This will help to find extra-solar planets, i.e. planets that are orbiting a star outside our solar-system. By refining this technology it might become possible to directly measure even very small changes in the expansion velocity of the universe.
Use of Doppler shift measurements in the search for planets. When a planet (red ball) orbits a star (yellow ball), the recoil it exerts gives rise to a periodic movement: at one time the star is moving towards the observer (above), and the light waves appear to be squeezed. This means the radiation is shifted towards higher frequencies, which is called a “blue shift”. If, on the other hand, the star is travelling away from the observer (see below), the waves seem to be stretched, resulting in a so-called “red-shift” towards lower frequencies. Graphic: Th. Udem, MPQ
A frequency comb is a light source with a comb-like spectrum. The frequency difference f_r between two neighbouring lines is always exactly the same. It is kept stable by comparing it with an atomic clock. The comb light is guided to the spectrograph in an optical fiber. The light is separated into its colours (i.e. its frequency components) by the spectrograph and imaged on the CCD detector. The comb-like spectrum appears as a row of dots of which each dot corresponds exactly to one line of the frequency comb. This "laser ruler" can now be used to calibrate the spectrograph.
Since their invention some ten years ago, laser frequency combs have proven to be a vital tool in many laser laboratories around the world. Originally developed for the investigation of the quantum world, they are now making their way into astronomy and astrophysics. A team of scientists from the Laser Spectroscopy Division of Professor Theodor W. Hänsch at the Max Planck Institute of Quantum Optics (Garching), in collaboration with the European Southern Observatory (ESO), the Instituto de Astrofísica de Canarias, and the Menlo Systems GmbH (Martinsried), has now modified the frequency comb technique in a way that it can be applied for the calibration of astronomical spectrographs (Nature, 31 May 2012, DOI:10.1038/nature11092).
The new instrument has been tested successfully with the High Accuracy Radial velocity Planet Searcher (HARPS), a spectrograph at the 3.6-metre-telescope at the La Silla Observatory in Chile. A tenfold improvement of precision was obtained as compared with traditional spectral lamp calibrators. This will greatly enhance the chances to find earth-like planets outside our solar system. Ultimately, the search for extra-solar planets shall answer the question whether our solar system is the only place in the universe that provides the conditions for life as we know it.
Hence, the amount of Doppler shift in the star’s spectrum is very small and can be detected only with the help of high precision measurement tools.
“Measuring” a physical parameter is equivalent to comparing it with a calibrated standard. So far the precision in frequency measurements was limited by slow drifts like aging of the calibration source, e.g. a thorium spectral lamp. The laser-frequency combs invented in the late nineties – for which Theodor W. Hänsch and John Hall were awarded the Nobel Prize in 2005 – have greatly improved the precision of frequency measurements. In 2005 MPQ and ESO decided to cooperate on the development of frequency combs for the calibration of astronomical spectrographs. As the first test measurements carried out at the VTT in Tenerife in 2008 turned out to be very promising, the scientists began to work on a frequency comb for the HARPS-spectrograph at the La Silla Observatory.
A frequency comb is emitted by a laser delivering light with a spectrum composed of many lines at constant intervals. By electronic feedback circuits each line is defined in reference to an atomic clock. By comparing the spectral lines of a star with the lines of this laser ruler, which does not change over time, it is possible to observe even very small variations of the star light, caused e.g. by an orbiting planet.
The adaption of the laser-frequency comb for its application in astronomical spectroscopy posed a few major technical challenges. Even precision spectrographs like HARPS provide a limited frequency resolution, typically of 105. Hence the lines of the frequency comb to be developed have to be spaced at intervals of more than 10 GHz, otherwise the spectrograph would not be able to resolve them. Furthermore, astronomical spectrographs operate in the visible spectral region.
In order to ensure robust and stable operation fibre-laser systems were chosen as the basis of the frequency comb. Fibre-laser systems, however, emit light in the infrared region, with spectral distances of a few 100 MHz. The scientists were able to change these properties by implementing a cascade of several spectral filters and using advanced fibres (developed by Philip Russell from the Max Planck Institute for the Science of Light, Erlangen). This resulted in a frequency comb with the desired mode spacing and a broad spectrum in the visible region. The calibration of the HARPS spectrograph with this frequency comb resulted in a sensitivity for velocity changes of 2.5 cm/s. This was demonstrated in a series of measurements in November 2010 and January 2011. By observing a star with a well known planet for a couple of nights the team could prove the high stability of the system over time.
For the near future the scientists pursue a task that is even more demanding than looking for planets. Astronomical observations have clearly shown that the universe is not static but instead expanding continuously. New results on the microwave background radiation and the observation of supernovae suggest that this expansion is accelerating over time. However, the change of the velocity is expected to be very small, of the order of annually one centimetre per second. Such precision is to be delivered by the next ESO-project, the European Extremely Large Telescope (E-ELT) which is planned to be constructed in Chile in the next decade. High precision frequency combs will be at the heart of its CODEX spectrograph, providing a calibration precision of one part per 300 billion – a feat equivalent to measuring the circumference of the Earth to half a millimetre. Olivia Meyer-StrengOriginal publication:
Nature, 31 May 2012, DOI:10.1038/nature11092Contact:
Dr. Olivia Meyer-Streng | Max-Planck-Institut
NASA spacecraft investigate clues in radiation belts
28.03.2017 | NASA/Goddard Space Flight Center
Researchers create artificial materials atom-by-atom
28.03.2017 | Aalto University
The Institute of Semiconductor Technology and the Institute of Physical and Theoretical Chemistry, both members of the Laboratory for Emerging Nanometrology (LENA), at Technische Universität Braunschweig are partners in a new European research project entitled ChipScope, which aims to develop a completely new and extremely small optical microscope capable of observing the interior of living cells in real time. A consortium of 7 partners from 5 countries will tackle this issue with very ambitious objectives during a four-year research program.
To demonstrate the usefulness of this new scientific tool, at the end of the project the developed chip-sized microscope will be used to observe in real-time...
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
28.03.2017 | Life Sciences
28.03.2017 | Information Technology
28.03.2017 | Physics and Astronomy