Tachyon Models Research Articles

Primordial features due to a step in the inflaton potential

Certain oscillatory features in the primordial scalar power spectrum are known to provide a better fit to the outliers in the cosmic microwave background data near the multipole moments of ℓ = 22 and 40. These features are usually generated by introducing a step in the popular, quadratic potential describing the canonical scalar field. Such a model will be ruled out, if the tensors remain undetected at a level corresponding to a tensor-to-scalar ratio of, say, r ≃ 0.1. In this work, in addition to the popular quadratic potential, we investigate the effects of the step in a small field model and a tachyon model. With possible applications to future datasets (such as PLANCK) in mind, we evaluate the tensor power spectrum exactly, and include its contribution in our analysis. We compare the models with the WMAP (five as well as seven-year), the QUaD and the ACBAR data. As expected, a step at a particular location and of a suitable magnitude and width is found to improve the fit to the outliers (near ℓ = 22 and 40) in all these cases. We point out that, if the tensors prove to be small (say, r≲0.01), the quadratic potential and the tachyon model will cease to be viable, and more attention will need to be paid to examples such as the small field models.

Tensor-to-scalar ratio in punctuated inflation

Recently, we have shown that scalar spectra with lower power on large scales and certain other features naturally occur in punctuated inflation, i.e. the scenario wherein a brief period of rapid roll is sandwiched between two stages of slow roll inflation. Such spectra gain importance due to the fact that they can lead to a better fit of the observed CMB anisotropies, when compared to the conventional, featureless, power law spectrum. In this paper, with examples from the canonical scalar field as well as the tachyonic models, we illustrate that, in punctuated inflation, a drop in the scalar power on large scales is always accompanied by a rise in the tensor power and, hence, an even more pronounced increase in the tensor-to-scalar ratio $r$ on these scales. Interestingly, we find that $r$ actually exceeds well beyond unity over a small range of scales. To our knowledge, this work presents for the first time, examples of single scalar field inflationary models wherein $r\ensuremath{\gg}1$. This feature opens up interesting possibilities. For instance, we show that the rise in $r$ on large scales translates to a rapid increase in the angular power spectrum, ${C}_{\ensuremath{\ell}}^{\mathrm{BB}}$, of the $B$-mode polarization of the CMB at the low multipoles. We discuss the observational implications of these results.

Holographic interacting tachyon model

We propose an interacting holographic tachyon model of dark energy. A correspondence between the tachyon energy density and the interacting holographic dark energy is established. As a result, we reconstruct the potential of the interacting holographic tachyon field and the dynamics of the tachyon field, in a flat Friedmann-Robertson-Walker background. We show that the evolution of the universe for −1 < w < 0 can be completely described by the resulting interacting tachyon model.

INTERACTING HOLOGRAPHIC TACHYON MODEL OF DARK ENERGY

We propose a holographic tachyon model of dark energy with interaction between the components of the dark sector. The correspondence between the tachyon field and the holographic dark energy densities allows the reconstruction of the potential and the dynamics of the tachyon scalar field in a flat Friedmann–Robertson–Walker universe. We show that this model can describe the observed accelerated expansion of our universe with a parameter space given by the most recent observational results.

Interacting agegraphic tachyon model of dark energy

Scalar-field dark energy models like tachyon are often regarded as an effective description of an underlying theory of dark energy. In this Letter, we implement the interacting agegraphic dark energy models with tachyon field. We demonstrate that the interacting agegraphic evolution of the universe can be described completely by a single tachyon scalar field. We thus reconstruct the potential as well as the dynamics of the tachyon field according to the evolutionary behavior of interacting agegraphic dark energy.

