Dark Matter Fluid Research Articles

Crossing the phantom divide in an interacting generalized Chaplygin gas

Unified generalized Chaplygin gas models assuming an interaction between dark energy anddark matter fluids have been previously proposed. Following these ideas, we consider aparticular relation between dark densities, which allows the possibility of a time varyingequation of state for dark energy that crosses the phantom divide at a recent epoch.Moreover, these densities decay throughout the evolution of the Universe, avoidinga big rip. We find also a scaling solution, i.e. these densities are asymptoticallyproportional in the future, which contributes to the solution of the coincidence problem.

The integrated Sachs-Wolfe effect in cosmologies with coupled dark matter and dark energy

The subject of this paper is the derivation of the integrated Sachs-Wolfe (iSW) effect in cosmologies with coupled dark matter and dark energy fluids. These couplings influence the iSW-effect in three ways: The Hubble function assumes a different scaling, the structure growth rate shows a different time evolution, and in addition, the Poisson equation, which relates the density perturbations to fluctuations in the gravitational potential, is changed, due to the violation of the scaling rho ~ a^{-3} of the matter density rho with scale factor a. Exemplarily, I derive the iSW-spectra for a model in which dark matter decays into dark energy, investigate the influence of the dark matter decay rate and the dark energy equation of state on the iSW-signal, and discuss the analogies for gravitational lensing. Quite generally iSW-measurements should reach similar accuracy in determining the dark energy equation of state parameter and the coupling constant.

One-loop quantum corrections to cosmological scalar field potentials

We study the loop corrections to potentials of complex or coupled real scalar fields used in cosmology to account for dark energy, dark matter, or dark fluid. We show that the SUGRA quintessence and dark matter scalar field potentials are stable against the quantum fluctuations, and we propose solutions to the instability of the potentials of coupled quintessence and dark fluid scalar fields. We also find that a coupling to fermions is very restricted, unless this coupling has a structure which already exists in the scalar field potential or which can be compensated by higher order corrections. Finally, we study the influence of the curvature and kinetic term corrections.

Cosmic microwave background anisotropies at second order: I

We present the computation of the full system of Boltzmann equations at second orderdescribing the evolution of the photon, baryon and cold dark matter fluids. These equationsallow one to follow the time evolution of the cosmic microwave background anisotropies atsecond order at all angular scales from the early epoch, when the cosmologicalperturbations were generated, to the present, through the recombination era. Thispaper sets the stage for the computation of the full second-order radiation transferfunction at all scales and for a generic set of initial conditions specifying the level ofprimordial non-Gaussianity. In a companion paper, we will present the computation ofthe three-point correlation function at recombination which is very relevant forthe issue of non-Gaussianity in the cosmic microwave background anisotropies.

Back to the primordial Universe by a Monge–Ampère–Kantorovich optimization scheme

A new method for reconstruction of the primordial density fluctuation field is presented. Various previous approaches to this problem rendered non-unique solutions. Here, it is demonstrated that the initial positions of dark matter fluid elements, under the hypothesis that their displacement is the gradient of a convex potential, can be reconstructed uniquely. In our approach, the cosmological reconstruction problem is reformulated as an assignment problem in optimization theory. When tested against numerical simulations, our scheme yields excellent reconstruction on scales larger than a few megaparsecs.

Galactic halos of fluid dark matter

Dwarf spiral galaxies, and in particular the prototypical DDO 154, are known to be completely dominated by an unseen component. The putative neutralinos, so far the favored explanation for the astronomical dark matter, fail to reproduce the well measured rotation curves of those systems because these species tend to form a central cusp whose presence is not supported by observation. We have considered here a self-coupled charged scalar field as an alternative to neutralinos and investigated whether a Bose condensate of that field could account for the dark matter inside DDO 154 and more generally inside dwarf spirals. The size of the condensate turns out to be precisely determined by the scalar mass m and self-coupling $\ensuremath{\lambda}$ of the field. We find actually that for ${m}^{4}/\ensuremath{\lambda}\ensuremath{\sim}50--75{\mathrm{eV}}^{4}$ the agreement with the measurements of the circular speed of DDO 154 is impressive, whereas it lessens for larger systems. The cosmological behavior of the field is also found to be consistent, though marginally, with the limits set by big bang nucleosynthesis on the effective number of neutrino families. We conclude that classical configurations of a scalar and self-coupled field provide a possible solution to the astronomical dark matter problem and we suggest further directions of research.

