4.2 Unobserved predictions
Apart from the above puzzling coincidences, the concordance
CDM model also has a few more
concrete empirical challenges to address, in the sense of having made a few predictions in contradiction with
observations (with the caveat in mind that the model itself is not always that predictive on small scales).
These include the following non-exhaustive list:
- The bulk flow challenge. Peculiar velocities of galaxy clusters are predicted to be on
the order of 200 km/s in the
CDM model. These can actually be measured by studying
the fluctuations in the CMB generated by the scattering of the CMB photons by the
hot X-ray-emitting gas inside clusters (the kinematic SZ effect). This yields an observed
coherent bulk flow of order 1000 km/s (5 times more than predicted) on scales out to at
least 400 Mpc [221
]. This bulk flow challenge appears not only in SZ studies but also in
galaxy studies [483]. A related problem is the collision velocity larger than 3100 km/s for
the merging bullet cluster 1E0657-56 at
, much too high to be accounted for by
CDM [249
, 455]. These observations would seem to indicate that the attractive force
between DM particles is enhanced compared to what
CDM predicts, and changing CDM
into WDM would not solve the problem.
- The high-z clusters challenge. Observation of even a single massive cluster at high
redshift can falsify
CDM [331]. In this respect the existence of the galaxy cluster XMMU
J2235.3-2557 [368] with a mass of of
at
, even though not sufficient
to rule out the model, is very surprising and could indicate that structure formation is actually
taking place earlier and faster than in
CDM (see also [420] on the Shapley supercluster
and the Sloan Great Wall).
- The Local Void challenge. The Local Volume is composed of 562 known galaxies at distances
smaller than 8 Mpc from the center of the Local Group, and the region known as the “Local
Void” hosts only 3 of them. This is much less than the expected
20 for a typical similar
void in
CDM [350
]. What is more, in the Local Volume, large luminous galaxies are
over-represented by a factor of 6 in the underdense regions, exactly opposite to what is expected
from
CDM. This could mean that the Local Volume is just a statistical anomaly, but it could
also point, in line with the two previous challenges, towards more rapid structure formation,
allowing sparse regions to more quickly form large galaxies cleaning their environment, making
the galaxies larger and the voids emptier at early times [350].
- The missing satellites challenge. It has long been known that the model predicts an
overabundance of dark subhalos orbiting Milky-Way–sized galaxies compared to the observed
number of satellite galaxies around the Milky Way [329]. This is a different problem from the
above-predicted overabundance of small galaxies in voids. It has subsequently been suggested
that stellar feedback and heating processes limit baryonic growth, that re-ionisation prevents
low-mass dark halos from forming stars, and that tidal forces from the host halo limit growth
of the dark-matter sub-halos and lead to their truncation. This important theoretical effort has
led recent semi-analytic models to predict a reduced number of
100 to 600 faint satellites
rather than the original thousands. Moreover, during the past 15 years 13 “new” and mostly
ultra-faint satellite galaxies have been found in addition to the 11 previously-known classical
bright ones. Since these new galaxies have been largely discovered with the Sloan Digital Sky
Survey (SDSS), and since this survey covered only one fifth of the sky, it has been argued that
the problem was solved. However, there are actually still missing satellites on the low mass
and high mass end of the mass function predicted by “
CDM+re-inoisation” semi-analytic
models. This is best illustrated on Figure 2 of [239
] showing the cumulative distribution for the
predicted and observationally-derived masses within the central 300 pc of Milky Way satellites.
A lot of low-mass satellites are still missing, and the most massive predicted subhaloes are
also incompatible with hosting any of the known Milky Way satellites [73, 75, 74]. This is
the modern version of the missing satellites challenge. An obvious but rather discomforting
way-out would be to simply state that the Milky Way must be a statistical outlier, but this is
contradicted by the study of [447] on the abundance of bright satellites around Milky Way-like
galaxies in SDSS. Another solution would be to change from CDM to WDM [252] (it is actually
one of the only listed challenges that such a change would probably immediately solve).
