An alternative to the cosmological 'concordance model'

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 {\em 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 problem is that the Hubble constant is required to be rather low: $H_0 \simeq 46$ km/s/Mpc; 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 ($Ω_X \sim 0.1$) 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-$α$ 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 {\em direct} evidence for a non-zero cosmological constant.

Figures (6)

• Figure 1: The temperature power spectrum for the best-fit power-law $\Lambda$CDM model (dotted black line) from Spergel et al. (2003), and for our broken-power-law model with $\Omega_\Lambda = 0$ (solid blue line), compared to data from WMAP and other experiments at small scales --- CBI (cbi), ACBAR (acbar), BOOMERanG (boom2) and VSA (vsa). Note the linear scale in $l$ for $l > 200$.
• Figure 2: Velocity versus luminosity-distance for Type Ia supernovae (filled circles), S-Z clusters (open circles) and gravitational lens time-delay systems (filled triangles), with z $>$ 0.05. All curves shown correspond to flat models and are labelled with the Hubble parameter in km/s/Mpc.
• Figure 3: CMB angular power spectra for models with $\Omega_{\rm m} = 1$, $H_0 = 46$ km/s/Mpc and $\Omega_{\rm b} h^2 = 0.019$ normalised to $\sigma_8=1$ with different power law indices for the primordial fluctuations. Note the constancy of the amplitude around $\ell \sim 900$
• Figure 4: The temperature power spectrum for the best-fit power-law $\Lambda$CDM model (dotted black line) from Spergel et al. (2003), and for our broken-power-law models (both having $\Omega_\Lambda = 0$) with $\Omega_\nu = 0.12$ (dot-dashed blue line) and $\Omega_{\rm Q}=0.12$ (solid green line), compared to data from WMAP and other experiments (vsacbiacbarboom2).
• Figure 5: The temperature-polarization (TE) cross power-spectrum and the polarization (EE) power spectrum for our E-deS models with $\Omega_\nu = 0.12$ (dot-dashed blue line) and $\Omega_{\rm Q}=0.12$ (solid green line), both with $\tau = 0.1$, compared to the concordance $\Lambda$CDM model (dotted line). The thin lines are obtained adopting the higher optical depth ($\tau = 0.17$) suggested by the WMAP fit to the TE data (Kogut et al. 2003).