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We present what we know on nucleosynthesis in the Universe and hypotheses that have been made in this regard. A brief description of the Universe's evolution during its different stages is offered, indicating which are the periods and mechanisms of element formation. A critical prospective on future research is formulated to validate, modify, or reject the hypotheses formulated. These will involve joint observations that encompass finer measurements of cosmic background radiation, galaxy clusters, and gravitational waves produced by neutron star collisions. The information thus obtained will be combined with restrictions given by theoretical models. Perhaps many current doubts will be clarified, but new questions will arise., 26 pages, 10 figures. Title and abstract also in Spanish

A method to set constraints on the parameters of extended theories of gravitation is presented. It is based on the comparison of two series expansions of any observable that depends on H(z). The first expansion is of the cosmographical type, while the second uses the dependence of H with z furnished by a given type of extended theory. When applied to f(R) theories together with the redshift drift, the method yields limits on the parameters of two examples (the theory of Hu and Sawicki (2007), and the exponential gravity introduced by Linder (2009)) that are compatible with or more stringent than the existing ones, as well as a limit for a previously unconstrained parameter., Comment: 9 pages, 2 figures; v2: modifications in title and presentation, results unchanged

Second part of a didactic sequence of activities on some topics of Astronomy, related mainly with the real shape of the Earth, the gravitational interactions between our planet and other celestial bodies, and the resulting movement of the rotational axis of the Earth, leading to the precession of the equinoctial points. All elements are given for the construction of a simplified Sun-Earth system model, with a zodiacal band surrounding it, in order to show in a pedagogical manner how precession involves a shift of the zodiacal constellations in time., Article in Spanish, PDF document; Published version with high resolution images available at http://cms.iafe.uba.ar/gangui/didaastro/ch/ . Posted here as a contribution to the International Year of Astronomy 2009