Limits of time in cosmology
  We provide a discussion of some main ideas in our project about the physical
foundation of the time concept in cosmology. It is standard to point to the
Planck scale (located at $\sim 10^{-43}$ seconds after a fictitious "Big Bang"
point) as a limit for how far back we may extrapolate the standard cosmological
model. In our work we have suggested that there are several other (physically
motivated) interesting limits -- located at least thirty orders of magnitude
before the Planck time -- where the physical basis of the cosmological model
and its time concept is progressively weakened. Some of these limits are
connected to phase transitions in the early universe which gradually undermine
the notion of 'standard clocks' widely employed in cosmology. Such
considerations lead to a 'scale problem' for time which becomes particularly
acute above the electroweak phase transition (before $\sim 10^{-11}$ seconds).
Other limits are due to problems of building up a cosmological reference frame,
or even contemplating a sensible notion of proper time, if the early universe
constituents become too quantum. This 'quantum problem' for time arises e.g. if
a pure quantum phase is contemplated at the beginning of inflation at, say,
$\sim 10^{-34}$ seconds.
