In its exuberant childhood, the Universe created galaxies that were vast star-making machines, astronomers reported on Wednesday.

Using a brand-new telescope in Chile's Atacama desert, the team snared light that took more than 12 thousand million years to reach them.

It came from massive galaxies in the distant cosmos which churned out a thousand stars per year, compared with just one per year for our own languid spiral galaxy, the Milky Way.

Astrophysicists have a passionate interest in how the Universe developed after the "Big Bang" some 13.7 billion years ago.

They have long known about so-called starburst galaxies, which convert vast reservoirs of cosmic gas and dust into stars at a frenzied rate.

Observations of 18 ancient galaxies now suggest this phenomenon occurred when the Universe was under two billion years old -- a whole billion years earlier than thought.

"The more distant the galaxy, the further back in time one is looking, so by measuring their distances we can piece together a timeline of how vigorously the Universe was making new stars at different stages of its 13.7-billion-year history," said Joaquin Vieira of the California Institute of Technology, or Caltech.

Galactic distance is measured by so-called redshift.

The Universe is expanding, which means that lightwaves become "stretched" as their source recedes.

Older -- and thus more distant -- starlight has a telltale signature of a deeper redness.

The astronomers used the Atacama Large Millimeter Array (ALMA), a collaboration between Europe, North America, Japan and Taiwan, to get images of the star clusters and their redshift fingerprint.

The light, helpfully magnified by the gravitational force of galaxies in the foreground, enabled them to double the number of known starburst galaxies with a high redshift of more than four.

Two of the galaxies had a redshift of 5.7, meaning that the cosmos was experiencing a stellar baby boom only a billion years after the Big Bang.

The venerable pair are not only among the oldest galaxies ever found. Light from one of the two pointed to the presence of water, the most distant observation of this precious substance ever made.

The study is published in Nature and the Astrophysical Journal, coinciding with the official inauguration of ALMA, a complex that will ultimately comprise 66 giant antennae, sited at an altitude of 5,000 metres (16,250 feet) in Chile's remote Chajnantor Plateau.