Gravitational Lensing Limits on Cold Dark Matter and Its Variants

Abstract

Standard $Ω_0=1$ cold dark matter (CDM) needs $0.27 < σ_8 < 0.63$ ($2σ$) to fit the observed number of large separation lenses, and the constraint is nearly independent of $H_0=100h^{-1}\kms$ Mpc$^{-1}$. This range is strongly inconsistent with the COBE estimate of $σ_8=(2.8\pm0.2)h$. Tilting the primordial spectrum $\propto k^n$ from $n=1$ to $0.3 \ltorder n \ltorder 0.7$, using an effective Hubble constant of $0.15 \ltorder Γ=h \ltorder 0.30$, or reducing the matter density to $0.15 \ltorder Ω_0 h \ltorder 0.3$ either with no cosmological constant ($λ_0=0$) or in a flat universe with a cosmological constant ($Ω_0+λ_0=1$) can bring the lensing estimate of $σ_8$ into agreement with the COBE estimates. The models and values for $σ_8$ consistent with both lensing and COBE match the estimates from the local number density of clusters and correlation functions. The conclusions are insensitive to systematic errors except for the assumption that cluster core radii are singular. If clusters with $ρ\propto(r^2+s^2)^{-1}$ have core radii exceeding $s = 15h^{-1}σ_3^2$ kpc for a cluster with velocity dispersion $σ=10^3σ_3 \kms$ then the estimates are invalid. There is, however, a fine tuning problem in making the cluster core radii large enough to invalidate the estimates of $σ_8$ while producing several lenses that do not have central or ``odd images.'' The estimated completeness of the current samples of lenses larger than $5\parcs0$ is 20\%, because neither quasar surveys nor lens surveys are optimized to this class of lenses.