**Thursday 24. January, 2 pm, Norbert Straumann** (Zuerich)

**Titel:**
**On the cosmological constant problems and the astronomical evidence
for a homogeneous energy density with negative pressure**

__Abstract:__

The cosmological constant problems, as well as the astronomical evidence
for a cosmologically significant homogeneous exotic energy density with
negative pressure, are reviewed. After a short history of the cosmological
term it is explained why the (effective) cosmological constant is expected
to obtain contributions from short-distance physics corresponding to an
energy scale at least as large as the Fermi scale. The actual tiny value
of the cosmological constant by particle physics standards represents,
therefore, one of the deepest mysteries of present-day fundamental physics.
In a second part I shall discuss recent astronomical evidence for a cosmologically
significant vacuum energy density or an effective equivalent, called quintessence.
Cosmological models, which attempt to avoid the disturbing cosmic coincidence
problem, are also briefly reviewed.

**Please notice:**
**Norbert Straumann will give his talk in the seminar room in the
central building (1st floor).**