The age of the Universe is a fundamental parameter in cosmology. Its
value inferred by the Hubble constant and globular cluster ages
differed
significantly (although they seem to converge in recent works). Thomas,
Tremaine & I devised a method in calculating
the age and mass of the Local Group (LG) by tracing the orbits of LG
dwarfs as test particles in the Lagrangian Potential of the Local
Group, by arguing that: had they not originated from the
Lagrangian Points L4 and L5 in the Big Bang, they would have been
captured by the Milky Way and M31 by now. The advantages of this
method over the least-action method (Peebles 1989) are that
complicated orbits of more nearby dwarfs can be modelled, and that
there is less freedom in the choice of parameters. This method is
successful in predicting the radial velocities of most LG dwarfs to
within 
. Integration of 6
dwarf orbits yielded an age and
mass of the Local Group of 
Gyrs and 
respectively (Hau
1992; Hau, Thomas & Tremaine, in
prep). The latter shows that the mass of the Local Group is about 10
times that of the mass inferred by the light.
Figure 5: Left: Orbits of NGC 6822 emerging
from the
Lagrangian Points L4 and L5 at the Big Bang, in the co-moving frame of
the Local Group in which the Milky way and M31 (left & right
crosses)
are stationary and their centre of mass is at the origin.
Right: The difference in the predicted and observed radial velocity
plotted against the
age of the Universe 
.