Gravitational astrophysics

We have known for some time that, like the surface of a busy swimming pool, spacetime is awash with waves generated by the local and distant motions of mass and that, in principle, much of this activity can be reconstructed by analysing the waveforms. However, instrumentation with a reasonable chance of directly detecting these gravitational waves has only become available within the past year, with the LIGO detectors now running at design sensitivity. Here we review the burgeoning field of observational gravitational astrophysics: using gravitational wave detectors as telescopes to help answer a wide range of astrophysical questions from neutron star physics to cosmology. The next generation of ground-based telescopes should be able to make extensive gravitational observations of some of the more energetic events in our local Universe. Looking only slightly further ahead, the space-based LISA observatory will reveal the gravitational Universe in phenomenal detail, supplying high quality data on perhaps thousands of sources, and tackling some of the most fascinating questions in contemporary astronomy. Observational astrophysics is often presented as the ultimate remote sensing problem. There is no possibility of travelling to the majority of targets that attract our attention to interact with them and carry out experiments. Furthermore many of our most closely studied targets have long since entirely ceased to exist, and can be studied only by reconstruction via their radiation fields. The tools at our disposal, though powerful, are currently limited in scope. Indeed, the vast majority of what we know about the Universe comes from a studying a single measure: the second moment (variance) of the electric component of the electromagnetic radiation field. This single statistic has delivered nearly all the imaging, spectroscopy, and multi-wavelength astrophysics that shapes our current view of the Universe. We give it many names, such as apparent magnitude, fringe visibility and photon counts, but essentially we are studying a statistic of the ensemble electric field from many independent charged particles in a remote environment. Astrophysics based on the electromagnetic field will always be so. Although large-scale coherent electromagnetic processes do exist (in for example astrophysical masers and pulsar radio emission) the majority of what we see is the result of the random motions of charged particles in similar environments and the nature of the electromagnetic interaction means that small-scale emission mechanisms dominate. The field itself is also random but can be characterised by its statistics, themselves dependent on the distribution of environmental conditions in the source. It is from these statistics, rather than the field itself, that we learn about the environment. Of course this may seem a rather perverse way of describing observational astrophysics, but it highlights some of the limitations of our current techniques and gives a flavour of what we may be missing. There are already forms of nonelectromagnetic astronomy, notably based on cosmic ray and neutrino particle counts, but the last (as far as we know) truly great challenge in observational astrophysics is to measure the gravitational field from distant sources. This is not a new topic. The first piece of direct gravitational astronomy was carried out in 17978 by Henry Cavendish following his determination of the Gravitational Constant G and subsequent measurement of the mass of the Earth via its gravitational pull. Admittedly the Earth is something of a soft target for gravitational astronomy, but Cavendish’s exquisitely sensitive experiment set the tone for what was to come. Unfortunately, direct measurements of the static gravitational field from distant sources is not a practical proposition, but the detection of gravitational waves, generated by accelerating masses in a manner similar to the way electromagnetic waves are generated by accelerating charges, is practical. What is more, mass all has the same gravitational sign and tends to stick together to generate the large coherent bulk motions that we see in, for example, binary stellar orbits. These motions in turn generate powerful, coherent gravitational waves that directly reflect this bulk motion. Whereas electromagnetic radiation is usually generated