The modified gravity with F(R,G) Lagrangian, G is the Gauss-Bonnet invariant, is considered. It is shown that the phantom-divide-line crossing and the deceleration to acceleration transition generally occur in these models. Our results coincide with the known results of f(R)-gravity and f(G)-gravity models. The contribution of quantum effects to these transitions is calculated, and it is shown that in some special cases where there are no transitions in classical level, quantum contributions can induce transitions. The quantum effects are described via the account of conformal anomaly.Comment: 11 pages, LaTeX, a paragraph added, to be appeared in Phys. Rev.
We explore the inflationary phase of a scalar field with a kinetic term non-minimally coupled to gravity. We find that one of the slow-roll conditions is naturally consequence of the equation of motion of the scalar field. Thus, slow-roll conditions impose fewer constraints on potentials than other inflationary models. Moreover, it is demonstrated that the inflationary phase can be described by just one slow-roll parameter. By investigating the metric perturbations, it is shown that except for one potential, almost all potentials have the same pattern in the (n s , r) plane. We provide an exact solution for the exceptional case. The exact solution represents the condensed scalar field and results in an accelerated expansion.
The generalized Gauss-Bonnet theory, introduced by Lagrangian F (R, G), has been considered as a general modified gravity for explanation of the dark energy. G is the Gauss-Bonnet invariant. For this model, we seek the situations under which the late-time behavior of the theory is the deSitter space-time. This is done by studying the two-dimensional phase space of this theory, i.e. the R − H plane. By obtaining the conditions under which the de-Sitter space-time is the stable attractor of this theory, several aspects of this problem have been investigated. It has been shown that there exist at least two classes of stable attractors: the singularities of the F (R, G), and the cases in which the model has a critical curve, instead of critical points. This curve is R = 12H 2 in R − H plane. Several examples, including their numerical calculations, have been discussed.
The first multi-color CCD photometric study of 27 δ Scuti stars is presented, which was performed over the three observing years. We obtained the maximum times and magnitude changes in the observation period for each star. The ephemeris of our δ Scuti stars was calculated based on the Markov chain Monte Carlo (MCMC) method, using the observed times of maxima and the period of star oscillations. We used Gaia EDR3 parallax for calculating the absolute magnitude of δ Scuti stars. The precise fundamental physical parameters of all studied stars, such as mass, radius, luminosity, and temperature, were estimated. The pulsation modes of stars were investigated according to their Periodogram, indicating they are all in radial pulsation modes. Since the period changing of pulsating variable stars indicates the stellar evolution, the Period-Luminosity (𝑃 − 𝐿) relation was calculated and discussed. Moreover, we present new 𝑃 − 𝐿 relations for fundamental and overtone modes; Machine Learning Classification was used for this purpose.
We investigate the f (R) theory of gravity with broken diffeomorphism due to the change of the coefficient in front of the total divergence term in the (3+ 1)-decomposition of the scalar curvature. We perform the canonical analysis of this theory and show that its consistent, i.e. with no unphysical degrees of freedom, form is equivalent to the low-energy limit of the non-projectable f (R) Hořava-Lifshitz theory of gravity. We also analyze its cosmological solutions and show that the de Sitter solution can be obtained also in the case of this broken symmetry. The consequences of the proposed theory on the asymptotic solutions of a few specific models in the cosmological context are also presented.
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