Interacting quintessence solution to the coincidence problem

We show that a suitable interaction between a scalar field and a matter fluid in a spatially homogeneous and isotropic spacetime can drive the transition from a matter dominated era to an accelerated expansion phase and simultaneously solve the coincidence problem of our present Universe. For this purpose we study the evolution of the energy density ratio of these two components. We demonstrate that a stationary attractor solution is compatible with an accelerated expansion of the Universe. We extend this study to account for dissipation effects due to interactions in the dark matter fluid. Finally, type Ia supernovae and primordial nucleosynthesis data are used to constrain the parameters of the model.

Tidal debris of dwarf spheroidals as a probe of structure formation models

Recent observations suggest that Carina and other nearby dwarf spheroidal galaxies (dSphs) are surrounded by unbound stars tidally stripped by the Milky Way. We have run high-resolution N-body simulations of dwarf galaxies orbiting within the Milky Way halo to determine if such observations can be explained with dark matter potentials like those implied by current structure formation models. We show that tidal forces acting on dwarfs with constant density cores or with cuspy profiles having a low concentration parameter c≲ 7) lead to flat outer stellar density profiles like that of Carina for a variety of orbital configurations. On the contrary, it is more difficult to remove stars from cuspy dark matter haloes with concentrations as high as predicted by cold dark matter (CDM) models at the mass scale of dwarf galaxies (c > 10), and the data can only be reproduced assuming nearly radial orbits. Our simulations show that Carina is losing mass at a fractional rate <0.1 Gyr−1 and its mass-to-light ratio could be inflated by at most a factor of 2 due to unbound stars projected along the line of sight. We follow the evolution of the tidal debris within a triaxial clumpy cold dark matter Milky Way halo which causes differential precession and small-scale heating of the stellar streams. This renders them useless as a dynamical tracer of the Galactic potential in CDM cosmogonies. Models with warm dark matter (WDM) or collisional fluid dark matter (FDM) produce dwarf haloes with lower central densities than CDM and would be consistent with the observed tidal tails even for orbits with eccentricities as low as indicated by current data on nearby dwarf spheroidals. Galactic haloes in models with FDM are expected to be smooth and spherical and would be favoured by the detection of cold coherent streams such as that associated with the Sagittarius dwarf spheroidal. In this respect the distribution of stellar streams on the plane of the sky can provide a novel test of the nature of the dark matter. On the other end, the streams could be similarly dispersed in lumpy CDM haloes and in smoother, triaxial WDM haloes.

Stationary dark energy with a baryon-dominated era: Solving the coincidence problem with a linear coupling

We show that all cosmological models with an accelerated stationary global attractor reduce asymptotically to a dark energy field with an exponential potential coupled linearly to a perfect fluid dark matter. In such models the abundance of the dark components reaches a stationary value and therefore the problem of their present coincidence is solved. The requirement of a vanishing coupling of the baryons in order to pass local gravity experiments induces the existence of an intermediate baryon-dominated era. We discuss in detail the properties of these models and show that to accommodate standard nucleosynthesis they cannot produce a microwave background consistent with observations. We conclude that, among stationary models, only a time-dependent coupling or equation of state might provide a realistic cosmology.

Interacting quintessence

We demonstrate that a suitable coupling between a quintessence scalar field and a pressureless cold dark matter (CDM) fluid leads to a constant ratio of the energy densities of both components which is compatible with an accelerated expansion of the Universe.

