Constraining the primordial spectrum of metric perturbations from gravitino and moduli production

We consider the production of gravitinos and moduli fields from quantum vacuum fluctuations induced by the presence of scalar metric perturbations at the end of inflation. We obtain the corresponding occupation numbers, up to first order in perturbation theory, in terms of the power spectrum of the metric perturbations. We compute the limits imposed by nucleosynthesis on the spectral index $n_s$ for different models with constant $n_s$. The results show that, in certain cases, such limits can be as strong as $n_s<1.12$, which is more stringent than those coming from primordial black hole production.Comment: 16 pages, LaTeX, 5 figures. Corrected figures, new references included. Final version to appear in Phys. Rev.

Estudio de la idoneidad de un nuevo hormigón estructural fabricado con áridos ligeros reforzados con fibra de carbono sinterizados a partir de residuos

The suitability of three new lightweight aggregates containing carbon fiber residues (CAs) as components in structural lightweight concrete has been studied. Prismatic concrete specimens were prepared using these CAs as a coarse fraction. Additional specimens of normal-weight aggregate, commercial lightweight aggregate and mortar were prepared for comparison. The CA-concrete samples (CACs) have yielded compressive strength values between 35 and 55 MPa as well as low density and thermal conductivity results. Furthermore, the CACs have displayed the highest ratios of mechanical strength over density and the thermal conductivity, which means that there is a better balance between their mechanical and physical properties than in the other samples studied. These results indicate that the new CAs could have great potential for use in structural lightweight concrete, also complying with the principles of the Circular Economy.Este estudio pretende comprobar la idoneidad de unos novedosos áridos ligeros sinterizados con residuos de fibra de carbono (CAs) en la fabricación de hormigón ligero estructural. Se prepararon probetas prismáticas de hormigón, utilizando estos CAs como fracción gruesa, comparándose a su vez con probetas fabricadas con un árido convencional, un árido ligero comercial y mortero. Las muestras de hormigón con los áridos CA (en adelante CAC) han dado lugar a valores de resistencia a compresión entre 35 y 55 MPa, así como a resultados bajos de densidad y conductividad térmica, mostrando además las ratios más altas al relacionar estos tres parámetros. Esto indicaría por tanto un mejor equilibrio entre las propiedades mecánicas y físicas que los obtenidos en las otras muestras estudiadas. Estos resultados apuntan a que los nuevos CAs podrían tener un gran potencial para su uso en hormigón ligero estructural, cumpliendo además los principios de la Economía Circular

Unstable Heavy Majorana Neutrinos and Leptogenesis

We propose a new mechanism producing a non-vanishing lepton number asymmetry, based on decays of heavy Majorana neutrinos. If they are produced out of equilibrium, as occurs in preheating scenario, and are superpositions of mass eigenstates rapidly decaying, their decay rates contains interference terms provided the mass differences $\Delta m$ are small compared to widths $\Gamma$. The resulting lepton asymmetry, which is the analogue of the time-integrated CP asymmetry in $B^0-\bar{B}^0$ system, is found to be proportional to $\Delta m/\Gamma$.Comment: 18 pages, latex, revised version to be published in Phys. Rev.

The equivalence theorem and the production of gravitinos after inflation

We study the high-energy equivalence between helicity 1/2 gravitinos and goldstinos in order to calculate the production of gravitinos in time-dependent scalar and gravitational backgrounds. We derive this equivalence for equations of motion, paying attention to some subtleties, mainly due to external sources, that are not present in the standard proofs. We also propose the Landau gauge as a simplifying alternative to the usual gauge choices, both for practical calculations and in the equivalence theorem proof.Comment: 13 pp. 2 figures. Final and shorter version to appear in Phys. Rev. D. References and minor errata correcte

