Scientists in Switzerland and the United States have managed to unravel one of the oldest mysteries known to humankind - the age of the universe.
The researchers used a simple new technique based on gravitational lensing, which allows scientists to probe distant parts of the cosmos, to determine that the universe is 13-and-a-half billion years old.
The results of their research will be published in "The Astrophysical Journal Letters", Zurich University announced on Wednesday.
Knowing the age of the universe and realising that it had a beginning is one of the most profound discoveries of modern science.
Since the 1920s scientists have spent years trying to date the Big Bang. It is now generally accepted that the age can be calculated by inversing the expansion rate of the universe, known as the Hubble constant.
The Swiss-US team's new method, first suggested in the 1970s, involved measuring the time difference between images observed deep in space.
"This is an important step. Our method is simple, cheap in time and money, and allows us to be much more precise in our calculation," Andrea Maccio, a researcher from the Institute for Theoretical Physics at Zurich University, told swissinfo.
Other techniques to compute the age of the universe include using the Hubble space telescope, observing supernovae – stars that suddenly gain greatly in brightness because of an explosion - or cosmic microwave background.
"Gravitational lensing gets close to their accuracy and, with more data, has the potential to surpass it," said Maccio.
Gravitational lensing is an astronomical trick, first predicted by Albert Einstein. The gravitational fields of large foreground objects in space bend and focus the light of distant galaxies, acting like a zoom lens.
Analysing changes in the light of these "lenses" reveals important information for scientists.
Since 1979, and especially since the advent of the Hubble space telescope, dozens of multiple-image gravitational lenses have been discovered and mapped. In these systems, the background light sources are most often quasars - black hole galaxies known for their flaring properties.
As a distant quasar flares, so do the different "mirage" images produced by the lensing galaxy. But the light, depending on the length of the path it has taken, flares at different times.
These "time delays" between mirages can vary from weeks to months, according to the galaxy, and represent the key to measuring the age of the universe.
Over a one-year period, the Swiss-based team studied ten gravitational lenses, each with two to four images, measuring time delay and other key data, which were entered into one of the fastest computers in the world at Zurich University.
"Up to now no one was able to apply this method to the observational data available because there wasn't enough," said Maccio.
Previous studies used only one, or a small number of lenses, with less certain results. By making simultaneous calculations with a much larger number sources, the scientists were able to come up with a much more predictable measurement.
Over the next year they plan to continue their work, analysing and collecting data on five more lens systems to demonstrate the importance of their technique and to improve their calculation.
swissinfo, Simon Bradley
The idea of the Big Bang – or creation of the universe – is a direct result of Edwin Hubble's work in the 1920s. Hubble discovered that other galaxies are speeding away from ours and the universe is expanding.
His observations led to the "Hubble constant", a mathematical quantity which measures the speed at which galaxies are travelling apart.
Using this constant and information about the expansion of the universe from observing the distances at which galaxies move away from us we can determine the age of the universe.
According to recent calculations, the universe is expanding at a rate of 70 kilometres per second per megaparsec. A megaparsec, or a million parsecs, is equal to about 3.26 million light years.
One "light year" is the distance that light travels in a year: 9,460,528,404,879 kilometres.
Light from the Sun takes 8 minutes to reach Earth.
The furthest distance that astronomers have ever seen is about 12 billion light years away, from a time just after the Big Bang.
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