Cosmic Dawn: How the Universe’s Lights Went On

A deep look into distant space reveals the moment the lights came on in the universe

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Astronomers call it the cosmic dawn, and it’s nearly as poetic as it sounds. This is the time, a few hundred million years after the Big Bang, when the first stars and galaxies burst into existence, bringing light to the cosmos and putting an end to the period known, just as evocatively, as the dark ages.

While scientists know the cosmic dawn must have happened — today’s universe is, after all, filled with glittering stars — exactly when and how it unfolded has largely been a mystery. Did it all occur at once, like a burst of cosmic fireworks, or did it happen more gradually? Were the earliest galaxies alone in the universe, or did giant black holes lurk among them? The full answers to these questions will probably have to wait for the James Webb Space Telescope, currently slated to go into orbit in 2018.

But thanks to the aging but still-vibrant Hubble, the veil of over the distant past is at least starting to part. A team of observers led by Caltech’s Richard Ellis  just announced that they have discovered seven galaxies dating from as early as 350 million years post-Big Bang. “This is the deepest archaeological dig into the universe we’ve ever had,” said Harvard astrophysicist Avi Loeb at a press conference.

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Among the objects uncovered in this dig was a galaxy that might turn out to be the most distant ever found, though that still needs to be verified. “Most people get excited about the farthest galaxy,” admitted Ellis at the press conference, “but the fact that we’re doing a census of early galaxies is even more interesting.” One reason is that simply by counting galaxies at earlier and earlier times in cosmic history, you can see if the numbers drop off abruptly at some point, suggesting how suddenly or gradually the stars came into existence.

The answer seems to be that they arrived gradually. The seven just-detected galaxies date from about 400 to 600 million years after the Big Bang, and, said Ellis, “if we compare the number we see here to a census taken at about a billion years after the Big Bang, you see a smooth decline in numbers as you look earlier and earlier.” The take-home message: the emergence of galaxies was a drawn-out affair.

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Another message, which confirms what theorists like Loeb have long believed, is that these earliest galaxies were much smaller, feebler and more tightly packed than the galaxies we see today. In fact, today’s galaxies, including the Milky Way, were almost certainly created by  these small galaxies merging, yielding one huge clump of stars out of many smaller groupings.

The small early galaxies also produced something else. The stars within them were made only of hydrogen and helium, the primordial elements created in the Big Bang The nuclear furnaces in their cores, however, transmuted those elements into heavier atoms, including carbon and oxygen and silicon, the building blocks of planets and people. “Life,” said Loeb, “could not exist until these heavy elements were produced. When that first, fast-burning generation of stars died and spewed their essence into interstellar space, the basic ingredients for life were part of the mix.

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Finally, the galaxies discovered by Ellis and his colleagues must have contributed, at least, to a kind of cosmic cleanup that took place during the cosmic dawn. When the universe went to black about 400,000 years after the Big Bang, protons and electrons combined to form the very first hydrogen atoms, whose light-absorbing properties led to a sort of great fog descending over everything. Some powerful energy source eventually broke the atoms apart once again — effectively blowing the fog away and allowing the dawn to break. At least some of that energy came in the form of ultraviolet light from the earliest stars, but it’s not clear that there were enough of them to do the job entirely. Super-massive black holes might also have been part of the mix, for example. This new census is the start, at least, of finding the answer.

It’s only the start, though. Powerful though the Hubble is, it can’t see very far into the infrared region of the electromagnetic spectrum, and these earliest galaxies are mostly visible in that  range. But infrared will be the Webb telescope’s sweet spot, and the Webb’s larger size — its light-gathering mirror will be more than 21 ft. (6.4 m) across, compared with Hubble’s 7.8 ft. (2.4 m) — makes it more than seven times more sensitive to faint light.

That means the Webb will almost certainly be able to finish the job the Hubble has started. “Our survey,” said Ellis, “may be as deep as Hubble can look. But I can now safely predict that the James Webb Space Telescope will find many galaxies from this stage in the history of the universe, and many from even farther back.”

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