Cosmic mimicry: is LCDM a Braneworld in disguise?

Abstract

For a broad range of parameter values, braneworld models display a remarkable property which we call cosmic mimicry. Cosmic mimicry is characterized by the fact that, at low redshifts, the Hubble parameter in the braneworld model is virtually indistinguishable from that in the LCDM (Λ+cold dark matter) cosmology. An important point to note is that the Ωm parameters in the braneworld model and in the LCDM cosmology can nevertheless be quite different. Thus, at high redshifts (early times), the braneworld asymptotically expands like a matter-dominated universe with the value of Ω_m inferred from the observations of the local matter density. At low redshifts (late times), the braneworld model behaves almost exactly like the LCDM model but with a renormalized value of the cosmological density parameter Ω_m^LCDM. The value of Ω_m^LCDM is smaller (larger) than Ω_m in the braneworld model with positive (negative) brane tension. The redshift which characterizes cosmic mimicry is related to the parameters in the higher-dimensional braneworld Lagrangian. Cosmic mimicry is a natural consequence of the scale dependence of gravity in braneworld models. The change in the value of the cosmological density parameter (from Ω_m at high z to Ω_m^LCDM at low z) is shown to be related to the spatial dependence of the effective gravitational constant G_eff in braneworld theory. A subclass of mimicry models lead to an older age of the universe and also predict a redshift of reionization which is lower than z_reion ⋍17 in the LCDM cosmology. These models might therefore provide a background cosmology which is in better agreement both with the observed quasar abundance at z≳ 4 and with the large optical depth to reionization measured by the Wilkinson microwave anisotropy probe.