Although it was born in the fireball brilliance of the Big Bang, the universe spent much of its infancy in the dark. Clouds of primordial particles expanded and cooled forming atoms–hydrogen, mostly that were opaque to light. Galaxies did not yet exist. Even stars did not yet exist. These are known as the cosmic Dark Ages. That era is shrouded in mystery, since scientists literally cannot see what was happening then.
Then gravity did its work. Gas collapsed into bright stars, larger clumpings of matter collected into proto-galaxies, and the universe began to light up. Astronomers have worked out the general theory of how this probably happened. Now at last they are seeing it for real, due to data coming from the brand-new Atacama Large Millimeter/submillimeter Array, or ALMA.
My friend Govert Schilling has written a great explainer about what ALMA is and how it works. This telescope array in Chile’s Atacama desert is a remarkable tool for exploring deep space–so remarkable that it has just yielded groundbreaking work on the Dark Ages even while it is still under construction.
Aiming 16 of its dishes at a series of distant galaxies, ALMA picked up faint millimeter-wave emission–shorter than FM radio waves, longer than the infrared rays from a heat lamp–from carbon monoxide molecules in those galaxies. Carbon monoxide is significant for two reasons. First, it is the second-most common molecule in the universe, behind molecular hydrogen; it therefore produces a relatively strong signal. But more important, carbon monoxide radiates especially in the locations where new stars are being born. When an infant galaxy begins forming stars, the carbon monoxide floodlights suddenly switch on.
These baby galaxies are very difficult to see in visible light. They tend to be shrouded in gas and dust, keeping the Dark Ages out of sight and making them undetectable to conventional telescopes. The radiation from carbon monoxide can still get through, however. That’s because the carbon monoxide “shines” by giving off radiation with a wavelength of 3 millimeters (about 1/10th of an inch), about 5,000 times as long as visible light. Such long waves basically jump right over bits of dust and continue on their way to us. Ordinary telescopes cannot detect such waves. The Hubble Space Telescope cannot see such waves. But ALMA can.
Which bring us back to ALMA’s mind-bending new discovery. A team of astronomers led by Joaquin Vieira, a post-doc at Caltech, zeroed in on 26 suspected distant galaxies. ALMA detected strong carbon monoxide emission in 23 of them–the telltale sign of a galaxy bursting with newborn stars. Moreover, by measuring how the radiation had been stretched by the expansion of the universe, Vieira and his colleagues were able to calculate the distances to these starburst galaxies and found that they were substantially farther away than expected. Most of them were about 12 billion light years away, meaning we are seeing them as they were 12 billion years ago.
Since the observable universe is just 13.7 billion light years old, these galaxies were lighting up the Dark Ages when the universe was less than 2 billion years old–a billion years earlier than astronomers expected. The most distant of the galaxies detected by ALMA were fully aglow with stars when the universe was just 1 billion years old.
OK, let’s step back for a moment and think about what that really means. The first chapters of cosmic history must have been extremely eventful, going from Big Bang to fog to Dark Ages to first stars to full-fledged galaxies in just the first 7% of the age of the universe. The first chapter of ALMA’s history is pretty impressive too. In its very first week of active operation, ALMA is already revealing new details about about the early history of galaxies like our Milky Way, and about the remarkable path that led from a soup of atomic particles to the sun, the Earth, and to you.
I can’t wait to see what ALMA will tell us next.
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