Axion perturbation spectra in string cosmologies

We discuss the semi-classical perturbation spectra produced in the massless fields of the low energy string action in a pre big bang type scenario. Axion fields may possess an almost scale-invariant spectrum on large scales dependent upon the evolution of the dilaton and moduli fields to which they are coupled. As an example we calculate the spectra for three axion fields present in a truncated type IIB model and show that they are related with at least one of the fields having a scale-invariant or red perturbation spectrum. In the simplest pre big bang scenario this may be inconsistent with the observed isotropy of the microwave background. More generally, relations between the perturbation spectra in low energy string cosmologies reflect the symmetries of the theory.Comment: 9 pages, latex with epsf, 1 figure. Revised estimate of amplitude of density perturbations and extended discussion of possible conflict with isotropy of cosmic microwave background. To appear in Physics Letters

Modified gravity with negative and positive powers of the curvature: unification of the inflation and of the cosmic acceleration

The modified gravity, which eliminates the need for dark energy and which seems to be stable, is considered. The terms with positive powers of the curvature support the inflationary epoch while the terms with negative powers of the curvature serve as effective dark energy, supporting current cosmic acceleration. The equivalent scalar-tensor gravity may be compatible with the simplest solar system experiments.Comment: 23 pages, 3 figures, discussion is extended, references added, version to appear in PR

On the Generation of a Scale-Invariant Spectrum of Adiabatic Fluctuations in Cosmological Models with a Contracting Phase

In Pre-Big-Bang and in Ekpyrotic Cosmology, perturbations on cosmological scales today are generated from quantum vacuum fluctuations during a phase when the Universe is contracting (viewed in the Einstein frame). The backgrounds studied to date do not yield a scale invariant spectrum of adiabatic fluctuations. Here, we present a new contracting background model (neither of Pre-Big-Bang nor of the Ekpyrotic form) involving a single scalar field coupled to gravity in which a scale-invariant spectrum of curvature fluctuations and gravitational waves results. The equation of state of this scalar field corresponds to cold matter. We demonstrate that if this contracting phase can be matched via a nonsingular bounce to an expanding Friedmann cosmology, the scale-invariance of the curvature fluctuations is maintained. We also find new background solutions for Pre-Big-Bang and for Ekpyrotic cosmology, which involve two scalar fields with exponential potentials with background values which are evolving in time. We comment on the difficulty of obtaining a scale-invariant spectrum of adiabatic fluctuations with background solutions which have been studied in the past.Comment: 8 pages, revised version without the section on perturbations, matching the version published on Phys. Rev. D. For cosmological perturbations in the two field model see astro-ph/021127

Evolution of cosmological perturbations in non-singular string cosmologies

In a class of non-singular cosmologies derived from higher-order corrections to the low-energy bosonic string action, we derive evolution equations for the most general cosmological scalar, vector and tensor perturbations. In the large scale limit, the evolutions of both scalar and tensor perturbations are characterised by conserved quantities, the usual curvature perturbation in the uniform-field gauge and the tensor-type perturbed metric. The vector perturbation is not affected, being described by the conservation of the angular momentum of the fluid component in the absence of any additional dissipative process. For the scalar- and tensor-type perturbations, we show how, given a background evolution during kinetic driven inflation of the dilaton field, we can obtain the final power spectra generated from the vacuum quantum fluctuations of the metric and the dilaton field during the inflation.Comment: 11 pages, 2 figures, submitted to Phys. Rev.

Time and Amplitude of Afterpulse Measured with a Large Size Photomultiplier Tube

We have studied the afterpulse of a hemispherical photomultiplier tube for an upcoming reactor neutrino experiment. The timing, the amplitude, and the rate of the afterpulse for a 10 inch photomultiplier tube were measured with a 400 MHz FADC up to 16 \ms time window after the initial signal generated by an LED light pulse. The time and amplitude correlation of the afterpulse shows several distinctive groups. We describe the dependencies of the afterpulse on the applied high voltage and the amplitude of the main light pulse. The present data could shed light upon the general mechanism of the afterpulse.Comment: 11 figure

Adiabatic and Isocurvature Perturbations for Multifield Generalized Einstein Models

