FRW Universe

1
FRW Universe The Hot Big Bang
2
Adiabatic Expansion
From the Friedmann equations, it is
straightforward to appreciate that cosmic
expansion is an adiabatic process
In other words, there is no external power
responsible for pumping the tube
3
Adiabatic Expansion
Translating the adiabatic expansion into the
temperature evolution of baryonic gas and
radiation (photon gas), we find that they cool
down as the Universe expands
4
Adiabatic Expansion
Thus, as we go back in time and the volume of the
Universe shrinks accordingly, the temperature of
the Universe goes up. This temperature behaviour
is the essence behind what we commonly denote as
Hot Big Bang
From this evolution of temperature we can thus
reconstruct
the detailed
Cosmic Thermal History
5
The Universe the Hot Big Bang

• Timeline the Cosmic Thermal History

6
Equilibrium Processes
Throughout most of the universes history (i.e.
in the early universe), various species of
particles keep in (local) thermal equilibrium via
interaction processes
Equilibrium as long as the interaction rate Gint
in the cosmos thermal bath, leading to Nint
interactions in time t, is much larger than the
expansion rate of the Universe, the Hubble
parameter H(t)
7
Brief History of Time
8
Reconstructing Thermal History Timeline

• 
  Strategy
• To work out the thermal history of the Universe,
  one has to evaluate at each cosmic time which
  physical processes are still in equilibrium. Once
  this no longer is the case, a physically
  significant transition has taken place. Dependent
  on whether one wants a crude impression or an
  accurately and detailed worked out description,
  one may follow two approaches
• Crudely
• Assess transitions of particles out of
  equilibrium, when they decouple from
• thermal bath. Usually, on crude argument
• Strictly
• evolve particle distributions by
  integrating the Boltzmann equation

9
Thermal History Interactions

• Particle interactions are mediated by
  gauge bosons photons for the electromagnetic
  force, the W bosons for weak interactions, and
  gluons for the strong force (and gravitons for
  the gravitational force). The strength of the
  interaction is set by the coupling constant,
  leading to the following dependence of the
  interaction rate G, on temperature T
• mediated by massless gauge boson (photon)
• (ii) mediated by massive gauge boson (W/-
  ,Z0)

10
History of the Universe in Four Episodes I.
On the basis of the 1) complexity of the
involved physics and 2) our knowledge of the
physical processes we may broadly distinguish
four cosmic episodes
(I)
Origin universe ???
t lt 10-43 sec

• fundamental physics
• totally
• unknown

Planck Era
11
History of the Universe in Four Episodes II.

• ?tot
• curvature/
• flatness
• ?b (nb/n?)
• exotic
• dark matter
• primordial
• fluctuations

(II)
10-43 lt t lt 10-3 sec

• fundamental
• physics
• poorly known
• speculative

VERY early universe
Products
12
History of the Universe in Four Episodes III.
(III)

• primordial
• nucleo-
• synthesis
• blackbody
• radiation
• CMB

10-3 lt t lt 1013 sec
Standard Hot Big Bang Fireball
fundamental microphysics known very well
Products
13
History of the Universe in Four Episodes IV.
(IV)

• structure
• formation
• stars,
• galaxies
• clusters

t gt 1013 sec
Post (Re)Combination universe

• complex macrophysics
• Fundamentals known
• complex interplay

Products
14
EpisodesThermal History
Planck Epoch

t lt 10-43 sec
Phase Transition Era
10-43
sec lt t lt 105sec Hadron Era

t
10-5 sec Lepton Era

10-5 sec lt t lt 1 min
Radiation Era

1 min lt t lt379,000 yrs Post-Recombination Era

t gt 379,000 yrs
GUT transition electroweak transition quark-hadron
transition
muon annihilation neutrino decoupling electron-pos
itron annihilation primordial nucleosynthesis
radiation-matter equivalence recombination
decoupling
Structure Galaxy formation Dark Ages
Reionization Matter-Dark Energy transition
15
Thermal HistoryEpisode by Episode
Planck Epoch

t lt 10-43 sec

• In principle, temperature T should rise to
  infinity as we probe earlier and earlier into the
  universes history
• However, at that time the energy of the particles
  starts to reach values where quantum gravity
  effects become dominant. In other words, the de
  Broglie wavelength of the particles become
  comparable to their own Schwarzschild radius.

