218 Dark Matter
An intermediate category is constituted by possible sterile neutrinos and by the
gravitino, which is a SUSY partner of the graviton. These have been calledwarm dark
matter(WDM). Both HDM and WDM are now ruled out by computer simulations of
the galaxy distribution in the sky. WDM is also ruled out by the CMB detection of early
re-ionization at푧>11. We shall therefore not discuss these alternatives further.
9.6 The Cold Dark Matter Paradigm
The휆CDM paradigm (also written휆CDM or LCDM) is based on all the knowledge
we have assembled so far: the FLRW model with a spatially flat geometry, BBN and
thermodynamics with a known matter inventory including dark energy (cf. Chapter
11) of unknown origin but known density; inflation-caused linear, adiabatic, Gaussian
mass fluctuations accompanying the CMB anisotropies with a nearly scale-invariant
Harrison–Zel’dovich power spectrum; growth by gravitational instability from푡equntil
recombination, and from hot gas to star formation and hierarchical clustering.
The new element in this scenario is collisionless DM, which caused matter domi-
nation to start much earlier than if there had been only baryons. The behavior of DM
is governed exclusively by gravity (unless we discover any DM interactions with mat-
ter or with itself), whereas the formation of the visible parts of galaxies involves gas
dynamics and radiative processes.
While the CMB temperature and polarization anisotropies measure fluctuations
at recombination, the galaxy distribution measures fluctuations up to present times.
Cosmic shear in weak lensing is sensitive to the distribution of DM directly, but it
leaves a much weaker signal than do clusters.
Currently observable bright galaxies are more than 170 billion in number, out to
230 Mpc. The comoving future visibility limit is about 19 Gpc counting how far a
photon can travel from the Big Bang to the infinite future. The total number of galaxies
that one will eventually be able to see is then 400 billion, and the absolute limit in
redshift is푧< 1 .69 [12].
Distributions of galaxies in two-dimensional pictures of the sky show that they form
long filaments separating large underdense voids with diameters up to 60ℎ−^1 Mpc.
Figure 9.4 shows such a map out to 2.74 Gyr [20]. The image reveals a wealth of detail,
including linear supercluster features, often nearly perpendicular to the line of sight.
The largest structure is the Sloan Great Wall of galaxies 1. 37 ℎ−^1 billion lightyears
long, 80% longer than the previously known Great Wall.
Hierarchical Scenarios. Early CDM models (without an훺휆component) produced
galaxies naturally, but underproduced galaxy clusters and supergalaxies of mass scale
1015 푀⊙. This was an example of a bottom–top scenario, where small-scale structures
were produced first and large-scale structures had to be assembled from them later.
Although there was not time enough in this scenario to produce large-scale structures
within the known age of the Universe, the scenario could be improved by the intro-
duction of a new degree of freedom, the cosmological constant.
The opposite ‘top–bottom’ scenario was predicted by HDM models where the first
structures, supergalaxies, formed by neutrino clouds contracting into pancakes which