Analytic study of cosmological perturbations in a unified model of dark matter and dark energy with a sharp transition

Abstract

We study cosmological perturbations in a model of unified dark matter and dark energy with a sharp transition in the late-time universe. The dark sector is described by a dark fluid which evolves from an early stage at redshifts z > zC when it behaves as cold dark matter (CDM) to a late time dark energy (DE) phase (z < zC ) when the equation of state parameter is w = −1 + , with a constant which must be in the range 0 < < 2/3. We show that fluctuations in the dark energy phase suffer from an exponential instability, the mode functions growing both as a function of comoving momentum k and of conformal time η. In order that this exponential instability does not lead to distortions of the energy density power spectrum on scales for which we have good observational results, the redshift zC of transition between the two phases is constrained to be so close to zero that the model is unable to explain the supernova data.

New scalar field quartessence

“…This leads to an instability in the growth of density fluctuations in the Dark-Energy phase (see e.g. [10] for a review, and [44] for a recent study in a particular model) . In order to be consistent with data on the power spectrum of large-scale structure and the cosmic microwave background anisotropies, the time at the onset of Dark Energy domination must be so close to the present time that the models cannot explain the supernova data [44].…”

“…[10] for a review, and [44] for a recent study in a particular model) . In order to be consistent with data on the power spectrum of large-scale structure and the cosmic microwave background anisotropies, the time at the onset of Dark Energy domination must be so close to the present time that the models cannot explain the supernova data [44]. This problem can be avoided in generalised fluid models, where c 2 s = w and where c 2 s is tuned to be close to zero in the Dark-Energy phase.…”

Unified Dark Matter, Dark Energy and baryogenesis via a “cosmological wetting transition”

“…3 In voids, the expectation value of χ 2 redshifts according to the law in Eq. (11), and condition (13) becomes…”

“…The field ϕ might actually be of importance in yet another context: If one couples the gradient ∂ µ ϕ linearly to the baryon current, j µ B , (thus violating CP) then, at early times (before ϕ starts to oscillate around ϕ 0 ),φ has the function of a "chemical potential" tuning the matterantimatter asymmetry during baryogenesis [4]; see also [5]. 2 The model discussed in this paper has various advantages over other models of unified Dark Matter and Dark Energy; (see [9] for a review of early work on unified models): Since the square of the speed of sound, c 2 s , is negligibly small at early times (t < t c ), structure formation proceeds as in the standard ΛCDM model up to the time t c when the phase transition takes place, and since, after the transition, c 2 s = 1 in the bulk, no dangerous instabilities for fluctuations at late times arise, as they do in certain quartessence models [10]; (see the discussion in [11]).…”

f(R,∇μ1R,…,∇Cuzinatto1, Melo2, Medeiros3 et al. 2019Phys. Rev. D16040View full textAdd to dashboardCite

“…Dark energy hints that general relativity may not be the final theory of the gravitational interaction -although it is possible to explain it via a cosmological constant [17] or exotic matter components. The last solution is the so-called modified matter approach [18] whose particular models are: quintessence [19][20][21], k-essence [22,23], and unified models of dark matter and dark energy [24][25][26][27][28][29][30]. Dark energy could also be explained by the modified gravity approach, i.e.…”