For starters, contrary to prior less-detailed studies, Ostriker and Gnedin found that reionization happens suddenly -- about 500 million years after the Big Bang -- preceded by slow reheating as matter coalesces. "One moment it's the dark ages," says Gnedin, "then shortly after it's bright. There's no transition. It takes about 50 million years, which from the point of view of stellar lifetime or the age of the universe is almost instantaneous." If the universe is a 45-year-old person, says Gnedin, reionization happened when she was two-years-old and took about two months.

Another important new finding adds to what we know about how elements scatter through the universe. "We are stardust, billion-year-old carbon," goes the lyric from Joni Mitchell's pop paean to Woodstock. As humans we have in common that we're made from elements heavier than hydrogen and helium -- carbon, oxygen, and others -- formed only in supernovas and cast into interstellar space by supernova explosions. But there's also heavy elements in intergalactic space, vast expanses where there are no stars, galaxies, or supernovas. How did they get there?

Until Ostriker and Gnedin's simulations, it was an unanswered question. Physicists postulated that supernovas somehow blasted matter the enormous distances required to deposit it between the galaxies. The simulations show another way. Almost immediately after reionization, stars begin to gather in small groups, forerunners of present-day galaxies. Drawn by each other's gravity, some of these protogalaxies merge, and in the swirling tango of merger, some of their gas breaks away, thrown into space at high velocities.

"This is new physics," says Gnedin. "Heavy elements have a choice now. They can drive or take the bus. If they want to go into open space, they can take the standard vehicle -- the supernova -- or they can take this merger mechanism. Before there was no choice. This mechanism wasn't noticed till now, and you can notice it only with a simulation. It's a good example of how large-scale computation can give a qualitatively new result."

Another conclusion from the simulations is that these protogalaxies formed at the end of the dark ages are similar in their fundamental internal properties to galaxies we see around us. "It's like comparing a child to an adult," says Gnedin. "Except for size, everything is the same -- two hands, two legs, a face, a nose, ears. The same internal structure. There was no a priori reason for this to be so." Protogalaxies, in other words, could have been tadpoles. Why the internal properties of galaxies are as they are isn't well understood, and other than simulations, says Gnedin, there was no way to predict this child-as-father-to-the-man relationship.

These three frames represent formation of the first stars in the universe. Evolution during this reionization period leads to large-scale structure that resembles the current universe. Color (blue through red) corresponds to the age of the universe (.21 to 1.35 billion years) when the stars form. "Galaxies are all in these filaments," says Ostriker,"and we can see in the physics exactly why this happens. It's basically that shocks form, and then gas cools in those shocks and condenses into stars."