A super telescope has begun the most detailed survey of the Universe ever undertaken.
The aim of the five-year programme is to shed light on Dark Energy - the mysterious force thought to drive an accelerated expansion of the Universe.
The instrument effectively contains 5,000 mini-telescopes. Each one can image a galaxy every 20 minutes.
In just one year scientists will have surveyed more galaxies than all the other telescopes in the world combined.
What is Dark Energy?
The Big Bang theory of the creation of the Universe originally predicted that its expansion would slow down, and that it would possibly begin to contract as a result of the pull of gravity.
However, in 1998, astronomers were shocked to discover that not only was the Universe continuing to expand, but that this expansion was also accelerating.
The most widely held view is that something is counteracting the pull of gravity - and that something has been termed Dark Energy.
It has been calculated that Dark Energy makes up most of the Universe. Indeed, the atoms that build planets, stars and galaxies probably account for just 5%.
Prof Ofer Lahav, from University College London, is taking part in the project. He said scientists still knew next to nothing about Dark Energy 20 years after its discovery.
"It is just embarrassing to live in a Universe where you only know 5% of it," he told BBC News.
"The nature of Dark Energy, and what it is, may well lead to a revolution in physics - the whole of physics!"
What will the new project examine?
An international team of researchers will use a device called the Dark Energy Spectroscopic Instrument (DESI). It has been retrofitted on to the 4m Mayall telescope at the Kitt Peak observatory in Arizona, US.
Inside DESI are 5,000 optical fibres, each acting as a mini-telescope. This enables the instrument to capture light from 5,000 different galaxies simultaneously, precisely to map their distance from Earth, and gauge how much the Universe expanded as this light travelled to Earth.
In ideal conditions, DESI can cycle through a new set of 5,000 galaxies every 20 minutes.
The further DESI looks into space, the further back in time it sees. This is because of the time it takes for light to reach Earth. The instrument can see objects 10 billion light-years away, which are therefore 10 billion years in the past.
There have been other similar projects, but DESI will cover a much larger volume of space and will measure the acceleration of the expansion of the Universe three times more accurately than ever before.
The team has drawn on previous surveys to create a guide map of the Universe from which 35 million galaxies will be selected as targets for DESI.
How does Dark Energy push the Universe apart?
The key seems to be a force called vacuum pressure which is caused by fluctuations in the fabric of space-time at a sub-atomic level.
Calculations suggest that this vacuum pressure should be an unimaginably larger number (1 with 120 zeroes after it) than the force astronomers actually see pushing galaxies apart.
One possibility is that the vacuum pressure was much, much larger in the early Universe and now has dwindled to its current level.
If, however, astronomers discover that the vacuum pressure has stayed the same, then some more speculative theories come into play.
One suggests that our Universe is one of many in a massive "multiverse". Ours has a tiny vacuum pressure while others may have the much larger pressure.
But matter - and ultimately life - can only exist in a Universe like ours, with a low vacuum pressure.
But could the answer lie elsewhere?
Another possibility is that the current theory of gravity is incomplete. Unlike other fundamental forces, it does not have an opposite force - akin to the positive and negative charges of electricity.
DESI is able to test the current theory of gravity, developed by Albert Einstein more than 100 years ago, in unprecedented detail because it can see so far back into the past.
Astronomers will be able to see, blow by blow and in exquisite detail, how gravity operated over that time to bring together particles to form the planets, stars and galaxies we see.
This "movie" will enable cosmologists to check if Einstein's theory of gravity and space-time (General Relativity) is correct. If not then the concept of Dark Energy might not be required. Instead, what would be needed is a more complete theory of gravity that explains the accelerating Universe.
The DESI Collaboration involves researchers at 25 institutions from the US, UK, France, Switzerland, and Spain.
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