Towards the first direct detection of Gravitational Waves
Gravitational waves are tiny ripples in spacetime, created by any moving mass (or energy concentration in other words), as predicted by Einstein's General Theory of Relativity. The detection of gravitational waves will signify the start of Gravitational Wave Astronomy, opening a completely new observational window to the Universe, just like the first detection of radio waves from space which gave birth to Radio Astronomy.
Being actually perturbations of spacetime itself, these waves propagate unattenuated throughout the Universe carrying the information of the dynamic events that created them. In this way, we will be able to probe regions of space inaccesible through the various electromagnetic radiation bands which are either distorted or completely blocked due to the intervening matter. Because the amplitude of gravitational waves is extremelly small, the most promising sources are highly relativistic, just like black holes which cannot be seen at all in the electromagnetic spectrum!
The pulsar group at the University of Manchester is playing a crucial role in the global effort for the first direct detection of gravitational waves. In order to achieve this, we are using high precision pulsar timing observations of millisecond pulsars, the long term rotational stability of which challenges the stability of the best atomic clocks ever constructed. By observing an array of such millisecond pulsars spread throughout our Galaxy, we will be able to detect irregularities in their pulses which will have been created by individual gravitational wave sources or a stochastic gravitational wave background. Pulsar Timing Arrays, are sensitive to gravitational waves at the nHz scale, expected to be created by astrophysical sources, such as super massive black hole binaries (merging galaxies), and by cosmological sources such as decaying cosmic string loops, phase transitions in the early Universe and inflation.
Artistic impression of a Pulsar Timing Array
The pulsar group at the University of Manchester is part of the European Pulsar Timing Array (EPTA), where we combine high precision pulsar timing data data from the five largest radio telescopes in Europe (Lovell , Effelsberg in Germany, Nancay in France, WSRT in the Netherlands and SRT in Italy) for gravitational wave detection purposes. Moreover, the pulsar group plays a key role in the Large European Array for Pulsars (LEAP project), the puspose of which is to coherently combine all these five telescopes in order to form a 200-m equivalent dish telescope. Pulsar timing data from the LEAP project are expected to offer a significant boost towards the final goal of the first direct detection of gravitational waves. EPTA is also one of the three members of the International Pulsar Timing Array (IPTA).
The EPTA/LEAP telescopes
There is also theoretical work undertaken within the group, mostly focussed on the possible gravitational wave sources for PTAs and especially cosmic strings. Cosmic strings are exotic networks of one-dimensional topological defects expected to be created during the symmetry breakings that occured in the phase transitions of the early Universe. These networks follow a scaling evolution along the expansion of the Universe, something which is achieved with the creation of cosmic string loops. These loops start to decay immediately after their formation, emitting gravitational waves and creating a stochastic gravitational wave background. Even more interesting, PTAs are likely to probe the emission originating from cosmic strings during the radiation to matter era transition, when the Universe was only ~23.000 years old, much earlier than what we can reach with electromagnetic radiation (~270.000 years)!
The cosmic population of oscillating cosmic string loops decays creating a stochastic gravitational wave background. On the right you can see the contributions of the radiation (red) and matter era (blue) loops. Pulsar Timing arrays typically probe the emission originating from the radiation to matter era transition.