RECONSTRUCTING THE POTENTIALS FOR THE QUINTESSENCE AND TACHYON DARK ENERGY, FROM THE HOLOGRAPHIC PRINCIPLE

We propose holographic quintessence and tachyon models of dark energy. The correspondence between the quintessence and tachyon energy densities with the holographic density allows the reconstruction of the potentials and the dynamics for the quintessence and tachyon fields, in the flat FRW background. The proposed infrared cutoff for the holographic energy density works for two cases of the constant α: for α &lt; 1 we reconstruct the holographic quintessence model in the region before the ω = -1 crossing for the EoS parameter. The cosmological dynamics for α &gt; 1 is also reconstructed for the holographic quintessence and tachyon models.

Cosmological tachyon condensation

We consider the prospects for dark matter/energy unification in k-essence type cosmologies. General mappings are established between the k-essence scalar field, the hydrodynamic and braneworld descriptions. We develop an extension of the general relativistic dust model that incorporates the effects of both pressure and the associated acoustic horizon. Applying this to a tachyon model, we show that this inhomogeneous variable Chaplygin gas does evolve into a mixed system containing cold dark matter like gravitational condensate in significant quantities. Our methods can be applied to any dark energy model, as well as to mixtures of dark energy and traditional dark matter.

Tachyon cosmology, supernovae data, and the big brake singularity

We compare the existing observational data on type Ia supernovae with the evolutions of the Universe predicted by a one-parameter family of tachyon models which we have introduced recently [Phys. Rev. D 69, 123512 (2004)]. Among the set of the trajectories of the model which are compatible with the data there is a consistent subset for which the Universe ends up in a new type of soft cosmological singularity dubbed big brake. This opens up yet another scenario for the future history of the Universe besides the one predicted by the standard {lambda}CDM model.

Open inflationary universe with tachyonic matter

We study a one-field open inflationary model in a universe dominated by tachyon matter [1, 2]. We work in the scenario of the one-bubble universe where bubble formation is described by the Coleman-De Lucia (CDL) instanton [3]. The conditions of existence and basic characteristics of the CDL instanton were determinated for the tachyon model using analytical and numerical approaches. Also, we study the Lorentzian regime, that is, the period of inflation after tunnelling has occurred. In particular we focus on the viability of the model in order to generate a consistent inflationary scenario, compatible with observations.

Emergent universe and the phantom tachyon model

In this work, I have considered that the universe is filled with normal matter and a phantom field (or tachyonic field). If the universe is filled with a scalar field, Ellis et al have shown that an emergent scenario is possible only for k = +1, i.e. for a closed universe. Here I have shown that the emergent scenario is possible for a closed universe if the universe contains the normal tachyonic field. But for a phantom field (or tachyonic field), the negative kinetic term can generate the emergent scenario for all values of k(=0, ± 1). From recently developed statefinder parameters, the behaviour of different stages of the evolution of the emergent universe has been studied. The static Einstein universe and the stability analysis have been briefly discussed for both phantom and tachyon models.

Dynamics in nonlocal cosmological models derived from string field theory

A general class of nonlocal cosmological models is considered. A new method for solving nonlocal Friedmann equations is proposed, and solutions of the Friedmann equations with nonlocal operator are presented. The cosmological properties of these solutions are discussed. Especially indicated is the p-adic cosmological model in which we have obtained a nonsingular bouncing solution and string field theory tachyon model in which we have obtained full solution of nonlocal Friedmann equations with w=-1 at large times. The possibility of obtaining a realistic value of the cosmological constant from nonlocal cosmological models is also discussed.

Holographic tachyon model of dark energy

In this Letter we consider a correspondence between the holographic dark energy density and tachyon energy density in FRW universe. Then we reconstruct the potential and the dynamics of the tachyon field which describe tachyon cosmology.

Instability of holographic dark energy models

We investigate the difference between holographic dark energy, Chaplygin gas, and tachyon model with constant potential. For this purpose, we examine their squared speeds of sound which are evaluated to zeroth order in perturbation theory and hence depends only on time. We find that the squared speed for holographic dark energy is always negative when choosing the future event horizon as the IR cutoff, while those for Chaplygin gas and tachyon are non-negative. This means that the perfect fluid for holographic dark energy is classically unstable. Hence the holographic interpretation for Chaplygin gas and tachyon is problematic.

