'Laser frequency comb' to measure the acceleration of the Universe

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Munich, September 5 : Scientists have demonstrated a Noble Prize-winning technology called a 'laser frequency comb', which may be used to measure the acceleration of the Universe.

Astronomers use instruments called spectrographs to spread the light from celestial objects into its component colours, or frequencies, in the same way water droplets create a rainbow from sunlight.

They can then measure the velocities of stars, galaxies and quasars, search for planets around other stars, or study the expansion of the Universe. A spectrograph must be accurately calibrated so that the frequencies of light can be correctly measured.

This is similar to how we need accurate rulers to measure lengths correctly.

In the present case, a laser provides a sort of ruler, for measuring colours rather than distances, with an extremely accurate and fine grid.

New, extremely precise spectrographs will be needed in experiments planned for the future European Extremely Large Telescope (E-ELT), which is being designed by ESO, the European Southern Observatory.

These new spectrographs will need to be calibrated with even more accurate 'rulers'. In fact, they must be accurate to about one part in 30 billions.

"We'll need something beyond what current technology can offer, and that's where the laser frequency comb comes in," explained team member Constanza Araujo-Hauck from ESO.

The project aims to measure the recently discovered acceleration of the universe directly, by following the velocities of distant galaxies and quasars over a 20-year period.

This would let astronomers test Einstein's general relativity and the nature of the recently discovered, and mysterious, dark energy.

"We have to measure the movement of these distant galaxies to a few centimeters per second, and follow this over decades. These speeds are barely faster than a snail's pace, and the laser frequency comb is absolutely crucial for this," said team member Antonio Manescau, from ESO.

The new calibration technique comes from the combination of astronomy and quantum optics, in a collaboration between researchers at ESO and the Max Planck Institute for Quantum Optics.

It uses ultra-short pulses of laser light to create a 'frequency comb'- light at many frequencies separated by a constant interval - to create just the kind of precise 'ruler' needed to calibrate a spectrograph.

After successful tests in the MPQ laboratory in 2007, the team has successfully tested a prototype device using the laser comb at the VTT (Vacuum Tower Telescope) solar telescope in Tenerife, measuring the spectrum of the Sun in infrared light.

The results are already impressive, and the technique promises to achieve the accuracy needed to study some big astronomical questions, such as the search for Earth-like planets or the way the Universe expands.

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