Number counts in homogeneous and inhomogeneous dark energy models

Free access article

Issue		A&A Volume 450, Number 3, May II 2006


Page(s)		899 - 907
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361:20053706

A&A 450, 899-907 (2006)
DOI: 10.1051/0004-6361:20053706

Number counts in homogeneous and inhomogeneous dark energy models

N. J. Nunes¹, A. C. da Silva^{2, 3} and N. Aghanim²

¹ School of Physics and Astronomy, 116 Church Street S.E., University of Minnesota, Minneapolis, Minnesota 55455, USA
e-mail: nunes@physics.umn.edu
² IAS - CNRS, Bâtiment 121, Université Paris-Sud, 91405 Orsay, France
³ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-764 Porto, Portugal

(Received 27 June 2005 / Accepted 6 October 2005)

Abstract
In the simple case of a constant equation of state, redshift distribution of collapsed structures may constrain dark energy models. Different dark energy models having the same energy density today but different equations of state give quite different number counts. Moreover, we show that introducing the possibility that dark energy collapses with dark matter ("inhomogeneous" dark energy) significantly complicates the picture. We illustrate our results by comparing four dark energy models to the standard $\Lambda$ -model. We investigate a model with a constant equation of state equal to -0.8, a phantom energy model and two scalar potentials (built out of a combination of two exponential terms). Although their equations of state at present are almost indistinguishable from a $\Lambda$ -model, both scalar potentials undergo quite different evolutions at higher redshifts and give different number counts. We show that phantom dark energy induces opposite departures from the $\Lambda$ -model as compared with the other models considered here. Finally, we find that inhomogeneous dark energy enhances departures from the $\Lambda$ -model with maximum deviations of about 15% for both number counts and integrated number counts. Larger departures from the $\Lambda$ -model are obtained for massive structures which are rare objects making it difficult to statistically distinguish between models.

Key words: cosmology: cosmological parameters -- galaxies: clusters: general

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