Holographic tachyon model

We propose in this Letter a holographic model of tachyon dark energy. A connection between the tachyon scalar-field and the holographic dark energy is established, and accordingly, the potential of the holographic tachyon field is constructed. We show that the holographic evolution of the universe with c⩾1 can be described completely by the resulting tachyon model in a certain way.

Tachyon dark energy models: Dynamics and constraints

We explore the dynamics of dark energy models based on a Dirac-Born-Infeld (DBI) tachyonic action, studying a range of potentials. We numerically investigate the existence of tracking behavior and determine the present-day value of the equation of state parameter and its running, which are compared with observational bounds. We find that tachyon models have quite similar phenomenology to canonical quintessence models. While some potentials can be selected amongst many possibilities and fine-tuned to give viable scenarios, there is no apparent advantage in choosing a DBI scalar field instead of a Klein-Gordon one.

Exact scaling solutions and fixed points for general scalar field

We show that the most general dark energy model that possesses a scaling solution ρϕ∝an is the k-essence model, which includes both of the quintessence and tachyon models. The exact scaling solutions are then derived. The potential that gives the tracking solution in which dark energy exactly tracks the background matter field is the inverse squared potential. The quintessence field with exponential potential can be obtained from the k-essence field with the inverse squared potential. We also find the fixed points and study their main properties, whereby the scalar field dominant fixed point is identified.

Curvaton reheating in tachyonic inflationary models

The curvaton reheating in a tachyonic inflationary universe model with an exponential potential is studied. We have found that the energy density in the kinetic epoch, has a complicated dependencies of the scale factor. For different scenarios, the temperature of reheating is computed. These temperature result to be analogous to those obtained in the standard case of the curvaton scenario.

Inflation from geometrical tachyons

We propose an alternative formulation of tachyon inflation using the geometrical tachyon arising from the time dependent motion of a BPS $D3$-brane in the background geometry due to $k$ parallel $NS$5-branes arranged around a ring of radius $R $. Due to the fact that the mass of this geometrical tachyon field is $\sqrt{2/k} $ times smaller than the corresponding open-string tachyon mass, we find that the slow roll conditions for inflation and the number of e-foldings can be satisfied in a manner that is consistent with an effective 4-dimensional model and with a perturbative string coupling. We also show that the metric perturbations produced at the end of inflation can be sufficiently small and do not lead to the inconsistencies that plague the open string tachyon models. Finally we argue for the existence of a minimum of the geometrical tachyon potential which could give rise to a traditional reheating mechanism.

WMAP BOUNDS ON BRANEWORLD TACHYONIC INFLATION

We analyze the implications of the Wilkinson Microwave Anisotropy Probe (WMAP) results for a braneworld tachyonic model of inflation. We find that WMAP bounds on ns allow us to constrain significantly the parameter space of the model; in particular, extremely weak string coupling is required, gs~10-15. Moreover, our analysis shows that the running of the scalar spectral index is within the bounds determined by WMAP for the allowed range of model parameters; however, it is not possible to obtain ns&gt;1 on large scales and ns&lt;1 on small scales.

Tachyons, scalar fields, and cosmology

We study the role that tachyon fields may play in cosmology as compared to the well-established use of minimally coupled scalar fields. We first elaborate on a kind of correspondence existing between tachyons and minimally coupled scalar fields; corresponding theories give rise to the same cosmological evolution for a particular choice of the initial conditions but not for any other. This leads us to study a specific one-parameter family of tachyonic models based on a perfect fluid mixed with a positive cosmological constant. For positive values of the parameter one needs to modify Sen's action and use the sigma process of resolution of singularities. The physics described by this model is dramatically different and much richer than that of the corresponding scalar field. For particular choices of the initial conditions the universe, that does mimick for a long time a de Sitter-like expansion, ends up in a finite time in a special type of singularity that we call a "big brake". This singularity is characterized by an infinite deceleration.