Brane Inflation from Rotation of D4 Brane

In this paper, a inflationary model from the rotation of D4-brane is constructed. We show that for a very wide rage of parameter, this model satisfies the observation and find that regarded as inflaton, the rotation of branes may be more nature than the distance between branes. Our model offers a new avenue for brane inflation.Comment: 6 pages, no figure

Hybrid Inflation Exit through Tunneling

For hybrid inflationary potentials, we derive the tunneling rate from field configurations along the flat direction towards the waterfall regime. This process competes with the classically rolling evolution of the scalar fields and needs to be strongly subdominant for phenomenologically viable models. Tunneling may exclude models with a mass scale below 10^12 GeV, but can be suppressed by small values of the coupling constants. We find that tunneling is negligible for those models, which do not require fine tuning in order to cancel radiative corrections, in particular for GUT-scale SUSY inflation. In contrast, electroweak scale hybrid inflation is not viable, unless the inflaton-waterfall field coupling is smaller than approximately 10^-11.Comment: 17 pages, 2 figure

Uncertainties of predictions in models of eternal inflation

In a previous paper \cite{MakingPredictions}, a method of comparing the volumes of thermalized regions in eternally inflating universe was introduced. In this paper, we investigate the dependence of the results obtained through that method on the choice of the time variable and factor ordering in the diffusion equation that describes the evolution of eternally inflating universes. It is shown, both analytically and numerically, that the variation of the results due to factor ordering ambiguity inherent in the model is of the same order as their variation due to the choice of the time variable. Therefore, the results are, within their accuracy, free of the spurious dependence on the time parametrization.Comment: 30 pages, RevTeX, figure included, added some references and Comments on recent proposal (gr-qc/9511058) of alternative regularization schemes, to appear in Phys. Rev.

Non-equilibrium Goldstone phenomenon in tachyonic preheating

The dominance of the direct production of elementary Goldstone waves is demonstrated in tachyonic preheating by numerically determining the evolution of the dispersion relation, the equation of state and the kinetic power spectra for the angular degree of freedom of the complex matter field. The importance of the domain structure in the order parameter distribution for the quantitative understanding of the excitation mechanism is emphasized. Evidence is presented for the very early decoupling of the low-momentum Goldstone modes.Comment: 14 LaTeX pages, 5 figures, version published in Phys. Rev.

Lambda-inflation and CMB anisotropy

We explore a broad class of three-parameter inflationary models, called the $\Lambda$-inflation, and its observational predictions: high abundance of cosmic gravitational waves consistent with the Harrison-Zel'dovich spectrum of primordial cosmological perturbations, the non-power-law wing-like spectrum of matter density perturbations, high efficiency of these models to meet current observational tests, and others. We show that a parity contribution of the gravitational waves and adiabatic density perturbations into the large-scale temperature anisotropy, T/S $\sim 1$, is a common feature of $\Lambda$-inflation; the maximum values of T/S (basically not larger than 10) are reached in models where (i) the local spectrum shape of density perturbations is flat or slightly red ($n_S{}_\sim^< 1$), and (ii) the residual potential energy of the inflaton is near the GUT scale ($V_0^{{1/4}} \sim 10^{16} GeV$). The conditions to find large T/S in the paradigm of cosmic inflation and the relationship of T/S to the ratio of the power spectra, $r$, and to the inflationary $\gamma$ and Hubble parameters, are discussed. We argue that a simple estimate, T/S$\simeq 3r\simeq 12\gamma \simeq (\frac{H}{6\times 10^{13}{\rm GeV}})^2$, is true for most known inflationary solutions and allows to relate straightforwardly the important parameters of observational and physical cosmology.Comment: 29 pages, 3 figures include

Can we avoid dark energy?

The idea that we live near the centre of a large, nonlinear void has attracted attention recently as an alternative to dark energy or modified gravity. We show that an appropriate void profile can fit both the latest cosmic microwave background and supernova data. However, this requires either a fine-tuned primordial spectrum or a Hubble rate so low as to rule these models out. We also show that measurements of the radial baryon acoustic scale can provide very strong constraints. Our results present a serious challenge to void models of acceleration.Comment: 5 pages, 4 figures; minor changes; version published in Phys. Rev. Let

Generalized Slow Roll Conditions and the Possibility of Intermediate Scale Inflation in Scalar-Tensor Theory

Generalized slow roll conditions and parameters are obtained for a general form of scalar-tensor theory (with no external sources), having arbitrary functions describing a nonminimal gravitational coupling F(\phi), a Kahler-like kinetic function k(\phi), and a scalar potential V(\phi). These results are then used to analyze a simple toy model example of chaotic inflation with a single scalar field \phi and a standard Higgs potential and a simple gravitational coupling function. In this type of model inflation can occur with inflaton field values at an intermediate scale of roughly 10^{11} GeV when the particle physics symmetry breaking scale is approximately 1 TeV, provided that the theory is realized within the Jordan frame. If the theory is realized in the Einstein frame, however, the intermediate scale inflation does not occur.Comment: 14 pages, no figs. Accepted to Classical and Quantum Gravit

Corrections to the apparent value of the cosmological constant due to local inhomogeneities

Supernovae observations strongly support the presence of a cosmological constant, but its value, which we will call apparent, is normally determined assuming that the Universe can be accurately described by a homogeneous model. Even in the presence of a cosmological constant we cannot exclude nevertheless the presence of a small local inhomogeneity which could affect the apparent value of the cosmological constant. Neglecting the presence of the inhomogeneity can in fact introduce a systematic misinterpretation of cosmological data, leading to the distinction between an apparent and true value of the cosmological constant. We establish the theoretical framework to calculate the corrections to the apparent value of the cosmological constant by modeling the local inhomogeneity with a $\Lambda LTB$ solution. Our assumption to be at the center of a spherically symmetric inhomogeneous matter distribution correspond to effectively calculate the monopole contribution of the large scale inhomogeneities surrounding us, which we expect to be the dominant one, because of other observations supporting a high level of isotropy of the Universe around us. By performing a local Taylor expansion we analyze the number of independent degrees of freedom which determine the local shape of the inhomogeneity, and consider the issue of central smoothness, showing how the same correction can correspond to different inhomogeneity profiles. Contrary to previous attempts to fit data using large void models our approach is quite general. The correction to the apparent value of the cosmological constant is in fact present for local inhomogeneities of any size, and should always be taken appropriately into account both theoretically and observationally.Comment: 16 pages,new sections added analyzing central smoothness and accuracy of the Taylor expansion approach, Accepted for publication by JCAP. An essay based on this paper received honorable mention in the 2011 Essay Context of the Gravity Research Foundatio