A:4. When were the first stars, black holes and galaxies born?
Following the fireball of the hot Big Bang, the Universe expanded and cooled to the point where the normal matter, comprised almost entirely of hydrogen and helium, could take a neutral atomic form. This is the epoch of recombination, when the radiation which we currently detect as the cosmic microwave background was generated. There then followed a cosmic "dark age" until eventually the first stars lit up the Universe anew.
It is likely that the first stars, were massive objects with short lifetimes, which through their explosive destruction in supernovae and/or gamma-ray bursts, chemically enriched their surroundings. Rapid star-formation in the newly forming galaxies and the production of both stellar-mass and galaxy-size black-holes are further ingredients within this particular melting pot. One consequence of the birth of luminous sources and their associated hard radiation fields is that the neutral hydrogen of the Universe will be converted back to an ionized form during the "Epoch of Re-ionization". A better understanding of the cosmic Dark Age, first light and the re-ionization of the Universe are key goals of modern astronomy.