If z is a scalar, result is a scalar double precision lookback time. If z is an array, result is a double precision array with the lookback time for each z value.
If H0 is specified, the result of calc_t is given in seconds (or in years, if the keyword /year is used, see below). Otherwise, it is internally set to 1 so the result is the lookback time in terms of H0 (see eq. 2).
If the keyword /year is used the result is given in years instead of seconds. This of course only works if H0 is specified, otherwise the result is the lookback time in terms of H0.
The function calc_t returns -1000 if the passed parameters describe a bouncing universe (see section V below).