Based on a dispersive approach, we apply the inverse amplitude method to unitarize one-loop SU͑2͒ and SU͑3͒ chiral perturbation theory. Numerically, we find that this unitarization technique yields the correct complex analytic structure in terms of cuts and poles. Indeed, using the chiral parameter estimates obtained from low-energy experiments we obtain the poles associated with the (770) and K*(982) resonances. Just by fixing their actual masses we obtain a parametrization of the and K phase shifts in eight different channels. With this fit we have then calculated several low-energy phenomenological parameters estimating their errors. Among others, we have obtained the chiral parameters and scattering lengths, which can be relevant for future experiments. ͓S0556-2821͑97͒00217-8͔

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We show that, in the context of brane-world scenarios with low tension tau=f(4), massive brane fluctuations (branons) are natural dark matter candidates. We calculate the present abundances for both hot (warm) and cold branons in terms of the branon mass M and the tension scale f. The results are compared with the current experimental bounds on these parameters. We also study the prospects for their detection in direct search experiments and comment on their characteristic signals in the indirect ones.

We apply the one-loop results of the SU (3) L × SU (3) R ChPT suplemented with the inverse amplitude method to fit the available experimental data on ππ and πK scattering. With esentially only three parameters we describe accurately data corresponding to six different channels, namely (I, J) = (0, 0), (2, 0), (1, 1), (1/2, 0), (3/2, 0) and (1/2, 1). In addition we reproduce the first resonances of the (1, 1) and (1/2, 1) channel with the right mass corresponding to the ρ and the K * (892) particles.

In this work we consider the possibility of describing the current evolution of the universe, without the introduction of any cosmological constant or dark energy (DE), by modifying the Einstein-Hilbert (EH) action. In the context of the fR gravities within the metric formalism, we show that it is possible to find an action without cosmological constant which exactly reproduces the behavior of the EH action with cosmological constant. In addition the fR action is analytical at the origin having Minkowski and Schwarzschild solutions as vacuum solutions. The found fR action is highly nontrivial and must be written in terms of hypergeometric functions but, in spite of looking somewhat artificial, it shows that the cosmological constant, or more generally the DE, is not a logical necessity. ÿ0:03 and t 0 t today . The main problem of this kind of description is that the fitted value seems to be about 55 orders of magnitude smaller than expected (the cosmological constant problem). The second type of explanations consider a dynamical DE by introducing a new scalar field. Finally the third one is trying to explain the cosmic acceleration as a consequence of new gravitational physics [3]. EH action modifications have been widely considered in the literature [4], firstly to describe inflation, and more recently to describe the current cosmic speed-up, or even both cosmological eras simultaneously.The simplest way of modifying EH action is by adding some function fR with the required properties (see [5] for a recent review on fR gravities). For example in [6] it was introduced a gravitational model where fR ÿ 4 =R, being the total gravitational action proportional to R ÿ 4 =R. This proposal has very interesting cosmological properties and triggered a lot of work on fR gravities applied to cosmology. However this kind of actions with negative powers of the curvature has the very serious drawback of not having vacuum solutions with vanishing curvature. For instance in the mentioned model the vacuum constant curvature solution is R 3 p 2 . Thus, even if one succeeds in reproducing cosmic acceleration, paradigmatic GR vacuum solutions assumed to play a major role in any fundamental theory of gravity, such as Minkowski or Schwarzschild, are excluded. Other fR functions recently considered in the literature face similar problems and moreover could be in conflict with Solar System experiments [7] while some other models could agree with Supenovae data [8].In this work we address the issue of finding a fR gravity able to reproduce the current cosmic speed-up without any cosmological constant but having R 0 as vacuum solution. From a more formal point of view we are seeking for a fR gravity having the same FriedmannRobertson-Walker (FRW) solution as the standard EH action with cosmological constant for nonrelativistic matter (p 0), but being analytical at R 0. Clearly the fR expansion at R 0 must start at the R 2 term to avoid having cosmological constant or to redefine the Newton constant.In order to consider such as the standar...

We compute the mean free path and shear viscosity in the color-flavor locked (CFL) phase of dense quark matter at low temperature T , when the contributions of mesons, quarks and gluons to the transport coefficients are Boltzmann suppressed. CFL quark matter displays superfluid properties, and transport phenomena in such cold regime are dominated by phonon-phonon scattering. We study superfluid phonons within thermal field theory and compute the mean free path associated to their most relevant collision processes. Small-angle processes turn out to be more efficient in slowing transport phenomena in the CFL matter, while the mean free path relevant for the shear viscosity is less sensitive to collinear scattering due to the presence of zero modes in the Boltzmann equation. In analogy with superfluid He 4 , we find the same T power law for the superfluid phonon damping rate and mean free path. Our results are relevant for the study of rotational properties of compact stars, and correct wrong estimates existing in the literature.

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In the context of f (R) theories of gravity, we study the evolution of scalar cosmological perturbations in the metric formalism. Using a completely general procedure, we find the exact fourth-order differential equation for the matter density perturbations in the longitudinal gauge. In the case of sub-Hubble modes, the expression reduces to a second-order equation which is compared with the standard (quasi-static) equation used in the literature. We show that for general f (R) functions the quasi-static approximation is not justified. However, for those functions adequately describing the present phase of accelerated expansion and satisfying local gravity tests, it provides a correct description for the evolution of perturbations.