Narrow Results

We assume that universe is dominated by non-relativistic matter and tachyon field and reconstruct the potential of tachyon field directly from the effective equation of state (EOS) of dark energy. We apply the method to four known parametrizations of equation of state and discuss the general features of the resulting potentials.

In this paper we study the statefinder parameters for the tachyon dark energy model. There are two kinds of stable attractor solutions in this model. The statefinder diagrams characterize the properties of the tachyon dark energy model. Our results show that the evolving trajectories of the attractor solutions lie in the total region and pass through the LCDM fixed point, which is different from other dark energy model.

In this paper we study the statefinder parameters for the coupled tachyon dark energy model which has two types of stable scaling solutions. It is found that the evolving trajectories of the solutions are different in the statefinder parameters plane. These trajectories lie in the two regions r > 1, s < 0 and r < 1, s > 0, and pass through the LCDM fixed point. So, through the statefinder diagnostic, we not only characterize the properties of the coupled tachyon dark energy model, but also show the difference from other dark energy model.

In this paper we investigate a tachyon field model in cosmology, provided its interaction with the quintessence or phantom fields. The model takes into account this interaction beyond the usual approach, in which the interaction is phenomenologically described by the energy flow between the matter components. In our model, the interaction of tachyon field with a canonical scalar field is taken into account through the interaction potential in the total Lagrangian of the system, like in the case of two or more canonical scalar fields. We obtain the different types of exact solution for the model by employing the so-called "first order formalism" procedures.

In this paper, we have researched tachyon field, k-essence and quintessence dark energy (DE) models for Friedmann–Robertson–Walker (FRW) universe with varying G and $\mathrm{\Lambda }$ in f(R, T) gravitation theory. The theory of f(R, T) is proposed by Harko et al. [Phys. Rev. D84, 024020, 2011]. In this theory, R is the Ricci scalar and T is the trace of energy–momentum tensor. For the solutions of field equations, we have used linearly varying deceleration parameter (LVDP), the equation of state (EoS) and the ratio between $\mathrm{\Lambda }$ and Hubble parameter. Also, we have discussed some physical behavior of the models with various graphics.

In this research, we have investigated tachyon and $k$-essence dark energy (DE) candidates with varying $G$ and $\mathrm{\Lambda }$ for Marder universe in $f(R,T)$ gravitation theory [Harko et al., Phys. Rev. D84, 024020 (2011)]. Here, $f(R,T)$ is the arbitrary function of Ricci scalar ($R$) and the trace of $T_{ik}$. To solve $f(R,T)$ gravity field equations, three different approximations have been used such as cosmological term, equation of state (EoS) parameter and the anisotropy feature of Marder universe. Also, we have discussed some physical results with various graphics.

We present an inflationary solution of the early universe considering tachyon field. The technique of Zhuravlev and Chervon to obtain inflationary cosmological models without restrictions on a scalar field potential is employed here. We note that like the scalar field, the inflationary solution obtained here with tachyon field does not depend on the potential. However, unlike the scalar field, inflation with the tachyon field is obtained for restricted values of the field to begin with. We present the potential for which one gets inflation. Unlike the scalar field potential, the tachyonic potential is not regular at all values of the field. The solution obtained here with tachyon field is new.

By studying a tachyon field on the DGP brane model, in order to embed the 4D standard Friedmann equation with a brane tachyon field in the 5D bulk, we present the metric of the 5D space–time. Then, adopting the inverse square potential of the tachyon field, we obtain an expanding universe with a power law on the brane and an exact 5D solution.

The tachyon field in cosmology is studied in this paper by applying the generating function method to obtain exact solutions. The equation of state parameter of the tachyon field is , which can be expressed as a function in terms of the redshift z. Based on these solutions, we propose some tachyon-inspired dark energy models to explore the properties of the corresponding cosmological evolution. The explicit relations between the Hubble parameter and redshift enable us to test the models with SNeIa data sets easily. In this paper, we employ the SNeIa data with the parameter measured from the SDSS and the shift parameter from WMAP observations to constrain the parameters in our models.

