There are some gravitational theories in which the ordinary energy-momentum conservation law is not valid in the curved spacetime. Rastall gravity is one of the known theories in this regard which includes a non-minimal coupling between geometry and matter fields. Equipped with the basis of such theory, we study the properties of traversable wormholes with flat asymptotes. We investigate the possibility of exact solutions by a source with the baryonic matter state parameter. Our survey indicates that Rastall theory has considerable effects on the wormhole characteristics. In addition, we study various case studies and show that the weak energy condition may be met for some solutions. We also give a discussion regarding to traversability of such wormhole geometry with phantom sources.
We extend the idea of mimetic gravity to a Randall-Sundrum II braneworld model. As for the 4-dimensional mimetic gravity, we isolate the conformal degree of freedom of 5dimensional gravity in a covariant manner. We assume the bulk metric to be made up of a non-dynamical scalar field Φ and an auxiliary metricG AB so that G AB =G CD Φ ,C Φ ,DGAB where A, B, ... are the bulk spacetime indices. Then we show that the induced conformal degree of freedom on the brane as an induced scalar field, plays the role of a mimetic field on the brane. In fact, we suppose that the scalar degree of freedom which mimics the dark sectors on the brane has its origin on the bulk scalar field, Φ. By adopting some suitable mimetic potentials on the brane, we show that this brane mimetic field explains the late time cosmic expansion in the favor of observational data: the equation of state parameter of this field crosses the cosmological constant line in near past from quintessence to phantom phase in a redshift well in the range of observation. We show also that this induced mimetic scalar field has the capability to explain initial time cosmological inflation. We study parameter space of the models numerically in order to constraint the models with Planck2015 data set. PACS: 98.80.k, 95.36.+x, 95.35.+d
Considering some modified Newtonian potentials and the Hubble law in writing the total energy of a test mass located at the edge of a flat Friedmann–Robertson–Walker (FRW) universe, we obtain several modified Friedmann equations. Interestingly enough, our study shows that the employed potentials, while some of them have some successes in modeling the spiral galaxies rotation curves, may also address an accelerated universe. This fact indicates that dark energy and dark matter may have some common origins and aspects.
Supernova explosion is a phenomenon described very well by the laws of quantum mechanics meaning that the Heisenberg uncertainty principle (HUP) restricts the achievable information from this source, and indeed, the accuracy of measurements on Hubble parameter by using this source is bounded by HUP. On the other hand, cosmic microwave background (CMB) stores quantum gravity (QG) effects dominant in the early universe. Hence, its physics is supposed to be under the influence of the modified forms of HUP (obtained in the QG scenarios). This means that the most accurate H 0 measurements, by using this source, may meet modified forms of HUP instead of HUP itself. Therefore, photons coming from these sources satisfy different uncertainty principles. Here, we show that the difference between these two regimes (or equally, the difference between the uncertainty principles) establishes an eternal discrepancy between the H 0 values obtained by these sources. Consequently, more accurate observations and estimations on the value of Hubble parameter may help us find out the values of QG parameters.
The publication of this article was funded by SCOAP 3 .Considering the Einstein field equations in Lyra manifold and applying the unified first law of thermodynamics as well as the Clausius relation to the apparent horizon of FRW universe, we find the entropy of apparent horizon in Lyra manifold. In addition, the validity of second law of thermodynamics and its generalized form are also studied. Finally, we use the first law of thermodynamics in order to find the horizon entropy of static spherically symmetric spacetimes. Some results of considering (anti)de-Sitter and Schwarzschild metrics have also been addressed.
Following our recent work on braneworld mimetic gravity, in this paper, we study an extension of braneworld mimetic gravity to the case that the gravitational sector on the brane is modified in the spirit of [Formula: see text] theories. We assume the physical 5D bulk metric in the Randall–Sundrum II braneworld scenario consists of a 5D scalar field (which mimics the dark sectors on the brane) and an auxiliary 5D metric. We find the 5D Einstein’s field equations and the 5D equation of motion of the bulk scalar field in this setup. By using the Gauss–Codazzi equations, we obtain the induced Einstein’s field equations on the brane. Finally, by adopting the FRW background, we find the Friedmann equation on the brane in this [Formula: see text] mimetic braneworld setup.
By using the Kaniadakis statistics, we discuss the modifications of Newtonian gravity and radial velocity profile in the light of Verlinde's formalism for gravitational entropy.After considering the implications of κ-statistics on the gravitational potential, it is shown that an accelerated universe may be obtained by considering the Friedmann first equation in this non-extensive statistics.
By using the Kaniadakis statistics, we discuss the modifications of Newtonian gravity and radial velocity profile in the light of Verlinde’s formalism for gravitational entropy. After considering the implications of [Formula: see text]-statistics on the gravitational potential, it is shown that an accelerated universe may be obtained by considering the Friedmann first equation in this nonextensive statistics.
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