Gravitational waves from coalescing binaries

The Nikhef compact binary coalescence data analysis subgroup is focused on developing analysis pipelines, to be used on detections of gravitational wave signals from inspiraling and colliding binary neutron stars and black holes by the Advanced Virgo/LIGO network, and extracting science from them in a variety of ways. In addition, we continue to develop the science case for the 3rd generation Eintein Telescope.

Binary neutron stars have given us the first (indirect) evidence of the existence of gravitational waves. The most famous binary neutron star system is the Hulse-Taylor pulsar, which was shown to be losing orbital energy at exactly the rate one would expect if gravitational waves are emitted in the way general relativity predicts. This earned their discoverers a Nobel Prize in 1993.

With the exception of cosmology, the observed binary pulsars provide the only empirical access we have to the dynamics of spacetime. However, this only gets us to the weak field regime; the strong-field dynamics of general relativity are entirely untested. For this we need direct detection of gravitational waves.

Systems consisting of two compact objects (neutron stars and/or black holes) which have lost so much orbital energy through gravitational wave emission that they are on the verge of merger, are the most promising candidates for a first detection. Through the study of the gravitational wave signals, we will gain access to some of the most interesting processes predicted by general relativity, including  gravitational self-interaction, black hole spins affecting the orbital motion through extreme frame dragging, and the ultra-strong gravitational fields occurring when the two objects merge to form a single black hole. Seeing these effects directly will allow for the most stringent tests of general relativity. The data analysis subgroup at Nikhef has developed a Bayesian method which would allow one to this in a very generic manner, enabling us to search for generic violations of general relativity, irrespective of their detailed nature. To complete this effort, increasingly more realistic waveform models will have to be incorporated into our Bayesian data analysis scheme, and this is a major focus of our activities.  

A detailed knowledge of the interiors of neutron stars is still lacking. With gravititational waves, we will be able to infer the elusive equation of state because of tidal effects influencing the motion of binary neutron stars, and through the dependence of the merger itself on the equation of state. At one extreme one can consider a 'soft' equation of state, in which case the merger of the two neutron stars will be followed by prompt formation of a black hole. On the other hand, with a 'hard' equation of state, the merger results in the formation of a fast-spinning, bar-shaped object, and it will take more time for a black hole to form. The ability to measure the neutron star equation of state will greatly increase our understanding of the behavior of bulk matter at supra-nuclear densities.

As gravitational waves travel from source to observer, they get affected by the changing spacetime curvature of the expanding Universe. By using binary coalescences as cosmic distance markers we will be able infer the contents of the Universe by their effect on its evolution. Currently this is done with so-called 'standard candles' such as Type Ia supernovae, whose intrinsic luminosity (through which distance is measured) needs to be calibrated by more nearby standard candles, which themselves require calibration, leading to a 'cosmic distance ladder'. By contrast, signals from coalescing binaries are self-calibrating: the distance can be inferred directly from the gravitational wave signal itself. Such events will allow us to measure cosmological parameters like the Hubble constant, and later on also the density of matter, the density of dark energy, and the dark energy equation of state as accurately as with other methods while being free of the systematics associated with a cosmic distance ladder. This will allow for a completely independent test of the prevailing cosmological paradigm.