Evidence for Acceleration from Type Ia Supernovae
 The astronomers plotted (Mm) versus redshift (z),
which is equivalent to plotting distance versus velocity.
 The data favours a universe whose expansion is
accelerating so that the expansion speed increases with time!
 This is opposite to what was expected!
 How the expansion changes depends on the matter content of the
Universe, and what type of matter dominates. Einstein equations give us
the deceleration parameter
q = &Omega_{rad} + 1/2 &Omega_{m}  &Omega_{&Lambda}
when q > 0 expansion decelerates, while it accelerates if q < 0
 Neglecting &Omega_{rad}, we conclude that
Universe was decelerating while &Omega_{m} > 2 &Omega_{&Lambda} and accelerating afterwards.
Note, in this formula we consider all &Omega's as functions of time, not just
the values at the present moment. For example
&Omega_{m}=&rho_{m}(t)/&rho_{crit}(t), etc.
You can see that to find whether Universe accelerates or not at a particular
redshift, one does not need to know how critical density changes with time.
 Note that ordinary matter always causes deceleration. One need an
unusual type of energy which density does not decrease with expansion
to have acceleration ! This is very strong argument for existenceof
"dark energy"

