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| ![]() | © CAMBRIDGE UNIVERSITY PRESS 2000 |
7.4 Growth of structure: clusters, walls, and voids
We still have not answered the question that we asked at the end of
Section 7.1: how did our Universe, which
was highly uniform at the
time of recombination, develop the galaxy clusters, and the huge walls
and voids that we see in
Figure 7.3? These structures are
visible at
the present time t0 because the density of luminous
matter in
them is a few times greater than in the surrounding regions. If we
assume that galaxies trace the mass density, then in the language of
Equation 7.25, the fractional change in density
| (t0)|
1.
Earlier, density variations must have been far smaller. At the time of
recombination, the time when matter became neutral and
transparent to photons of the cosmic background radiation, its
temperature was Trec
3000 K. Since then, the
radiation has been redshifted by an amount
1 + zrec
3000/2.7
1100.
Equation 7.28 tells us that if density fluctuations present at
time trec were to develop into the clusters, walls and
voids that we observe today, they must have been at least as large as
(trec) ~
10-3. If the Universe has a low density, with
0 ~ 0.1,
then we would need
(trec)
10-2, since
these structures must have reached
~ 1 by the time they
stopped growing, at z ~ 1 /
0.
What effect would variations in the matter density have on the cosmic
background radiation as we observe it today? In Equation 7.27,
we assumed that the perturbation
refers to the total density,
with the radiation and matter remaining well mixed; these are called
adiabatic fluctuations.
To reach us from an overdense region,
radiation has to climb out of a deeper gravitational potential. In
doing this, it suffers a gravitational redshift proportional to
g, the excess depth of
the potential: its
temperature T changes by
T, where
T / T ~
g / c2.
If the dense region has radius R, then its excess mass
M =
4
R3
/3.
At these early times, the average density
is very
nearly equal to the critical density of Equation 7.17, so we can
write
![]() | (7.40) |
The dense region subtends an angle on the sky of
~
R / dA, where dA is the
angular diameter distance, which we will define in Section 8.3.
So the increment to the gravitational potential is
g /c2
-
2.
A more careful calculation gives
T / T
-
2 / 3; the radiation
reaching us from denser regions has a lower temperature.
The Cosmic Background Explorer satellite COBE made maps with
angular resolution
7° or 0.1 radians.
At the time of recombination, a region of this size contains a mass
~ 1020
M
. The
temperature variations on that huge scale
were no more than a few parts in 105. Extrapolating to
the smaller fluctuations that were to make galaxy clusters, we have
(trec)
10-3 on those
scales. If the density is close to the critical value, with
0
1, such tiny ripples are
only just large enough to
generate the galaxy clusters that we see today.
If
0 ~ 0.1, they are
grossly insufficient. In Section 1.5 we saw that
the measured amount of deuterium and other light elements implies
that neutrons and protons can account for no more than
0 = 0.1. Unless the
Universe contains other forms of matter,
the density fluctuations at the time of recombination are far too
small to make the walls and voids that we observe today.
The difficulty might be avoided if the Universe had begun with its matter arranged in a slightly irregular pattern, but with the radiation almost completely uniform: these are isothermal or isocurvature fluctuations. They are popular with some cosmologists, but we do not know of any process that would distribute the matter in this way. Others postulate that most matter in the Universe consists of the weakly interacting massive particles, or WIMPs, that we discussed in Section 2.3. To see why WIMPs could be useful, we must ask what forces could prevent a gas cloud from becoming ever denser, as gravity draws matter inward.
Objects like stars are supported by gas pressure, which counteracts
the inward pull of gravity. The larger a body is, the more likely it
is that gravity will win the fight against the outward forces that
hold it up. In life, the giant insects of horror movies would be
crushed by their own weight.
A large cloud of gas collapses if its gravitational potential energy
outweighs the kinetic energy in its random internal motions
and the thermal motion of its atoms.
Using Equation 3.31 for the
potential energy PE of a sphere of radius r filled with gas of
density and sound
speed cs, we can compare it with
the thermal energy KE:
![]() | (7.41) |
If we have |PE| > KE, then the cloud's diameter 2r must exceed a lower limit:
![]() | (7.42) |
where the length J
is called the Jeans length.
A more careful analysis shows that when gas is compressed, both
its pressure and the inward force of gravity are increased.
Higher internal pressure tends to cause expansion, while the
the extra gravity pulls inward. If a gas cloud's diameter is less
than
J, the
additional pressure more than offsets the increased gravity: the cloud
re-expands. In a larger cloud gravity wins, and collapse ensues.
In the early radiation-dominated phase of the Universe, the density
and sound speed cs in Equation 7.42 are those of a gas of
photons: the density is r
= aB T4 /
c2, and cs = c /
3. So the Jeans length
![]() | (7.43) |
it grows as T-2 or as
2 (t) as
the Universe expands.
