This is the third sequel in series of Big Bang and the one which gives birth to very first creation within our universe. If you missed first two sequels, here they are:

Big Bang I: Dawn of Time

Big Bang II: First cry of baby Universe



Structure formation in the Big Bang model proceeds hierarchically, with smaller structures forming before larger ones of course. The first structures to form were quasars, which are thought to be bright, early active galaxies, and population III stars. Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult (more details below).


One of the key realizations made by cosmologists in the 1970s and 1980s was that the majority of the matter content of the universe was composed not of atoms, but rather a mysterious form of matter known as dark matter. Dark matter interacts through the force of gravity, but it is not composed of baryons and it is known with very high accuracy that it does not emit or absorb radiation. It may be composed of particles that interact through the weak interaction, such as neutrinos, but it cannot be composed entirely of the three known kinds of neutrinos (although some have suggested it is a sterile neutrino).




Recently, astronomers found two immense clouds of pristine gas, nearly 12 billion light years away - clouds that astronomers suspect are the stuff from which the first stars were born. If you were to take a space ship and go back to that point in cosmic time, you'd find that the sky is really ablaze with them - so many of them that their combined output is very significant. These early objects are in fact very, very small - about a tenth the diameter of the Milky Way. And yet, they are forming stars more prodigiously than the galaxy we live in today.


Discoveries such as these come as scientists focus on studying the gaseous "fingerprints" the early galaxies left on their surroundings - what's called the intergalactic medium (IGM). We know from these observations that after about one billion years all of this gas is ionized, and it's the galaxies that we think did this. If we can map the distribution of galaxies and the hydrogen gas between them, in the IGM, we can see the relationship between the places where the galaxies are and the regions that were ionized. But before we get to galaxies we need to get to the stars.


Results from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) released in February 2003 show that the first stars formed when the universe was only about 200 million years old. Observations reveal that tiny clumps of matter formed in the baby universe; to WMAP, these clumps are seen as tiny temperatures differences of less than one-millionth of a degree. Gravity then pulled in more matter from areas of lower density and the clumps grew. After about 200 million years of this clumping, there was enough matter in one place that the temperature got high enough for nuclear fusion to begin - providing the engine for stars to glow. The intense radiation they emit reionizes the surrounding universe. From this point on, most of the universe is composed of plasma.



The new simulation indicates that star creation occurred much earlier than originally thought - when the universe was only 30 million years old. It also suggests that it took about another 370 million years for the first galaxy as massive as our own Milky Way to form. Astronomers suspect that the first star formed in a dense cloud of dark matter and gas. They also think that many present-day galaxies are the products of mergers between much smaller galaxies during the early days of our universe. Estimating when the first star formed, however, has been difficult. That's because even the most powerful supercomputers can only simulate small portions of the universe at a time.


Even by stellar standards, the primordial star was a monster. It likely had a mass of about 100 times that of our Sun and it would have spewed out vast amounts of energetic radiation, especially in the ultraviolet range. Had human eyes been around to see it, it would have appeared blue-violet in color. The first star shone brighter than most stars in existence today and it zipped through its stellar life in only 2 million to 3 million years, compared to the several-billion-year lifetimes that some of today's stars have. Our Sun is middle-aged now and has been around for 4.6 billion years. Scientists think that when it spent its fuel, the first star exploded in a titanic stellar cataclysm called a supernova, flinging heavy elements forged during the star's lifetime into space, setting the stage for the next generation of stars. After a short time, stars began appearing in greater abundance throughout the universe. The second generation stars likely formed within about a million years after the first. Within five million years, there were about 100 stars; within ten million years, 10000 celestial orbs of fire were lighting up the heavens. Unlike that first star, which was made up mostly of hydrogen and helium, the stars that came after contained heavier elements, such as carbon and iron. Light once emitted by the first star might still be detectable. If the first star was indeed massive and produced in its death a huge supernova explosion or gamma ray burst, then we might have a chance to see the explosion with the instruments planned for the coming decade.


This epoch, called reionization, is believed to last between 150 million to 1 billion years after Big Bang. In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the "dark ages", and is the second of two major phase changes of gas in the universe. As the majority of baryonic matter is in the form of hydrogen, reionization usually refers to the reionization of hydrogen gas. The primordial helium in the universe experienced the same phase changes, but at different points in the history of the universe, and is usually referred to as Helium reionization.




