arXiv subjects

Iiro Vilja

Publications and source records attributed to Iiro Vilja.

At least 37 records · Page 2Linked to original sources

arXiv2016

Jeans Analysis of Bok globules in $f(R)$ gravity

We examine the effects of $f(R)$ gravity on Jeans analysis of collapsing dust clouds. We provide a method for testing modified gravity models by their effects on star formation as the presence of $f(R)$ gravity is found to modify the limit for collapse. In this analysis we add perturbations to a de Sitter background. Depending on the characteristics of a chosen $f(R)$ model, the appearance of new limits is possible. The physicality of these limits is further examined. We find the asymptotic Jeans masses for $f(R)$ theories compared to standard Jeans mass. Through this ratio, the effects of the $f(R)$ modified Jeans mass for viable theories are examined in molecular clouds. Bok globules have a mass range comparable to Jeans masses in question and are therefore used for comparing different $f(R)$ models. Viable theories are found to assist in star formation.

astro-ph.CO↗

arXiv2016

f(R) gravity constraints from gravitational waves

The recent LIGO observation sparked interest in the field of gravity wave signals. Besides the gravity wave observation the LIGO collaboration used the inspiraling black hole pair to constrain the graviton mass. Unlike general relativity, $f(R)$ theories have a characteristic non-zero mass graviton. We apply the constraint on the graviton mass to viable $f(R)$ models to find the effects on model parameters. We find it possible to constrain the parameter space with the gravity wave based observations. We make a case study for the popular Hu-Sawicki model and find a parameter bracket. The result generalizes to other $f(R)$ theories and can be used to contain the parameter space.

astro-ph.CO↗

arXiv2016

Inhomogeneity of the $Λ$LTB models

The Lemaítre-Toman-Bondi (LTB) models have reported to suffer from incompatibility with cosmological observations and fine-tuning of the observer's location. Further analysis of these issues indicates that they could be resolved by models that are compatible with the supernova Ia data, but less inhomogeneous than those that have been presented in the literature so far. We study if such models exist by employing the degrees of freedom of the LTB models in a novel manner. We discovered two scenarios which may meet the expectations, but extensive numerical and analytical investigation showed them inviable. We extended our studies to the $Λ$LTB models, which generalizes the LTB models by including a non-zero cosmological constant $Λ$ in Einsteins equations. This adds an additional degree of freedom for the earlier scenarios and introduces a new scenario capable of meeting the expectations. However, extensive numerical and analytical investigation reveals that inclusion of $Λ$ does not enhance the viability of the models. We identify the lack of degrees of freedom to be the reason for the unviability. However, the method presented here can be generalized to models including more degrees of freedom, like the Szekeres models, which have more promise to overcome the issues in the LTB models.

astro-ph.CO↗

arXiv2015

Can a void mimic the $Λ$ in $Λ$CDM?

We investigate Lemaître-Tolman-Bondi (LTB) models, whose early time evolution and bang time are homogeneous and the distance - redshift relation and local Hubble parameter are inherited from the $Λ$CDM model. We show that the obtained LTB models and the $Λ$CDM model predict different relative local expansion rates and that the Hubble functions of the models diverge increasingly with redshift. The LTB models show tension between low redshift baryon acoustic oscillation and supernova observations and including Lyman-$α$ forest or cosmic microwave background observations only accentuates the better fit of the $Λ$CDM model compared to the LTB model. The result indicates that additional degrees of freedom are needed to explain the observations, for example by renouncing spherical symmetry, homogeneous bang time, negligible effects of pressure, or the early time homogeneity assumption.

astro-ph.CO↗

arXiv2015

Is scalar-tensor gravity consistent with polytropic stellar models?

We study the scalar field potential $V(ϕ)$ in the scalar-tensor gravity with self-consistent polytropic stellar configurations. Without choosing a particular potential, we numerically derive the potential inside various stellar objects. We restrict the potential to conform to general relativity or to $f(R)$ gravity inside and require the solution to arrive at SdS vacuum at the surface. The studied objects are required to obtain observationally valid masses and radii corresponding to solar type stars, white dwarfs and neutron stars. We find that the resulting scalar-tensor potential $V(ϕ)$ for the numerically derived polytrope that conforms to general relativity, in each object class, is highly dependent on the matter configuration as well as on the vacuum requirement at the boundary. As a result, every stellar configuration arrives at a potential $V(ϕ)$ that is not consistent with the other stellar class potentials. Therefore, a general potential that conforms to all these polytropic stellar classes could not be found.

