We introduce a new model for dark energy in the Universe in which a small cosmological constant is generated by ordinary electromagnetic vacuum energy. The corresponding virtual photons exist at all frequencies but switch from a gravitationally active phase at low frequencies to a gravitationally inactive phase at higher frequencies via a Ginzburg

We discuss discrimination techniques and their quantitative efficiency that allow the separation of high energy electron/photon and muon primaries originated in cosmic ray extensive air showers impinging the ground. The experimental setup used in this study is detailed elsewhere in this volume (Sanchez et al.). Several approaches are analyzed

Fluorescent X-rays from the surfaces of airless planetary bodies in the inner solar system have been measured by instruments on several spacecraft. X-ray emission follows photoionisation by incident solar X-rays and charged particles and reveals the elemental composition of the surface. Analyses of X-ray spectra obtained by orbiting spacecraft, use

The low-frequency peaked BL Lac (LBL) object BL Lacertae was observed with the MAGIC telescope from August to December 2005 (22.2 hrs), and from July to September 2006 (26.0 hrs). A very high energy (VHE) gamma-ray signal was discovered with a 5.1 sigma excess in the 2005 data. Above 200 GeV, an integral flux of approximate by 3% of the Crab flux

The MAGIC telescope has observed very high energy gamma-ray emission from the BL Lac object PG 1553+113 in 2005 and 2006 at an overall significance is 8.8 sigma in 18.8h. The light curve shows no significant flux variations on a daily time-scale. The flux level during 2005 was, however, significantly higher compared to 2006. The differential energy

We review the state of the art of the derivation of star formation histories from color-magnitude diagrams reaching the oldest main sequence turnoffs. From the observational point of view, high quality CMDs are being obtained for galaxies over virtually the whole Local Group. However, the retrieval of star formation histories from these data using