The first phase change of hydrogen in the universe was recombination, which occurred at what we call a redshift z = 1100 (380000 years after the Big Bang), due to the cooling of the universe to the point where the rate of combination of an electron and proton to form neutral hydrogen was higher than the ionization rate of hydrogen. The universe was opaque before recombination because photons scatter off free electrons (and, to a significantly lesser extent, free protons), but it became transparent as more and more electrons and protons combined to form hydrogen atoms. While electrons in neutral hydrogen (or other atoms or molecules) can absorb photons of some wavelengths by going to an excited state, a universe full of neutral hydrogen will be relatively opaque only at those wavelengths, and transparent over most of the spectrum. The Dark Ages start at that point, because there are no light sources yet other than the gradually darkening cosmic background radiation.



Remember that cool picture taken by WMAP showing CMB (see above)? It shows a smooth, homogeneous universe with density anisotropies (term to describe the uneven temperature distribution of CMB) of one part in 105. However, when we observe the universe today we find large structure and density fluctuations. Galaxies, for instance, are 106 times more dense than the universe's mean density. The current belief is that the universe was built in a bottom-up fashion, meaning that the small anisotropies of the early universe acted as gravitational seeds for the structure we see today. Overdense regions attract more matter, while underdense regions attract less, and thus these small anisotropies we see in the CMB become the large scale structures we observe in the universe today.


Now, imagine an overdense region of the primordial plasma. While this overdensity gravitationally attracts matter towards it, the heat of photon-matter interactions creates a large amount of outward pressure. These counteracting forces of gravity and pressure create oscillations, analogous to sound waves created in air by pressure differences. Consider a single wave originating from this overdense region in the center of the plasma. This region contains dark matter, baryons and photons. The pressure results in a spherical sound wave of both baryons and photons moving with a speed slightly over half the speed of light outwards from the overdensity. The dark matter only interacts gravitationally and so it stays at the center of the sound wave, the origin of the overdensity. Before decoupling, the photons and baryons move outwards together. After decoupling the photons are no longer interacting with the baryonic matter so they diffuse away. This relieves the pressure on the system, leaving a shell of baryonic matter at a fixed radius. This radius is often referred to as the sound horizon. Without the photo-baryon pressure driving the system outwards, the only remaining force on the baryons is gravitational. Therefore, the baryons and dark matter (still at the center of the perturbation) form a configuration which includes overdensities of matter both at the original site of the anisotropy and in a shell at the sound horizon. The ripples in the density of space continue to attract matter and eventually galaxies have formed in a similar pattern, therefore one would expect to see a greater number of galaxies separated by the sound horizon than by nearby length scales. This particular configuration of matter occurred at each anisotropy in the early universe, and therefore the universe is not composed of one sound ripple, but many overlapping ripples. As an analogy, imagine dropping many pebbles into a pond and watching the resulting wave patterns in the water. It is not possible to observe this preferred separation of galaxies on the sound horizon scale by eye, but one can measure this signal statistically by looking at the separations of large numbers of galaxies.


The second phase change occurred once objects started to form in the early universe energetic enough to ionize neutral hydrogen. As these objects formed and radiated energy, the universe went from being neutral back to being an ionized plasma, between 150 million and one billion years after the Big Bang (at a redshift 6 < z < 20). By now, however, matter has been diluted by the expansion of the universe, and scattering interactions are much less frequent than before recombination. Thus a universe full of low density ionized hydrogen will remain transparent, as is the case today.


While observations have come in which narrow the window during which the epoch of reionization could have taken place, it is still uncertain which objects provided the photons that reionized the IGM. To ionize neutral hydrogen, an energy larger than 13.6 eV is required, which corresponds to photons with a wavelength of 91.2 nm or shorter. This is in the ultraviolet part of the electromagnetic spectrum, which means that the primary candidates are all sources which produce a significant amount of energy in the ultraviolet and above. How numerous the source are must be also considered, as well as the longevity, as protons and electrons will recombine if energy is not continuously provided to keep them apart.  With these constraints, it is expected that quasars and first generation stars were the main sources of energy!



So, in summary, the universe went through an initial heat wave over 13 billion years ago when energy from early massive stars ionized cold interstellar hydrogen from the big bang. This epoch we call reionization because the hydrogen nuclei were originally in an ionized state shortly after the big bang. We can say that during this long phase the first quasars and stars form from gravitational collapse, and the intense radiation they emit reionizes the surrounding universe. From this point on, most of the universe goes from being neutral back to being composed of ionized plasma and galaxies which form on the way. In next 3 sequels I will go through quasars, stars and galaxies - each individually. In 2010, Hubble found that it would take another 2 billion years before the universe produced sources of ultraviolet radiation with enough energy to do the heavy lifting and reionize the primordial helium that was also cooked up in the big bang. This radiation didn't come from stars, but rather from quasars. In fact the epoch when the helium was being reionized corresponds to a transitory time in the universe's history when quasars were most abundant.



Related posts:

Big Bang I: Dawn of time

Big Bang II: First cry of baby Universe


Credits: Space Telescope Science Institute, Wikipedia, arXiv