Tachyonization of the ΛCDM cosmological model

In this work a tachyonization of the ΛCDM model for a spatially flat Friedmann–Robertson–Walker space–time is proposed. A tachyon field and a cosmological constant are considered as the sources of the gravitational field. Starting from a stability analysis and from the exact solutions for a standard tachyon field driven by a given potential, the search for a large set of cosmological models which contain the ΛCDM model is investigated. By the use of internal transformations two new kinds of tachyon fields are derived from the standard tachyon field, namely, a complementary and a phantom tachyon fields. Numerical solutions for the three kinds of tachyon fields are determined and it is shown that the standard and complementary tachyon fields reproduces the ΛCDM model as a limiting case. The standard tachyon field can also describe a transition from an accelerated to a decelerated regime, behaving as an inflationary field at early times and as a matter field at late times. The complementary tachyon field always behaves as a matter field. The phantom tachyon field is characterized by a rapid expansion where its energy density increases with time.     

Moving branes in the presence of background tachyon fields

We compute the boundary state associated with a moving Dp-brane in the presence of the open string tachyon field as a background field. The effect of the tachyon condensation on the boundary state is discussed. It leads to a boundary state associated with a lower-dimensional moving D-brane or a stationary instantonic D-brane. The former originates from condensation along the spatial directions and the latter comes from the temporal direction of the D-brane worldvolume. Using the boundary state, we also study the interaction amplitude between two arbitrary Dp ₁- and Dp ₂-branes. The long-range behavior of the amplitude is investigated, demonstrating an obvious deviation from the conventional form, due to the presence of the background tachyon field.     

Non-supersymmetric orientifolds of Gepner models

Starting from a previously collected set of tachyon-free closed strings, we search for N=2$N=2$ minimal model orientifold spectra which contain the standard model and are free of tachyons and tadpoles at lowest order. For each class of tachyon-free closed strings – bulk supersymmetry, automorphism invariants or Klein bottle projection – we do indeed find non-supersymmetric and tachyon free chiral brane configurations that contain the standard model. However, a tadpole-cancelling hidden sector could only be found in the case of bulk supersymmetry. Although about half of the examples we have found make use of branes that break the bulk space–time supersymmetry, the resulting massless open string spectra are nevertheless supersymmetric in all cases. Dropping the requirement that the standard model be contained in the spectrum, we find chiral tachyon and tadpole-free solutions in all three cases, although in the case of bulk supersymmetry all massless spectra are supersymmetric. In the other two cases we find truly non-supersymmetric spectra, but a large fraction of them are nevertheless partly or fully supersymmetric at the massless level.     

On effective actions of non-BPS branes and their higher derivative corrections

By calculating various disk level S-matrix elements and studying in details their momentum expansions, we have extracted some of the couplings in tachyon DBI action and Wess–Zumino terms of the non-BPS branes, and their higher derivative corrections. In particular, we have found that there is exact consistency between field theory and string theory tachyon pole of S-matrix element of one RR and three tachyons provided that one takes into account the fact that the tachyon vertex operator in 0 picture to be along the Pauli matrix σ₁${\sigma }_1$ whereas the tachyon in −1 picture to be along the σ₂${\sigma }_2$ direction. This internal CP factors should be included in the tachyon DBI part of the effective action.     

Tachyon warm-inflationary universe model in the weakly dissipative regime

The warm-inflationary universe model in a tachyon field theory is studied in the weakly dissipative regime. We develop our model for an exponential potential and the dissipation parameter Γ=Γ ₀=constant. We describe scalar and tensor perturbations for this scenario.     

Changes in the hyperfine interactions in the Fe80Nb3Cu1B16 metallic glass under tensile loading

The influence of tensile stress on the changes in hyperfine parameters obtained by Mössbauer spectroscopy of the ferromagnetic amorphous NANOPERM-type Fe₈₀Nb₃Cu₁B₁₆ alloy has been investigated. The bulk changes are obtained by Mössbauer spectroscopy in a transmission geometry using γ-rays; the surface properties are studied in a scattering geometry using conversion electrons. The Mössbauer spectra are analyzed by two distributions of hyperfine inductions, which result in high-field and low-field components corresponding with a model of clusters formed by predominantly iron atoms and intermediate phase containing mainly the Nb, Cu and B atoms. The obtained results are completed by the bulk and surface magnetic measurements, with observations of surface morphology by AFM and microstructure by SEM. The investigations yield approximately linear increase of hyperfine parameters and slight deterioration of the bulk as well as surface magnetic characteristics with tensile loading.     

