Is scalar-tensor gravity consistent with polytropic stellar models?

We study the scalar field potential $V(\phi)$ 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 $V(\phi)$ 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 $V(\phi)$ 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.Comment: 6 pages, 2 figures, text revised and some concepts clarified, results unchange

Short-range correlations in binary alloys: Spin model approach to Agc_cPd1−c_{1-c}

Short-range correlations in Ag-Pd alloys are investigated by analyzing the {\em ab initio} total energy of fcc based random Ag$_c$Pd$_{1-c}$. Since the information on the atomic interactions is incorporated in the energetics of alloys it is possible with a suitable model, Bethe-Peierls-Weiss model is used in the present work, to invert the problem, i.e.\ to obtain information on the short-range correlation from the total energy of a random system. As an example we demonstrate how site correlations can be extracted from random alloy data. Bethe-Peierls-Weiss model predicts positive first neighbor correlator and mixing energy for substitutional face centered cubic (fcc) Ag-Pd alloys at low temperature which can be related to the optimal structures of Ag$_{0.5}$Pd$_{0.5}$.Comment: 6 page

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.Comment: 22 pages, 8 figure

Induced Nucleation in Weak First Order Phase Transition

We study induced nucleation by considering the accumulation rate of shrinking subcritical bubbles. We derive the probability for a collection of subcritical bubbles to form a critical bubble, and argue that this mechanism could well play a role in electroweak phase transitions if the Higgs is heavy.Comment: 11 pages in latex, HU-TFT-94-41 and TURKU-FL-P1

Particle decay in expanding Friedmann-Robertson-Walker universes

The lack of energy conservation introduces new particle processes in curved spacetime that are forbidden in flat space. Therefore one has to be very cautious about using the results calculated in Minkowskian space in early universe applications. This is true for particle decay rates in particular, which need to be calculated using quantum field theory in curved spacetime. Previous studies are usually restricted to using minimal or conformal coupling for the decaying particle, while using a more general coupling would give deeper insight into particle decay. This paper presents the results we obtained for a massive particle decaying in a general power-law universe with arbitrary coupling to gravity. We find that depending on the value of the gravitational coupling, the effect of gravitation may either strengthen or weaken the decay. The analysis further reveals that, apart from radiation dominated universe, there are values of the coupling constant for which the decay rate is exactly Minkowskian for all universe types. Because the decay rate may be considerably modified in curved space, these issues need to be considered when doing precise cosmological calculations.Comment: 9 pages, 5 figure

Decay of a Massive Particle in a Stiff Matter Dominated Universe

In the presence of a gravitational field decay rates may significantly differ from flat space equivalent. By studying mutually interacting quantum fields the decay rates can be calculated on a given spacetime. This paper presents the calculation of the transition probability for the decay of a massive scalar particle in a stiff matter dominated universe. We find that due to the precence of a gravitational field a finite correction to the transition probability is added which depends inversely on the mass. Moreover the decay rate is smaller and lifetime of the particles is longer compared to flat space. The mass dependence is such that the lifetime of lighter particles is prolonged more compared to heavier particles. This result may be of significance when studying cosmological situations involving stiff matter.Comment: 6 pages, 1 figur

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.Comment: 10 pages, in the revised version added discussion on different modes to section 2, some typos and terminology correcte

Decaying Massive Particle in Matter and Radiation Dominated Eras

According to the standard model of cosmology, the early universe has been dominated by radiation or non-relativistic matter in several eras of its history. However, many cosmological calculations involving particle processes are commonly done using Minkowskian results for them, although, for more precise treatment, quantum field theory in curved spacetime is needed. This paper aims to fill this gap by presenting decay rates for matter and radiation dominated universes in this more precise treatment. We provide a study of the average decay rates for a process where a conformally coupled massive scalar field decays into massless scalar particles. It is found that the presence of a curved spacetime modifies the Minkowskian result considerably for early times but asymptotically only by an additive term proportional to the inverse of mass and interaction time. Thus, the correction is small for large time scales, but on the time scales of the order of $m\sim t$, the relative correction term may be of importance.Comment: 6 pages, 2 figure

Inhomogeneity of the Λ\LambdaLTB models

The Lema\'itre-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 $\Lambda$LTB models, which generalizes the LTB models by including a non-zero cosmological constant $\Lambda$ 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 $\Lambda$ 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.Comment: 15 pages including 1 appendix and 4 figure

Jeans Analysis of Bok globules in f(R)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.Comment: 22 pages, 1 figure, new version 23.8.2016: added discussion and corrected typo