Scientific Seminars

One more step toward the Hubble constant

Gustav Tammann
Inst. of Physics, Basel University

2013-01-16    14:00    Brera - Cupola Fiore

The Hubble constant Ho is the present (local) cosmic expansion rate, freed of all peculiar motions. The expansion is best traced - out to very large distances - by a Hubble diagram [log(redshift) versus apparent magnitude] of supernovae of type Ia (SNeIa), that are excellent standard candles. Ho follows then if also their absolute magnitudes M are known. The latter are generally determined from the Cepheid distances of a few local SNeIa. Unfortunately the results differ betwee 62 < Ho < 75 [km s-1 Mpc-1] due to a different treatment of metallicity effects and internal absorption. A new fully independent way is provided by the tip of the Red-Giant branch (TRGB) that is a very powerful distance indicator. TRGB distances of 6 local SNeIa are now available. They confirm a value of Ho = 64.3 within 4%. The value is also in agreement with other distance indicators.