gr-qc↗

arXiv2014

Inhomegeneous cosmological models and fine-tuning of the initial state

Inhomogeneous cosmological models are often reported to suffer from a fine-tuning problem because of the observer's location. We study if this is a generic feature in the Lemaître-Tolman (LT) models, by investigating if there are models with freedom in the initial state. In these cases, the present fine-tuned location would be evolved from a non-fine-tuned initial state and thus vanishing the problem. In this paper, we show that this is not a generic problem and we give the condition when the LT models do not have fine-tuned initial state. The physical meaning of this condition, however, requires more investigation. We investigate if this condition can be found from a special case: homogeneous models with matter, dark, and curvature density as parameters. We found that with any reasonable density values, these models do not satisfy this condition and thus do not have freedom in the initial state. We interpret this to be linked with the fine-tuning problem of the initial state of the homogeneous models, when the early time inflation is not included to them. We discuss of the condition in the context of non-homogeneous models.

astro-ph.CO↗

arXiv2013

Stability of the scale parameter: Hu-Sawicki model

The evolution of the scale parameter in the Hu-Sawicki model is examined. We search the parameter area for instabilities. It turns out the parameter area of physically meaningful evolution is non-existent. For greater stability a stabilising term such as R^2 should be added. With the addition, the area mainly coincides with phantom models. We also find that the evolution of the scale parameter can be examined through a linear expansion and not only in the Hu-Sawicki case. The oscillatory behaviour found in the evolution raises questions of the physicality of the model. The behaviour can also cause current observations of the Hubble parameter to be misleading. This paper has been withdrawn by the authords due to a critical error in equation (10)

astro-ph.CO↗

arXiv2013

Baryon asymmetry and dark matter from soft leptogenesis

The framework for soft leptogenesis minimally extended with a DM sector is studied. A heavy singlet neutrino superfield acts as the source for (s)lepton asymmetry and by coupling to the singlet DM superfield it produces a DM particle density through decays. The nature of DM generated is twofold depending on whether the Yukawa and DM couplings are either small or large. With sufficiently small Yukawa and DM couplings DM annihilations into MSSM particles are slow and as a consequence all DM particles form the DM component. The solutions to Boltzmann equations are given and the dependence between the DM masses and coupling are presented in this weak coupling regime. Also, the behavior of the efficiency of producing asymmetric DM is determined with weak couplings. We note that a different outcome arises if the couplings are larger because then the ADM component is dominant due to the effectiveness of DM decays into the MSSM sector.

hep-ph↗

arXiv2011

Numerical study of leptogenesis in a 5D split fermion model with bulk neutrinos

We study numerically a 5D hybrid model which incorporates a split fermion scenario and bulk neutrinos. We perform a Monte Carlo analysis of the model in order to find the regions in the parameter space allowing for realization of the leptogenesis. We find that higher order Yukawa terms must be included in order the model to produce a CP violation and net baryon number sufficient for the creation of the observed baryon asymmetry of the Universe.

hep-ph↗

arXiv2011

Leptogenesis in B-L gauged SUSY with MSSM Higgs sector

We study a modified version of a $U(1)_{B-L}$ gauged MSSM that was recently shown to produce a new source of leptogenesis through the CP asymmetry of sneutrinos and antisneutrinos (K.S. Babu, Y. Meng, Z. Tavartkiladze, Phys Lett B681:37 (2009)). By taking all superpotential terms and couplings between the MSSM Higgs and $B-L$ scalar sectors into account we find that the model allows a large enough CP violation to explain the observed baryon number to entropy ratio. Monte Carlo analysis shows that a large amount of CP violation can be produced in the decays of the $B-L$ Higgs bosons and that there are two dominating channels that drive CP violation.

hep-ph↗

arXiv2010

Thermal leptogenesis in a 5D split fermion scenario with bulk neutrinos

We study the thermal leptogenesis in a hybrid model, which combines the so called split fermion model and the bulk neutrino model defined in five dimensional spacetime. This model predicts the existence of a heavy neutrino pair nearly degenerate in mass, whose decays might generate a CP violation large enough for creating the baryon asymmetry of the universe through leptogenesis. We investigate numerically the constraints this sets on the parameters of the model such as the size of the compactified fifth dimension.