16
Thermal History Planck Epoch
Once the de Broglie wavelength is smaller than
the corresponding Schwarzschild radius, the
particle has essentially become a quantum black
hole de Broglie
wavelength

Schwarzschild radius
These two mass scales define the epoch of quantum
cosmology, in which the purely deterministic
metric description of gravity by the theory of
relativity needs to be augmented by a theory
incorporating quantum effects quantum gravity.
17
Thermal History Planck Epoch
On the basis of the expressions of the de Broglie
wavelength and the Schwarzschild radius we may
infer the typical mass scale, length scale and
timescale for this epoch of quantum cosmology


Planck Mass

Planck Length

Planck Time Because our
physics cannot yet handle quantum black holes,
i.e. because we do not have any viable theory of
quantum gravity we cannot answer sensibly
questions on what happened before the Planck
time. In other words, we are not able to probe
the ultimate cosmic singularity some ideas of
how things may have been do exist
18
Planck Transition
? In the Planck epoch, before the universe is 1
hundred-million-trillion-trillionth (10-44) sec
old, the density reaches values higher than
?1094 g/cm3 and temperatures in excess of T
1032 K. ? Quantum fluctuations of spacetime,
on the scale of the Planck scale and Planck time
are now of cosmic magnitude. Space and time are
inextricably and discontinuously. As was pictured
by J. Wheeler, spacetime under these conditions
looks like a chaotic foam. ? Spacetime is a
foam of quantized black holes, and space and time
no longer exist in the sense that we would
understand. There is no now and then, no
here and there, for everywhere is torn into
discontinuities. ? Then, due to the cosmic
expansion, temperatures drop below T1032 K,
and the unified superforce splits into a force
of Gravity and a GUT force
Gravity
Unified Superforce
Grand Unified Force
19
Thermal HistoryEpisode by Episode
Phase Transition Era
10-43
sec lt t lt 10-5 sec

• The universe is filled by a plasma of
  relativistic particles
• quarks,
  leptons,
• gauge
  bosons, Higgs bosons,
• During this epoch, as the universe expands and
  cools down, it undergoes various phase
  transitions, as a result of
• Spontaneous Symmetry
  Breaking

20
Thermal HistoryEpisode by Episode
Phase Transition Era

10-43 sec lt t lt 10-5 sec

• We may identify three major phase transitions
  during this era
• ? GUT transition
  z 1027-1029
• ? Electroweak transition
  z 1015
• ? Quark-Hadron
  transition z 1011-1012 (t10-5s)
  

21
GUT Transition
T 1014 1016 GeV
1027 1029 K
z 1027 1029

• Before this transition, at Tgt1014-1016 GeV, there
  was one unified GUT force, i.e. strong, weak and
  electromagnetic force equally strong (note
  gravity is a different case).
• Also, the universe did not have a net baryon
  number (as many baryons as antibaryons).
• At the GUT transition, supposedly through the
  Higgs mechanism, the unified GUT force splits
  into forces, the strong force and the electroweak
  force

22
GUT Transition
Strong Force
GUT
Electroweak Force

• Baryon non-conserving processes
• It is possible that the origin of the
  present-day excess of matter over antimatter
  finds its origin in the GUT phase transition.
• Inflationary Epoch
• It is conceivable that the GUT transition
  may be identified with the phase transition that
  gave rise to a rapid exponential de Sitter
  expansion, in which the universe expanded by 60
  orders of magnitude (and in which its horizon
  shrank accordingly). Primordial density
  perturbations, the seeds of cosmic structure, may
  have been generated during this episode.

23
Electroweak Transition
T 300 GeV
3 x 1015 K
z 1015

• At this energy scale, the electroweak force
  splits into the electromagnetic force and the
  weak force .

Electromagnetic Force
Electroweak
Weak Force

• All the leptons acquire masses (except possibly
  neutrinos),
• intermediate vector bosons give rise to
  massive bosons W, W- and
• Z0, and photons.

