We investigate a model of interacting dark matter and dark energy given by a modified holographic Ricci cutoff in a spatially flat Friedmann-Robertson-Walker space-time. We consider a nonlinear interaction consisting of a significant rational function of the total energy density and its first derivative homogeneous of degree one and show that the effective one-fluid obeys the equation of state of a relaxed Chaplygin gas. So that, the Universe is dominated by pressureless dark matter at early times and undergoes an accelerated expansion in the far future driven by a strong negative pressure. We apply the $\chi${}2-statistical method to the observational Hubble data and the Union2 compilation of SNe~Ia for constraining the cosmological parameters and analyze the feasibility of the modified holographic Ricci cutoff. By using the new Om diagnostic method, we find that the effective model differs substantially from the $\Lambda${}-CDM one, because it gets the accelerated expansion faster than the $\Lambda${}-CDM model. Finally, a model with a third component decoupled from the interacting dark sector is presented for studying bounds on the dark energy at early times.

We introduce the Lagrangian for a multiscalar-field configuration in an $N$-dimensional internal space endowed with a constant metric $Q_{ik}$, and generalize the quintom cosmological scenario. We find the energy momentum tensor of the model and show that the set of dual transformations, which preserve the form of the Einstein equations in the Friedmann-Robertson-Walker cosmology, is enlarged. We show that the stability of the power-law solutions leads to an exponential potential which is invariant under linear transformations in the internal space. Moreover, we obtain the general exact solution of the Einstein-Klein-Gordon equations for that potential. There exist solutions that cross the phantom divide and solutions that blow up at a finite time, exhibiting a superaccelerated behavior and ending in a big rip. We show that the quintom model with a separable potential can be identified with a mixture of several fluids. This framework includes the $\Lambda \mathrm{CDM}$ model, a bouncing model, and a setting sourced by a cosmic string network plus a cosmological constant. Then we concentrate on the case where the dimension of the internal quintessence sector $N_q$ exceeds the dimension of the internal phantom sector $N_{\mathrm{ph}}$. For $(N_q,N_{\mathrm{ph}})=(2,1)$ the dark energy density derived from the 3-scalar field crosses the phantom divide, and its negative component plays the role of the negative part of a classical Dirac field.

Spherically symmetric thin-shell wormholes are studied within the framework of Einstein-Born-Infeld theory. We analyze the exotic matter content, and find that for certain values of the Born-Infeld parameter the amount of exotic matter on the shell can be reduced in relation to the Maxwell case. We also examine the mechanical stability of the wormhole configurations under radial perturbations preserving the spherical symmetry. In addition, in the Appendix the repulsive or attractive character of the wormhole geometries is briefly discussed.

We investigate a spatially flat Friedmann-Robertson-Walker universe that has an interacting dark matter, a modified holographic Ricci dark energy (MHRDE), plus a third, decoupled component that behaves as a radiation term. We consider a nonlinear interaction in the dark component densities and their derivatives up to second order. We apply the $\chi${}2 method to the observational Hubble data for constraining the cosmological parameters, and analyze the amount of dark energy in the radiation era for both MHRDE and holographic Ricci dark energy models. The former is consistent with the bound $\Omega${}x(z$\simeq${}1100)<0.1 reported for the behavior of dark energy at early times, while the latter does not fulfill it.

We construct traversable thin-shell wormholes which are asymptotically Ads/dS applying the cut and paste procedure for the case of an acoustic metric created by a relativistic Bose-Einstein condensate. We examine several definitions of the flare-out condition along with the violation or not of the energy conditions for such relativistic geometries. Under reasonable assumptions about the equation of state of the matter located at the shell, we concentrate on the mechanical stability of wormholes under radial perturbation preserving the original spherical symmetry. To do so, we consider linearized perturbations around static solutions. We obtain that dS acoustic wormholes remain stable under radial perturbations as long as they have small radius; such wormholes with finite radius do not violate the strong/null energy condition. Besides, we show that stable Ads wormhole satisfy some of the energy conditions whereas unstable Ads wormhole with large radii violate them.