3.4. Problems with DM
A number of potential problems with the neutrino dominated
universe had emerged by about 1983, however.
-
From studies both of nonlinear clustering
[48,
50]
(comoving length scale
10 Mpc) and of
streaming velocities
[51]
in the linear regime ( > 10
Mpc), it follows that
supercluster collapse must have occurred recently:
zsc 0.5 is
indicated and in any case zsc < 2
[48].
However, the best limits on galaxy ages coming from globular
clusters and other stellar populations indicated that galaxy
formation took place before z
3.
Moreover, if quasars are associated with galaxies, as is
suggested by the detection of galactic luminosity around
nearby quasars and the apparent association of more distant
quasars with galaxy clusters, the abundance of quasars at z > 2
was also inconsistent with the ``top-down'' neutrino
dominated scheme in which superclusters form first: zsc
> zgalaxies.
-
Numerical simulations of the nonlinear ``pancake'' collapse
taking into account dissipation of the baryonic matter showed
that at least 85% of the baryons are so heated by the
associated shock that they remain unable to
condense, attract neutrino halos, and eventually form
galaxies [5,
52].
This was a problem for the hot DM scheme for two
reasons. With the primordial nucleosynthesis constraint
b
0.1, there would be
difficulty having enough
baryonic matter condense to form the luminosity that we
actually observe. And, where are the X-rays from the
shock-heated pancakes
[53]?
-
The neutrino picture predicts
[54]
that there should be a factor of
~ 5 increase in M / Mb between large galaxies
(M ~ 1012
M) and large
clusters (M
1014
M), since the
larger clusters,
with their higher escape velocities, are able to trap a
considerably larger fraction of the neutrinos. Although
there is some indication that the mass-to-light ratio
M/L increases with M, the ratio of total to
luminous mass M / Mlum is probably a better indicator of
the value of M /Mb, and
it is roughly the same for galaxies with large halos
and for rich clusters.
These problems, while serious, would perhaps not have been fatal
for the hot DM scheme. But an even more
serious problem for HDM arose from the low
amplitude of the CMB fluctuations detected by the COBE satellite,
(T / T)rms =
(1.1 ± 0.2) x 10-5 smoothed on an angular
scale of about 10°
[55].
Although HDM and CDM both have the
Zel'dovich spectrum shape (P(k)
k) in the long-wavelength
limit, because of the
free-streaming cutoff the amplitude of the HDM spectrum must be
considerably higher in order to form any structure by the present.
With the COBE normalization, the HDM spectrum is
only beginning to reach nonlinearity at the present epoch.
Thus the evidence against standard hot DM is convincing. At very
least, it indicates that structure formation in a neutrino-dominated
universe must be rather more complicated than in the standard
inflationary picture.
The main alternative that has been considered is cosmic strings plus
hot dark matter. Because the strings would continue to seed structure
up until the present, and because these seeds are in the nature of
rather localized fluctuations, hot DM would probably work better with
string seeds than cold DM. However, strings and other cosmic defect
models are now essentially ruled out
[56,
57]
because they predict that the
cosmic microwave background would have an angular power spectrum
without the pronounced (doppler/acoustic/Sakharov) peak at angular
wavenumber l ~ 220 that now appears to be clearly indicated by the
data, along with secondary peaks at higher l.