Accurate fundamental parameters for Lower Main Sequence Stars

We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the InfraRed Flux Method to multi-band photometry. Our study is based on 104 stars for which we have excellent BVRIJHK photometry, excellent parallaxes and good metallicities. Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement. A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in literature shows that systematic effects still exist in the calibrations at the level of a few percent. Our InfraRed Flux Method temperature scale is 100K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3% smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology. Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices. We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well chosen set of G and K dwarfs would go a long way to addressing this problem.Comment: 34 pages, 20 figures. Accepted by MNRAS. Landscape table available online at http://users.utu.fi/luccas/IRFM

Dark energy: the absolute electric potential of the universe

Is there an absolute cosmic electric potential?. The recent discovery of the accelerated expansion of the universe could be indicating that this is certainly the case. In this essay we show that the consistency of the covariant and gauge invariant theory of electromagnetism is truly questionable when considered on cosmological scales. Out of the four components of the electromagnetic field, Maxwell's theory only contains two physical degrees of freedom. However, in the presence of gravity, one of the "unphysical" states cannot be consistently eliminated, thus becoming real. This third polarization state is completely decoupled from charged matter, but can be excited gravitationally thus breaking gauge invariance. On large scales the new state can be seen as a homogeneous cosmic electric potential, whose energy density behaves as a cosmological constant.Comment: 9 pages, 2 figures. Essay selected for "Honorable Mention" in the 2009 Awards for Essays on Gravitation (Gravity Research Foundation

Cosmological magnetic fields from inflation in extended electromagnetism

In this work we consider an extended electromagnetic theory in which the scalar state which is usually eliminated by means of the Lorenz condition is allowed to propagate. This state has been shown to generate a small cosmological constant in the context of standard inflationary cosmology. Here we show that the usual Lorenz gauge-breaking term now plays the role of an effective electromagnetic current. Such a current is generated during inflation from quantum fluctuations and gives rise to a stochastic effective charge density distribution. Due to the high electric conductivity of the cosmic plasma after inflation, the electric charge density generates currents which give rise to both vorticity and magnetic fields on sub-Hubble scales. Present upper limits on vorticity coming from temperature anisotropies of the CMB are translated into lower limits on the present value of cosmic magnetic fields. We find that, for a nearly scale invariant vorticity spectrum, magnetic fields $B_{\lambda}> 10^{-12}$ G are typically generated with coherence lengths ranging from sub-galactic scales up to the present Hubble radius. Those fields could act as seeds for a galactic dynamo or even account for observations just by collapse and differential rotation of the protogalactic cloud.Comment: 5 pages, 2 figures. Final version to appear in Phys. Rev.

Electromagnetic nature of dark energy

Out of the four components of the electromagnetic field, Maxwell's theory only contains two physical degrees of freedom. However, in an expanding universe, consistently eliminating one of the "unphysical" states in the covariant (Gupta-Bleuler) formalism turns out to be difficult to realize. In this work we explore the possibility of quantization without subsidiary conditions. This implies that the theory would contain a third physical state. The presence of such a new (temporal) electromagnetic mode on cosmological scales is shown to generate an effective cosmological constant which can account for the accelerated expansion of the universe. This new polarization state is completely decoupled from charged matter, but can be excited gravitationally. In fact, primordial electromagnetic quantum fluctuations produced during electroweak scale inflation could naturally explain the presence of this mode and also the measured value of the cosmological constant. The theory is compatible with all the local gravity tests, it is free from classical or quantum instabilities and reduces to standard QED in the flat space-time limit. 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. Possible signals, discriminating this model from LCDM, are also discussed.Comment: 10 pages, 2 figures. Contribution to the Proceedings of Invisible Universe International Conference, UNESCO, Paris, June 29-July 3, 200