The consistency of the constraint with the evolution equations for spatially inhomogeneous and irrotational silent (SIIS) models of Petrov type I, demands that the former are preserved along the timelike congruence represented by the velocity of the dust fluid, leading to an infinite set of non-trivial constraints. This fact has been used to conjecture that the resulting models correspond to the spatially homogeneous (SH) models of Bianchi type I, at least for the case where the cosmological constant vanish. By exploiting the full set of the constraint equations as expressed in the 1+3 covariant formalism and using elements from the theory of the spacelike congruences, we provide a direct and simple proof of this conjecture for vacuum and dust fluid models, which shows that the Szekeres family of solutions represents the most general class of SIIS models. The suggested procedure also shows that, the uniqueness of the spatially inhomogeneous and irrotational models of Petrov type D is not affected by the presence of a non-zero pressure fluid and one should consider more general (silent or not) configurations in order to allow a broader class of solutions apart from the SH models of Bianchi type I.
We numerically investigate density perturbations generated in the smooth hybrid new inflation model, a kind of double inflation model that is designed to reproduce the running spectral index suggested by the WMAP results. We confirm that this model provides the running spectral index within 1sigma range of the three year WMAP result. In addition, we find a sharp and strong peak on the spectrum of primordial curvature perturbation at small scales. This originates from amplification of fluctuation in the first inflaton fields due to parametric resonance, which takes place in the oscillatory phase between two inflationary regime. Formation probability of primordial black holes (PBHs) is discussed as a consequence of such peak.
This is a sequel to a previous detailed study of quantum corrections to cosmological correlations. It was found there that except in special cases these corrections depend on the whole history of inflation, not just on the behavior of fields at horizon exit. It is shown here that at least in perturbation theory these corrections can nevertheless not be proportional to positive powers of the Robertson--Walker scale factor, but only at most to powers of its logarithm, and are therefore never large.
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In a first part the scope of classical thermodynamics and statistical mechanics is discussed in the broader context of formal dynamical systems, including computer programmes. In this context classical thermodynamics appears as a particular theory suited to a subset of all dynamical systems. A statistical mechanics similar to the one derived with the microcanonical ensemble emerges from dynamical systems provided it contains, 1) a finite non-integrable part of its phase space which is, 2) ergodic at a satisfactory degree after a finite time. The integrable part of phase space provides the constraints that shape the particular system macroscopical properties, and the chaotic part provides well behaved statistical properties over a relevant finite time. More generic semi-ergodic systems lead to intermittent behaviour, thus may be unsuited for a statistical description of steady states. Following these lines of thought, in a second part non-extensive hierarchical systems with statistical scale-invariance and power law interactions are explored. Only the virial constraint, consistent with their microdynamics, is included. No assumptions of classical thermodynamics are used, in particular extensivity and local homogeneity. In the limit of a large hierarchical range new constraints emerge in some conditions that depend on the interaction law range. In particular for the gravitational case, a velocity-site scaling relation is derived which is consistant with the ones empirically observed in the fractal interstellar medium.
A host of dark energy models and non-standard cosmologies predict an enhanced Hubble rate in the early Universe: perfectly viable models, which satisfy Big Bang Nucleosynthesis (BBN), cosmic microwave background and general relativity tests, may nevertheless lead to enhancements of the Hubble rate up to many orders of magnitude. In this paper we show that strong bounds on the pre-BBN evolution of the Universe may be derived, under the assumption that dark matter is a thermal relic, by combining the dark matter relic density bound with constraints coming from the production of cosmic-ray antiprotons by dark matter annihilation in the Galaxy. The limits we derive can be sizable and apply to the Hubble rate around the temperature of dark matter decoupling. For dark matter masses lighter than 100 GeV, the bound on the Hubble-rate enhancement ranges from a factor of a few to a factor of 30, depending on the actual cosmological model, while for a mass of 500 GeV the bound falls in the range 50-500. Uncertainties in the derivation of the bounds and situations where the bounds become looser are discussed. We finally discuss how these limits apply to some specific realizations of non-standard cosmologies: a scalar-tensor gravity model, kination models and a Randall-Sundrum D-brane model.
We study possibilities for lowering the decay constants of superstring axions. In the heterotic Calabi-Yau compactification, a localized model-dependent axion can appear at a nearly collapsing 2-cycle. The effect of flux can be used for generating warp factor suppression of the axion decay constant. We also point out that the hidden sector instanton potential much higher than the QCD instanton potential picks up the larger effective axion decay constant as that of the QCD axion. However, we show that this can be converted easily because the hidden sector gauginos lower the hidden sector instanton potential contribution, and thus there exists a possibility that the decay constant of the QCD axion is much smaller than the string scale.
The problem of assigning probabilities to vacua is notoriously ambiguous in the global description of eternal inflation. The local point of view is preferred by holography, and it naturally picks out a simple probability measure. This requires no ambiguous choices, such as which time slices to use, or how to weight the volume occupied by a vacuum. The local viewpoint also cuts off the weight carried by exponentially large slow-roll expansion factors or lifetimes.
