"Artist's impression of the expected dark matter distribution around the Milky Way" by ESO/L. Calçada / CC BY 4.0

The Galaxy That Grew Up Too Fast

May 20, 2020

By Dennis Overbye

In the early days of the Hubble Space Telescope, astronomers, eager to see how far out in space — and how far back in time — their new instrument could peer, pointed it at an empty tract of sky. What returned was an image of space littered with what the astronomer Alan Dressler of the Carnegie Observatories called “train wrecks”: irregular, fragmented clouds of stars known as protogalaxies, flecks of starlight scattered like orphan jigsaw puzzle pieces across the primordial heavens.

The scene fit nicely with the growing consensus of how the universe had evolved over cosmic time: Small bits of matter — gas, dust and starlight — slowly assembled themselves into ever larger structures, eventually resulting in majestic spiral galaxies, like our own Milky Way, 100,000 light-years across and home to hundreds of billions of stars.

But a new discovery suggests that this vision of cosmic growth may need revision. On Wednesday, radio astronomers using the mighty Altacama Large Millimeter Array, or ALMA, radio telescope in Chile announced that they had discovered a cloud of gas located on the distant shores of time. It appears to be an infant galaxy similar in size to its grown-up counterpart, our own Milky Way, and dates to a time when the universe was only 1.5 billion years old, one-tenth its current age.

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