The Jeans mass MJ is the amount of matter in
a sphere of diameter
J:
![]() | (7.44) |
where m
refers only to the matter density.
Since
m
decreases as
-3,
the Jeans mass grows as MJ
3 (t); the mass
enclosed in a sphere of diameter
J
increases as the Universe becomes more diffuse.
At the time teq when the density of matter is equal to
that in radiation, the temperature was Teq, and
m =
r =
aB T4eq /
c2; so we can write
![]() | (7.45) |
If equality occurs at the redshift 1 + zeq = 24 000
0 h2
of Problem 7.9, then at earlier times the Jeans mass can be written as
![]() | (7.46) |
At the time of equality, the Jeans mass has grown much larger than the
mass of a galaxy cluster: according to Equation 1.24, it is about
the mass that we would find today in a huge cube (50 /
0
h2) Mpc on a side. This is approximately the spatial
scale of some
of the largest voids and complexes of galaxy clusters in
Figure 7.3.
Overdense regions with masses below MJ could not
collapse before the time of recombination.
Instead, radiation gradually diffused out of them, taking the ionized
gas with it, and damping out small irregularities.
By the time that the gas became
neutral and transparent, matter concentrations with
M
2 x 1012
(
0
h2)-5/4
M
would be smoothed away.
The very hugest structures would also have been unable to grow.
Before time t, light and gravitational influences could not
propagate beyond the horizon
H (t) given by
Equation 7.23. So larger structures were unable to collapse
until the horizon expanded to encompass them.
Until the time of matter-radiation equality, the mass of baryons
within the horizon was no more than
MH (teq)
1015 (
0
h2)-2
M
.
Variation or uniformity on larger scales must reflect
conditions at early times, before the cosmic expansion had begun to
follow Equation 7.20.
According to the inflationary theory,
quantum fluctuations in the field responsible for the vacuum energy
VAC leave
their imprint as irregularities in the
density of matter and radiation. After inflation ends, most versions
of the theory predict that on large scales the power spectrum
P(k)
k, and
that the random phase hypothesis, on which
Equation 7.4 depends, should be valid.
By a redshift zrec ~ 1100 when the temperature
Trec 3000
K, hydrogen atoms had recombined, and
the pressure of radiation was removed. The
sound speed drops to that of the matter:
![]() | (7.47) |
During recombination, the Jeans mass of Equation 7.44 falls abruptly by a factor of 1014! Just afterward, we have
![]() | (7.48) |
Radiation continues to transfer heat to the matter,
keeping their temperatures roughly equal until z ~ 100.
Since the radiation temperature Trec
(t),
the decreasing temperature offsets the drop in density
m to
keep the Jeans mass nearly constant, about equal to
the mass of a globular cluster.
If the first dense objects formed with roughly this mass,
they could subsequently have merged to build up larger bodies.
Once it is no longer receiving heat, the matter cools according to
as Tm
-2. To
see why, think of the perfect gas law relating temperature to volume, or
recall that expansion reduces the random speeds of atoms
according to Equation 7.24. So the Jeans mass falls
further; gas pressure is far too feeble to affect the collapse
of anything as big as a galaxy.
If the only matter in the Universe is the `normal' matter of neutrons and protons, collectively called baryons, then Equation 7.45 tells us that only very large matter concentrations can grow at early times. These huge structures are generally not spherical; as they collapse, they shrink fastest along their shortest direction, forming a `pancake' shape (Figure 7.9). Since gas pressure is not important, collapse occurs on roughly the free-fall timescale tff of Equation 3.23. By Equation 6.36, compressing the gas decreases its cooling time. If it is initially dense enough to cool within the free-fall time, the cloud rapidly loses energy as it shrinks, reducing the Jeans mass. Smaller fragments of the dense pancake can then collapse successively, to become galaxy clusters and individual galaxies. Where sheets intersect, large clusters of galaxies would form. This picture is called the top-down model, since the largest structures in the Universe would be made first.
But if WIMPs accounted for most of the matter, smaller objects could
collapse and become dense at an earlier stage. To see why, we can
recalculate the minimum mass of a region that will collapse under its
own gravity, by analogy with Equation 7.44. WIMPs are unaffected
by radiation pressure. So where the density of WIMPs is
w
and the random speed is cw, a dense region would fall in on
itself if it contained a mass larger than
![]() | (7.49) |
While the WIMPs are relativistic, their Jeans mass is high; but as
soon as the speed cw drops appreciably below c
/ 3,
the Jeans mass starts to fall. Lumps of WIMPs with the mass of
galaxies or galaxy clusters could start to collapse well before
comparably sized chunks of normal
matter. As it escaped from the contracting clouds of WIMPs, the
radiation would have taken the normal matter with it. So both of
these should be quite evenly spread at recombination, and
we would expect very little variation in the temperature of
the cosmic background radiation. Later, as the
matter became neutral, and was freed of the radiation pressure, it would
fall into the already-dense clumps of WIMPs. Fluctuations
in the density of normal matter could then grow much more
rapidly than Equation 7.27 would allow, building up the
galaxies and clusters that we see today.