Tachyon emission from white-holes

Investigations are made about the motion of a radially outward propagating tachyon which is created in the singularity with the white-hole. The problem of confinement or escape of such a tachyon from a white-hole is discussed. It is shown that the confinement or escape of the tachyon depends on the maximum radius of the white-hole and also on a parameterk (defined in the text) associated with the momentum of the tachyon. Also it is shown that when a tachyon escapes it always escapes beofre the white-hole has expanded to half its Schwarzschild radius.     

Spin polarization in low-energy electron diffraction: Surface analysis of Pt(111)

Transverse and longitudinal spin polarization components arising in the diffraction of unpolarized low-energy electrons from Pt(111) have been studied as functions of energy and diffraction geometry. Experimental data measured by means of a Mott detector are in good agreement with theoretical results obtained by relativistic LEED calculations. A detailed investigation of the selective sensitivity of the calculated polarization profiles to the surface relaxation δ₁₂ and to various non-structural model features firstly supports an ion-core potential involving an energy-dependent exchange approximation (by discriminating against two band structure potentials), and secondly establishes the geometry of Pt(111) as unreconstructed with a possible slight outward relaxation of the topmost atomic layer (δ₁₂ = 0.5% ± 1.0% of the bulk interlayer distance). A surface Debye temperature is found close to the bulk value 230 K.     

Gravitational collapse with tachyon field and barotropic fluid

A particular class of space-time, with a tachyon field, $\phi $ , and a barotropic fluid constituting the matter content, is considered herein as a model for gravitational collapse. For simplicity, the tachyon potential is assumed to be of inverse square form i.e., $V(\phi )\sim \phi ^{-2}$ . Our purpose, by making use of the specific kinematical features of the tachyon, which are rather different from a standard scalar field, is to establish the several types of asymptotic behavior that our matter content induces. Employing a dynamical system analysis, complemented by a thorough numerical study, we find classical solutions corresponding to a naked singularity or a black hole formation. In particular, there is a subset where the fluid and tachyon participate in an interesting tracking behaviour, depending sensitively on the initial conditions for the energy densities of the tachyon field and barotropic fluid. Two other classes of solutions are present, corresponding respectively, to either a tachyon or a barotropic fluid regime. Which of these emerges as dominant, will depend on the choice of the barotropic parameter, $\gamma $ . Furthermore, these collapsing scenarios both have as final state the formation of a black hole.     

Tachyons, Lamb shifts and superluminal chaos

An elementary account on the origins of cosmic chaos in an open and multiply connected universe is given; there is a finite region in the open 3-space in which the world-lines of galaxies are chaotic, and the mixing taking place in this chaotic nucleus of the universe provides a mechanism to create equidistribution. The galaxy background defines a distinguished frame of reference and a unique cosmic time order; in this context superluminal signal transfer is studied. Tachyons are described by a real Proca field with negative mass square, coupled to a current of subluminal matter. Estimates on tachyon mixing in the geometric optics limit are derived. The potential of a static point source in this field theory is a damped periodic function. We treat this tachyon potential as a perturbation of the Coulomb potential, and study its effects on energy levels in hydrogenic systems. By comparing the induced level shifts to high-precision Lamb shift measurements and QED calculations, we suggest a tachyon mass of 2.1 keV/c² and estimate the tachyonic coupling strength to subluminal matter. The impact of the tachyon field on ground state hyperfine transitions in hydrogen and muonium is investigated. Bounds on atomic transition rates effected by tachyon radiation as well as estimates on the spectral energy density of a possible cosmic tachyon background radiation are derived. Received 13 August 1999 and Received in final form 7 February 2000     

Ward identities of W∞ symmetry and higher-genus amplitudes in 2D string theory

The Ward identities of the W_∞ symmetry in two-dimensional string theory in the tachyon background are studied in the continuum approach. We consider amplitudes different from 2D string ones by the external leg factor and derive the recursion relations among them. The recursion relations have non-linear terms which give relations among the amplitudes defined on different genus. The solutions agree with the matrix model results even in higher genus. We also discuss the differences of the roles of the external leg factor between the c_M = 1 model and the c_m < 1 model.     

Dynamic Equations for Tachyon Gas

Dynamics of the tachyon gas is considered. It is interesting in the relation, that dark matter phenomenon is explained freely by existence of the tachyon gas. Tachyons have two unexpected properties: (1) a single tachyon cannot be detected and (2) the tachyon gas can be detected by its gravitational field. Although molecules (tachyons) of the tachyon gas moves with superluninal velocities, the mean motion of these molecules appears to be less, than the speed of the light. The tachyon gas properties differs from those of usual gas. The pressure of the tachyon gas is very high. It is not isotropic and depends on the gravitational potential. As a result the tachyon gas may form huge halos around galaxies. These halos have very large and almost constant density. This circumstance can explain the law of star velocities at the periphery of a galaxy. Properties of the tachyon gas admit one to consider it as a dark matter.     

