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The hierarchical clustering inherent in Lambda-Cold Dark Matter cosmology (ΛCDM) seems to produce many of the observed characteristics of large-scale structure. But some glaring problems still remain, including the over-prediction (by a factor 10) of the number of dwarf galaxies within the virialized population of the local group. We carry out a detailed calculation of the dwarf halo evolution incorporating the effects of a hypothesized dark-matter decay, D→D′+l, where D is the unstable particle, D′ is the more massive daughter particle and l is the other, lighter (or possibly massless) daughter particle. This process preferentially heats the smaller halos, expanding them during their evolution and reducing their present-day circular velocity. We find that this mechanism can account very well for the factor 4 deficit in the observed number of systems with velocity 10–20 km s-1 compared to those predicted by the numerical simulations, if Δm/mD′∼5–7×10-5, where Δm is the mass difference between the initial and final states. The corresponding lifetime τ cannot be longer than ∼30 Gyr, but may be as short as just a few Gyr. [ABSTRACT FROM AUTHOR]

We show that perhaps already with the next generation of long-baseline interferometers at submm-wavelengths we will able to image the shadow of the black hole in the Galactic Center. To a distant observer, the event horizon casts a relatively large “shadow” with an apparent diameter of ∼10 gravitational radii due to bending of light by the black hole, nearly independent of the black hole spin or orientation. The predicted angular size for the Galactic Center black hole is ∼30 μarcseconds, a mere factor two smaller than the highest currently achieved resolution with VLBI techniques. Taking into account scatter-broadening of the image in the interstellar medium and the finite achievable telescope resolution, we show that the shadow of Sgr A* can be observed at suitably high frequencies. The main problems are possible optical depth effects for an ADAF model and Doppler boosting for a jet model. This has an influence on which dynamic range and which observing frequency is ultimately required to prove or disprove the existence of an event horizon. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]