A fluid os strings as a viable candidate to the dark side of the universe

Abstract

We investigate the possibility that part of the dark matter is not made out of the usual cold dark matter (CDM) dustlike particles, but is under the form of a fluid of strings with barotropic factor $w_s = -1/3$ of cosmic origin. To this aim, we split the dark matter density parameter in two terms and investigate the dynamics of a spatially flat universe filled with baryons, CDM, fluid of strings and dark energy, modeling this latter as a cosmological constant or a negative pressure fluid with a constant equation of state $w < 0$. To test the viability of the models and to constrain their parameters, we use the Type Ia Supernovae Hubble diagram and the data on the gas mass fraction in galaxy clusters. We also discuss the weak field limit of a model comprising a significant fraction of dark matter in the form of a fluid of strings and show that this mechanism makes it possible to reduce the need for the elusive and up to now undetected CDM. We finally find that a model comprising both a cosmological constant and a fluid of strings fits very well the data and eliminates the need of phantom dark energy thus representing a viable candidate to alleviate some of the problems plaguing the dark side of the universe.