The sound of colliding black holes - and how to filter out the noise of the Universe from it

Meeting of experts at the AEI from 6 to 9 July, 2009

June 25, 2009

Scientists from all over the world are looking forward to the first direct detection of gravitational waves. With the aid of the gravitational wave astronomy that will begin once this has been achieved it will be possible to learn a great deal about the still unknown 96% of the universe. For the first time, it will be possible to listen to and observe the universe using a new frequency spectrum. In addition to high-precision detector technology, as well as theoretical and experimental basic research in numerous fields, two fields research are of particular importance in order to be able to hear and understand the sounds of the universe: numerical relativity and data analysis.

• Numerical relativity is required to accurately predict the expected gravitational wave signals.

• For gravitational wave research, new methods of data analysis are being developed to filter the tiny gravitational wave signals out of large amounts of data.

During the Numerical Relativity and Data Analysis Meeting (NRDA) 2009, around 80 international scientists from the areas of numerical relativity and data analysis will be present at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) from 6 to 9 July, 2009 to discuss the latest developments in both areas and to intensify their cooperation. For example, one question will be how to reliably filter out the sound of two colliding black holes from the clutter of noises in the universe.

We cordially invite you to participate in the conference and to engage in conversations with the scientists. For registration please contact:

Susanne Milde, Tel.: 0331 - 583 93 54, Email:

Background Information

In recent years, pioneering progress has been made in ​​numerical relativity and data analysis. On the one hand, the prediction of gravitational wave signals from various astrophysical sources has become more and more accurate; on the other hand, the analysis of the immense amounts of data produced by gravitational wave observatories has become faster and more reliable. Now is the time to link the information and methods of both research areas. The AEI scientists belong to some of the world's leading research groups in both areas.

Numerical relativity at AEI

The achievements of the Numerical Relativity Group led by Prof. Luciano Rezzolla, include:

  • the calculation of so-called burst signals, which are signals from collapsing neutron stars, for the very first time. This represents the solution of a problem that had been discussed for more than 40 years and therefore constitutes a milestone in the direct detection of gravitational waves.
  • the first completely relativistic simulation of a binary neutron star merger. This provides answers to questions such as: what exactly happens in this process, which ultimately gives rise to a black hole? How much energy is released? Is this the cause of the enigmatic gamma-ray bursts that take place?

Data analysis at AEI

At AEI there are two research groups that are involved in different aspects of data analysis:

  • In the Gravitational Wave Analysis Group (led by Dr. Maria Alessandra Papa), the data provided by gravitational wave detectors is investigated and evaluated, using the powerful computer cluster Morgane. In particular, customised methods for analysing the data are developed. The area of research of ​​Dr. Badri Krishnan work is the better understanding of the sources of gravitational waves. In addition, the scientists are concerned with a key astrophysical question: What new findings will the observation of gravitational waves bring to light?
  • The scientists in the Department of Observational Relativity and Cosmology at the AEI site in Hannover, Germany, operate the world's fastest and largest computer cluster that primarily analyses data from gravitational wave detectors: ATLAS. This is undertaken under the direction of Prof. Bruce Allen. All the data currently available in the US (LIGO) and Europe (GEO600 and Virgo) observatories is collected here. ATLAS is expected to confirm the first direct measurement of gravitational waves.

The task of analysing the extremely weak gravitational wave signals and separating them from the data flood is a task that scientists from all over the world are working on. They have joined together in the "LIGO-Virgo Scientific Cooperation" (LSC-V) and have agreed to a complete exchange of recorded data. In addition, all scientific results are published jointly. The LSC-V consortium is headed by Dr. Maria Alessandra Papa of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute).

The reliability and speed of the data analysis is tested in the context of so-called "mock data challenges" or even sample evaluations. Simulated gravitational wave signals are injected into the detector data. Up to now these artificial signals have been reliably identified.

The ATLAS Cluster is the world's largest resource for the analysis of gravitational waves. It unites about 1700 individual computers. It was created under the leadership of Prof. Bruce Allen, Director at the Max Planck Institute for Gravitational Physics.

Detection of gravitational waves
The direct proof of the gravitational waves predicted by Albert Einstein - tiny distortions of space-time - remains one of the most important open questions of modern science. Their direct observation will lead to an era of gravitational wave astronomy and provide completely new insights into our universe. With the help of gravitational waves, it will be possible to look back into the first billionth of a second of the universe and thereby solve many riddles about its origins. Observation methods that have been used up to now have been unable to achieve these insights.