WIMPs massive enough that their sound speed cw fell below the speed of light long before the time teq of matter-radiation equality are called cold dark matter. If these account for most of the mass in the Universe, then the first structures to collapse might have been of galactic mass, or smaller. Galaxies themselves would be built from these smaller fragments; at the present, each should have a massive dark halo made largely of WIMPs. We call this the bottom-up picture because galaxies form early, and then fall together to form clusters and larger structures. WIMPs with masses of only a few electron-volts remain relativistic until the time of recombination, and are called hot dark matter. They behave much like photons; the Jeans mass stays high, and we again have top-down galaxy formation.
Figure 7.10 shows results from a computer simulation, following the way that gravity amplifies small initial ripples in an expanding universe of cold dark matter. The figure shows a stage of the calculation representing the present day; note the profusion of dense small structures. The densest regions, shown in the side boxes, have ceased to expand and have fallen back on themselves. Hydrogen gas would accumulate there, cooling to form groups of luminous galaxies.
Neither the top-down nor the bottom-up picture is entirely
satisfactory. The top-down model insists that the enormous wall and
void structures must form earlier than individual galaxies. But both
galaxies themselves, and quasars, which are
active galactic nuclei, are seen at z ~ 5; see Chapter 8. A
simple argument tells us that the walls of Figure 1.18 could not
have been made at such an early stage. At the time when the cosmic
expansion has been reversed locally, so that collapse is
just about to start, such a region must be denser than average -
otherwise it would still be expanding. We will see in Section 8.3
that the present density of a galaxy or cluster is at least 8 times
greater than it was when its expansion had just been halted. A region
that started to fall inward at z = 5, when the average density was
53 = 125 times more than at the present day, should now be at least
125 x 8 = 1000 times denser than average. The walls and
filaments are only a few times denser than their surroundings, so they
can have begun their collapse only in the very recent past, at
z 2.
The very large walls and voids that we see in Figure 7.3 are too big to be affected by the cold dark matter WIMPs. In bottom-up galaxy formation, matter clumps much more strongly on small scales of a few megaparsecs than on large scales. We can use models like Figure 7.10 to predict the power spectrum P(k), defined by Equation 7.3, and compare that with measured values.
The simplest inflationary theory of cosmology predicts that the
Universe should be flat, with
= 1 exactly. The left panel of
Figure 7.11 shows the corresponding power
spectrum, when we
choose the parameters to reproduce the observed clustering on scales
of k-1 ~ 8 h-1 Mpc. When the curve
is scaled to agree with
what is required to fit COBE's observations of the microwave
background, the clustering is far too strong on the scales of a few
megaparsecs that are characteristic of galaxy groups and clusters.
Astronomers have tried to `rescue' this simple picture, by postulating that
the galaxies we see are not a fair sample of all the matter
in the Universe. If galaxies formed only in
exceptionally dense regions, while the large voids were filled with dark
matter and perhaps diffuse gas, then the true density fluctuations
(x, t) would be
much smaller than what we deduce from
counting the luminous galaxies. This is the hypothesis of biassed
galaxy formation; there is little independent evidence in its favor.
Another approach is to decrease the assumed density of the
Universe; weakening gravitational forces suppresses the clumping
of the densest small-scale regions. The right panel
of Figure 7.11 shows what the
0 = 0.4 model of
Figure 7.10 predicts; the peak of the power spectrum
moves to larger scales, at k-1 ~ 100
h-1 Mpc.
The smooth curve is now a fairly good match to the observed clustering
of luminous galaxies.
In this model, space is negatively curved. We will see in
Section 8.3 that a fixed angle on the sky now corresponds to a larger
linear distance, so the limits we derive from the microwave background
observations move to smaller wavenumber k.
The calculations do not include dissipation of energy in the
normal matter, which acts to increase the density of galaxies on scales
k-1
1 Mpc;
so the model curve falls below the measured power spectrum there.
A further option is to postulate that fluctuations on small mass scales were weaker to begin with: this is known as a `tilted perturbation spectrum'. This hypothesis suffers the same defect as biassed galaxy formation: nobody knows when it should happen, or how to predict its form. Some astronomers advocate a combination of top-down and bottom-up models; they postulate a mixture of hot and cold dark matter, making galaxies with the aid of the cold dark matter, and large structures with the hot dark matter. Others feel that this idea is implausibly contrived.