In the context of Finsler-Randers theory we consider, for a first time, the cosmological scenario of the varying vacuum. In particular, we assume the existence of a cosmological fluid source described by an ideal fluid and the varying vacuum terms. W
e determine the cosmological history of this model by performing a detailed study on the dynamics of the field equations. We determine the limit of General Relativity, while we find new eras in the cosmological history provided by the geometrodynamical terms provided by the Finsler-Randers theory.
We investigate the dynamical features of a large family of running vacuum cosmologies for which $Lambda$ evolves as a polynomial in the Hubble parameter. Specifically, using the critical point analysis we study the existence and the stability of sing
ular solutions which describe de-Sitter, radiation and matter dominated eras. We find several classes of $Lambda(H)$ cosmologies for which new analytical solutions are given in terms of Laurent expansions. Finally, we show that the Milne universe and the $R_{h}=ct$ model can be seen as perturbations around a specific $Lambda(H)$ model, but this model is unstable.
We study for the first time the dynamical properties and the growth index of linear matter perturbations of the Finsler-Randers (FR) cosmological model, for which we consider that the cosmic fluid contains matter, radiation and a scalar field. Initia
lly, for various FR scenarios we implement a critical point analysis and we find solutions which provide cosmic acceleration and under certain circumstances we can have de-Sitter points as stable late-time attractors. Then we derive the growth index of matter fluctuations in various Finsler-Randers cosmologies. Considering cold dark matter and neglecting the scalar field component from the perturbation analysis we find that the asymptotic value of the growth index is $gamma_{infty}^{(FR)}approxfrac {9}{16}$, which is close to that of the concordance $Lambda$ cosmology, $gamma^{(Lambda)} approxfrac{6}{11}$. In this context, we show that the current FR model provides the same Hubble expansion with that of Dvali, Gabadadze and Porrati (DGP) gravity model. However, the two models can be distinguished at the perturbation level since the growth index of FR model is $sim18.2%$ lower than that of the DPG gravity $gamma^{(DGP)} approx frac{11}{16}$. If we allow pressure in the matter fluid then we obtain $gamma_{infty}^{(FR)}approxfrac{9(1+w_{m})(1+2w_{m})}{2[8+3w_{m}% (5+3w_{m})]}$, where $w_{m}$ is the matter equation of state parameter. Finally, we extend the growth index analysis by using the scalar field and we find that the evolution of the growth index in FR cosmologies is affected by the presence of scalar field.
In the context of generalised Brans-Dicke cosmology we use the Killing tensors of the minisuperspace in order to determine the unspecified potential of a scalar-tensor gravity theory. Specifically, based on the existence of contact symmetries of the
field equations, we find four types of potentials which provide exactly integrable dynamical systems. We investigate the dynamical properties of these potentials by using a critical point analysis and we find solutions which lead to cosmic acceleration and under specific conditions we can have de-Sitter points as stable late-time attractors.
The rate of structure formation in the Universe is different in homogeneous and clustered dark energy models. The degree of dark energy clustering depends on the magnitude of its effective sound speed $c^{2}_{rm eff}$ and for $c_{rm eff}=0$ dark ener
gy clusters in a similar fashion to dark matter while for $c_{rm eff}=1$ it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, $w_{rm d}=const$ and $w_{rm d}=w_0+w_1left(frac{z}{1+z}right)$ with $c_{rm eff}$ as a free parameter and we try to constrain the dark energy effective sound speed using current available data including SnIa, Baryon Acoustic Oscillation, CMB shift parameter ({em Planck} and {em WMAP}), Hubble parameter, Big Bang Nucleosynthesis and the growth rate of structures $fsigma_{8}(z)$. At first we derive the most general form of the equations governing dark matter and dark energy clustering under the assumption that $c_{rm eff}=const$. Finally, performing an overall likelihood analysis we find that the likelihood function peaks at $c_{rm eff}=0$, however the dark energy sound speed is degenerate with respect to the cosmological parameters, namely $Omega_{rm m}$ and $w_{rm d}$.
We study the dynamics of the scalar field FLRW flat cosmological models within the framework of the Unified Dark Matter (UDM) scenario. In this model we find that the main cosmological functions such as the scale factor of the Universe, the scalar fi
eld, the Hubble flow and the equation of state parameter are defined in terms of hyperbolic functions. These analytical solutions can accommodate an accelerated expansion, equivalent to either the dark energy or the standard $Lambda$ models. Performing a joint likelihood analysis of the recent supernovae type Ia data and the Baryonic Acoustic Oscillations traced by the SDSS galaxies, we place tight constraints on the main cosmological parameters of the UDM cosmological scenario. Finally, we compare the UDM scenario with various dark energy models namely $Lambda$ cosmology, parametric dark energy model and variable Chaplygin gas. We find that the UDM scalar field model provides a large and small scale dynamics which are in fair agreement with the predictions by the above dark energy models although there are some differences especially at high redshifts.
In this paper we present the first results from an on-going serendipitous survey aiming to identify X-ray selected `normal galaxies (i.e. not AGN dominated) by combining archival XMM-Newton data with the Sloan Digital Sky Survey. In the first 4.5deg2
of this program we have identified a total of 11 `normal galaxy candidates (8 of them with optical spectroscopy) with fluxes f_X(0.5-8keV)~1e-15-1e-13cgs. These sources are selected to have low X-ray--to--optical flux ratio (logf_X/f_{opt}<-2), soft X-ray spectral properties and optical spectra, when available, consistent with the presence of a stellar ionising continuum. These sources comprise both early and late type systems at redshifts z<0.2 with luminosities L_X(0.5-8keV)~1e39-1e42cgs. This dataset provides the first tight constraint on the surface density of X-ray selected `normal galaxies at relatively bright fluxes spanning two orders of magnitude (1e-15 - 1e-13cgs). The slope of the `normal galaxy logN-logS in the above flux range is estimated -1.4pm0.3 consistent with the euclidean prediction. We also discuss the prospects of `normal galaxy studies at X-ray wavelengths using both our continuously expanding survey and future X-ray missions.
In this work we present combined optical and X-ray cluster detection methods in an area near the North Galactic Pole area, previously covered by the SDSS and 2dF optical surveys. The same area has been covered by shallow ($sim 1.8$ deg$^{2}$) XMM-{em
Newton} observations. The optical cluster detection procedure is based on merging two independent selection methods - a smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the SAS v.5.2 package. The final optical sample counts 9 candidate clusters with richness of more than 20 galaxies, corresponding roughly to APM richness class. Three, of our optically detected clusters are also detected in our X-ray survey.
