Freezing out early dark energy

Abstract

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.

Dark energy scaling from dark matter to acceleration

“…5, in our MCMC fitting of the model to data we will only consider the range log a t ∈ [−3.3, −2.0]. We do not extend the range to later times since the growth of cosmic structure rather than the CMB will impose the main constraints there, and the trend is already clear as discussed later in this section; furthermore similar late-time constraints were shown by [10] for another transition model. Given that large τ moves the model closer and closer to ΛCDM, we only consider the range τ ∈ [0.33, 1.2].…”

“…In order for this to work, there must be a period where the dark energy density rapidly declines relative to the matter -what [10] called freeze out behavior -in order to satisfy both CMB constraints on the dark matter density and satisfy the growth of structure constraints during the matter dominated era. Essentially this means that the dark energy must gain a positive equation of state, such as during kination, a period of kinetic dominated dynamics where w = +1.…”

“…Due to the rapidity of the transition, if the transition takes place sufficiently before recombination then the dark energy does not affect the dark matter component at recombination, and the CMB power spectra should show little effect. (Note that [10] considered transitions after recombination to avoid disturbing the CMB power spectra.) Nevertheless for the earlier history of the universe there can be a tie between the components of the dark sector, possibly alleviating the coincidence problem.…”

“…Moreover, so as not to disagree with formation of galaxies and clusters, the dark energy must quickly fade away from the early universe into the matter dominated era where structure grows. Recently, [10] investigated a model where such a transition occurred after recombination. However, they kept the fluctuation sound speed in the dark energy to be the speed of light, reducing the effect of perturbations, and adopted a purely phenomenological model for the density evolution.…”

Early dark energy and its interaction with dark matter

“…EDE models can potentially alleviate the coincidence problem. Furthermore, they can influence the cosmic microwave background [3][4][5][6][7][8][9], big-bang nucleosynthesis [10] and large-scale structure formation [11][12][13][14][17][18][19]. For now, it would be fair to say that there are no strong observational constraints on the EDE models, and it is especially difficult to discriminate EDE models which have w = −1 at present from the ΛCDM model.…”