24
Quark-Hadron Transition
T 0.2 GeV
1012 K t
10-5 sec

• Above this temperature, matter in the universe
  exists in the form of a quark-gluon plasma. Below
  this temperature, isolated quarks cannot exist,
  and become confined in composite particles called
  hadrons.They combine into (quark confinement)
• ? baryons
  quark triplet
• ? mesons
  quark-antiquark pairs
• Also, 1) QCD chiral symmetry breaking
• 2) axion acquires mass
  
• (axion most
  popular candidate for Cold Dark Matter)

25
Thermal HistoryEpisode by Episode
Hadron Era
t 10-5 sec 300
gt T gt 130 MeV

• The hadrons formed during the quark-hadron
  transition are usually short-lived particles
  (except for protons neutrons). Therefore, there
  is only a brief period in which the hadrons
  flourish.
• Although called Hadron Era, hadrons do not
  dominate the energy density.
• Pion-pion interactions are very important.
  Towards the end of hadron era, p and p-
  annihilate, p0 decay into photons.

26
Thermal HistoryEpisode by Episode
Lepton Era

10-5 sec lt t lt 1 min 130 gt
Tgt 0.5MeV 1012 K gt T gt 5x109 K

• At the beginning of the lepton era, the universe
  comprises
• ? photons,
• ? baryons (small number)
• ? leptons electrons
  positrons e-, e, muons µ, µ- , taus t and t-
  
• 
  electron, muon and tau neutrinos
• 
  

27
Thermal HistoryEpisode by Episode
Lepton Era
10-5 sec lt t lt 1
min 130 gt Tgt 0.5MeV 1012
K gt T gt 5x109 K

• Four major events occur during the lepton era
• ? Annihilation muons
  T 1012 K
• ? Neutrino Decoupling
  T 1010.5 K z 1010
• ? Electron-Positron
  Annihilation Tlt 109 K z 109,
  t1 min
• ? Primordial
  Nucleosynthesis T 109 K
  t 200 sec (3 min)

28
Neutrino Decoupling
T 1010.5 K
t 10-5 sec, z 1010

• Weak interactions, e.g.
• get so slow that neutrinos decouple from
  the e, e-, ? plasma. Subsequently , they proceed
  as a relativistic gas with its own temperature T?
  .
• Because they decouple before the
  electron-positron annihilation, they keep a
  temperature T? which is lower than the photon
  temperature T? (which gets boost from released
  annihilation energy )
• The redshift of neutrino decoupling, z1010,
  defines a surface of last neutrino scattering,
  resulting in a Cosmic Neutrino Background with
  present-day temperature T1.95 K. A pity it is
  technically not feasible to see it !

29
Electron-PositronAnnihilation
T lt 109 K t
1 min, z 109

• Before this redshift, electrons and photons are
  in thermal equilibrium. After the
• temperature drops below T109 K, the
  electrons and positrons annihilate, leaving a sea
  of photons.
• As they absorb the total entropy s of the e,
  e-, ? plasma, the photons acquire a temperature
  T? gt neutrino temperatureT? .

30
Electron-PositronAnnihilation
T lt 109 K t
1 min, z 109

• At this
  redshift the majority of photons of the
• Cosmic
  Microwave Background are generated.
• These photons keep on being scattered back and
  forth until z 1089, the epoch of recombination.
  
• Within 2 months after the fact, thermal
  equilibrium of photons is restored by a few
  scattering processes
• ? free-free scattering
• ? Compton scattering
• ? double Compton scattering
• The net result is the perfect blackbody CMB
  spectrum we observe nowadays.

!
!
31
Primordial Nucleosynthesis
T 109 K 0.1 MeV
t 200 sec 3 min

• At the end of these first three minutes we
  find an event that provides us with the first
  direct probe of the Hot Big Bang, the
  nucleosynthesis of the light chemical elements,
  such as deuterium, helium and lithium.
• The prelude to this event occurs shortly before
  the annihilation of positrons and electrons. The
  weak interactions coupling neutrons and protons
• can no longer be sustained when the
  temperature drops belowT 109 K, resulting in a
  
• Freeze-out of Neutron-Proton ratio

32
Primordial Nucleosynthesis

• Note that from the ratio Nn/Np 1/6 we can
  already infer that if all neutrons would get
  incorporated into 4He nuclei, around 25 of
  the baryon mass would involve Helium ! Not far
  from the actual number ...
• After freeze-out of protons and neutrons, a
  number of light element nucleons forms through a
  number of nuclear reactions involving the
  absorption of neutrons and protons
• ? Deuterium
• ? 3He
• ? 4He
• and traces of 7Li and 9Be