The expansion instability of a toroidal current ring in low-beta magnetized plasma is investigated. Qualitative agreement is obtained with experiments on spheromak expansion and with essential properties of solar coronal mass ejections (CMEs), unifying the two apparently disparate classes of fast and slow CMEs.
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A semi-classical analysis of vacuum energy in the expanding spacetime suggests that the cosmological term decays with time, with a concomitant matter production. For early times we find, in Planck units, $\Lambda \approx H^4$, where H is the Hubble parameter. The corresponding cosmological solution has no initial singularity, existing since an infinite past. During an infinitely long period we have a quasi-de Sitter, inflationary universe, with $H \approx 1$. However, at a given time, the expansion undertakes a phase transition, with H and $\Lambda$ decreasing to nearly zero in a few Planck times, producing a huge amount of radiation. On the other hand, the late-time scenario is similar to the standard model, with the radiation phase followed by a dust era, which tends asymptotically to a de Sitter universe, with vacuum dominating again.
The gravitational wave signals from coalescing Supermassive Black Hole Binaries are one of the prime targets for the Laser Interferometer Space Antenna (LISA). With optimal data processing techniques the LISA observatory should be able to detect these black hole mergers anywhere in the Universe. The challenge is to find ways to dig the signals out of a combination of instrument noise and the large foreground from stellar mass binaries in our own galaxy. The standard procedure of matched filtering against a bank of templates can be computationally prohibitive, especially when the black holes are spinning or the mass ratio is large. Here we develop an alternative approach based on annealed Markov Chains that is orders of magnitude cheaper than a grid search. We demonstrate our approach on a simulated LISA data stream that contains the signal from a binary system of Schwarzschild Black Holes, embedded in instrument noise and a foreground containing 26 million galactic binaries. The annealed chains are able to accurately recover the 9 parameters that describe the black hole binary without first having to remove any of the bright sources in the foreground.
Assuming the Hubble parameter is a continuous and differentiable function of comoving time, we investigate necessary conditions for quintessence to phantom phase transition in quintom model. For power-law and exponential potential examples, we study the behavior of dynamical dark energy fields and Hubble parameter near the transition time, and show that the phantom-divide-line w=-1 is crossed in these models.
The gravitino is a promising candidate for cold dark matter. We study cosmological constraints on scenarios in which the gravitino is the lightest supersymmetric particle and a charged slepton the next-to-lightest supersymmetric particle (NLSP). We obtain new results for the hadronic nucleosynthesis bounds by computing the 4-body decay of the NLSP slepton into the gravitino, the associated lepton, and a quark-antiquark pair. The bounds from the observed dark matter density are refined by taking into account gravitinos from both late NLSP decays and thermal scattering in the early Universe. We examine the present free-streaming velocity of gravitino dark matter and the limits from observations and simulations of cosmic structures. Assuming that the NLSP sleptons freeze out with a thermal abundance before their decay, we derive new bounds on the slepton and gravitino masses. The implications of the constraints for cosmology and collider phenomenology are discussed and the potential insights from future experiments are outlined. We propose a set of benchmark scenarios with gravitino dark matter and long-lived charged NLSP sleptons and describe prospects for the Large Hadron Collider and the International Linear Collider.
We consider the coupling of quintessence to observable matter in supergravity and study the dynamics of both supersymmetry breaking and quintessence in this context. We investigate how the quintessence potential is modified by supersymmetry breaking and analyse the structure of the soft supersymmetry breaking terms. We pay attention to their dependence on the quintessence field and to the electroweak symmetry breaking, ie the pattern of fermion masses at low energy within the Minimal Supersymmetric Standard Model (MSSM) coupled to quintessence. In particular, we compute explicitly how the fermion masses generated through the Higgs mechanism depend on the quintessence field for a general model of quintessence. Fifth force and equivalence principle violations are potentially present as the vacuum expectation values of the Higgs bosons become quintessence field dependent. We emphasize that equivalence principle violations are a generic consequence of the fact that, in the MSSM, the fermions couple differently to the two Higgs doublets. Finally, we also discuss how the scaling of the cold dark and baryonic matter energy density is modified and comment on the possible variation of the gauge coupling constants, among which is the fine structure constant, and of the proton-electron mass ratio
The string $\alpha^\prime$-correction to the usual Einstein action comprises a Gauss-Bonnet integrand multiplied by non-trivial functions of the modulus field $\chi$ and/or the dilaton field $\phi$. We discuss how the presence of such terms in the four dimensional effective action can explain several novel phenomena, such as a four-dimensional flat Friedmann-Robertson-Walker universe undergoing a cosmic inflation at the early epoch, as well as a cosmic acceleration at late times. The model predicts, during inflation, spectra of both density perturbations and gravitational waves that may fall well within the experimental bounds. The model therefore provides a unified approach for explaining the early and late time accelerating phases of the universe.
