Gravitational wave detection using pulsars

The origin and evolution of the Universe as governed by the laws of gravity is currently best described by Einstein's highly successful theory of gravity, the theory of general relativity (GR). In GR (and in other relativistic theories of gravity), space and time are combined to form “spacetime” which is curved in the presence of mass. It is the curvature of space-time that itself determines how masses move. To date, GR has passed all observational tests with flying colours. The most recent experimental confirmation of GR came from the direct detection of gravitational waves (GWs) by laser interferometers. GWs are wave propagations of spacetime ripples that result when spacetime curvature is perturbed by accelerating masses (for non-spherically symmetric motions). GWs propagate through the Universe virtually unaffected by absorption and scattering, being therefore able to carry information to our detectors from processes happening far in the Universe's past. Their detection is technologically challenging due to their very small amplitudes.

The pulsar binary system PSR J0737–3039A/B. The system is emitting gravity waves as they orbit around their common centre of mass (John Rowe Animation/Australia Telescope National Facility, CSIRO).

The first detection, a GW signal (GW150914) created by the collision of two black holes of about 30 solar masses, was announced by the LIGO/Virgo collaboration on February 11th 2016, making the 100th-year anniversary of Einstein's 1916 paper predicting GWs a trully special one. This was the result from decades of work setting up this huge experiment and rightfully earned the 2017 Nobel Prize in Physics. This came 24 years after the 1993 Nobel prize was awarded for the first discovery of a binary pulsar which led to the first indirect confirmation of the existence of GWs, by demonstrating that the binary's orbital decay was in remarkable agreement with the predicted energy loss from GW emission. The European Pulsar Timing Array project and its partner collaborations have the ambitious aim to make the first direct detections of GWs in the nanohertz frequency regime, probing completely new physical processes of cosmological origin. This is to be achieved by observations of radio pulsars, this special class of astrophysical objects that led to the first (indirect) verification of the existence of GWs.

Pulsars are fast rotating, magnetized, compact and massive objects known as a neutron stars, which emits a narrow beam of radio emission along open magnetic field lines. As the pulsar rotates, it acts like a cosmic lighthouse and pulsar radiation directed to Earth can be observed once per rotation, producing a narrow pulse and hence a natural beacon for a terrestrial observer. Because pulsars are massive and compact (the mass of 1.4 Suns is concentrated in a sphere of only 20 km diameter), they represent massive flywheels in space, whose rotation and repetition rate can hardly be disturbed. This makes pulsars very precise cosmic clocks. It is therefore in principle possible to detect fluctuations in the pulse arrival times caused by propagating GWs. In practice, due to the very small expected amplitudes, only millisecond pulsars have so low levels of intrinsic noise that are capable of possibly detecting such small fluctuations (predicted to be below 200 nanoseconds). Because it is virtually impossible to say with certainty whether a candidate GW signal is or not intrinsic pulsar or instrumental noise, we simultaneously look for the same signals using Pulsar Timing Arrays (PTAs), ensemble of millisecond pulsars covering as many sky locations as possible.

The rewards for a successful detection of nanohertz GWs are immense and would have enormous consequences. Such GWs are expected from inspiralling supermassive black-hole binaries and cosmological gravitational-wave backgrounds and their detection will provide unique observational constraints on cosmological models and theories for formation of the observed large-scale structure Universe. It would also enable tests of GR and alternative gravity theories in the radiative regime in several, unprecedented ways. As most relativistic theories of gravity conjecture the existence of gravitational waves, the predictions of GR for GW properties, such as their polarization modes, propagation velocity and the mass of graviton, can be compared to those of alternative theories.