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But as the universe expanded, it finally cooled down enough to allow atoms to form and light to shine out across open space. The accidental discovery of that light back in the 1960s convinced astronomers that the Big Bang was a real event, not just a theoretical construct.
That first detection of the remnants of the Big Bang was crude, but a series of increasingly sophisticated instruments, culminating in the Wilkinson Microwave Anisotropy Probe (WMAP) satellite in 2003, have laid bare the structure of the 400,000-year-old cosmos--only a few hundred-thousandths of its present age--in surprising detail. This was the baby picture Loeb referred to. At that point, the universe was still a very simple place. "You can summarize the initial conditions," says Loeb, "on a single sheet of paper." Some regions were a tiny bit denser than average and some a little more sparse. Most of the stuff in it--then and still today--was the mysterious dark matter that nobody has yet identified, largely because it doesn't produce light of any sort. The rest was mostly hydrogen, with a bit of helium mixed in. So far, the universe hadn't done much of anything.
THE FIRST STARS
At the start of the dark ages, there were no galaxies, no stars, no planets. Even if there had been, we wouldn't be able to spot them. That's because hydrogen-gas clouds are nearly opaque to visible light; no ordinary telescope will ever be able to see what happened afterward. Yet somehow the matter that started as a sea of individual atoms managed to transform itself into something more. So back in the early 1990s, Loeb began lobbying theorists to make a major push to deduce through computer simulations how the first stars formed. The plan was to re-create the young universe digitally, plug in equations for the relevant physics and see what must have happened.
At first, the simulations agree, gravity was the only force at work. Regions of higher density drew matter to them, becoming denser still--a pattern preserved to this day in the distribution of galaxies, with huge clusters where there were high-density regions back then and great voids in between. Eventually, clouds of hydrogen became so dense that their cores ignited with the fires of thermonuclear reactions--the sustained hydrogen-bomb explosions, in essence, that we know as stars. But whereas the familiar stars of the Milky Way are mostly similar in mass to the sun, these first stars were, on average, gigantic--at least 25 times as massive as the sun and ranging as much as 100 times as massive, if not more. A star that big burns very hot, shining perhaps a million times brighter than the sun and generating a wind of particles that pushes the surrounding gases outward, keeping them from collapsing on their own to form new stars. The very first galaxies in the young universe may well have been microgalaxies, as theorist Mike Norman of the University of California at San Diego calls them: each one a single, huge, superhot star, surrounded by a halo of hydrogen.