E. Høg

The velocity field and 3-D structure of the Universe

The velocity field of matter in the Universe is dominated by the Hubble expansion, and the 3-dimensional structure is characterized by sheets with high density of visible matter separated by large voids. A superposed large-scale streaming of the luminous matter is only poorly indicated because distance determinations independent of the Hubble law are too uncertain. Very accurate tracers of the distance (±5%), the Type la Supernova«, are available, but they have been too sparsely observed to be useful in this context. Detection with Gaia, followed by ground-based monitoring, can give distances for about 50000 supernovae up to 500 Mpc or z = 0.10. This will greatly enhance our knowledge of the structure and velocities of the Universe. The underlying dark matter and dark energy will emerge from obscurity.

The Treatment of Double Stars in the Hipparcos-Tycho Programme

The Tycho Catalogue1 contains astrometric data and (B, V) photometry for more than one million stars. The double stars with separations wider than about 3 arcsec appear as two separate entries as if they were two single stars. Between 1.5 and 3 arcsec, the two components are possibly also separate entries, but the photometric data is somewhat less accurate. Double stars with separations as close as 0.4 arcsec were also detected in the Tycho observations, and they are flagged in the Catalogue.