
500 Classical Mechanics
© 2010 Taylor & Francis Group, LLC
applying Newtonian gravitation. A uid with zero pressure moving according to Newton’s laws of
motion and gravitation is known as a Newtonian dust. We now consider a Newtonian dust of uni-
form density ρ(t), which is in a state of uniform expansion; the only force on it is gravity. We will
work with a co-moving coordinate system, which is carried along with the expansion. Because the
expansion is uniform, the position vector
of a uid particle (a galaxy) at any time can be written
in terms of the co-moving distance, which we call
:
rt stc() ()= (14.23)
where s(t) is known as the ...