, and we show that they completely describe any deviations from isotropy. We then perform a Fisher analysis in order to quantify the constraining power of future galaxy surveys. As an example, we show that the SKA2 would be able to detect anisotropic rotational velocities with amplitudes as low as 1% of that of the vorticity generated during shell-crossing in standard dark matter scenarios.

Comment on “Scalar-tensor gravity coupled to a global monopole and flat rotation curves”

The recent paper by Lee and Lee [Phys. Rev. D 69, 127502 (2004)] may strongly leave the impression that astronomers have established that the rotation curves of spiral galaxies are flat. We show that the old paradigm of flat rotation curves lacks, today, any observational support and following it at face value leads to intrinsically flawed alternatives to the standard dark matter scenario. On the other hand, we claim that the rich systematics of spiral galaxy rotation curves, which reveals, in the standard Newtonian gravity framework, the phenomenon of dark matter, in alternative scenarios, works as a unique benchmark.

Galactic Metric, Dark Radiation, Dark Pressure, and Gravitational Lensing in Brane World Models

In the brane world scenario, the four-dimensional effective Einstein equation has extra terms that arise from the embedding of the 3-brane in the bulk. These nonlocal effects may provide an explanation for the dynamics of the neutral hydrogen clouds at large distances from the galactic center, which is usually explained by postulating the existence of the dark matter. We obtain the exact galactic metric, the dark radiation, and the dark pressure in the flat rotation curve region in the brane world scenario. Due to the presence of the bulk effects, the flat rotation curves could extend several hundred kiloparsecs. The limiting radius for which bulk effects are important is compared with the numerical values of the truncation parameter of the dark matter halos, obtained from weak-lensing observations. There is a relatively good agreement between the predictions of the model and observations. The deflection of photons in the flat rotation curve region is also considered. The bending angle predicted by the brane world models is much larger than that predicted by dark matter models. The angular radii of the Einstein rings are obtained in the small-angle approximation. The tangential shear is compared with the observational data obtained in the weak lensing of galaxies in the Red-Sequence Cluster Survey. The study of the light deflection by galaxies and the gravitational lensing could discriminate between the different dynamical laws proposed to model the motion of particles at the galactic level and the standard dark matter scenario.