Webb telescope to look for first light of cosmic dawn

Peering into deep space and billions of years back in time, the James Webb telescope promises to offer the clearest glimpse yet of the Universe’s cosmic dawn, when the earliest galaxies began to form.

The largest and most powerful telescope ever to be launched into space, which will take over from Hubble, will "directly observe a part of space and time never seen before", says Nasa. This is the Universe in its youth, just a few hundred million years after the Big Bang.

With "very lofty science goals in mind", Webb will look back 13.5 billion years to when the first galaxies evolved in the Universe, said Begonia Vila, instrument systems engineer at Nasa’s Goddard Space Flight Center in a briefing.

It aims to allow us to observe "how they change and evolve to get to the type of galaxy where we live in today," she said, as well as looking at the first stars and trying to detect "the type of elements that we identify as a sign of the life as we know it; water, carbon dioxide, methane".

Looking farther into space means looking farther back in time because of how long it takes for light to travel -- sunlight, for example, takes eight minutes to reach our eyes on Earth. Hubble reached its limit at 13.4 billion years, with the discovery of the oldest galaxy yet observed, GN-z11.

That ancient galaxy may have been an unprepossessing little dot but it was also "a surprise, with a luminosity that one did not expect at such a distance", said the Swiss astrophysicist Pascal Oesch, who first reported GN-z11.

Hubble, which was launched in 1990, looks mainly at visible light -- but Webb, set to launch on December 22, focuses on infrared. Light emitted by the very first luminous objects has been "redshifted" by the Universe’s continual expansion, according to Nasa, arriving today as infrared.

Webb, with its significantly greater sensitivity than Hubble, is expected to provide much more detailed images, which Oesch said "will allow us to explore this era in extraordinary detail". He is betting that "many, many more galaxies will then be revealed, but they will be much less luminous".

Its infrared capability will also allow Webb to penetrate the interstellar dust clouds that absorb starlight and hide them from Hubble’s view. This "makes it possible to see what is hidden in the clouds, the birth of stars and galaxies", said David Elbaz, an astrophysicist at the French Atomic Energy Commission.

Webb has been jointly developed by Nasa, the European Space Agency and the Canadian Space Agency. The ambition is to help explain a key stage in the evolution of the Universe, when "the lights went on, when the very first stars began to form", said Oesch.

After the Big Bang triggered the expansion of the Universe, it entered a virtually lightless period, the cosmic dark ages, where it was cast in a gaseous fog of hydrogen and helium swirled, making it opaque.

This period continued for hundreds of millions of years until the first stars began to form. These are thought to have been giants -- up to 300 times more massive than our Sun -- that burnt only for a few million years before exploding as supernovae.