- The satellites phase-space correlation challenge. In addition to the above challenge, the
distribution of dark subhalos around the Galaxy is also predicted by
CDM to be isotropic,
or quasi-isotropic. However, the Milky Way satellites are currently observed to be correlated
in phase-space: they lie within a seemingly rotation-supported disk [239
]. Young halo globular
clusters define the same disk, and streams of stars and gas, tracing the orbits of the objects
from which they are stripped, preferentially lie in this disk, too [347]. Since SDSS covered only
one fifth of the sky, it will be interesting to see whether future surveys such as Pan-Starrs will
confirm this state of affairs. Whether or not this phase-space correlation would be unique to
the Milky Way should also be carefully checked, the evidence in M31 being currently much
less convincing, with a richer and more complex satellite population [289]. But in any case,
the current distribution of satellites around the Milky Way is statistically incompatible with
the predictions of
CDM at a very high level of confidence, even when taking into account
the observational bias from SDSS [239
]. While this might perhaps have been explained by the
infall of a small group of galaxies that would have retained correlated orbits, this solution is
ruled out by the fact that no nearby groups are observed to be anywhere near as spatially
small as the disk of satellites [290]. Another solution might be that most Milky Way satellites
are actually not primordial galaxies but old tidal dwarf galaxies created in an early major
merger event, accounting for their presently-correlated phase-space distribution [346]. Note in
passing that if only one or two long-lived tidal dwarfs are created in each gas-dissipational
galaxy encounter, they could probably account for most of the dwarf galaxy population in the
Universe, leaving no room for small CDM subhalos to create galaxies, which would transform
the missing satellites challenge into a missing satellites catastrophe [239
].
- The cusp-core challenge. Another long-standing problem of
CDM is the fact that
the simulations of the collapse of CDM halos lead to a density distribution as a function
of radius,
, which is well fitted by a smooth function asymptoting to a central cusp
with slope
in the central parts [126, 332
], while observations clearly point
towards large constant density cores in the central parts [118
, 169
, 479]. Even though the latest
simulations [333
] rather point towards Einasto [133] profiles with
(with
slightly varying with halo mass, and
for a Milky Way-sized halo, meaning
that the slope is zero only very close to the nucleus [177], and is still
at 200 pc from the
center), fitting such profiles to observed galactic kinematical data such as rotation curves [88]
leads to values of
that are much smaller than simulated values (meaning that they have
much larger cores), which is another way of re-assessing the old cusp problem of
CDM. Note
that a change from CDM to WDM could solve the problem in dwarf galaxies, by leading to the
formation of small cores, but certainly not in large galaxies where large cores are needed from
observations. Thus, one has to rely on baryon feedback to erase the cusp from all galaxies. But
this is not easily done, as the adiabatic cooling of baryons in the center of dark matter halos
should lead to an even more concentrated dark matter distribution. A possibility would be that
angular momentum transfer from a rotating stellar bar destroys dark-matter cusps: however,
significant cusp destruction requires substantially more angular momentum than is realistically
available in stellar bars [89, 286]. Note also that not all galaxies are barred (e.g., M33 is not).
The state-of-the-art solution nowadays is to enforce strong supernovae outflows that move large
amounts of low-angular-momentum gas from the central parts and that “pull” on the central
dark matter concentration to create a core [176
], but this is still a highly fine-tuned process,
which fails to address the baryon fraction problem (see challenge 10 below).
- The angular momentum challenge. As a consequence of the merger history of galaxy disks
in a hierarchical formation scenario, as well as of the associated transfer of angular momentum
from the baryonic disk to the dark halo, the specific angular momentum of the baryons ends up
being much too small in simulated disks, which in turn end up much smaller than the observed
ones [4]. Similarly, elliptical systems end up too concentrated as well. Addressing this challenge
within the standard paradigm essentially relies on forming disks through late-time quiescent gas
accretion from large-scale filaments, with much less late-time mergers than presently predicted
in
CDM.
