EDP Sciences
Free access
Volume 412, Number 1, December II 2003
Page(s) 35 - 44
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20031425

A&A 412, 35-44 (2003)
DOI: 10.1051/0004-6361:20031425

An alternative to the cosmological "concordance model"

A. Blanchard1, M. Douspis2, M. Rowan-Robinson3 and S. Sarkar4

1  Laboratoire d'Astrophysique de l'Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France
2  Astrophysics, University of Oxford, Dennis Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
3  Astrophysics Group, Imperial College, Blackett Laboratory, Prince Consort Road, London SW7 2BW, UK
4  Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, UK

(Received 14 April 2003 / Accepted 30 July 2003)

Precision measurements of the cosmic microwave background by WMAP are believed to have established a flat $\Lambda$-dominated universe, seeded by nearly scale-invariant adiabatic primordial fluctuations. However by relaxing the hypothesis that the fluctuation spectrum can be described by a single power law, we demonstrate that an Einstein-de Sitter universe with zero cosmological constant can fit the data as well as the best concordance model. Moreover unlike a $\Lambda$-dominated universe, such an universe has no strong integrated Sachs-Wolfe effect, so is in better agreement with the low quadrupole seen by WMAP. The main concern is that the Hubble constant is required to be rather low: $H_0 \simeq 46$ km s -1 Mpc -1; we discuss whether this can be consistent with observations. Furthermore for universes consisting only of baryons and cold dark matter, the amplitude of matter fluctuations on cluster scales is too high, a problem which seems generic. However, an additional small contribution ( $\Omega_{\rm X} \sim 0.1$) of matter which does not cluster on small scales, e.g. relic neutrinos with mass of order eV or a "quintessence" with $w \sim 0$, can alleviate this problem. Such models provide a satisfying description of the power spectrum derived from the 2dF galaxy redshift survey and from observations of the Ly- $\alpha$ forest. We conclude that Einstein-de Sitter models can indeed accommodate all data on the large scale structure of the Universe, hence the Hubble diagram of distant type Ia supernovae remains the only direct evidence for a non-zero cosmological constant.

Key words: cosmology: theory -- cosmology: cosmic microwave background -- cosmology: large scale structure of Universe -- cosmology: cosmological parameters

Offprint request: A. Blanchard, Alain.Blanchard@ast.obs-mip.fr

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