f(T) modified teleparallel gravity as an alternative for holographic and new agegraphic dark energy models

In the present work, we reconstruct different f（T）-gravity models corre- sponding to the original and entropy-corrected versions of the holographic and new agegraphic dark energy models. We also obtain the equation of state parameters of the corresponding f（T）-gravity models. We conclude that the original holographic and new agegraphic f （T）-gravity models behave like the phantom or quintessence model, whereas in the entropy-corrected models, the equation of state parameter can justify the transition from the quintessence state to the phantom regime as indicated by the recent observations.

f(R,L(X))-gravity in the context of dark energy with power law expansion and energy conditions

The objective of this work is to generate a general formalism of f(R,L(X))-gravity in the context of dark energy under the framework of K-essence emergent geometry with the Dirac-Born-Infeld(DBI) variety of action,where R is the familiar Ricci scalar,L(X) is the DBI type non-canonical Lagrangian with X=1/2g^(μν)▽_(μ)φ▽_(ν)φ,and φ is the K-essence scalar field.The emergent gravity metric(G_(μν)) and the well known gravitational metric(g^(μν))are not conformally equivalent.We have constructed a modified field equation using the metric formalism in f(R,L(X))-gravity incorporating the corresponding Friedmann equations into the framework of the background gravitational metric,which is of Friedmann-Lemaitre-Robertson-Walker(FLRW) type.The solution of the modified Friedmann equations have been deduced for the specific choice of f(R,L(X)),which is of Starobinsky-type,using the power law expansion method.The consistency of the model with the accelerating phase of the universe has been shown when we restrict ourselves to consider the value of the dark energy density as φ^(2)=8/9=0.888 <1,which indicates that the present universe is dark-energy dominated.Graphical plots for the energy density(ρ),pressure(p),and equation of state parameter(ω) with respect to(w.r.t.) time(t) based on parametric values are interestingly consistent with the dark energy domination theory,and hence the accelerating features.We also highlight the corresponding energy conditions and constraints of the f(R,L(X)) theory with a basic example.

Cloud of strings in f(R) gravity

We derive the solution for a spherically symmetric string cloud configuration in a ＆dimensional spacetime in the framework of f（R） theories of gravity. We also analyze some thermodynamic properties of the joint black hole - cloud of strings solution. For its Hawking temperature, we found that the dependence of the mass with the horizon is significantly different in both theories. For the interaction of a black hole with thermal radiation, we found that the shapes of the curves are similar, but shifted. Our analysis generalizes some known results in the literature.

Charged black holes in the infinite derivative theory of gravity

The infinite derivative theory of gravity is a generalization of Einstein gravity with many interesting properties,but the black hole solutions in this theory are still not fully understood.In the paper,we concentrate on studying the charged black holes in such a theory.Adding the electromagnetic field part to the effective action,we show how the black hole solutions around the Reissner-Nordstrom metric can be solved perturbatively and iteratively.We further calculate the corresponding temperature,entropy and electrostatic potential of the black holes and verify the first law of thermodynamics.