- #1
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In Weinberg's Cosmology, the comoving coordinate described as "A particle at rest in these coordinates will, therefore, stay at rest, so these are co-moving coordinates"
Now when we write the proper distance
##s = a(t)\chi## where ##\chi## is the comoving coordinate.
Taking the time derivative
$$v =\frac{ds}{dt} = \dot{a(t)}\chi + a(t)\dot{\chi}$$
Here ##V_H = \dot{a(t)}\chi## which is due to Hubble flow and ##v_p= a(t)\dot{\chi}## is called the peculiar velocity. So according to the above description (the italic sentence), a particle will not stay at rest in these coordinates. In this sense, the comoving coordinates are not perfect (?)
To avoid this I guess we are assuming large comoving distances so that ##V_H \gg v_p##, so that we can ignore the peculiar velocity?
Now when we write the proper distance
##s = a(t)\chi## where ##\chi## is the comoving coordinate.
Taking the time derivative
$$v =\frac{ds}{dt} = \dot{a(t)}\chi + a(t)\dot{\chi}$$
Here ##V_H = \dot{a(t)}\chi## which is due to Hubble flow and ##v_p= a(t)\dot{\chi}## is called the peculiar velocity. So according to the above description (the italic sentence), a particle will not stay at rest in these coordinates. In this sense, the comoving coordinates are not perfect (?)
To avoid this I guess we are assuming large comoving distances so that ##V_H \gg v_p##, so that we can ignore the peculiar velocity?