Tomasz Denkiewicz

Barotropic index w-singularities in cosmology

We find an explicit cosmological model which allows a special type of cosmological singularity which we call a w-singularity. This singularity has the scale factor finite, the energy density and pressure vanishing, and the only singular behavior appears in a time-dependent barotropic index w(t). It is different from the type IV cosmological singularity in that it does not exhibit the divergence of the higher derivatives of the Hubble parameter and from the big brake since it does not fulfill the anti-Chaplygin gas equation of state. We also find an interesting duality between the w-singularities and the big-bang singularities. Physical examples of w-singularities appear in f(R), scalar field and brane cosmologies.

The conformal status of o = ‐3/2 Brans‐Dicke cosmology

Following recent fit of supernovae data to Brans-Dicke theory which favours the model with o = - 3/2 [1] we discuss the status of this special case of Brans-Dicke cosmology in both isotropic and anisotropic framework. It emerges that the limit o = -3/2 is consistent only with the vacuum field equations and it makes such a Brans-Dicke theory conformally invariant. Then it is an example of the conformal relativity theory which allows the invariance with respect to conformal transformations of the metric. Besides, Brans-Dicke theory with o = -3/2 gives a border between a standard scalar field model and a ghost/phantom model. In this paper we show that in o = -3/2 Brans-Dicke theory, i.e., in the conformal relativity there are no isotropic Friedmann solutions of non-zero spatial curvature except for k=-1 case. Further we show that this k=-1 case, after the conformal transformation into the Einstein frame, is just the Milne universe and, as such, it is equivalent to Minkowski spacetime. It generally means that only flat models are fully consistent with the field equations. On the other hand, it is shown explicitly that the anisotropic non-zero spatial curvature models of Kantowski-Sachs type are admissible in o = -3/2 Brans-Dicke theory. It then seems that an additional scale factor which appears in anisotropic models gives an extra deegre of freedom and makes it less restrictive than in an isotropic Friedmann case.

Exotic‐singularity‐driven dark energy

We discuss various types of exotic (non‐standard) singularities in the Universe: a Big‐Rip (BR or type I), a Sudden Future Singularity (SFS or type II), a Generalized Sudden Future Singularity, a Finite Scale Factor singularity (FSF or type III), a Big‐Separation (BS or type IV) and a w‐singularity. They are characterized by violation of all or some of the energy conditions which results in a blow‐up of all or some of the physical quantities: the scale factor, the energy density, the pressure, and the barotropic index. We relate the emergence of these singularities with physical theories (superstring, brane, higher‐order gravity, loop quantum cosmology). We show how the models involving exotic singularities may serve as dark energy by applying the observational data. In particular, we show that some of these exotic singularities (though being of a weak type according to relativistic definitions) may occur in the near future of the universe.

Cosmological tests of sudden future singularities

We discuss combined constraints, coming from the cosmic microwave background shift parameter R, the baryon acoustic oscillations distance parameter A, and from the latest type Ia supernovae data, imposed on cosmological models which allow sudden future singularities of pressure. We show that due to their weakness such sudden singularities may happen in the very near future and that at present they can mimic standard dark energy models.

Density perturbations in a finite scale factor singularity universe

Institute of Physics, University of Szczecin, Wielkopolska 15, 70-451 Szczecin, Poland andCopernicus Center for Interdisciplinary Studies, S lawkowska 17, 31-016 Krak´ow, Poland(Dated: February 16, 2012)We discuss evolution of density perturbations in cosmological models which admit finite scalefactor singularities. After solving the matter perturbations equations we find that there exists a setof the parameters which admit a finite scale factor singularity in future and instantaneously recovermatter density evolution history which are indistinguishable from the standard ΛCDM scenario.

Observational constraints on finite scale factor singularities

We discuss the combined constraints on a Finite Scale Factor Singularity (FSF) universe evolution scenario, which come from the shift parameter , baryon acoustic oscillations (BAO) , and from the type Ia supernovae. We show that observations allow existence of such singularities in the 2 ? 109 years in future (at 1? CL) which is much farther than a Sudden Future Singularity (SFS), and that at the present moment of the cosmic evolution, one cannot differentiate between cosmological scenario which allow finite scale factor singularities and the standard ?CDM dark energy models. We also show that there is an allowed value of m = 2/3 within 1? CL, which corresponds to a dust-filled Einstein-de-Sitter universe limit of the early time evolution and so it is pasted into a standard early-time scenario.

Redshift drift test of exotic singularity universes

We discuss how dynamical dark energy universes with exotic singularities may be distinguished from the standard

Variations of the fine-structure constant

\Lambda

\alpha

CDM model on the basis of their redshift drift signal, for which measurements both in the acceleration phase and in the deep matter era will be provided by forthcoming astrophysical facilities. Two specific classes of exotic singularity models are studied: sudden future singularity models and finite scale factor singularity models. In each class we identify the models which can mimic

in exotic singularity models

\Lambda