FNWI --- IMAPP Department of Astrophysics
Radboud University > Faculty of Science > Department of Astrophysics

Gravitational waves

Gravitational Waves (GWs) were predicted by Einstein almost 100 years ago in his Theory of General Relativity (GR). GWs are ripples in spacetime produced by large accelerating masses. A GW passing by periodically stretches and squeezes freely falling masses in space in the direction perpendicular to the direction of wave propagation. Detection of GWs will provide a direct proof of the theory of GR, and will open a completely new eye to the universe. It will provide a fresh look on compact binaries consisting of white dwarfs (WDs), neutron stars (NSs) and/or (supermassive) black holes (BHs). We want to know to what extent the knowledge we have from electromagnetic observations can help in the analysis of GW detections. We do this for Galactic WD binaries which are guaranteed sources (the verification binaries) to be observed by LISA, and for the BH-NS inspirals that LIGO/Virgo can observe. We also study recoiling supermassive BH binaries.

ESA is currently considering a smaller version of LISA as an European only mission: eLISA

Group members

Sources of GW

In order to detect the minute distortions in spacetime from distant sources, sensitive detectors that use laser interferometry have been designed which are basically Michelson interferometers. Few GW detectors in ground with 90 degree arms of 2-4 km will measure slight stretches in armlengths when a GW passes through. LIGO and Virgo are km-scale Michelson interferometers and have been taking data for a few years. Advanced LIGO and Virgo will be ten times more sensitive and are scheduled to start observing in 2015.A similar principle is applied to the future space-based GW detector like LISA. LISA will detect GWs by measuring fractional frequency shifts in the lasers exchanged between the three spacecrafts (with test masses in them) which are ∼ 5 million km apart. These shifts will have a contribution from the GWs and detector noise.

ligo-lisa.jpg Left: Sensitivity of LIGO/Virgo in high-frequency band compared to that of LISA in the low-frequency band. The observatories are designed to detect different types of sources

The LIGO (LA, USA) Artist’s view of LISA satellite in space

Ground-based detectors like LIGO and Virgo are sensitive to high-frequency (Hz - kHz) sources such as the mergers of NS/BH binaries and core collapse supernovae. These detectors are limited to low frequency below 1 Hz GW sources due to seismic noise and small armlenghts compared to the wavelenght of a GW source. Future space-based detector like LISA is sensitive to low-frequency (mHz - Hz). The low-frequency window is a rich band with signals from super massive black holes, and galactic compact binaries.

Artist's image of a capture of a stellar mass object by a massive black holeSimulation of binary mergerArtist's image of a compact binary emitting GWs

Recoiling Supermassive Blackholes

smbh.jpg It is believed that most galaxies harbour supermassive black holes (SMBHs) at their centres. When galaxies merge, their SMBHs may merge too, and the merger may lead to a recoil of the resulting black hole. Recently a high-velocity SMBH was discovered using X-ray observations. Chandra measurements suggest that the BH is not at the centre of its host galaxy making it a potential candidate of a recoiling SMBH. LISA will be able to observe SMBH mergers which will help us understand the mergers, and hence evolution of galaxies.

Right: Hubble Telescope image of a distant galaxy whose centre is marked by a white circle. The inferred position of the SMBH it hosts is indicated by the red circle.

GW journal club