33
Primordial Nucleosynthesis
? Heavier nuclei will not form anymore, even
though thermodynamically preferred at lower
temperatures when 4He had formed, the
temperature and density have simply below too low
for any significant synthesis. ? The precise
abundances of the light elements depends
sensitively on various cosmological parameters. ?
Particularly noteworthy is the dependence on the
ratio of baryons to photons (proportional to the
entropy of the universe), setting the neutrons
and protons available for fusion ? By
comparing the predicted abundances as function of
?, one can infer the density of baryons in the
universe, ?B (see figure).
34
Primordial Nucleosynthesis
? On the basis of the measured light element
abundances, we find a rather stringent limit on
the baryon density in the universe ?
This estimate of the baryon density from
primordial nucleosynthesis is in perfect
agreement with the completely independent
estimate of the baryon density from the second
peak in the angular power spectrum of the WMAP
temperature perturbations ? This should be
considered as a truly astonishing vindication of
the Hot Big Bang. ? Not that these nuclear
reactions also occur in the Sun, but at a
considerably lower temperature T 1.6 x 107 K.
The fact that they occur in the early universe
only at temperatures in excess of 109 K is due to
the considerably lower density in the early
universe
35
Thermal HistoryEpisode by Episode
Radiation Era
t gt 1
min T lt 5 x 109 K

• The radiation era begins at the moment of
  annihilation of electron-positron pairs.
• After this event, the contents of the universe is
  a plasma of photons and neutrinos, and matter
  (after nucleosynthesis mainly protons, electrons
  and helium nuclei, and of course the unknown
  dark matter).
• During this era, also called Plasma Epoch, the
  photons and baryonic matter are glued together.
  The protons and electrons are strongly coupled by
  Coulomb interactions, and they have the same
  temperature. The electrons are coupled to the
  radiation by means of Compton scattering. Hence,
  baryons and radiation are in thermal equilibrium.
  

36
Thermal HistoryEpisode by Episode
Radiation Era
t gt 1
min T lt 5 x 109 K

• Two cosmic key events mark the plasma era
• ? Radiation-Matter transition
  zeq2 x 104
  
• (equivalence
  matter-radiation)
• ? Recombination
  Decoupling z 1089 t 279,000
  yrs

37
Radiation-Matter Equality
zeq 2 x 104
? The time of matter-radiation equality
represents a crucial dynamical transition of the
universe. ? Before zeq the dynamics of the
universe is dominated by Radiation. After
equivalence Matter takes over as the dominant
component of the universe. ? Because
the energy density of radiation diminishes with
the fourth power of the expansion of the
universe, while the density of matter does so
with the third power, the ratio between radiation
and matter density is an increasing function of
a(t)
38
Radiation-Matter Equality
? The redshift zeq at which the radiation and
matter density are equal to each other can then
be inferred ? Because of the different
equation of state for matter and radiation (and
hence their different density evolution), the
universe changes its expansion behaviour
? This has dramatic consequences for
various (cosmic structure formation) processes,
and we can find back the imprint of this cosmic
transition in various phenomena. ? Note that
the universe underwent a similar transition at a
more recent date. This transition, the
Matter-Dark Energy Equality marks the epoch at
which dark energy took over from matter as
dynamically dominant component of the universe.
? radiation-dominated ? matter-dominated
39
Recombination Epoch
T 3000 K
zdec1089 (?zdec195)
tdec379.000 yrs

• Before this time, radiation and matter are
  tightly coupled through bremsstrahlung
• Because of the continuing scattering of
  photons, the universe is a fog.
• A radical change of this situation occurs once
  the temperature starts to drop below T3000 K.
  and electrons. Thermodynamically it becomes
  favorable to form neutral (hydrogen) atoms H
  (because the photons can no longer destory the
  atoms)
• This transition is usually marked by the word
  recombination, somewhat of a misnomer, as of
  course hydrogen atoms combine just for the first
  time in cosmic history. It marks a radical
  transition point in the universes history.
  

40
Recombination Epoch

• This happened 279,000 years after the Big
  Bang, according to the impressively accurate
  determination by the WMAP satellite (2003).

