Jose Beltran Jimenez

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.

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.

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.

The electromagnetic dark sector

Abstract We consider electromagnetic field quantization in an expanding universe. We find that the covariant (Gupta–Bleuler) method exhibits certain difficulties when trying to impose the quantum Lorenz condition on cosmological scales. We thus explore the possibility of consistently quantizing without imposing such a condition. In this case there are three physical states, which are the two transverse polarizations of the massless photon and a new massless scalar mode coming from the temporal and longitudinal components of the electromagnetic field. An explicit example in de Sitter space–time shows that it is still possible to eliminate the negative norm state and to ensure the positivity of the energy in this theory. The new state is decoupled from the conserved electromagnetic currents, but is non-conformally coupled to gravity and therefore can be excited from vacuum fluctuations by the expanding background. The cosmological evolution ensures that the new state modifies Maxwells equations in a totally negligible way on sub-Hubble scales. However, on cosmological scales it can give rise to a non-negligible energy density which could explain in a natural way the present phase of accelerated expansion of the universe.

Born-Infeld inspired modifications of gravity

General Relativity has shown an outstanding observational success in the scales where it has been directly tested. However, modifications have been intensively explored in the regimes where it seems either incomplete or signals its own limit of validity. In particular, the breakdown of unitarity near the Planck scale strongly suggests that General Relativity needs to be modified at high energies and quantum gravity effects are expected to be important. This is related to the existence of spacetime singularities when the solutions of General Relativity are extrapolated to regimes where curvatures are large. In this sense, Born–Infeld inspired modifications of gravity have shown an extraordinary ability to regularise the gravitational dynamics, leading to non-singular cosmologies and regular black hole spacetimes in a very robust manner and without resorting to quantum gravity effects. This has boosted the interest in these theories in applications to stellar structure, compact objects, inflationary scenarios, cosmological singularities, and black hole and wormhole physics, among others. We review the motivations, various formulations, and main results achieved within these theories, including their observational viability, and provide an overview of current open problems and future research opportunities.

Perturbations in electromagnetic dark energy

It has been recently proposed that the presence of a temporal electromagnetic field on cosmological scales could explain the phase of accelerated expansion that the universe is currently undergoing. The field contributes as a cosmological constant and therefore, the homogeneous cosmology produced by such a model is exactly the same as that of ΛCDM. However, unlike a cosmological constant term, electromagnetic fields can acquire perturbations which in principle could affect CMB anisotropies and structure formation. In this work, we study the evolution of inhomogeneous scalar perturbations in this model. We show that provided the initial electromagnetic fluctuations generated during inflation are small, the model is perfectly compatible with both CMB and large scale structure observations at the same level of accuracy as ΛCDM.

Cosmology with moving dark energy and the CMB quadrupole

We study the consequences of a homogeneous dark energy fluid having a nonvanishing velocity with respect to the matter and radiation large-scale rest frames. We consider homogeneous anisotropic cosmological models with four fluids (baryons, radiation, dark matter, and dark energy) whose velocities can differ from each other. Performing a perturbative calculation up to second order in the velocities, we obtain the contribution of the anisotropies generated by the fluids motion to the CMB quadrupole and compare with observations. We also consider the exact problem for arbitrary velocities and solve the corresponding equations numerically for different dark energy models. We find that models whose equation of state is initially stiffer than radiation, as for instance some tracking models, are unstable against velocity perturbations, thus spoiling the late-time predictions for the energy densities. In the case of scaling models, the contributions to the quadrupole can be non-negligible for a wide range of initial conditions. We also consider fluids moving at the speed of light (null fluids) with positive energy and show that, without assuming any particular equation of state, they generically act as a cosmological constant at late times. We find the parameter region for which the models considered could be compatible with the measured (low) quadrupole.

Flavonoids and cardiovascular diseases

Abstract Several epidemiological studies have found an inverse correlation between the dietary flavonoid intake and a reduced mortality from coronary heart disease and the incidence of stroke. We will focus our review on several mechanisms which have been suggested to explain these protective effects. o a) Antiatherogenic effects. Flavonoids together with others antioxidants constitute two lines of defense in protecting cells against injury owing to oxidation of LDL: 1) at the LDL level, by inhibiting LDL oxidation due to theirfree radical scavenger activity, and 2) at the cellular level, by protecting the cells directly, i.e., by increasing their resistance against the cytotoxic effect of oxidised LDL. Additionally, recent studies indicate that flavonoids can prevent the expression of adhesion and chemoattractant molecules. b) Antiaggregant effects. Flavonoids prevent platelet aggregation induced by several proaggregant stimuli although relatively high doses are required. Inhibition of platelet phosphodiesterases, inhibition of arachidonic acid metabolism and antioxidant effects have been suggested as possible mechanisms of action. c) Direct effects on vascular smooth muscle. The vasodilator effects of flavonoids in vitro is mainly endothelium-independent. The main mechanism of action seems to be related to their inhibitory effects on protein kinases. Some flavonoids, however, can produce endothelium-dependent contractile responses due to increased TXA 2 production. d) Antihypertensive effects. Little information about the effects of flavonoids on blood pressure is available. However, recently, the chronic oral administration of quercetin has been shown to exert potent antihypertensive effects

N-body simulations with a cosmic vector for dark energy

We present the results of a series of cosmological N-body simulations of a vector dark energy (VDE) model, performed using a suitably modified version of the publicly available gadget-2 code. The set-ups of our simulations were calibrated pursuing a twofold aim: (1) to analyse the large scale distribution of massive objects and (2) to determine the properties of halo structure in this different framework. We observe that structure formation is enhanced in VDE, since the mass function at high redshift is boosted up to a factor of 10 with respect to ? cold dark matter ( CDM), possibly alleviating tensions with the observations of massive clusters at high redshifts and early reionization epoch. Significant differences can also be found for the value of the growth factor, which in VDE shows a completely different behaviour, and in the distribution of voids, which in this cosmology are on average smaller and less abundant. We further studied the structure of dark matter haloes more massive than 5 x 1013 h-1 M?, finding that no substantial difference emerges when comparing spin parameter, shape, triaxiality and profiles of structures evolved under different cosmological pictures. Nevertheless, minor differences can be found in the concentrationmass relation and the two-point correlation function, both showing different amplitudes and steeper slopes. Using an additional series of simulations of a ?CDM scenario with the same and s8 used in the VDE cosmology, we have been able to establish whether the modifications induced in the new cosmological picture were due to the particular nature of the dynamical dark energy or a straightforward consequence of the cosmological parameters. On large scales, the dynamical effects of the cosmic vector field can be seen in the peculiar evolution of the cluster number density function with redshift, in the shape of the mass function, in the distribution of voids and on the characteristic form of the growth index (z). On smaller scales, internal properties of haloes are almost unaffected by the change of cosmology, since no statistical difference can be observed in the characteristics of halo profiles, spin parameters, shapes and triaxialities. Only halo masses and concentrations show a substantial increase, which can, however, be attributed to the change in the cosmological parameters.