hep-ph↗

arXiv2009

Curvaton decay into relativistic matter

We consider an inflationary curvaton scenario, where the curvaton decays into two non-interacting relativistic fluids and later during the cosmological evolution one of them becomes non-relativistic, forming dark matter component of the universe. We study the thermic properties and the generation of non-gaussianity in this three fluid curvaton model. By solving the evolution of the system and using several cosmological conditions we find that the allowed parameter space is strongly constrained. The naturalness of this curvaton scenario is also discussed.

astro-ph.CO↗

arXiv2009

Hamiltonian perturbation theory in f(R) gravity

Hamiltonian perturbation theory is used to analyse the stability of f(R) models. The Hamiltonian equations for the metric and its momentum conjugate are written for f(R) Lagrangian in the presence of perfect fluid matter. The perturbations examined are perpendicular to R. As perturbations are added to the metric and momentum conjugate to the induced metric instabilities are found, depending on the form of f(R). Thus the examination of these instabilities is a way to rule out certain f(R) models.

gr-qc↗

arXiv2008

Maximal symmetry and metric-affine f(R) gravity

The affine connection in a space-time with a maximally symmetric spatial subspace is derived using the properties of maximally symmetric tensors. The number of degrees of freedom in metric-affine gravity is thereby considerably reduced while the theory allows spatio-temporal torsion and remains non-metric. The Ricci tensor and scalar are calculated in terms of the connection and the field equations derived for the Einstein-Hilbert as wells as for f(R) Lagrangians. By considering specific forms of f(R), we demonstrate that the resulting Friedmann equations in Palatini formalism without torsion and metric-affine formalism with maximal symmetry are in general different in the presence of matter.

gr-qc↗

arXiv2008

Non-Gaussianity in three fluid curvaton model

The generation of non-gaussianity is studied in a three fluid curvaton model. By utilizing second order perturbation theory we derive general formulae for the large scale temperature fluctuation and non-gaussianity parameter, $f_{NL}$, that includes the possibility of a non-adiabatic final state. In the adiabatic limit we recover previously known results. The results are applied to a three fluid curvaton model where the curvaton decays into radiation and matter. We find that the amount of non-gaussianity decreases as the final state of the system becomes more adiabatic and that the generated non-gaussianity in the scenario is small, $|f_{NL}| \sim \mathcal{O}(1)$.

astro-ph↗

arXiv2008

Energy-momentum complexes in f(R) theories of gravity

Despite the fact that modified theories of gravity, in particular the f(R) gravity models have attracted much attention in the last years, the problem of the energy localization in the framework of these models has not been addressed. In the present work the concept of energy-momentum complexes is presented in this context. We generalize the Landau-Lifshitz prescription of calculating the energy-momentum complex to the framework of f(R) gravity. As an important special case, we explicitly calculate the energy-momentum complex for the Schwarzschild-de Sitter metric for a general f(R) theory as well as for a number of specific, popular choices of f(R).

gr-qc↗

arXiv2007

Constraints on the three-fluid model of curvaton decay

A three fluid system describing the decay of the curvaton is studied by numerical and analytical means. We place constraints on the allowed interaction strengths between the fluids and initial curvaton density by requiring that the curvaton decays before nucleosynthesis while nucleosynthesis, radiation-matter equality and decoupling occur at correct temperatures. We find that with a continuous, time-independent interaction, a small initial curvaton density is naturally preferred along with a low reheating temperature. Allowing for a time-dependent interaction, this constraint can be relaxed. In both cases, a purely adiabatic final state can be generated, but not without fine-tuning. Unlike in the two fluid system, the time-dependent interactions are found to have a small effect on the curvature perturbation itself due to the different nature of the system. The presence of non-gaussianity in the model is discussed.

astro-ph↗

arXiv2006

Spherically symmetric solutions of modified field equations in f(R) theories of gravity

Spherically symmetric static empty space solutions are studied in f(R) theories of gravity. We reduce the set of modified Einstein's equations to a single equation and show how one can construct exact solutions in different f(R) models. In particular, we show that for a large class models, including e.g. the f(R)=R-μ^4/R model, the Schwarzschild-de Sitter metric is an exact solution of the field equations. The significance of these solutions is discussed in light of solar system constraints on $f(R)$ theories of gravity.

astro-ph↗