Kavli Affiliate: W. L. Kimmy Wu

| First 5 Authors: Jannis Bielefeld, W. L. Kimmy Wu, Robert R. Caldwell, Olivier Dore,

| Summary:

A phenomenological model of dark energy that tracks the baryonic and cold

dark matter at early times but resembles a cosmological constant at late times

is explored. In the transition between these two regimes, the dark energy

density drops rapidly as if it were a relic species that freezes out, during

which time the equation of state peaks at +1. Such an adjustment in the dark

energy density, as it shifts from scaling to potential-domination, could be the

signature of a trigger mechanism that helps explain the late-time cosmic

acceleration. We show that the non-negligible dark energy density at early

times, and the subsequent peak in the equation of state at the transition,

leave an imprint on the cosmic microwave background anisotropy pattern and the

rate of growth of large scale structure. The model introduces two new

parameters, consisting of the present-day equation of state and the redshift of

the freeze-out transition. A Monte Carlo Markov Chain analysis of a

ten-dimensional parameter space is performed to compare the model with

pre-Planck cosmic microwave background, large scale structure and supernova

data and measurements of the Hubble constant. We find that the transition

described by this model could have taken place as late as a redshift z~400. We

explore the capability of future cosmic microwave background and weak lensing

experiments to put tighter constraints on this model. The viability of this

model may suggest new directions in dark-energy model building that address the

coincidence problem.

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