There are certain more or less well or badly determined integral constraints
on the past history of star formation in the universe. These include
- Cosmic baryon density
B, now fairly
well determined both from primordial deuterium
(O'Meara et al 2001)
and from the CMB fluctuation spectrum (e.g.
- Cosmic mass density of stars
rather less well determined as it
involves a combination of luminosity-density measurements with an IMF
and evolutionary population synthesis models.
- Extragalactic background light (EBL) intensity, now known within a
factor of 2 or so from COBE (FIRAS and DIRBE) and galaxy counts in the
optical and near IR (e.g.
Gispert, Lagache &
- Cosmic abundance of "metals",
Z, due to the
heavy-element content of stars, the interstellar medium and the
intergalactic medium, a quantity that is very poorly known and largely
a matter of guesswork. In this talk I shall nevertheless make some guesses,
so that at least one can see more easily how things relate to one another.
In particular, the metallicity
ZIGM of the intergalactic
medium has tended to be either neglected or underestimated in models of
the past star formation
rate, and it is of interest to ask about its relation with EBL.