Gravitational waves are one of the greatest predictions of Einstein's theory of general relativity. In general relativity, gravitation is explained through the curvature of spacetime. Massive objects bend spacetime, and the curvature of spacetime tells objects how to move. It is the influence of curved spacetime that we call gravity.
There is an ongoing effort to detect gravitational waves using interferometers on the ground and in space, and through accurate timing of pulsars. First detections are expected within the next few years and these observations have the potential to transform our understanding of astrophysics, cosmology and fundamental physics. Members of the IoA Gravity Group are involved with all aspects of this work on gravitational wave detection, including source modelling, data analysis and exploring the scientific exploitation of the data. To find out more about the group's activities, please click on one of these links.
Group Members - find out who we are.
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Research Activities - find out more about the research activities within the group.
The most common analogy used to describe gravity in the picture proposed by Einstein is to imagine spacetime as a stretched rubber sheet. If we were to roll a table tennis ball across the sheet it would move in a straight line, just like an object would travel in a straight line in the absence of gravity. Now, if we were to put a bowling ball in the middle of the sheet, it would stretch. This is the bending of spacetime due to gravity. If were to roll the table tennis ball across the sheet again, it would now follow a curved path. It is attracted towards the bowling ball because of its gravity.
The Earth orbits the Sun because of the curvature of spacetime. Credit: WGBH Boston.
When massive objects move, the curvature of spacetime must change to follow their new positions. It takes time for spacetime to react, as information can only propagate at the speed of light. There are therefore ripples in spacetime, just like there would be ripples on a pond if you were to disturb its surface. These ripples in spacetime are gravitational waves.
A more familiar wave is electromagnetic (EM) radiation or light. EM waves are oscillations of the electric and magnetic fields, whilst gravitational waves are oscillations of spacetime. EM waves are produced by accelerating charges, whilst gravitational waves are produced by accelerating masses. For gravitational waves we also need an asymmetry in the system to produce radiation, for example we need a binary system and not just a single (non-spinning) object; this can be considered as a consequence of the conservation of momentum.
Gravitational waves offer a unique probe into some of the most extreme systems in the Universe. They originate from merging supermassive black holes; from binary stars orbiting at close to the speed of light, and from the Big Bang itself. The challenge in gravitational wave astronomy is detecting the waves, and then disentangling the signals to extract the information they contain.
Gravitational waves are created by a wide range of phenomena, each of which can teach us something interesting about the Universe:
A simulation of the cosmic microwave background as measured by Planck. Credit: ESA
So far, we only have indirect evidence the existence for gravitational waves. Whilst we have not seen the waves themselves, we have measured the energy and angular momentum they carry away. We have observed a number of binary pulsars. A pulsar is a neutron star, a dead star that has collapsed down to a very dense state, that emits a period signal (it is observed to pulse). These signals are highly regular, in fact pulsars are some of the best clocks in nature, and this allows extremely precise measurements of their motion. Binary pulsars are systems where a pulsar orbits a companion, such as a white dwarf or neutron star (even another pulsar). We are lucky to find such wonderful systems. In 1974 Russell A. Hulse and Joseph H. Taylor made observations of the first known binary pulsar. They found that the orbital period changes with time, and this change is in exact agreement with the prediction from General Relativity for the loss of energy and angular momentum due to gravitational wave emission! Hulse and Taylor won the 1993 Nobel Prize in Physics for this fantastic result.
The orbital decay of the Hulse-Taylor binary pulsar (PSR B1913+16). The points are measured values, while the curve is the theoretical prediction for gravitational waves. Credit: Weisberg & Taylor (2005).
There is a global community of scientists and engineers currently working towards the first direct detection of gravitational waves. To visualise the effect of a gravitational wave passing imagine you have a ring of particles lying in a plane. When the wave passes through the ring it is stretched and squeezed, although the area enclosed remains the same. Detectors work by trying to measure the differences in length across a detector produced as a wave passes. The fractional changes in length are tiny, about one part in 1021, so the measurements are extremely difficult. That is the same as trying to measure the distance from the Earth to the Sun to the accuracy of the size of a hydrogen atom!
The effects of the two gravitational wave polarizations, plus (+) and cross (×), on a ring of particles. The wave would be travelling out of the screen.
Just like electromagnetic radiation, gravitational radiation comes in a range of frequencies. The frequency is set by the scale of the system producing the radiation. Different parts of the spectrum can be observed using different detectors.
The gravitational wave spectrum, sources and detectors. Credit: NASA.