The dynamical behavior of tachyon field with an inverse potential is investigated in loop quantum cosmology. It reveals that the late-time behavior of tachyon field with this potential leads to a power-law expansion. In addition, an additional barotropic perfect fluid with the adiabatic index 0 < γ < 2 is added and the dynamical system is shown to be an autonomous one. The stability of this autonomous system is discussed using phase plane analysis. There exist up to five fixed points with only two of them possibly stable. The two stable node (attractor) solutions are specified and their cosmological indications are discussed. For the tachyon dominated solution, the further discussion is stretched to the possibility of considering tachyon field as a combination of two parts which respectively behave like dark matter and dark energy.

Assuming that the e-folding number is just determined by the change of the scale factor, the tachyonic inflation theory in loop quantum cosmology (LQC) has been discussed. Considering the tachyon field with exponential potential and inverse quadratic potential, we find that the evolutionary pictures of super inflation are affected by the potentials and the initial conditions. However it cannot provide enough e-folding number, no matter which condition is chosen. Therefore a slow-rolling inflation is necessary. The e-folding number for slow-rolling inflation depends on the values of the parameter α of the exponential potential and the initial conditions. To get enough e-folding number, α should be small. Based on the slow-rolling inflation happens immediately when the super inflation ends, and the scale factor continuously grows during the whole inflation stage, we consider an e-folding number provided by the whole inflationary stage, and we find that it is easier to get enough e-folding number when the scale factor increases during all the inflation phase.

In this paper we investigate the observational signatures of Loop Quantum Cosmology (LQC) in the CMB data. First, we concentrate on the dynamics of LQC and we provide the basic cosmological functions. We then obtain the power spectrum of scalar and tensor perturbations in order to study the performance of LQC against the latest CMB data. We find that LQC provides a robust prediction for the main slow-roll parameters, like the scalar spectral index and the tensor-to-scalar fluctuation ratio, which are in excellent agreement within $1\sigma$ with the values recently measured by the Planck collaboration. This result indicates that LQC can be seen as an alternative scenario with respect to that of standard inflation.

By proposing a new cosmic fluid model of $-1\le \omega \le 0$ as an alternative to the generalized Chaplygin gas, we reexamine the role of Chaplygin gaslike fluid models in understanding dark energy and dark matter. Instead of as a unified dark matter, the fluid is suggested to be a mixture of unclustered dark energy and pressureless dark matter. Within such a scenario, the sub-horizon fluctuations of matter are stable and scale invariant, similar to those in standard $\mathrm{\Lambda }\text{CDM}$ model.

We investigate the observational signatures of quantum cosmology in the Cosmic Microwave Background data provided by Planck collaboration. We apply the warm inflationary paradigm with a tachyon scalar field to the loop quantum cosmology. In this context, we first provide the basic cosmological functions in terms of the tachyon field. We then obtain the slow-roll parameters and the power spectrum of scalar and tensor fluctuations, respectively. Finally, we study the performance of various warm inflationary scenarios against the latest Planck data and we find a family of models which are in agreement with the observations.

Cosmological solution to the gravitational field equations in the generalized Randall–Sundrum model for an anisotropic brane with Bianchi-I geometry and perfect fluid as matter sources has been considered. The matter on the brane is described by a tachyonic field. The solution admits inflationary era and at a later epoch the anisotropy of the universe washes out. We obtain two classes of cosmological scenario: in the first case, universe evolves from singularity and in the second case, universe expands without singularity.

In this work, we investigate a cosmological model with the tachyon and fermion fields with barotropic equation of state, where pressure $p$, energy density $\rho$ and barotropic index $\gamma$ are related by the relation $p=(\gamma -1)\rho$. We applied the tachyonization method which allows to consider cosmological model with the fermion and the tachyon fields, driven by special potential. In this paper, tachyonization model was defined from the stability analysis and exact solution standard of the tachyon field. Analysis of the solution via statefinder parameters illustrated that our model in fiducial points with deceleration parameter $q=0.5$ and statefinder $r=1$ corresponds to the matter-dominated universe (SCDM) but ends its evolution at a point in the future $(q=-1,r=1)$ which corresponds to the de Sitter expansion. Comparison of the model parameters with the cosmological observation data demonstrates that our proposed cosmological model is stable at barotropic index ${\gamma }_0=0.00744$.

