Journey through the Post-Recombination Universe: Structure Formation, Dark Ages to Reionization

1. Post-RecombinationUniverse:a seven-mile journeyof structure formationhistory

2. Gravitational Instability Quiescent Linear Evolution …

3. 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 6<z<20. 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.

4. Formation First Stars Simulation: V. Bromm et al.

5. 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.

6. 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.

7. 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 z>6.2 (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 20>z>15 … • Perhaps LOFAR, the new radiotelescope in Drenthe, will provide the answer and show what happened …

8. 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 M~1012 M0, are probably formed by a redshift of z~6.5-4. …However, this is a truly largely unsettled field, open for large strides in understanding

9. 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

10. Post-Recombination Era: Galaxy Formation to the present-day richness in galaxies, arguably the most prominent denizens of the cosmos poster: Z. Frei

11. Matter-Dark Energy Transition zMΛ ~ 0.3, tMΛ~ 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 Λ, i.e. it having p/ρ=-1 as equation of state, after zeq and before zMΛ the dynamics of the universe is dominated by Matter. After zMΛ 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 Λ), the ratio between dark energy and matter density increases with a(t) as :

12. Post-Recombination Era: Cluster Formation • As long as density perturbations manage to become highly nonlinear, δ>> 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 z~1, 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 …

13. 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 Ωm~0.3. simulation courtesy: V. Springel Weblike patterns formed through gravitational structure formation in a ΛCDM universe. We focus in on the cluster in the centre …

14. 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 Ωm~0.3. ● 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 …

15. Gas in the Cosmic Web … gas settles in the cosmic web: ● neutral hydrogen gas traces DM density Lyα forest ● shock-heated hot/warm intergalactic gas in filaments WHIM (major deposit of baryons in the Universe ? 50% ?)

16. 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 …

17. 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