• Major consequence of recombination
• Decoupling of
• Radiation Matter
• With the electrons and protons
• absorbed into hydrogen atoms, the
• Photons decouple from the plasma, their
• mean free path becoming of the order of
• the Hubble radius. The cosmic fog lifts

universe transparent

• The photons assume their long travel along the
  depths of the cosmos. Until some of them,
  Gigaparsecs further on and Gigayears later, are
  detected by telescopes on and around a small
  planet in some faraway corner of the cosmos

41
Recombination Decoupling

• In summary, the recombination transition and the
  related decoupling of matter and radiation
  defines one of the most crucial events in
  cosmology. In a rather sudden transition, the
  universe changes from

• Before zdec, zgtzdec
• universe fully ionized
• photons incessantly scattered
• pressure dominated by
• radiation

• After zdec, zltzdec
• universe practically neutral
• photons propagate freely
• pressure only by
• baryons
• (photon pressure negligible)

42
Recombination Decoupling

• Note that the decoupling transition occurs rather
  sudden at T3000 K, with a cosmic photosphere
  depth of only ?zdec195 (at z1089).
• The cosmological situation is highly exceptional.
  Under more common circumstances the
  (re)combination transition would already have
  taken place at a temperature of T104 K.
• Due to the enormous amount of photons in the
  universe, signified by the abnormally high cosmic
  entropy,
• even long after the temperature dropped
  below T 104 K there are still sufficient photons
  to keep the hydrogen ionized (i.e. there are
  still plenty of photons in the Wien part of the
  spectrum).
• Recombination therefore proceeds via a 2-step
  transition, not directly to the groundstate of
  hydrogen. The process is therefore dictated by
  the rate at which Lya photons redshift out of the
  Lya rest wavelenght. For n? /nB109 this
  occurs at

43
Recombination Epoch

• The photons that are currently reaching us,
  emanate from the
• Surface of
• Last Scattering
• located at a redshift of z1089.
• The WMAP measurement of the redshift of last
  scattering confirms the theoretical predictions
  (Jones Wyse 1985) of a sharply defined last
  scattering surface.
• The last scattering surface is in fact somewhat
  fuzzy, the photons arrive from a cosmic
  photosphere with a narrow redshift width of
  ?z195.

44
Recombination Epoch

• The photons emanating from the last scattering
  surface, freely propagating through our
  universe, define a near isotropic sea of
  radiation.
• Shortly after they were created at the time of
  electron-positron annihilation, z 109, the
  photon bath was thoroughly thermalized. It thus
  defines a most perfect blackbody radiation field
• Due to the cosmic expansion, the radiation field
  has in the meantime cooled down to a temperature
  of T2.725 K (/- 0.002 K, WMAP).
• This cosmic radiation we observe as the
• Cosmic Microwave
  Background

45
Recombination Epoch
Cosmic Microwave Background

• first discovered serendipitously by Penzias
  Wilson in 1965, and reported in their publication
  an excess measurement , without doubt should
  be regarded as one of the principal scientific
  discoveries of the 20th century.
• Its almost perfect blackbody spectrum is the
  ultimate proof of a hot and dense early phase
• the Hot Big Bang

the Nobel prize for the discovery of the CMB
followed in 1978
46
Recombination Epoch
Cosmic Microwave Background

• The amazingly precise blackbody nature of the CMB
  was demonstrated by the COBE satellite (1992).
• The spectral energy distribution in the figure is
  so accurately fit by a Planckian spectrum that
  the error bars are smaller than the thickness of
  the solid (blue) curve (see figure) !!!
• Note that the corresponding CMB photon number
  density is

47
Cosmic Microwave Background
The CMB is a fabulously rich treasure trove of
information on the primordial universe. In the
accompanying figure you see three milestones of
CMB research 1) The discovery of the CMB by
Penzias Wilson in 1965. 2) The COBE satellite
(1992), first discovery of primordial
perturbations. 3) WMAP (2003), detailed
temperature perturbations fix the universes
parameters.
Opening View onto the
Primordial Universe
48
Thermal HistoryEpisode by Episode
Post-Recombination Era
t gt 279,000 yrs
T lt 3000 K
After recombination/decoupling, while the
universe expands it gradually cools down
(baryonic matter faster than radiation once they
are entirely decoupled). We can identify various
major processes and transitions during these
long-lasting eons

• ? Structure Galaxy Formation
  z 1089-0
• ? Dark Ages
  z 1089-10/20
• ? Reionization
  z 20-6 ?
• ? Matter-Dark Energy
  transition z 0.3

