Cosmic mimicry: is LCDM a braneworld in disguise?

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For a broad range of parameter values, braneworld models display a remarkable property whichwe call cosmic mimicry. Cosmic mimicry is characterized by the fact that, at low redshifts, theHubble 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 theΩm parameters in the braneworld model and in the LCDM cosmology can neverthelessbe quite different. Thus, at high redshifts (early times), the braneworldasymptotically expands like a matter-dominated universe with the value ofΩm inferred from the observations of the local matter density. At low redshifts(late times), the braneworld model behaves almost exactly like the LCDMmodel but with a renormalized value of the cosmological density parameterΩmLCDM. The valueof ΩmLCDM is smaller(larger) than Ωm in the braneworld model with positive (negative) brane tension. The redshift which characterizescosmic mimicry is related to the parameters in the higher-dimensional braneworldLagrangian. Cosmic mimicry is a natural consequence of the scale dependence of gravity inbraneworld models. The change in the value of the cosmological density parameter (fromΩm athigh z to ΩmLCDM atlow z) is shown to be related to the spatial dependence of the effective gravitational constantGeff in braneworld theory. A subclass of mimicry models lead to an older age of the universeand also predict a redshift of reionization which is lower than in the LCDM cosmology. These models might therefore provide a background cosmologywhich is in better agreement both with the observed quasar abundance at and with the large optical depth to reionization measured by the Wilkinson microwaveanisotropy probe.