An alternative to the cosmological 'concordance model'
Résumé
Precision measurements of the cosmic microwave background byWMAP are believed to have established a flat $\Lambda$-dominateduniverse, seeded by nearly scale-invariant adiabatic primordialfluctuations. However by relaxing the hypothesis that the fluctuationspectrum can be described by a single power law, we demonstrate thatan Einstein-de Sitter universe with {\em zero} cosmological constantcan fit the data as well as the best concordance model. Moreoverunlike a $\Lambda$-dominated universe, such an universe has no strongintegrated Sachs-Wolfe effect, so is in better agreement with the lowquadrupole seen by WMAP. The main problem is that the Hubble constantis required to be rather low: $H_0 \simeq 46$~km/s/Mpc; we discusswhether this can be consistent with observations. Furthermore foruniverses consisting only of baryons and cold dark matter, theamplitude of matter fluctuations on cluster scales is too high, aproblem which seems generic. However, an additional small contribution($\Omega_X \sim 0.1$) of matter which does not cluster on smallscales, e.g. relic neutrinos with mass of order eV or a `quintessence'with $w \sim 0$, can alleviate this problem. Such models provide asatisfying description of the power spectrum derived from the 2dFgalaxy redshift survey and from observations of the Ly-$\alpha$forest. We conclude that Einstein-de Sitter models can indeedaccommodate all data on the large scale structure of the Universe,hence the Hubble diagram of distant Type Ia supernovae remains theonly {\em direct} evidence for a non-zero cosmological constant.
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