firstname = "Artyom"
lastname = "Astashenok"
email = "artyom.art@gmail.com"
affiliation = "Baltic Federal University"
city = "Kaliningrad"
country = "Russia"
passportname = ""
birthday = ""
citizenship = ""
passportnumber = ""
passportissued = ""
passportexpire = ""
workplace = ""
workaddress = ""
visadates = ""
talktitle = "Confronting dark energy models mimicking $\Lambda$CDM epoch with observational constraints"
section = "C - Gravitation and cosmology"
talkabstract = "We confront dark energy models which are currently similar to $\Lambda$CDM theory with observational data which include the SNe data, matter density perturbations and baryon acoustic oscillations data. 

The combined data analysis proves that DE models under consideration are as consistent as $\Lambda$CDM model. We demonstrate that growth of matter density perturbations may occur at sufficiently small background density but still before the possible disintegration of bound objects (like clusters of galaxies, galaxies, etc) in Big Rip, type III singularity, Little Rip or Pseudo-Rip universe.

One can rewrite the dark energy EoS (\ref{EoS-0}) in the following form:
$p_{D}=-\rho_{D}-g(\rho_{D})\,$
where $g(\rho_{D})$ is some function.

We study the following two class of models. The natural choice is
$g(\rho_{D})=\alpha^{2}\rho_{D0}\left(\frac{\rho_{D}}{\rho_{D0}}\right)^{\beta}$
where $\alpha$ and $\beta$ are dimensionless constants. If $\beta=1$ we have ordinary phantom energy model with constant EoS parameter $w=-1-\alpha^{2}$. For various $\beta$ the model describes three types of future universe evolution:

(a) Little Rip if $\beta\leq 1/2$,

(b) Big Rip if $1/2<\beta\leq 1$ and

(c) type III singularity if $\beta>1$.

The another interesting class of DE models is given by the equation
$g(\rho)=\alpha^{2}\rho_{D0}\left(1-\frac{\rho_{D}}{\rho_{f}}\right)^{\beta}, \quad 0<\rho_{D}<\rho_{f},\quad \beta \neq 0.$
Such dark fluid leads to the following variants of evolution:

(i) DE energy-density asymptotically tends to $\rho_{f}$ if $\beta>1/2$. Therefore, the universe expands  according to de Sitter law at $t\rightarrow\infty$ (Pseudo-Rip).

(ii)DE energy-density reaches $\rho_{f}$ for $t_{f}<\infty$ if $0<\beta\leq 1/2$.

(iii) type II singularity occurs if $\beta<0$. The second derivative of scale factor diverges while first derivative remains finite. 

The analysis of observational data shows that  there exists sufficiently wide region of parameters for each of DE model under discussion where these theories are not less viable than the standard $\Lambda$CDM model.

On the same time, DE models under consideration show qualitatively different future behaviour with Big Rip, type II and type III future singularities, Little Rip, Pseudo-Rip or Quasi-Rip evolution.
Nevertheless, current observational data cannot determine whether or not the universe will end in a future singularity.

It should be noted that the observational data for density perturbations are more sensitive to the deviation from standard
cosmological model in comparison with SNe data. Moreover, the account of density perturbations in Rip cosmology indicates to sharp growth of density at sufficiently small background density still before the possible disintegration of bound objects. This growth from our viewpoint can lead to possibility that future universe may split in the number of separate regions so that it becomes chaotic and never reaches the Rip singularity. Further consequences of above effect will be considered elsewhere.

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