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 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 ~ 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=0, can alleviate this problem. Such modelsprovide a satisfying description of the power spectrum derived fromthe 2dF galaxy redshift survey and from observations of theLy-alpha forest. We conclude that Einstein-de Sitter models canindeed accommodate all data on the large scale structure of theUniverse, hence the Hubble diagram of distant Type Ia supernovaeremains the only direct evidence for a non-zero cosmologicalconstant.