Unified dark matter: constraints from galaxies and clusters

Abstract

Unified dark matter models are appealing in that they describe the dark sector in terms of a single component. They however face problems when attempting to account for structure formation: in the linear regime, density fluctuations can become Jeans stable and oscillate rather than collapse, though it is possible that this difficulty may be circumvented by invoking nonlinear clustering. Here we examine the behaviour in the fully nonlinear regime, of collapsed objects that should mimic standard dark matter haloes. It is shown that the pressure gradient associated with the unified dark matter fluid should be significant in the outer parts of galaxies and clusters, and its effects obervable. In this case, no flat or falling rotation curve is possible for any (barotropic) equation of state with associated sound speed decreasing with density (a necessary condition if the fluid is to behave as pressureless matter at high density). The associated density profile is therefore also incompatible with that inferred in the outer part of clusters. For the prototypical case of the generalised Chaplygin gas, it is shown that this limits the values of the equation of state index α that are compatible with observations to α 0.0001 or α 2. This is in line from what is deduced from linear analysis. More generally, from the expected properties of dark matter haloes, constraints on the sound speed are derived. For the particular case of the generalised Chaplygin gas, this further constrains the index to α 10 −9 or α 6.7. For a unified dark matter fluid to mimic dark halo properties, therefore, it needs to have an equation of state such that the pressure gradients are either minimal or which decrease fast enough so as to be negligible at densities characteristic of the outer parts of haloes.

On the concordance of cosmological data in the case of the generalized Chaplygin gas

“…Negative values for α are also found for a flat universe by including gamma-ray bursts data [24], however, the reliability of these data is strongly debated. To be compatible with the rotation curves of galaxies, dark matter haloes consisting of pure generalised Chaplygin gas should satisfy α 0.0001 [25].…”

“…This generated again a large interest in the Chaplygin cosmologies and its variants, see e. g. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Most papers assume a flat universe when the allowed parameter space is estimated according to various cosmological data.…”

Hamilton-Jacobi formalism for Generalized Chaplygin Gas models

“…It is also worth mentioning [36] which has pointed out the importance of including an independent baryon fluid in the analyses involving a GCG. After these first works, many other authors have discussed the viability of GCG models in a fully consistent cosmological setup [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]. While a GCG cosmology is slightly disfavoured with respect to standard ΛCDM, models featuring small values of α (i.e.…”

Chaplygin gas halos

“…Indeed it is possible to construct unified dark matter models (UDM) associating both dark components with a single unknown substance. Concrete examples of effective UDM theories are provided by models based on the equation of state of the generalized Chaplygin gas (gCg) [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32].…”

“…[24], to qualify as a dark-matter component as well it should be able to form dark-matter like halos in galaxies and galaxy clusters to explain the measured angular velocity distribution of stars in galaxies and the non-virial motion of galaxies in clusters. In this paper we address the question how to model spherical gCg halos and derive their short-and long-range properties to allow comparison with observational constraints on dark matter and dark energy; earlier work on these topics can be found in [26,28,29].…”