- The pure disk challenge. Related to the previous challenge, large bulgeless thin disk galaxies
are extremely difficult to produce in simulations. This is because major mergers, at any time in
the galaxy formation process, typically create bulges, so bulgeless galaxies would represent the
quiescent tail of a distribution of merger histories for galaxies of the Local Volume. However,
these bulgeless disk galaxies represent more than half of large galaxies (with
)
in the Local Volume [178, 231]. Solving this problem would rely, e.g., on suppressing central
spheroid formation for mergers with mass ratios lower than 30% [228].
- The stability challenge. Round CDM halos tend to stabilize very low surface density
disks against the formation of bars and spirals, due to a lack of disk self-gravity [291
]. The
observation [282] of Low Surface Brightness (LSB) disk galaxies with strong bars and spirals is
thus challenging in the absence of a significant disk component of dark matter. What is more,
in the absence of such a disk DM component, the lack of disk self-gravity prevents the creation
of very-large razor-thin LSB disks, but these are observed [222, 260]. In the standard context,
these observations would tend to point towards an additional disk DM component, either a
CDM-one linked to in-plane accretion of satellites or a baryonic one in the form of molecular
gas.
- The missing baryons challenge(s). As mentioned above, constraints from the CMB imply
and
. However, our inventory of known baryons in the local Universe,
summing over all observed stars, gas, etc., comes up short of the total. For example, [42
]
estimate that the sum of stars and cold gas is only
5% of
. While there now seems
to be a good chance that many of the missing baryons are in the form of highly ionized gas
in the warm-hot intergalactic medium (WHIM), we are still far from being able to give a
confident account of where all the baryons reside. Indeed, there could be multiple distinct
reservoirs in addition to the WHIM, each comparable to the mass in stars, within the current
uncertainties. But there is another missing baryons challenge, namely the halo-by-halo missing
baryons. Indeed, each CDM halo can, to a first approximation, be thought of as a microcosm of
the whole. As such, one would naively expect each halo to have the same baryon fraction as the
whole Universe,
. On the scale of clusters of galaxies, this is approximately
true (but still systematically low), but for individual galaxies, observations depart from this
in a systematic way which we have yet to understand, and which has nothing to do with
the truncation radius. The ratio of the galaxy-detected baryon fraction over the cosmological
one,
, is plotted as a function of the potential well of the systems in Figure 2 [284
].
There is a clear correlation, less massive objects being much more dark-matter dominated than
massive ones. This correlation is a priori not predicted at all by
CDM, at least not with the
correct shape [273
]. This missing baryons challenge is actually closely related to the baryonic
Tully–Fisher relation, which we expand on in Section 4.3.1.
However, let us note that, while challenges 1 to 3 are not real smoking guns yet for the
CDM model,
challenges 4 to 10 are concerned with processes happening on kpc scales, for which it is fair to consider that
the model is not very predictive because the baryon physics should play a more important role, and this is
hard to take into account rigorously. However, it is not sufficient to qualitatively invoke handwavy baryon
physics to avoid confronting predictions of
CDM with observations. It is also mandatory to show that
the feedback from the baryons, which is needed to solve the observational problems, is what would
quantitatively happen in a physical galaxy. This, presently, is not yet the case for the aforementioned
challenges. However, these challenges are “model-dependent problems”, in the sense of being failed
predictions of a given model, but would not have appeared a priori surprising without the standard
concordance model at hand. This means that subtly changing some parameters of the model (like,
e.g., swapping CDM for WDM, making DM more self-interacting, etc.) might help solving at
least a few of them. But what is even more challenging is a set of observations that appear
surprising independently of any specific dark matter model, as they involve a fine-tuned relation
between the distribution of visible and dark matter. These are what we call hereafter “unpredicted
observations”.