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

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.

Remarks:

¹ STFC: Science and Technology Facilities Council

2 MPG: Max-Planck-Gesellschaft