49
Post-Recombination Era
Structure Formation
After decoupling, density perturbations
in the matter distribution gradually develop into
forming structures by means of the gravitational
instability mechanism. The origin of these
density perturbations is still an unsettled
issue. Their presence, however, has been proven
beyond doubt their imprint in the CMB
beautifully confirmed by COBE and WMAP.
Hidden in the depths of the very first instances
of the early universe, at present the most viable
suggestion is that it concerns quantum
fluctuations blown up to macroscopic proportions
in an inflationary phase of cosmic expansion. In
the later phases of more quiescent cosmic
expansion, density fluctuations, frozen while
they have the superhorizon scale assumed in
inflation, gradually enter the horizon (i.e it
is overtaken by it). From that instant on they
can start growing !
50
Gravitational Instability

• the tiny density perturbations in the
  early universe
• 
  correspond to
• GRAVITY PERTURBATIONS

51
Post-Recombination Era
Structure Formation
The gravity perturbations induce cosmic flows of
matter. High density regions start to contract
and finally collapse, assembling more and more
matter from their surroundings. On the other
hand, as matter is moving out of them, low
density regions turn into empty void regions.
Gradually, dependent on scale, we see the
emergence of cosmic structures. These days we
can simulate the characteristics of the process
through large computer simulations. Succesfull
confrontation with the observational reality has
given confidence in our understanding.
density field d(x,t)
gravity field g(x,t)
displaced mass structure forming
peculiar velocity v(x,t)
52
Post-Recombination Era
Structure Formation
It is important to realize the distinct
difference between the evolution of the dark
matter perturbations and those in baryonic
matter.
Dark Matter ?
Dark matter is the dominant gravitational
component of the universe, and thus also drives
the structure formation process. ? The
perturbations in the gravitationally dominant
(collisionless) dark matter component started
growing already after matter came to dominate
cosmic dynamics, i.e. after radiation-matter
equivalence.
Baryonic Matter ?
Fluctuations in baryonic matter were enable to
grow only once radiation pressure disappeared,
i.e. after decoupling. ? Baryonic
matter fluctuations staart to grow strongly
through infall into the gravitational potential
wells defined by the developing dark matter
perturbations.
53
Post-Recombination Era
Structure Formation
Gravitational Cosmic Structure Formation
involves a few typical characteristics. The
two most salient ones are
? hierarchical structure
formation ? anisotropic
collapse Nearly all viable
theories of cosmic structure formation concern a
hierarchical buildup of structure. Density
fluctuations on smaller scales have a higher
amplitude, and represent a region of stronger
excess gravity. They will therefore contract,
collapse and virialize faster than perturbations
on larger scales. When embedded within a larger
overdensity they will subsequently merge with
their surrounding peers, marking the collapse
phase of this entity. In all, it results in a
continuing process of smaller scale dark matter
halos merging into ever larger objects.
Hierarchical Structure Formation
54
Hierarchical Structure Formation
In the accompanying figure of an N-body
simulation, the hierarchically progressing
buildup of a dark matter halo is illustrated.
The hierarchical nature of structure
formation may be best appreciated from the
included movie (below, courtesy Virgo
consortium).
55
Anisotropic Collapse
? A salient characteristic of gravitational
collapse is that any small initial deviation from
sphericity of a collapsing cloud is magnified
by the corresponding gravitational field. Because
the corresponding gravitational acceleration is
stronger along the smallest axis than along the
medium axis, which in turn corresponds to a
stronger force than along the longest axis,
gravitational collapse generically gravitational
collapse proceeds along the following sequence
? collapse along smallest
axis planar geometry
wall ? collapse medium
axis elongated
filament
? full 3-D collapse
clump
clump/halo After having collapsed into
a clump, the object rapidly virializes and an
individual cosmic object emerges.
56
Anisotropic Collapse
? The tendency to collapse via anisotropic
configurations is the basic explanation for the
salient wall-like and filamentary structures seen
in the galaxy distribution. Reflecting the
underlying mass distribution on Megaparsec
scales, these structures are as yet in a rather
youthful, mildly nonlinear, evolutionary stage of
gravitational contraction. At earlier cosmic
epochs structure on smaller scales will have
proceeded along similar lines. ? The
anisotropic nature of collapse is augmented by
the surrounding inhomogeneous cosmic matter
distribution. The (anisotropic) tidal force field
accompanying such a matter distribution by itself
will induce anisotropic collapse. Filamentary
structures are the typical result of a
quadrupolar mass distribution, which explains the
close relationship between clusters and weblike
features in the Megaparsec cosmic galaxy
distribution. ? The formation of the cosmic
web is tellingly illustrated in the simulation
sequence shown below (courtesy A. Kravtsov).
57
Hierarchy Anisotropy
? The resulting evolution is that of clumps of
matter forming in a cosmic web, in which matter
gets channelled along planar and filamentary
structures towards the emerging halo. ? While
clumps have formed by hierarchical evolution,
they participate in the collapse of the
surroundings. In its early phases this proceeds
in a typical anisotropic fashion. ? It results
in a picture of merging clumps, moving along
filamentary pathways towards the highest density
concentration in their vicinity. ? This is
beautifully illustrated in the accompanying movie
(courtesy Virgo consortium)
58
Post-Recombination Era
? As long as there are no stars, the universe
becomes darker and darker (the CMB spectrum
gradually shifting from reddish at last
scattering down to longer and longer
wavelengths). This epoch has acquired the name
coined by M. Rees
Dark Ages
? Then, the lights go on, the first objects
emerge on the scene. These may include