We develop a theory of static BPS domain walls in stringy landscape and present a large family of BPS walls interpolating between different supersymmetric vacua. Examples include KKLT models, STU models, type IIB multiple flux vacua, and models with several Minkowski and AdS vacua. After the uplifting, some of the vacua become dS, whereas some others remain AdS. The near-BPS walls separating these vacua may be seen as bubble walls in the theory of vacuum decay. As an outcome of our investigation of the BPS walls, we found that the decay rate of dS vacua to a collapsing space with a negative vacuum energy can be quite large. The parts of space that experience a decay to a collapsing space, or to a Minkowski vacuum, never return back to dS space. The channels of irreversible vacuum decay serve as sinks for the probability flow. The existence of such sinks is a distinguishing feature of the landscape. We show that it strongly affects the probability distributions in string cosmology.
A new class of accelerating, exact, explicit and simple solutions of relativistic hydrodynamics is presented. The 1+1 dimensional solutions generalize certain aspects of both the Landau-Khalatnikov and the Hwa-Bjorken 1+1 dimensional solutions, and are valid and regular both inside and outside the lightcone. The rapidity distribution in each of these new solutions is finite. Generalization is also given for spherical collapses and subsequent expansions in arbitrary number of spatial dimensions. These solutions may describe spherical collapses or explosions of stars, and are utilized here for an advanced estimation of the initial energy density in relativistic heavy ion collisions.
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We show that the gravitino-overproduction problem is prevalent among inflation models in supergravity. An inflaton field \phi generically acquires (effective) non-vanishing auxiliary field G_\phi, if the K\"ahler potential is non-minimal. The inflaton field then decays into a pair of the gravitinos. We extensively study the cosmological constraints on G_\phi for a wide range of the gravitino mass. For many inflation models we explicitly estimate G_\phi, and show that the gravitino-overproduction problem severely constrains the inflation models, unless such an interaction as K = \kappa/2 |\phi|^2 z^2 + h.c. is suppressed (here z is the field responsible for the supersymmetry breaking). We find that many of them are already excluded or on the verge of, if \kappa \sim O(1).
We argue that in fat brane-world scenarios the light propagating in vacuum will, because of massive ``Kaluza--Klein'' (KK) excitations, experience a refraction. The motion of a photon inside a fat brane can be decomposed in the longitudinal and transverse directions with respect to the surface of the brane. Since the light observable propagation is related only with the longitudinal motion, the obsered speed of light depends on the value of the momentum transverse fraction contributing as the massive KK excitations. This is directly connected with the energy of the particles emitting the light, and hence with the frequency of the light itself. Using recent results on the arrival times of radiation of different energies from the measurements of gamma-ray bursters with known redshifts, we establish the limit M>620 TeV on the inverse thickness of the brane, and thus on the masses of the KK excitations. This limit exceeds by at least one order of magnitude the typical energy scale currently in use to characterize brane phenomena in the realm of future colliders.
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The current bounds on the PPN parameters gamma and beta are of the order of 10^-4-10^-5. Various missions aimed at improving such limits by several orders of magnitude have more or less recently been proposed like LATOR, ASTROD, BepiColombo and GAIA. They involve the use of various spacecraft, to be launched along interplanetary trajectories, for measuring the effects of the solar gravity on the propagation of electromagnetic waves. In this paper we explore the possibility of measuring the combination nu=(2+2gamma-beta)/3 of the post-Newtonian gravitoelectric Einstein perigee precession of a test particle to an accuracy of 10^-5-10^-6 with a pair of drag-free spacecraft in the Earth's gravitational field. It turns out that the latest gravity models from the dedicated CHAMP and GRACE missions would allow to reduce the systematic error of gravitational origin just to this demanding level of accuracy. In regard to the non-gravitational errors, the spectral noise density of the drag-free sensors required to reach such level of accuracy would amounts to 10^-8-10^-9 cm s^-2 Hz^-1/2 over very low frequencies. Although not yet obtainable with the present technologies, such level of compensation is much less demanding than those required for, e.g., LISA. As a by-product, an independent measurement of the post-Newtonian gravitomagnetic Lense-Thirring effect with a 0.9% accuracy would be possible as well. The forthcoming Earth gravity models from CHAMP and GRACE will further reduce the systematic gravitational errors in both of such tests.
The Dvali-Gabadadze-Porrati model introduces a parameter, the cross-over scale $r_c$, setting the scale where higher dimensional effects are important. In order to agree with observations and to explain the current acceleration of the Universe, $r_c$ must be of the order of the present Hubble radius. We discuss a mechanism to generate a large $r_c$, assuming that it is determined by a dynamical field and exploiting the quantum effects of the graviton. For simplicity, we consider a scalar field $\Psi$ with a kinetic term on the brane instead of the full metric perturbations. We compute the Green function and the 1-loop expectation value of the stress tensor of $\Psi$ on the background defined by a flat bulk and an inflating brane (self-accelerated or not). We also include the flat brane limit. The quantum fluctuations of the bulk field $\Psi$ provide an effective potential for $r_c$. For a flat brane, the 1-loop effective potential is of the Coleman-Weinberg form, and admits a minimum for large $r_c$ without fine tuning. When we take into account the brane curvature, a sizeable contribution at the classical level changes this picture and the potential develops a (minimum) maximum for the (non-) self-accelerated branch.
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