Annu. Rev. Astron. Astrophys. 1992. 30:
499-542 Copyright © 1992 by Annual Reviews. All rights reserved |
3.5 Growth of Linear Perturbations
In all the homogeneous and isotropic cosmologies, linear cold matter perturbations / grow at a rate that does not depend on their comoving spatial scale (e.g. Peebles 1980). An explicit expression for the amplitude of a growing perturbation (Heath 1977) is
where a' is the dummy integration variable, and da /
d is to be viewed
as a known function of a or a', in our case given explicitly by
Equation 9. Equation 28 is normalized so that the fiducial case of
M
= 1, = 0 gives the familiar
scaling (a) = a,
with coefficient unity.
Different values of M,
lead to different linear
growth factors
from early times (a
0) to the present (a = 1, da /
d = 1). Denoting
the ratio of the current linear amplitude to the fiducial case by
0(M, ) we have
(The remarkable approximation formula - good to a few percent in
regions of plausible M,
- follows from
Lahav et al 1991 and
Lightman &
Schechter 1990.)
Figure 7 shows
numerical values for 0
(M,
) for the region in the
(,
tot)
plane previously seen in Figures 1
and 4. One sees that as
M is reduced from
unity, both along the line
= 0 and along the line
tot
= 1, the growth of perturbations is
suppressed, but somewhat less suppressed in the
tot = 1 case. The
reason is that, for fixed
M, linear growth
effectively stopped at a
redshift (1 + z) = M-1 in the open case (when the
universe became
curvature dominated), but, more recently, at (1 + z) =
M-1/3 in
the flat case (when the universe became
dominated).
Figure 7. Growth factor for linear
perturbations, as contours in the
(M,
tot) plane,
normalized to unity for the case
M = 1,
= 0. There
is relatively less suppression of growth as
M is decreased along the
line tot = 1 than
along the line = 0; but for credible
values of M
the difference is not a large factor. Perturbation growth approaches
at the ``loiter line,'' but for credible
M it occurs at too small a
redshift to explain quasars (see
Figure 1).
To the right of the line
tot = 1 in
Figure 7, one sees values of
0
(M,
) that are greater than 1,
in fact approaching infinity at the loitering cosmology line (cf
Figure 1 and discussion
above). Loitering cosmologies allow the arbitrarily large growth of
linear perturbations, since the perturbations continue to grow during
the (arbitrarily long) loiter time.
Related to the growth of linear perturbations is the relation
between peculiar velocity v and peculiar acceleration g, or (as a
special case) the radial infall velocity vrad around a
spherical perturbation of radius
. These quantities
depend not directly on
Equation 28, but on its logarithmic derivative, the exponent in the
momentary power law relating
to a,
the relation being
where <> is the overdensity
averaged over the interior of the sphere
of radius
(Peebles 1980,
Section 14). One can calculate
Figure 8 plots f(z) for our standard
models A-E. One sees that, at
small redshifts, peculiar velocities depend almost entirely on
M and
are quite insensitive to . This
is because they are driven by the
matter perturbations in primarily the most recent Hubble time. Looking
back to redshifts z
1, however, the peculiar
velocities do start
depending on , allowing in principle for
observational tests (but see
Lahav et al 1991
for caveats).