## Abstract

We study the spherical collapse model in the presence of quintessence with negligible speed of sound. This case is particularly motivated for w < -1 as it is required by stability. As pressure gradients are negligible, quintessence follows dark matter during the collapse. The spherical overdensity behaves as a separate closed FLRW universe, so that its evolution can be studied exactly. We derive the critical overdensity for collapse and we use the extended Press-Schechter theory to study how the clustering of quintessence affects the dark matter mass function. The effect is dominated by the modification of the linear dark matter growth function. A larger effect occurs on the total mass function, which includes the quintessence overdensities. Indeed, here quintessence constitutes a third component of virialized objects, together with baryons and dark matter, and contributes to the total halo mass by a fraction ∼ (1+w)Ω _{Q}/Ω _{m}. This gives a distinctive modification of the total mass function at low redshift.

Original language | English |
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Article number | 027 |

Journal | Journal of Cosmology and Astroparticle Physics |

Volume | 2010 |

Issue number | 3 |

DOIs | |

State | Published - 2010 |

Externally published | Yes |

## Keywords

- Cluster counts
- Dark energy theory