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, $|f_{NL}| \sim \mathcal{O}(1)$.Comment: 10 pages, 2 figure

Static spherically symmetric perfect fluid solutions in f(R)f(R) theories of gravity

Static spherically symmetric perfect fluid solutions are studied in metric $f(R)$ theories of gravity. We show that pressure and density do not uniquely determine $f(R)$ ie. given a matter distribution and an equation state, one cannot determine the functional form of $f(R)$. However, we also show that matching the outside Schwarzschild-de Sitter-metric to the metric inside the mass distribution leads to additional constraints that severely limit the allowed fluid configurations.Comment: 5 page

Baryon and lepton number transport in electroweak phase transition

We consider the baryon number generation by charge transport mechanism in the electroweak phase transition taking properly into account thermal fluxes through the wall separating true and false vacuum in the spatial space. We show that the diffusion from the true vacuum to the false one has a large diminishing effect on the baryon number unless the wall velocity is near to, but less than, the speed of sound in the medium and the ratio between the collision rate and wall thickness is about 0.3. The maximum net baryon density generated is $\rho_B/s\simeq 0.2\times 10^{-10}$, where $s$ is the entropy density of the Universe. If the wall proceeds as a detonation, no baryon number is produced.Comment: 13 pages + 2 figures available on request, HU-TFT-94-15, TURKU-FL-P1

On the Nonequilibrium Effective Potential

Nonequilibrium phenomena of the phase transitions are studied. It is shown that due to finite relaxation time of the particle distributions, the use of scalar background dependent distribution functions is inconsistent.This observation may change the picture of rapid processes during the electroweak phase transition, like subcritical bubble formation and propagation of bubble walls.Comment: 12 pages (REVTeX) + 1 figur

Short-range correlations in binary alloys: Spin model approach to AgcAu1-c and AgcPd1-c

Short-range correlations in Ag-Au and Ag-Pd alloys are investigated by analyzing the ab initio total energy of face centered cubic (fcc) based random AgcAu1-c, and AgcPd1-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 negative (positive) first neighbor correlator for substitutional fcc Ag-Au and (Ag-Pd) alloys at low temperature which can be related to the optimal structures of Ag0.5Au0.5 (and Ag0.5Pd0.5). (C) 2014 Elsevier B.V. All rights reserved.</p

Vacuum stability in the singlet Majoron model

We study the vacuum stability of the singlet Majoron model using full renormalization group improved scalar potential and Monte Carlo techniques. We show that in the perturbative regime of the various free parameters, the vacuum stability requirement together with LEP limits is passed by 18% of the parameter space if the scale of new physics is 10 TeV and 6% if the scale is $10^{14}$ GeV. Moreover, if the baryogenesis condition for scalar couplings is required, no portion of the parameter space survives.Comment: 9 pages + 1 uuencoded figur

Cosmological expansion and the uniqueness of gravitational action

Modified theories of gravity have recently been studied by several authors as possibly viable alternatives to the cosmological concordance model. Such theories attempt to explain the accelerating expansion of the universe by changing the theory of gravity, instead of introducing dark energy. In particular, a class of models based on higher order curvature invariants, so-called $f(R)$ gravity models, has drawn attention. In this letter we show that within this framework, the expansion history of the universe does not uniquely determine the form of the gravitational action and it can be radically different from the standard Einstein-Hilbert action. We demonstrate that for any barotropic fluid, there always exists a class of $f(R)$ models that will have exactly the same expansion history as that arising from the Einstein-Hilbert action. We explicitly show how one can extend the Einstein-Hilbert action by constructing a $f(R)$ theory that is equivalent on the classical level. Due to the classical equivalence between $f(R)$ theories and Einstein-Hilbert gravity with an extra scalar field, one can also hence construct equivalent scalar-tensor theories with standard expansion.Comment: 4 page