Cosmic mimicry: is LCDM a braneworld in disguise?

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 (Lambda+cold dark matter) cosmology. An important point to note is that the Omega(m) 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 Omega(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. Omega(LCDM)(m). The value of Omega(LCDM)(m) is smaller (larger) than Omega(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 Omega(m) at high z to Omega(LCDM)(m) 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)similar to 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 greater than or similar to 4 and with the large optical depth to reionization measured by the Wilkinson microwave anisotropy probe.