Antonio L. Maroto

Brane-World Dark Matter

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.

Cosmic vector for dark energy

In this work we show that the presence of a vector field on cosmological scales could explain the present phase of accelerated expansion of the Universe. The proposed theory contains no dimensional parameters nor potential terms and does not require unnatural initial conditions in the early universe, thus avoiding the so-called cosmic coincidence problem. In addition, it fits the data from high-redshift supernovae with excellent precision, making definite predictions for cosmological parameters. Upcoming observations will be able to clearly discriminate this model from standard cosmology with cosmological constant.

Black holes inf(R)theories

In the context of f(R) theories of gravity, we address the problem of finding static and spherically symmetric black hole solutions. Several aspects of constant curvature solutions with and without electric charge are discussed. We also study the general case (without imposing constant curvature). Following a perturbative approach around the Einstein-Hilbert action, it is found that only solutions of the Schwarzschild-(anti) de Sitter type are present up to second order in perturbations. Explicit expressions for the effective cosmological constant are obtained in terms of the f(R) function. Finally, we have considered the thermodynamics of black holes in anti-de Sitter space-time and found that this kind of solution can only exist provided the theory satisfies R{sub 0}+f(R{sub 0})<0. Interestingly, this expression is related to the condition which guarantees the positivity of the effective Newtons constant in this type of theories. In addition, it also ensures that the thermodynamical properties in f(R) gravities are qualitatively similar to those of standard general relativity.

Cosmological electromagnetic fields and dark energy

We show that the presence of a temporal electromagnetic field on cosmological scales generates an effective cosmological constant which can account for the accelerated expansion of the universe. Primordial electromagnetic quantum fluctuations produced during electroweak scale inflation could naturally explain the presence of this field and also the measured value of the dark energy density. The behavior of the electromagnetic field on cosmological scales is found to differ from the well studied short-distance behavior and, in fact, the presence of a non-vanishing cosmological constant could be signalling the breakdown of gauge invariance on cosmological scales. The theory is compatible with all the local gravity tests, and is free from classical or quantum instabilities. Thus we see that, not only the true nature of dark energy can be established without resorting to new physics, but also the value of the cosmological constant finds a natural explanation in the context of standard inflationary cosmology. This mechanism could be discriminated from a true cosmological constant by upcoming observations of CMB anisotropies and large scale structure.

Photon spectra from WIMP annihilation

If the present dark matter in the Universe annihilates into standard model particles, it must contribute to the fluxes of cosmic rays that are detected on the Earth and, in particular, to the observed gamma-ray fluxes. The magnitude of such a contribution depends on the particular dark matter candidate, but certain features of the produced photon spectra may be analyzed in a rather model-independent fashion. In this work we provide the complete photon spectra coming from WIMP annihilation into standard model particle-antiparticle pairs obtained by extensive Monte Carlo simulations. We present results for each individual annihilation channel and provide analytical fitting formulas for the different spectra for a wide range of WIMP masses.

Cosmological evolution in vector-tensor theories of gravity

We present a detailed study of the cosmological evolution in general vector-tensor theories of gravity without potential terms. We consider the evolution of the vector field throughout the expansion history of the Universe and carry out a classification of models according to the behavior of the vector field in each cosmological epoch. We also analyze the case in which the Universe is dominated by the vector field, performing a complete analysis of the system phase map and identifying those attracting solutions which give rise to accelerated expansion. Moreover, we consider the evolution in a universe filled with a pressureless fluid in addition to the vector field and study the existence of attractors in which we can have a transition from matter domination to vector domination with accelerated expansion so that the vector field may play the role of dark energy. We find that the existence of solutions with late-time accelerated expansion is a generic prediction of vector-tensor theories and that such solutions typically lead to the presence of future singularities. Finally, limits from local gravity tests are used to get constraints on the value of the vector field at small (Solar System) scales.

Cosmic vector for dark energy: Constraints from supernovae, cosmic microwave background, and baryon acoustic oscillations

It has been recently shown that the presence of a vector field over cosmological scales could explain the observed accelerated expansion of the universe without introducing neither new scales nor unnatural initial conditions in the early universe, thus avoiding the coincidence problem. Here, we present a detailed analysis of the constraints imposed by SNIa, CMB and BAO data on the vector dark energy model with general spatial curvature. We find that contrary to standard cosmology, CMB data excludes a flat universe for this model and, in fact, predicts a closed geometry for the spatial sections. We see that CMB and SNIa Gold data are perfectly compatible at the 1-sigma level, however SNIa Union dataset exhibits a 3-sigma tension with CMB. The same level of tension is also found between SNIa and BAO measurements.

Cosmological and astrophysical limits on brane fluctuations

We consider a general brane-world model parametrized by the brane tension scale f and the branon mass M. For a low tension compared to the fundamental gravitational scale, we calculate the relic branon abundance and its contribution to the cosmological dark matter. We compare this result with the current observational limits on the total and hot dark matter energy densities and derive the corresponding bounds on f and M. Using the nucleosynthesis bounds on the number of relativistic species, we also set a limit on the number of light branons in terms of the brane tension. Finally, we estimate the bounds coming from the energy loss rate in supernovae explosions due to massive branon emission.

Branon search in hadronic colliders

In the context of the brane-world scenarios with compactified extra dimensions, we study the production of brane fluctuations (branons) in hadron colliders (p (p) over bar, pp, and e(+/-)p) in terms of the brane tension parameter f, the branon mass M, and the number of branons N. From the absence of monojet events at HERA and Tevatron (run I), we set bounds on these parameters and we also study how such bounds could be improved at Tevatron (run II) and the future LHC. The single-photon channel is also analyzed for the two last colliders.

Isotropy theorem for cosmological vector fields

We consider homogeneous Abelian vector fields in an expanding universe. We find a mechanical analogy in which the system behaves as a particle moving in three dimensions under the action of a central potential. In the case of bounded and rapid evolution compared to the rate of expansion, we show-by making use of the virial theorem-that for an arbitrary potential and polarization pattern, the average energy-momentum tensor is always diagonal and isotropic despite the intrinsic anisotropic evolution of the vector field. For simple power law potentials of the form V = lambda(A(mu)A(mu))(n), the average equation of state is found to be w = (n - 1)/(n + 1). This implies that vector coherent oscillations could act as natural dark matter or dark energy candidates. Finally, we show that under very general conditions, the average energy-momentum tensor of a rapidly evolving bounded vector field in any background geometry is always isotropic and has the perfect fluid form for any locally inertial observer.