The galaxies themselves became organized in the large-scale structure: the cosmic web consisting of galaxy walls, galaxy filaments, galaxy clusters, galaxy groups, and voids.
For an example galaxy cluster, see the figure below (local link / general link: galaxy_cluster_RXC_J0142.9+4438.html).
The rich got richer and the poor got poorer.
In the dense initial density fluctuations,
there were gravitational runaways of matter
(∼ 85 % of it being dark matter)
to form clumps (dense regions) called
dark matter halos
where the
proto galaxies formed.
In the less-dense initial density fluctuations,
the dark matter did NOT clump and
large voids formed.
The baryonic matter to 1st order
just follows the dark matter pulled
along by the dark matter's
gravity.
"Cold" in this context means moving slow relative to the
vacuum light speed c = 2.99792458*10**5 km/s ≅ 3*10**5 km/s: i.e., at nonrelativistic
velocities relative to the
local comoving frames.
Cold dark matter
is needed to get the clumping properties needed for the
observed large-scale structure.
Without cold dark matter,
the baryonic matter
would probably still clump to form
stars, but
galaxies and the rest of
large-scale structure
would look very different from what we see.
Note that there MAY be exotic
hot dark matter
(dark matter
moving at relativistic
velocities)
and warm dark matter
(at intermediate velocities),
but these dark matter
forms can only be of secondary importance in
structure formation.
Actually,
neutrinos forming the
cosmic neutrino background
(a relic of the
Big Bang from
before Big Bang nucleosynthesis (BBN))
were originally a form
of hot dark matter, but
they lost
kinetic energy in a manner
similar to that of
cosmologically redshifting
photons
and became a minor contribution to
cold dark matter.
Such N-body simulations
in the context of the
Λ-CDM model (AKA concordance model)
(our best current cosmological model),
have always done a good 1st order job in reproducing the statistical properties of
the observed
large-scale structure.
So to fully understand
large-scale structure,
we must understand how the
baryonic matter clumps into
observable galaxies, etc.
This is much more complicated than just studying
dark matter since
baryonic matter
interacts through pressure forces
as well as gravity.
The pressure forces
include
ideal gas law pressure,
radiation pressure,
and
magnetic pressure
(which if nothing else helps to launch
relativistic bipolar jets
from central supermassive black hole,
and thus provides AGN feedback
to structure formation).
We are trying to calculate simulated
large-scale structure
that as the SAME statistical properties as that of the
large-scale structure:
e.g., same average number of
galaxies of each type
per unit volume,
same average number of
galaxy clusters
per unit volume, etc.
Why CAN'T we calculate
our observable universe?
The initial density fluctuations left from the
Big Bang era
CANNOT be known.
We can only theorize their statistical properties, and so only calculate
the statistical properties of the
observable universe
that evolves from them.
But we have a good theory of those fluctuations since we do calculate
the statistical properties of the
observable universe
pretty well.
There is NO reason at present to believe that we will NOT
eventually match the
statistical properties of the
observable universe
to high accuracy provided
we can the right
overall cosmological model.
Do we have it now.
Probably NOT, at least NOT exactly.
See the next item.
There are some tensions for
structure formation, but
NO falsifications currently.
Digression on jargon:
A falsification
is a discrepancy between
theory
and observation sufficiently large that one judges the
theory to be wrong.
A tension
is a discrepancy that does NOT cause one to judge the
theory as wrong.
The discrepancy may be due
uncertainties
in the observations or in the application of the
theory.
Tensions
suggest there might be a problem with a
theory, but more work is
needed to show if that is true.
More work hopefully will cause the tensions
to go away OR turn them into
falsifications.
Either way, progress.
php require("/home/jeffery/public_html/astro/cosmol/large_scale_structure_keywords.html");?>
php require("/home/jeffery/public_html/astro/galaxies/galaxy_cluster_RXC_J0142_9_4438.html");?>