The detection of gravitational waves would have far-reaching effects in addition to further substantiation of the general theory of relativity. For the first time, it will be possible to take a look back into the "nursery" of the universe. Up to now observation of the sky has been restricted to the electromagnetic spectrum (for example, radio and X-ray telescopes, as well as the observation of visible light). The information that is available to us about the origin of the universe only goes back to 380,000 years after the Big Bang. Observations earlier than that are not now possible, since the universe only became transparent to electromagnetic radiation at that time. The various theories on the earlier universe have therefore remained experimentally unconfirmed. The direct measurement of gravitational waves would open up completely new possibilities in this regard, since it would presumably be possible to listen back to the first billionth of the first second that followed the Big Bang. With the help of gravitational wave astronomy, completely new fields of science would be opened up.

Status of currently operational gravitational wave observatories
A number of first-generation gravitational wave detectors are currently operating in Europe: German-British GEO600 observatory is operated by the AEI in the vicinity of Hannover and funded by STFC1, MPG2, as well as the state of Lower Saxony, while the French-Italian-Dutch Virgo project is located in Cascina in the vicinity of Pisa. The data from these measuring devices are combined with those of the three American LIGO interferometers. The entire data pool is currently being used to look for gravitational wave signals from astrophysical sources.

During the course of the next decade, all interferometric gravitational wave detectors will be upgraded to second-generation instruments. The sensitivity of Virgo and LIGO in lower frequencies (up to about one kilohertz) will be increased tenfold using technologies developed in Europe. In particular, GEO600 will do pioneering work in the field of broadband observation of high frequencies, here too through the development and use of new technologies. GEO600 is regarded as the think tank of gravitational wave research.

Neutron stars
Together with black holes, they are among the most fascinating objects of the universe. They constitute the final stage of star development and are the remains of supernova explosions. Neutron stars have a slightly larger mass than the sun (about 1.4 solar masses), but this is compressed into a perfect sphere of the size of a small city with a radius of 10-12 kilometres. They consist almost entirely of nuclear matter, mostly neutrons, which manifest a variety of extreme conditions. For example, the density of neutron stars is so high that a teaspoon thereof would weigh as much as the entire Alps. At the same time, the gravitational forces are so strong that the physical conditions of neutron stars are very similar to those of a black hole with comparable mass. Understandably enough, such conditions cannot be generated in laboratories on earth. This is why we currently know so little about these interesting structures.

The inner structure of neutron stars
Much of our knowledge about the size and mass of neutron stars has been obtained by satellite observations in the X-ray and gamma-ray range. Such measurements also provide us with information about behaviour of these compact objects in binary systems in which each star draws material away from the other. However, since we get this information from electromagnetic signals, we have no knowledge about the internal structure of neutron stars, but rather only about their surface.

In future, electromagnetic signals will not remain our only source of information about neutron stars. We know that binary neutron stars emit energy in the form of gravitational waves. Russell A. Hulse and Joseph H. Taylor received the Nobel Prize for Physics in 1993 for the long-term observation of the binary pulsar PSR 1913 + 16, through which they indirectly demonstrated the emission of gravitational waves. Such dual-star systems are among the strongest sources of gravitational waves and should be measurable with today's detectors - provided they are close enough. In contrast to electromagnetic radiation, gravitational waves will provide us with insight into the interior of neutron stars. Without exaggeration, the gravitational wave signal of a neutron star can be described as the "Rosetta Stone" for decoding its inner structure.

The calculation of gravitational wave signals emitted by merging neutron stars
Apart from the enormous experimental difficulties involved in the measurement of the gravitational wave signals of neutron stars, the calculation of these signals also poses a great challenge. However, if we were aware of the wave forms, then those engaged in experimental work could undertake a targeted search in the data. In order to theoretically predict such signals, gigantic supercomputers are necessary to numerically solve Einstein’s equations – a system of nonlinear, coupled differential equations – and the equations of relativistic hydrodynamics (these describe the motion of matter).

During the last few years, the Numerical Relativity Group at AEI has developed codes for the computation of such wave forms. In doing so, both binary black hole systems, as well as neutron stars are being investigated.


1 STFC: Science and Technology Facilities Council

2 MPG: Max-Planck-Gesellschaft

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