First-principles study on the neutral-to-ionic phase transition in charge-transfer complex tetrathiafulvalene-p-chloranil

We extend the simplex-in-cell (SIC) technique recently introduced in the context of collisionless dark matter fluids [1], [2] to the case of collisionless plasmas. The six-dimensional phase space distribution function f(x,v) is represented by an ensemble of three-dimensional manifolds, which we refer to as sheets. The electric potential field is obtained by solving the Poisson equation on a uniform mesh, where the charge density is evaluated by a spatial projection of the phase space sheets. The SIC representation of phase space density facilitates robust, high accuracy numerical evolution of the Vlasov–Poisson system using significantly fewer tracer particles than comparable particle-in-cell (PIC) approaches by reducing the numerical shot-noise associated with the latter. We introduce the SIC formulation and describe its implementation in a new code, which we validate using standard test problems including plasma oscillations, Landau damping, and two stream instabilities in one dimension. Merits of the new scheme are shown to include higher accuracy and faster convergence rates in the number of particles. We finally motivate and outline the efficient application of SIC to higher dimensional problems.

Primordial Non‐Gaussianity: Baryon Bias and Gravitational Collapse of Cosmic String Wakes

I compute the three-dimensional nonlinear evolution of gas and dark matter fluids in the neighborhood of cosmic string wakes that are formed at high redshift (z 2240) for a realistic scenario of wake formation. These wakes are the ones that stand out most prominently as cosmological sheets and are expected to play a dominant role in the cosmic string model of structure formation. Employing a high-resolution three-dimensional hydrodynamic code to evolve these wakes until the present day yields results for the baryon bias generated in the inner wake region. I find that today, wakes would be 1.5 h-1 Mpc thick and contain a 70% excess in the density of baryons over the dark matter density in their centers. However, high-density peaks in the wake region do not inherit a baryon enhancement. I propose a mechanism for this erasure of the baryon excess in spherically collapsed objects based on the geometry change around the collapsing region. Furthermore, I present heuristic arguments for the consequences of this work for large-scale structure in the cosmic string model and conclude that the peculiarities of wake formation are unlikely to have significant import for the discrepancy between power spectrum predictions and observations in this model. If one invokes the nucleosynthesis bound on Ωb, this could be seen as strengthening the case against Ωm = 1 or for low Hubble constants.

Generalized dilaton couplings to dark matter

Abstract Inspired by recent models of inflation within the Jordan-Brans-Dicke theory, namely the “generalized extended inflation” models (see Holman, Kolb, Vadas and Wang [Phys. Lett. B 250 (1990) 24; Phys. Rev. D 43 (1991) 995]), we propose generalized couplings of a dark matter component to the Jordan-Brans-Dicke, or dilaton , field. Assuming the dark matter component to be dominant today we use observational data on the rate of change of Newton's constant, as well as on its value during primordial nucleosynthesis, on the age of the Universe and on the present value of the Hubble constant to put limits on these couplings. Our results extended previous ones by Damour, Gibbons and Gundlach [Phys. Rev. Lett. 64 (1990) 123]. Unlike all previously considered models, the Newton constant would change with time even if the universe were dominated by an invisible radiation-like component (i.e., whose energy-momentum tensor has vanishing trace). Using a lagrangian approach we propose a natural coupling of the dilaton field to a perfect fluid dark matter component.

Inflationary cosmological universe containing dark matter and many charged galaxies

Within the framework of Einstein's theory, cosmological universes are considered that contain three types of "fluids." A neutral cosmological fluid (dark matter), which is present everywhere, determines the overall time evolution of the universe. The second type consists of charged matter that constitutes the cores of galaxies. The electromagnetic fields generated by the charged matter make up the third kind of fluid, which is evidently null. An exact cosmological solution is furnished that provides for an early inflationary period and contains many charged black holes as galactic cores.