Peering back 12 billion years using the Herschel space telescope to produce far-infrared images of the sky, a team led by researchers at Cardiff University in the UK have been able to observe the very early formation of galaxies and compare them to galaxies that have formed much more recently.

The results showed that stars were forming inside galaxies much faster in the past compared to today, and that this rapid star birth is using up more and more of the cosmic dust that is ubiquitous in the universe.

Cosmic dust is comprised of tiny solid particles that are found everywhere in space between the stars. The dust and the gas in the universe is the raw material out of which stars and galaxies form.

Though this blanket of material is key to the formation of stars and galaxies, it also acts as a sponge, absorbing almost half of the light emitted by stellar objects and making them impossible to observe with standard optical telescopes.

The Herschel space telescope, launched in 2009, provided with the perfect tool for probing this hidden universe.

Owing to a collection of sensitive instruments, mirrors and filters, the Herschel telescope had the capacity to detect the dust through the far-infrared emission it emits, unveiling the existence of stars and galaxies hidden by the dust.

"We were surprised to find that we didn't need to look far in the past to see signs of galaxy evolution, said Professor Steve Eales, from the Cardiff University.

"Our results show that the reason for this evolution is that galaxies used to contain more dust and gas in the past, and the universe is gradually becoming cleaner as the dust is used up," Eales said.

After seven years of work analysing the images from the Herschel telescope, researchers released a large catalogue of the sources of far-infrared radiation in this 'hidden universe'.

The team's survey of the sky, called the Herschel Astrophysical Terahertz Large Area Survey (Herschel ATLAS), has unveiled the details of over half a million galaxies, many of which have been viewed as they were over 12 billion years ago, just shortly after the big bang.

The team is hopeful that this unprecedented catalogue of sources will be a vital tool for astronomers wishing to understand the detailed history of galaxies and the wider cosmos.

"The exciting thing about our survey is that it encompasses almost all of cosmic history, from the violent star-forming systems full of dust and gas in the early universe, that are essentially galaxies in the process of formation, to the much more subdued systems we see around us today," said Elisabetta Valiante, from Cardiff University.

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