Post-Recombination Universe: a seven-mile journey of structure formation history

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Post-RecombinationUniversea seven-mile
journeyof structure formationhistory
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Gravitational Instability
Quiescent Linear Evolution
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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.
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Formation First Stars
Simulation V. Bromm et al.
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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.
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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.
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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

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Post-Recombination Era
Galaxy Formation
While gas falls into the potential wells
of galaxy-sized dark matter halos, 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
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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
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Post-Recombination Era
Galaxy Formation
to the present-day richness in galaxies,
arguably the most prominent denizens of the
cosmos
poster Z. Frei
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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 epoch 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
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Post-Recombination Era
Cluster Formation

• 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),
• 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
• 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

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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
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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 many 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
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Gas in the Cosmic Web


gas settles in the cosmic web ?
neutral hydrogen gas traces DM density
Lya
forest ? shock-heated hot/warm
intergalactic gas in filaments
WHIM
(major deposit of baryons in the Universe ? 50
?)
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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
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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