In this paper, we study the effects of the presence of the tachyon field around the black hole. We show that in presence of the tachyon field, unlike the ordinary canonical scalar field, the time evolution of the black hole mass depends on the potential of this field. By considering several types of potential, we study the behavior of the black hole mass and its time evolution and find some interesting results. We find that the presence of the tachyon field causes the accretion of the mass into the black hole. We also show that with linear and hilltop potentials, in some ranges of the parameters space, the mass of the black hole can decrease even without any Hawking radiation.

In this paper, we study the dynamics of a scalar–tensor model of dark energy in which a scalar field that plays the role of dark energy, non-minimally coupled to the Gauss–Bonnet invariant in four dimensions. We utilize the dynamical system method to extract the critical points of the model and to conclude about their stability, we investigate the sign of the corresponding eigenvalues of the perturbation matrix at each point numerically. For exponential form of the scalar field potential and coupling function, we find five stable points among the critical points of the autonomous system. We also find four scaling attractor solutions with the property that the ratio of dark energy to dark matter density parameters are of order one. These solutions give the hope to alleviate the well-known coincidence problem in cosmology.

This study investigates a bulk viscous fluid anisotropic cosmological model with $f(Q)$ gravity. Here, $Q$ stands for the nonmetricity factor that drives gravitational interaction. We reconstructed the associated parameters with Tsallis holographic dark energy (THDE). We have solved the modified Einstein’s field equations by considering the bulk viscosity factor $\xi ={\xi }_0+{\xi }_{1}H+{\xi }_2(Ḣ+H^2)$. Under the viscous and nonviscous THDE frameworks, we have obtained the expressions of $f(Q)$ using the power-law form of expansion. We have investigated the nature of various energy conditions for the stability analysis. The positive behavior of DEC and WEC indicates the model’s validation; on the other hand, SEC is violating, indicating the universe’s accelerated expansion. We have also investigated the reconstructed EoS parameter ${\omega }_{\mathrm{\text{rec}},T}$ for bulk viscosity and obtained the one that lies in both quintessence and phantom regions. We also discussed the correspondence of the tachyon scalar field with THDE energy density in $f(Q)$ gravity. This correspondence permits the reconstruction of potentials and dynamics for scalar field models describing accelerated expansion.

Holographic dark energy (HDE) models are significantly different from standard dark energy (DE) models since they are based on holographic principles rather than mentioning a term in Lagrangian. Nojiri et al. [Barrow entropic dark energy: A member of generalized holographic dark energy family, Phys. Lett. B 825 (2022) 136844] proposed a generalized Barrow HDE (BHDE) model depending on particle horizon and future horizon, where the infra-red cut-off is considered as a usual cut-off. In this paper, we have revisited the generalized BHDE adopting the Granda–Oliver cut-off as the standard cut-off for the model. We have generalized BHDE behaviors with two different cut-offs, future horizon $L_f$ and particle horizon $L_p$. The holographic cut-off is extended to depend on $L_{\mathrm{\text{GO}}}=L_{\mathrm{\text{GO}}}(L_p,{\dot{L}}_p,{\ddot{L}}_p,\dots ,L_f,{\dot{L}}_f,\dots ,a)$, where a is the scale factor. Using this formalism, we demonstrated that the Barrow entropic DE model is equivalent to the generalized HDE model, where two ways are used to compute the respective holographic cut-off: first, in terms of particle horizon and its derivative, and second, future horizon and its derivative. We use 57 observational data points to determine the current Hubble constant $H_0$. We have studied the behavior of few quantities, such as DE density $({\rho }_{\mathrm{\text{de}}})$, pressure $(p_{\mathrm{\text{de}}})$, equation of state (EoS) parameter under the observational data. Here, we have to find the EoS parameter for generalized HDE, equivalent to Barrow entropic DE model. Besides this, we have also discussed k-essence and tachyon DE models.