Astroparticle Physics

16 Glossary 335(c) The value of the velocity of light forms the basis for the definitionof the length unit meter. It takes 1/299 792 458 seconds forlight in vacuum to pass 1 meter. The velocity of light in vacuum isthe same in all frames of reference.Concentration of galaxies in the direction of the constellation Virgin.A particle that exists only for an extremely brief instant of time inan intermediate process. For virtual particles the Lorentz-invariantmass does not coincide with the rest mass. Virtual particles withnegative mass squared are called space-like, those with a positivemass squared time-like. Virtual particles can exist for timesallowed by the Heisenberg’s uncertainty principle.Charged gauge quanta of weak interactions. These quanta participatein so-called charged-current processes. These are interactionsin which the electric charge of the participating particles changes.Probability waves upon which quantum mechanics is founded.The weak interaction acts on all fermions, e.g., in the decay ofhadrons. It is responsible for the beta decay of particles and nuclei.In charged-current interactions the quark flavour is changed, whilein neutral-current interactions the quark flavour remains the same.A star that has exhausted most or all of its nuclear fuel and hascollapsed to a very small size. The stability of a white dwarf is notmaintained by the radiation or gas pressure as with normal stars butrather by the pressure of the degenerate electron gas. If the whitedwarf has a mass larger than 1.4 times the solar mass, also thispressure is overcome and the star will collapse to a neutron star.Weakly Interacting Massive Particles are candidates for dark matter.Wilkinson Microwave Anisotropy Probe launched in 2001 to measurethe fine structure of the cosmic background radiation.A proposed channel of space-time connecting distant regions ofthe universe. It is not totally inconceivable that wormholes mightprovide the possibility of time travel.The w parameter is defined as the ratio of pressure over energydensity (w ≡ P/ϱ). In models with a cosmological constant thepressure of the vacuum density P equals exactly the negative ofthe energy density ϱ; i.e., w = −1, in contrast to quintessencemodels, where −1