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Register a new userVariable sound speed in interacting dark energy models
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We consider a self-consistent and physical approach to interacting dark energy models described by a Lagrangian, and identify a new class of models with variable dark energy sound speed. We show that if the interaction between dark energy in the form of quintessence and cold dark matter is purely momentum exchange this generally leads to a dark energy sound speed that deviates from unity. Choosing a specific sub-case, we study its phenomenology by investigating the effects of the interaction on the cosmic microwave background and linear matter power spectrum. We also perform a global fitting of cosmological parameters using CMB data, and compare our findings to $Lambda$CDM.
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The possibility of reconstruction of Lagrangian for the scalar field dark energy with constant effective sound speed $c_s$ is analyzed. It is found that such reconstruction can be made with accuracy up to an arbitrary constant. The value of $c_s$ is estimated together with other dark energy parameters ($Omega_{de}$, $w_0$, $c_a^2$) and main cosmological ones on the basis of data including Planck-2013 results on CMB anisotropy, BAO distance ratios from recent galaxy surveys, galaxy power spectrum from WiggleZ, magnitude-redshift relations for distant SNe Ia from SNLS3 and Union2.1 compilations, the HST determination of the Hubble constant. It is shown that no value of $c_s$ from the range [0,1] is preferred by the used data because of very weak influence of dark energy perturbations on the large scale structure formation and CMB temperature fluctuations.
We consider two models of interacting dark energy, both of which interact only through momentum exchange. One is a phenomenological one-parameter extension to $w$CDM, and the other is a coupled quintessence model described by a Lagrangian formalism. Using a variety of high and low redshift data sets, we perform a global fitting of cosmological parameters and compare to $Lambda$CDM, uncoupled quintessence, and $w$CDM. We find that the models are competitive with $Lambda$CDM, even obtaining a better fit when certain data sets are included.
We study cosmological models with interaction between dark energy (DE) and dark matter (DM). For the interaction term $Q$ in cosmic evolution equations, there is a model-independent degeneracy-breaking (D-B) point when $Q_{1}$ (a part of $Q$) equals to zero, where the interaction can be probed without degeneracy between the constant DE equation of state (EoS).
Recent measurements of the Cosmic Microwave Anisotropies power spectra measured by the Planck satellite show a preference for a closed universe at more than $99 %$ Confidence Level. Such a scenario is however in disagreement with several low redshift observables, including luminosity distances of Type Ia Supernovae. Here we show that Interacting Dark Energy (IDE) models can ease the discrepancies between Planck and Supernovae Ia data in a closed Universe. Therefore IDE cosmologies remain as very appealing scenarios, as they can provide the solution to a number of observational tensions in different fiducial cosmologies. The results presented here strongly favour broader analyses of cosmological data, and suggest that relaxing the usual flatness and vacuum energy assumptions can lead to a much better agreement among theory and observations.
Within the framework of DBI non-canonical scalar field model of dark energy, we study the growth of dark matter perturbations in the both linear and non-linear regimes. In our DBI model, we consider the anti-de Sitter warp factor $f(phi)=f_0, phi^{-4}$ with constant $f_0>0$ and assume the DBI dark energy to be clustered and its sound speed $c_s$ to be constant. For a spatially flat FRW universe filled with pressureless dark matter and DBI dark energy, we first obtain the evolutionary behaviors of the background quantities. Our results show that in our DBI model, the universe starts from a matter dominated epoch and approaches to the de Sitter universe at late times, as expected. Also the DBI potential behaves like the power law one $V(phi)propto phi^n$. In addition, we use the Pseudo-Newtonian formalism to obtain the growth factor of dark matter perturbations in the linear regime. We conclude that for smaller $c_s$ (or $f_0$), the growth factor of dark matter is smaller for clustering DBI model compared to the homogeneous one. In the following, in the non-linear regime based on the spherical collapse model, we obtain the linear overdensity $delta_c(z_c)$, the virial overdensity $Delta_{rm vir}(z_c)$, overdensity at the turn around $zeta(z_c)$ and the rate of expansion of collapsed region $h_{rm ta}(z)$. We point out that for the smaller $c_s$ (or $tilde{f}_0$), the values of $delta_c(z_c)$, $Delta_{rm vir}(z_c)$, $zeta(z_c)$ and $h_{rm ta}(z)$ in non-clustering DBI models deviate more than the $Lambda$CDM compared to the clustering DBI. Finally, with the help of spherical collapse parameters we calculated the relative number density of halo objects above a given mass and conclude that the differences between clustering and homogeneous DBI models are more pronounced for higher-mass halos at high redshift.
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