Measuring the acceleration of the universe
According to Theodor Hänsch, director at the Max Planck Institute for Quantum Optics (MPQ) in Germany and one of the winners of the 2005 Nobel Prize in Physics, a laser frequency comb can now be used as a basis for new astronomical instruments. Telescopes using this method, which could 'answer crucial questions, such as the search for Earth-like planets or the way the Universe expands, have come a step closer with the first demonstration at the telescope of a new calibration system for precise spectrographs.' 'It looks as if we are on the way to fulfill one of astronomers' dreams,' says Hänsch. But read more...
You can see above a sketch of the experimental setup used at the VTT (Vacuum Tower Telescope) solar telescope in Tenerife in March 2008. By superimposing the frequency comb with light from a celestial body -- in this case, the Sun -- one can effectively calibrate its emission or absorption spectrum against an atomic clock. (Credit: see below)
On the left is an "artist's impression of the laser comb developed for astronomy. Such a laser comb is necessary to act as a 'ruler' for calibrating the new, extremely precise spectrographs that will be needed in the future to search for Earth-like planets or measure the expansion of the Universe. To test this laser comb, a team of scientists went to the telescope and analysed the light coming from the Sun." (Credit:ESO) Here is a link to other versions of this picture.
This research work involved 12 scientists from Germany and Australia, including Professor Theodor W. Hänsch and other researchers at MPQ and at the European Southern Observatory (ESO) to name a few.
Let's see how astronomers are studying the evolution 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."
And how this new method is supposed to improve astronomers' work? "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 -- a feat equivalent to measuring the circumference of the Earth to about a millimetre!"
This sounds good, but what about actual results? "After successful tests in the MPQ laboratory in 2007, the team have successfully tested a prototype device using the laser comb at the VTT (Vacuum Tower Telescope) solar telescope in Tenerife, on 8 March 2008, 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 these big astronomical questions. 'In our tests in Tenerife, we have already achieved beyond state-of-the-art accuracy. Now we are going to make the system more versatile, and develop it even further,' says team member Tilo Steinmetz, from Menlo Systems GmbH, a spin-off company from the Max Planck Institute, which was founded to commercialise the frequency comb technique."
This research work has been published in Science under the title "Laser Frequency Combs for Astronomical Observations" (Volume 321, Issue 5894, Pages 1335-1337, September 5, 2008). Here is a link to the abstract. "A direct measurement of the universe's expansion history could be made by observing in real time the evolution of the cosmological redshift of distant objects. However, this would require measurements of Doppler velocity drifts of 1 centimeter per second per year, and astronomical spectrographs have not yet been calibrated to this tolerance. We demonstrated the first use of a laser frequency comb for wavelength calibration of an astronomical telescope. Even with a simple analysis, absolute calibration is achieved with an equivalent Doppler precision of 9 meters per second at 1.5 micrometers—beyond state-of-the-art accuracy. We show that tracking complex, time-varying systematic effects in the spectrograph and detector system is a particular advantage of laser frequency comb calibration. This technique promises an effective means for modeling and removal of such systematic effects to the accuracy required by future experiments to see direct evidence of the universe's putative acceleration."
Here is an additional link to a document including the full paper and its supporting information (PDF format, 18 pages, 5.68 MB). The top illustration of this post has been extracted from this document.
For additional details, here are some other links. "Laser Frequency Combs for Astronomical Observations" contains lots of links and pictures while this paper from ESO Messenger (Issue 129, September 2007) is named "Future Wavelength Calibration Standards at ESO: the Laser Frequency Comb."
Finally, you should read a New Scientist article, "Laser 'comb' used to disentangle Sun's light" (Jeff Hecht, September 4, 2008). Here is the first paragraph. "'Combs' of laser light have taken a key step from the laboratory to the observatory, where they have been used to observe the light spectrum of the Sun. In the future, they may be able to spot Earth-like planets orbiting other stars, and probe the expansion history of the cosmos with unprecedented accuracy."
Sources: European Southern Observatory (ESO) news release, August 26, 2008; and various websites
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