? first
generation stars

(population III, extremely matter
poor)
?
108 M0 dark matter halos,

the first (dwarf) galactic
entities
?
supermassive black holes
? As yet, not entirely clear what the events
have been towards the end of the Dark Ages, nor
which were the first objects and, indeed, not
exactly when this happened. Reasonable estimates
now have it occurring between 6ltzlt20. What is
clear is that at some point a burst of
non-primordial light/radiation emitted by the
first generation of stars or from AGNs started to
ionize the surrounding neutral gas.
59
Formation First Stars
Simulation V. Bromm et al.
60
Post-Recombination Era
? Very soon the universe undergoes a
phase transition. The sources of non-primordial
light rapidly ionize the gas throughout the whole
universe. ? This is know as the
Epoch of
Reionization ? In the accompanying
movie, a simulation by N. Gnedin, one can observe
the sudden
transition in which the universe gets ionized
throughout. The movie shows how reionization
fronts propragate through the universe and
collide, leaving the universe highly ionized our
everywhere (except some places of high optical
depth). Four panels top left showing the neutral
hydrogen fraction, the bottom ones the gas
density and temperature.
61
Post-Recombination Era
? Very soon the universe undergoes a
phase transition. The sources of non-primordial
light rapidly ionize the gas throughout the whole
universe. ? This is know as the
Epoch of
Reionization ? In the accompanying
movie, a simulation by N. Gnedin, one can observe
the sudden
transition in which the universe gets ionized
throughout. The movie shows how reionization
fronts propragate through the universe and
collide, leaving the universe highly ionized our
everywhere (except some places of high optical
depth). Four panels top left showing the neutral
hydrogen fraction, the bottom ones the gas
density and temperature.
62
Epoch of Reionization

• The end of the dark ages, the formation of the
  first generation of stars, and the epoch of
  reionization are currently central themes of
  interest in cosmological research.
• As yet, estimates of when this occurs vary
• There is a firm lower limit from the spectra of
  high redshift quasars. Quasars at zgt6.2 (SDSS)
  have started to detect the first traces of
  neutral hydrogen amidst the sea of ionized
  hydrogen.
• WMAP managed to estimate the optical depth for
  the CMB radiation, due to the reionized medium it
  is passing through. It yielded the surprising
  result, as yet not really understood, that the
  first stars may have litted the skies in between
  20gtzgt15
• Perhaps LOFAR, the new radiotelescope in Drenthe,
  will provide the answer and show what happened

63
Post-Recombination Era
Galaxy Formation
While gas falls into the potential wells
of galaxy-sized dark matter halso, and starts to
settle, we will witness the formation of galaxies
as stars light up. After the very first
generation of stars, the extremely metal-poor
Population III stars, the formation of galaxies
is probably accompanied by violent bursts of star
formation. As this true first generation
of stars illuminates the skies, the galactic
lifecycle sets into gears. In a continuing
process, stars form from gas, enriching it with
their nuclear burning products, from which in
turn new stars will form with richer abundances
of heavy elements. The first large
galaxies, ie. of masses M1012 M0, are probably
formed by a redshift of z6.5-4. However, this
is a truly largely unsettled field, open for
large strides in understanding
64
Post-Recombination Era
Galaxy Formation
An impression of the galaxy formation history of
the universe may be obtained from

