# A milestone for the direct detection of gravitational waves

Signals of collapsing neutron stars calculated completely for the first time and the problem of bursts, discussed for more than 40 years, solved

Scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Potsdam-Golm have now calculated for the first time a complete gravitational wave signal that is emitted when a neutron star collapses into a rotating black hole - a milestone for the numerical simulation of Einstein's equations. The calculations carried out by Prof. Luciano Rezzolla and his colleagues have now been published in the Physical Review Letters. They are an important part of the precise analysis of the flood of data recorded by gravitational wave detectors and will greatly facilitate the search for gravitational waves.

In the Numerical Relativity group led by Prof. Rezzolla, cosmic catastrophes and the gravitational waves they generate are simulated on state-of-the-art supercomputers. Their current research shows that after the collapse of a neutron star, a deformed and rotating black hole is created that oscillates and emits gravitational waves. Like a bell after ringing, the sound of which is becoming ever quieter, the amplitude of the gravitational waves continues to decrease. “We are thus finally able to obtain information about the behaviour of a black hole in the so-called 'ring-down phase' after the gravitational collapse,” says Prof. Rezzolla, who carried out the calculations together with Dr. Luca Baiotti.

To realize the extensive calculation with the available computer capacity, Rezzolla and Baiotti used the so-called “Mesh-Refinement” method. Numerical calculations of this kind are performed on so-called meshes. The smaller the cells of the grid are defined, the more accurate the calculation becomes. Mesh-Refinement is a method that allows to refine the grid specifically at those points where the greatest possible accuracy is required. Other areas are calculated on rougher grids and therefore require less computing power.

## Collapsing neutron stars cause strong gravitational waves

Neutron stars are formed from massive stars that explode as supernovae. The stellar core condenses into an extremely compact formation of about 1.4 solar masses, which consists almost entirely of nuclear matter, mainly neutrons. If neutron stars continue to grow, they can reach a critical mass above which the gravitational force becomes so great that it outweighs the nuclear forces. The neutron star then collapses and a black hole is formed. The gravitational waves generated by this collapse are particularly strong. Their signals will probably be measured with the new generation of gravitational wave detectors.

## Bursts - a problem discussed for more than 40 years has been solved

Since the gravitational wave signal only appears for a very short time (a few milliseconds), it is called a burst signal. The burst signal is a classical problem that has been discussed for more than 40 years. The calculations now being carried out at the AEI show a complete burst signal for the first time - thus closing a major knowledge gap. In all previous calculations, either unrealistic physical conditions were used as a basis or only initial ranges of the signal were simulated, since sufficient computer capacities for the three-dimensional calculation of the complex systems were lacking. Furthermore, scientists today have an advanced knowledge of neutron stars.

## Signals that gravitational wave detectors are looking for

In addition to the lightning signals, modern gravitational wave detectors are used to search for other signals. These include

- periodic signals as emitted by pulsars or rotating neutron stars, and
- “inspiral” signals, for example from binary star systems or binary black hole systems, whose objects move towards each other on spiral orbits.

## The detection of gravitational waves

The direct observation of gravitational waves - tiny distortions of space-time - predicted by Albert Einstein in 1916 is still one of the most important open questions in modern science. They are created, for example, by the collision of black holes, stellar explosions or the collapse of a neutron star into a black hole. The two US physicists Hulse and Taylor were awarded the Nobel Prize for Physics in 1993 for their indirect detection of gravitational waves.

Today, finally, gravitational wave detectors, large laser interferometers such as the German-British GEO600 project, are sensitive enough to measure the weak signals of gravitational waves.