An alternative to the cosmological “concordance model”

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

Corresponding author: A. Blanchard, Alain.Blanchard@ast.obs-mip.fr

Received: 14 April 2003
Accepted: 30 July 2003

Abstract

Precision measurements of the cosmic microwave background by WMAP are believed to have established a flat Λ-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 Λ-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:  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 () of matter which does not cluster on small scales, e.g. relic neutrinos with mass of order eV or a “quintessence” with , 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-α 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.