Constraining late-time transitions in the dark energy equation of state

Abstract

One of the most compelling goals of observational cosmology is the characterisation of the properties of the dark energy component thought to be responsible for the recent acceleration of the universe, including its possible dynamics. In this work we study phenomenological but physically motivated classes of models in which the dark energy equation of state can undergo a rapid transition at low redshifts, perhaps associated with the onset of the acceleration phase itself. Through a standard statistical analysis we have used low-redshift cosmological data, coming from Type Ia supernova and Hubble parameter measurements, to set constraints on the steepness of these possible transitions as well as on the present-day values of the dark energy equation of state and in the asymptotic past in these models. We have also studied the way in which these constraints depend on the specific parametrisation being used. Our results confirm that such late-time transitions are strongly constrained. If one demands a matter-like pre-transition behaviour, then the transition is constrained to occur at high redshifts (effectively in the matter era), while if the pre-transition equation of state is a free parameter then it is constrained to be close to that of a cosmological constant. In any case, the value of dark energy equation of state near the present day must also be very similar to that of a cosmological constant. The overall conclusion is that any significant deviations from this behaviour can only occur in the deep matter era, so there is no evidence for a transition associated with the onset of acceleration. Observational tools capable of probing the dynamics of the universe in the deep matter era are therefore particularly important.

Fine-structure constant constraints on late-time dark energy transitions

“…Several phenomenological parametrisations of the dark energy equation of state have been proposed and studied, which allow for a fast transition, for example [21,22,23,24,25,26]; these were compared in [5] and found to be qualitatively similar. In what follows we will use that of Linder & Huterer [23], since it leads to an analytic form for the Friedmann equation (which somewhat simplifies the statistical analysis).…”

“…(3) We start by revisiting [5] and constrain this model using low-redshift background cosmology data, specifically the recent catalogue of Type Ia Pantheon supernovas [12] (while [5] used the earlier Union2.1 catalogue [6]) together with the compilation of measurements of the Hubble parameter by Farooq et al [7]. The Hubble constant is always analytically marginalized, following the procedure detailed in [27].…”

“…the resulting posterior, due to the effect of prior volume weighting.) To facilitate the comparison with our earlier work [5], we fix the present-day value of the matter density to be Ω m = 0.3, in agreement with both CMB and low-redshift data [14,12,28]. Note that one of our goals is to determine how well these models can be constrained by lowredshift data alone, and that the inclusion of the astrophysical data is not expected to significantly affect the preferred value of Ω m [15].…”

“…As a further check of the impact of our assumptions on the final results, we also repeat the analysis for two more specific cases of this parametrisation. The first particular case corresponds to fixing w i = 0; this will ensure a transition (from w = 0 to about w = −1), which the data will push to high redshifts [5], though note that this choice implies that at early times we will have an additional matterlike component. In other words, such models have a non-zero amount of early dark energy [29], which is constrained by Cosmic Microwave Background experiments such as Planck to sub-percent level [4].…”

“…First, we use the latest Pantheon supernova sample [12] which spans a larger redshift range. Second, the presentday value of the Hubble parameter is always analytically marginalized; while the analysis in [5] suggests that keeping it fixed has a relatively small impact, marginalization is a safer option given the ongoing controversy regarding its value [13,14]. And thirdly (and most significantly), we include in the analysis recent astrophysical and local tests of the stability of the fine-structure constant α-see [15] for a recent review.…”