a census of Galaxies in the Hubble Deep Field. In
the accompanying sequel of images these are shown
in a sequence or increasing z.
Courtesy C. Driver
65
Post-Recombination Era
Galaxy Formation
to the present-day richness in galaxies,
arguably the most prominent denizens of the
cosmos
poster Z. Frei
66
Matter-Dark Energy Transition
zM? 0.3, tM? 7 Gyr
? Comparable to the matter-radiation transition
at zeq 2 x 104, the universe undergoes another
crucial dynamical transition at a far mor recent
epoch the instant Dark Energy starts to take
over from Matter the dominance over the dynamics
of the universe. ? Assuming for the moment
that Dark Energy corresponds to the regular
Cosmological Constant ?, i.e. it having p/?-1
as equation of state, after zeq and before zM?
the dynamics of the universe is dominated by
Matter. After zM? Dark Energy takes over as the
dominant component of the universe ?
Because the energy density of matter diminishes
with the third power of the expansion of the
universe, while the dark energy density remains
constant (i.e. if it corresponds to a constant
?), the ratio between dark energy and matter
density increases with a(t) as
67
Post-Recombination Era
Cluster Formation

• While the majority of galaxies seems to have been
  assembled at high redshifts, be it that
  observations indicate they keep on evolving
  vigorously down to redshifts of z1, the more
  modest density perturbations on larger scales
  continue to evolve also
• As long as density perturbations manage to
  become highly nonlinear, d gtgt 1, by the
  redshift z at which structure ceases to grow
  (because the universe entered its free
  expansion phase),
• they will manage to decouple from the
  Hubble expansion , contract and collapse,
  virialize and turn into a genuine cosmic object.
  In this view, clusters of galaxies are the most
  massive, and most recently, fully collapsed
  structures in our universe. On even larger
  scales we still see the structure residing in the
  dynamically youthful stages of anisotropic
  contraction the Cosmic Web

68
Post-Recombination Era Cluster and
Structure Formation

? On Megaparsec scales we see the formation of
an intriguing weblike pattern in the matter
distribution. Filaments are the most
characteristic features in this distribution,
with matter being transported along the filaments
towards the high density clusters of galaxies
which have primarily formed at the intersections
of various filaments (see background image, and
zoom-in on next page). ? Indications have it
that most clusters were in place by z 1 (a few
massive clusters have even been seen at higher
redshifts), which agrees with the expectation
that major developments in the growth of cosmic
structure will cease at such a redshift in a
universe with ?m0.3.
simulation courtesy V. Springel Weblike patterns
formed through gravitational structure formation
in a ?CDM universe. We focus in on the cluster in
the centre
69
Post-Recombination Era Cluster and
Structure Formation
? On Megaparsec scales we see the formation of
an intriguing weblike pattern in the matter
distribution. Filaments are the most
characteristic features in this distribution,
with matter being transported along the filaments
towards the high density clusters of galaxies
which have primarily formed at the intersections
of various filaments (see background image). ?
Indications have it that most clusters were in
place by z 1 (a few massive clusters have even
been seen at higher redshifts), which agrees with
the expectation that major developments in the
growth of cosmic structure will cease at such a
redshift in a universe with ?m0.3. ?
Structure has been recognized on all scales
smaller than a hundred Megaparsec. Above that
scale primordial density perturbations were too
small in amplitude to have evolved substantially
in a Hubble time (and before structure stopped
growing). On all other scales we see a baffling
variety and wealth of structure, emerging through
the gravitational collapse of primordial
fluctuations

simulation courtesy V. Springel at the
intersection of the filaments, a majestic rich
cluster formed
70
Post-Recombination Era The
last five billion years

While the universe moved itself into a
period of accelerated exponential expansion as it
came to be dominated by Dark Energy, stars and
galaxies proceeded with their lives. Stars died,
new and enriched ones arose out of the ashes.
Alongside the newborn stars, planets emerged
One modest and average yellowish star, one
of the two hundred billion denizens of a rather
common Sb spiral galaxy called Milky Way,
harboured a planetary system of around 9 planets
a few of them rocky, heavy clumps with loads of
heavy elements One of them bluish, a
true pearl in the heavens
71
This planet, Earth it is called, became
home to remarkable creatures some of which
evolved sophisticated brains. The most complex
structures in the known universe Some
of them started using them to ponder about the
world in which they live Pythagoras,
Archimedes, Albert Einstein were their names
they took care of an astonishing feat they
found the universe to be understandable, how
truly perplexing ! A universe thinking
about itself and thinking it understands

Post-Recombination Era The
last five billion years