Interacting Tachyon Dark Energy Model in Gauss-Bonnet Cosmology

We investigate the tachyon scalar filed model of dark energy in the framework of Gauss-Bonnet cosmology. We consider a spatially non-flat universe containing interacting tachyon dark energy with pressureless dark matter. We obtain the equation of state and deceleration parameters. We also reconstruct the potential and the dynamics for the tachyon scalar field model, which describe accelerated expansion of the universe.     

Tachyon trajectories in Schwarzschild’s space-time

The motion of a tachyon in the empty Schwarzschild solution outside a massm is discussed. It is shown that a tachyon falling radially inwards never reaches the space-time singularity at the origin. Instead, it is bounced back at a point inside the Schwarzschild radius. The causal and non-causal aspects of such a bounce are considered. It is shown that a tachyon dropped from a radial co-ordinate <2.56m always airives before it went in whereas a tachyon dropped from a radial co-ordinate >3.27m always arrives later than its starting time. The more general case of a tachyon with a finite angular momentum is also analyzed. The possible astrophysical consequences of the presence of tachyons near condensed or collapsing objects and black holes are qualitatively discussed.     

Cosmological Implications of Interacting Polytropic Gas Dark Energy Model in Non-flat Universe

The polytropic gas model is investigated as an interacting dark energy scenario. The cosmological implications of the model including the evolution of EoS parameter w _Λ, energy density Ω_Λ and deceleration parameter q are investigated. We show that, depending on the parameter of model, the interacting polytropic gas can behave as a quintessence or phantom dark energy. In this model, the phantom divide is crossed from below to up. The evolution of q in the context of polytropic gas dark energy model represents the decelerated phase at the early time and accelerated phase later. The singularity of this model is also discussed. Eventually, we establish the correspondence between interacting polytropic gas model with tachyon, K-essence and dilaton scalar fields. The potential and the dynamics of these scalar field models are reconstructed according to the evolution of interacting polytropic gas.     

Interaction of moving branes with background massless and tachyon fields in superstring theory

Using the boundary state formalism, we study a moving Dp-brane in a partially compact space-time in the presence of background fields: the Kalb-Ramond field B _μν, a U(1) gauge field A _α, and the tachyon field. The boundary state enables us to obtain the interaction amplitude of two branes with the above back-ground fields. The branes are parallel or perpendicular to each other. Because of the presence of background fields, compactification of some space-time directions, motion of the branes, and the arbitrariness of the dimensions of the branes, the system is rather general. Due to the tachyon fields and velocities of the branes, the behavior of the interaction amplitude reveals obvious differences from the conventional behavior.     

Dark energy cosmology with tachyon field in teleparallel gravity

We construct a tachyon teleparallel dark energy model for a homogeneous and isotropic flat universe in which a tachyon as a non-canonical scalar field is non-minimally coupled to gravity in the framework of teleparallel gravity. The explicit form of potential and coupling functions are obtained under the assumption that the Lagrangian admits the Noether symmetry approach. The dynamical behavior of the basic cosmological observables is compared to recent observational data, which implies that the tachyon field may serve as a candidate for dark energy.     

Electromagnetic radiation of a charged tachyon moving in a dispersive medium

At the present time a number of studies [1–6] have considered the possible properties of ultrarelativistic tachyon particles. In particular, [5, 6] were dedicated to analysis of the radiation of charged tachyons in a vacuum. The goal of the present study is to analyze the radiation emitted by charged tachyons in an immobile dispersive medium. We will consider the following problem: a charged particle moves with velocity v in an immobile dispersive medium and at time t=0 decays to neutral particles and a charged tachyon which moves at a velocity v₀. The tachyon in turn, because of collisions with neutral particles of the medium, at time t= forms a charged particle (electron) also moving with velocity v.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 130–134, June, 1978.In conclusion, the author expresses his sincere thanks to Professor Ya. P. Terletskii for his interest and aid in the study.     

On modified tachyon DBI action

Recently a modification of the tachyon DBI action has been proposed in which the tachyon carries the internal CP matrix σ₁${\sigma }_1$ and σ₂${\sigma }_2$. In this paper, we find the momentum expansion of the disk level S-matrix element of four tachyons and one gauge field in superstring theory and show that the first and second order terms of the expansion are in perfect agreement with the above tachyon DBI action.