Low energy effective field theories motivated by string theory will likely contain several scalar moduli fields which will be relevant to early Universe cosmology. Some of these fields are expected to couple with non-standard kinetic terms to gravity. In this paper, we study the splitting into adiabatic and isocurvature perturbations for a model with two scalar fields, one of which has a non-standard kinetic term in the Einstein-frame action. Such actions can arise, e.g., in the Pre-Big-Bang and Ekpyrotic scenarios. The presence of a non-standard kinetic term induces a new coupling between adiabatic and isocurvature perturbations which is non-vanishing when the potential for the matter fields is nonzero. This coupling is un-suppressed in the long wavelength limit and thus can lead to an important transfer of power from the entropy to the adiabatic mode on super-Hubble scales. We apply the formalism to the case of a previously found exact solution with an exponential potential and study the resulting mixing of adiabatic and isocurvature fluctuations in this example. We also discuss the possible relevance of the extra coupling in the perturbation equations for the process of generating an adiabatic component of the fluctuations spectrum from isocurvature perturbations without considering a later decay of the isocurvature component.Comment: 11 pages, 3 figures, one equation corrected, typos fixed, conclusions unchange

Linear and non-linear perturbations in dark energy models

I review the linear and second-order perturbation theory in dark energy models with explicit interaction to matter in view of applications to N-body simulations and non-linear phenomena. Several new or generalized results are obtained: the general equations for the linear perturbation growth; an analytical expression for the bias induced by a species-dependent interaction; the Yukawa correction to the gravitational potential due to dark energy interaction; the second-order perturbation equations in coupled dark energy and their Newtonian limit. I also show that a density-dependent effective dark energy mass arises if the dark energy coupling is varying.Comment: 12 pages, submitted to Phys. Rev; v2: added a ref. and corrected a typ

Fermion Electric Dipole Moments in Supersymmetric Models with R-parity Violation

We analyze the electron and neutron electric dipole moments induced by R-parity violating interactions in supersymmetric models. It is pointed out that dominant contributions can come from one-loop diagrams involving both the bilinear and trilinear R-parity odd couplings, leading to somewhat severe constraints on the products of those couplings.Comment: Revtex, 19pp, four figures in axodraw.st

Revisiting Cardassian Model and Cosmic Constraint

In this paper, we revisit the Cardassian model in which the radiation energy component is included. It is important for early epoch when the radiation cannot be neglected because the equation of state (EoS) of the effective dark energy becomes time variable. Therefore, it is not equivalent to the quintessence model with a constant EoS anymore. This situation was almost overlooked in the literature. By using the recent released Union2 557 of type Ia supernovae (SN Ia), the baryon acoustic oscillation (BAO) from Sloan Digital Sky Survey and the WiggleZ data points, the full information of cosmic microwave background (CMB) measurement given by the seven-year Wilkinson Microwave Anisotropy Probe observation, we constrain the Cardassian model via the Markov Chain Monte Carlo (MCMC) method. A tight constraint is obtained: $n= -0.0479_{- 0.0732- 0.148}^{+ 0.0730+ 0.142}$ in $1,2\sigma$ regions. The deviation of Cardassian model from quintessence model is shown in CMB anisotropic power spectra at high l's parts due to the evolution of EoS. But it is about the order of 0.1% which cannot be discriminated by current data sets. The Cardassian model is consistent with current cosmic observational data sets.Comment: 6 pages, 5 figures, match the published versio

Temperature-dependent magnetization in diluted magnetic semiconductors

We calculate magnetization in magnetically doped semiconductors assuming a local exchange model of carrier-mediated ferromagnetic mechanism and using a number of complementary theoretical approaches. In general, we find that the results of our mean-field calculations, particularly the dynamical mean field theory results, give excellent qualitative agreement with the experimentally observed magnetization in systems with itinerant charge carriers, such as Ga_{1-x}Mn_xAs with 0.03 < x < 0.07, whereas our percolation-theory-based calculations agree well with the existing data in strongly insulating materials, such as Ge_{1-x}Mn_x. We comment on the issue of non-mean-field like magnetization curves and on the observed incomplete saturation magnetization values in diluted magnetic semiconductors from our theoretical perspective. In agreement with experimental observations, we find the carrier density to be the crucial parameter determining the magnetization behavior. Our calculated dependence of magnetization on external magnetic field is also in excellent agreement with the existing experimental data.Comment: 17 pages, 15 figure