Production of spin 3/2 particles from vacuum fluctuations

We study the production of spin 3/2 particles in homogeneous scalar and gravitational backgrounds from the mode-mixing Bogolyubov method. Considering only helicity $\pm 3/2$ states we can reduce the problem to a standard Dirac fermion calculation and apply the standard techniques in a straightforward way. As an example we consider a supergravity inflationary model and calculate the spectrum of gravitinos created during preheating at the end of inflation.Comment: 4 pages, RevTeX, 1 figure. New comments and references added. Final version to appear in Phys.Rev.Let

Particle production from nonlocal gravitational effective action

In this paper we show how the nonlocal effective action for gravity, obtained after integrating out the matter fields, can be used to compute particle production and spectra for different space-time metrics. Applying this technique to several examples, we find that the perturbative calculation of the effective action up to second order in curvatures yields exactly the same results for the total number of particles as the Bogolyubov transformations method, in the case of masless scalar fields propagating in a Robertson-Walker space-time. Using an adiabatic approximation we also obtain the corresponding spectra and compare the results with the traditional WKB approximation.Comment: 22 pages, LaTeX, no figures. Corrected version with new comments and results. To appear in Phys. Rev.

Thermalization After Inflation and Reheating Temperature

We present a detailed examination of thermalization after inflation for perturbative inflaton decay. Different interactions among particles in the plasma of inflaton decay products are considered and it will be shown that 2 -> 2 scatterings and particle decay are the important ones. We show that thermalization occurs after decays dominate scatterings, and that depending on the typical mass scale of inflaton decay products, different situations may arise. In particular, thermalization may be delayed until late times, in which case the bounds from thermal gravitino production on supersymmetric models of inflation are considerably relaxed. We will also consider the case where the observable sector consists only of the MSSM matter content, and point out that flat directions with large vevs may result in earlier thermalization of the plasma and push the reheat temperature towards its upper limit.Comment: 18 pages, LaTeX, a few references added and revise

Brane oscillations and the cosmic coincidence problem

We show that, under general assumptions, in six-dimensional brane-world models with compactified large extra dimensions, the energy density of brane oscillations scales as that of cold dark matter and its present value is compatible with observations. Such value is obtained from the only dimensional scale in the theory, namely, the fundamental scale of gravity in six dimensions $M_6\sim 1$ TeV, without any fine-tuning or the introduction of additional mass scales apart from the large size of the extra dimensions. It has been suggested that the same kind of models could provide also the correct magnitude of the cosmological constant. This observation can be relevant for the resolution of the cosmic coincidence problem in the brane-world scenario.Comment: 5 pages, RevTeX. Comments on the renormalization of the branon mass included. Final version to appear in Phys.Rev.D (R

Cosmic magnetic fields and dark energy in extended electromagnetism

We discuss an extended version of electromagnetism in which the usual gauge fixing term is promoted into a physical contribution that introduces a new scalar state in the theory. This new state can be generated from vacuum quantum fluctuations during an inflationary era and, on super-Hubble scales, gives rise to an effective cosmological constant. The value of such a cosmological constant coincides with the one inferred from observations as long as inflation took place at the electroweak scale. On the other hand, the new state also generates an effective electric charge density on sub-Hubble scales that produces both vorticity and magnetic fields with coherent lengths as large as the present Hubble horizon.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish Relativity Meeting 2010, Granada, Spain, 6-10 September 201

Renormalization of the nonequilibrium dynamics of fermions in a flat FRW universe

We derive the renormalized equations of motion and the renormalized energy-momentum tensor for fermions coupled to a spatially homogeneous scalar field (inflaton) in a flat FRW geometry. The fermion back reaction to the metric and to the inflaton field is formulated in one-loop approximation. Having determined the infinite counter terms in an $\bar{MS}$ scheme we formulate the finite terms in a form suitable for numerical computation. We comment on the trace anomaly which is inferred from the standard analysis. We also address the problem of initial singularities and determine the Bogoliubov transformation by which they are removed.Comment: 26 pages, LaTe