Better hearing with widely-spaced ears

The presence of an observatory in Japan, Australia or India would dramatically increase the probability of measuring gravitational waves

Detectors in the US, Germany and Italy are lying in wait to gather evidence that would unveil one of Albert Einstein’s last secrets: gravitational waves. Up to now, it has not been possible to detect these ripples in the curvature of space-time directly. However, if the available detectors were to be distributed differently across the globe, the chance of detecting the gravitational waves would increase more than twofold. This is the conclusion reached in a new study by Bernard F. Schutz, Director at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Golm. A further improvement in the detection process could also be achieved through the construction of additional gravitational wave observatories.

Many methods of studying the universe are available to us, most of which are based on the analysis of electromagnetic radiation from space. This involves the examination of photos from different eras, so to speak, which were taken on different wavelengths. Given that the traditional methods of space observation are blind to various phenomena, our perceptive spectrum would be extended considerably if we could also find a way of using our ears in this process.

Gravitational waves provide information about star explosions, collisions between black holes and neutron stars, and even about the Big Bang. Their frequencies do not lie in the electromagnetic range but in the acoustic range. These dents in space-time move at the speed of light and make the universe resonate.

Researchers have already constructed “telescope ears” for the detection of gravitational waves in Germany (GEO600), at two locations in the United States (LIGO) and in Italy (Virgo). These gravitational wave detectors measure and evaluate data together in a network. The observatories in the US and Italy are now being upgraded to enable the direct detection of gravitational waves for the first time and will recommence operation from 2016, taking measurements of ten times greater sensitivity than it is possible to do at present.

Up to now, the scientists have assumed that they would be able to observe an average of 40 melting neutron stars or black holes annually. Bernard F. Schutz’s study now reveals that a total of 160 such events could in fact be observed per year. However, this cannot be achieved with the current spatial distribution of the detectors. What is needed is a measuring instrument on the other side of the world – an “ear” at the “back of the head”, as it were.

The measuring sensitivity of a detector network depends on the sensitivity of the individual detectors and their position on earth. In his study, Bernard F. Schutz demonstrates how this relationship can be characterised by three figures for any network: the distance from which the gravitational wave in the sky can be perceived by the individual detector; the minimum signal-to-noise ratio at which gravitational wave detection is possible; the geometric arrangement of the detectors in the network.

“The transfer of one of the existing LIGO instruments from the US to Australia alone would increase the detection rate by a factor of two to four and provide far more accurate information about the events observed,” says Schutz. If – as planned – gravitational wave detectors commence operation in Japan, Australia and India, the scientists will be able to observe around 370 astronomical events annually - a figure that should increase to 500 per year once operation has become routine. The benefits gained from the improvement in measurement accuracy will fully offset the necessary investment.

“A new gravitational wave detector in Japan, whose construction was decided on last year, would further increase the sensitivity and reliability of the detector network and, moreover, enable the observation of a larger proportion of the sky,” says Schutz. “Not only would we be more certain than ever before of being able to measure Einstein’s waves directly, we would also obtain completely new information about neutron stars and gamma-ray bursts. This would mark the advent of a whole new type of astronomy.”

Background information

The direct detection of Einstein's gravitational waves is still one of the most important open questions in modern science. Their direct observation, the scientists hope, will not only underpin the general theory of relativity, especially for extreme gravitational fields around black holes, but also herald the era of gravitational wave astronomy and thus provide completely new insights into our universe: For the first time, it would then be possible to take a look into the very early "nursery" of the universe.

Since previous cosmological observations of the sky have been limited to the electromagnetic spectrum, information about the origin of the universe will only reach us from the period of about 380,000 years after the Big Bang. Development phases further back in time remain hidden from observation because light and matter previously interacted continuously with each other, and the universe only became transparent to electromagnetic radiation after this time. The various theories about the earlier universe have thus not yet been experimentally confirmed. If gravitational waves were measured directly, it would probably be possible to look back to the first quadrillionth of a second after the Big Bang and thus provide completely new insights into our universe.

Gravitational wave research is a worldwide effort, since complete information about many of the gravitational wave sources can only be obtained with several measuring instruments working simultaneously at widely spaced locations. For this reason, scientists around the world have been working closely together for a long time. They share technological research and findings, theoretical advances, and data analysis methods and tools.

The currently active observatories:

• GEO600: The German-British observatory is located near Hanover and is operated by researchers from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute AEI) and Leibniz University Hanover as well as the British universities of Glasgow, Cardiff and Birmingham. The GEO project is funded by the Max Planck Society, the State of Lower Saxony, the Volkswagen Foundation, and the British Science and Technologies Facilities Council (STFC). GEO works closely with the Cluster of Excellence QUEST (Centre for Quantum Engineering and Space-Time Research) in Hannover. Further information: http://www.geo600.de
• Virgo: French-Italian-Dutch project with 3 km long laser arms in Cascina near Pisa. From the beginning, this project also aims at measurements at particularly low frequencies. Virgo is financed by CNRS (Centre national de la recherche scientifique) and INFN (Istituto Nazionale de Fisica Nucleare). Further information: http://www.virgo.infn.it/
• The US LIGO detectors are one 2 km and one 4 km instrument in Hanford, Washington State and one 4 km instrument in Livingston, Louisiana. The LIGO project developed and operated by the California Institute of Technology (CalTech) and the Massachusetts Institute of Technology (MIT) is funded by the National Science Foundation (NSF). Further information: http://www.ligo.caltech.edu/