Gravitational waves are one 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.

                                   

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. The 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.

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Astrophysical sources of gravitational waves

Gravitational waves are created by a wide range of phenomena, each of which can teach us something interesting about the Universe:

• Galactic compact binaries: Compact binaries are made up of at least one white dwarf or neutron star which orbits close to its companion. Such sources are so common in the Galaxy that they may begin to form a background of noise. Observations of these sources will help us to understand stellar population models and the evolution of stars. Some known Galactic binaries, called verification binaries, should be detectable within a few hours of operation of a space-borne detector, and will allow us to test that it is working. The binary systems slowly inspiral as gravitational waves carry away energy and momentum. Eventually the two objects merge. Neutron star-neutron star mergers are a potential candidate for short gamma rays bursts, one of the most energetic processes in the Universe.
                             
  An artist's impression of a gamma-ray burst. Credit: NASA.
• Black hole mergers: One of the ways that galaxies evolve is through mergers. There is evidence to suggest that a supermassive black hole (SMBH), a black hole with a mass of over a million times the mass of the Sun, lurks at the centre of most galaxies. When two galaxies merge, the SMBHs in their centres can also spiral in together. The gravitational radiation emitted when they collide will be some of the loudest events in the Universe. More energy is emitted as gravitational radiation from one SMBH merger than as light from all the stars in the visible Universe. Measuring these mergers would tell us many interesting things about the properties of black holes, allowing us to test our understanding of GR, as well as informing our understanding of galaxy evolution.
• Extreme-mass-ratio inspirals: In the core of galaxies, compact objects such as white dwarfs, neutron stars or black holes, may travel towards the SMBH at the centre of as a consequence of scattering form other objects. If they get close enough, they will start to inspiral as their orbits shrink due to the loss of energy and angular momentum carried away by gravitational waves. These are known as extreme-mass-ratio inspirals (EMRIs) on account of the huge difference in mass between the SMBH and the orbitting compact object. The inspiral is slow, meaning that we can observe gravitational waves emitted over hundreds of thousand of orbits. This allows us to build up an immensely detailed picture of the spacetime of the SMBH. These events would allow us to do fundamental physics by probing precisely the strong gravitational field about the SMBH, and, should we observe enough, we will be able to learn more about the stellar systems in the centre of galaxies.
                               
  An artist's impression of the spacetime of an extreme-mass-ratio inspiral: a smaller black hole orbits about a supermassive black hole. Credit: NASA.
• The Big Bang: When the Universe was very young it underwent a period of very rapid expansion. Tiny fluctuations in spacetime would have been greatly stretched during this period and could still exist today as a background of gravitational waves. This could be detected by studying the polarisation patterns in the cosmic microwave background (CMB). With current instruments it is unlikely, but not impossible that we shall be able to measure the background. However, a positive detection would allow us to better understand the mechanism that drove early inflation of the Universe and probe extremely high energy physics. The gravitational wave background would allow us to see right back to the Big Bang, much further than we can see using EM radiation.

                                 
  A simulation of the cosmic microwave background as measured by Planck. Credit:  ESA

• Phase transitions: As the Universe evolves from its early state it goes through a number of phase transitions which can be associated with symmetry breaking or decoupling of forces. These transitions can create lead to several different types of gravitational radiation. As an analogy, imagine cooling water so that it begins to form ice. This is a phase transition too. Ice begins to form as small crystals that grow outwards. The same can happen in the Universe, small pockets undergo the transition and these expand out as a bubble. For certain types of transitions, gravitational waves would be emitted when bubbles collide. In other cases, topological defects are created following the transition. The analogy would be when two crystals of ice grow together, but their structures are not quite aligned, so that that there is a clear boundary, a defect or domain wall. For spacetime, two examples of topological defects are cosmic strings and domain walls; the former are 1D strings of cosmic length, whereas domain walls are 2D. Such defects are expected to be rare as they have been diluted in space by the expansion of the Universe. However, they have a unique gravitational wave signal, which should make them easy to identify. Such a detection would be an exciting discovery of exotic physics.
                                 
  A numerical simulation of the network of cosmic strings. When they intersect, they chop of small loops which decay by emitting gravitational waves. Credit: Allen & Shellard (1990).

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Detection of gravitational waves

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.

• Extremely low frequency (ELF) regime: The lowest frequency gravitational radiation has wavelengths of order of the size of the Universe. It can be probed by instruments such as the Planck satellite which looks at polarisation of the CMB. Gravitational radiation is encoded in the B-mode polarization. The Planck collaboration will announce its first measurements in 2013.
• Very low frequency (VLF) regime: Gravitation radiation with periods of order of a few years can be measured with patient and precise observations of a network of millisecond pulsars. Pulsar timing arrays (PTAs) make use of the extremely precise regularity of pulsar signals. Periodic variances in the arrival times of pulsar signals indicate that spacetime between the Earth and the pulsars are is being distorted be the passage of a gravitational wave. Using many pulsars together allows us to pin down the signal. There are several groups monitoring pulsars currently: the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA) in Australia. These collaborate together as the International Pulsar Timing Array (IPTA). Since we are looking for gravitational waves with periods of years, it requires years of observations to detect them. It is hoped that this will soon be the case. Our ability to observe pulsars will be greatly improved following the completion of the Square Kilometre Array (SKA), the world's largest radio telescope.
                  
  An artist's impression of an array pulsars emitting radiation, which, combined with the rotation of the pulsars, produces the characteristic lighthouse effect. The background of gravitational waves distorts the spacetime between pulsar and Earth, which can be registered as a change to the expected pulse arrival-times. Credit: David Champion.
• Low frequency (LF) regime: Many of the most interesting sources could be observed using a space-borne detector. The favoured design for such a detector is the Laser Interferometer Space Antenna (LISA). No such mission is currently funded, and so the exact configuration is still to be decided. The technology necessary to make such precision measurements in space shall be demonstrated by LISA Pathfinder, due for launch in 2014. After this, it is hoped that a full LISA mission will be soon to follow.
• High frequency (HF) regime: Ground based detectors may provide the first direct detection of gravitational waves. There is a global network of detectors currently being developed. The Laser Interferometer Gravitational-Wave Observatory (LIGO) is made up of several detectors, one in Livingstone, Louisiana, and two in Hanford, Washington. Each detcotr has two 4 km long arms. The detectors are currently being upgraded to Advanced LIGO, which should have the necessary sensitivity to make the first detections. This is due to begin operations in 2014. There are also plans to place one of the Hanford detectors at a site in India (LIGO-India); having a southern hemisphere site would greatly improve our ability to locate the source of gravitational waves. Collaborating with LIGO is the French-Italian Virgo, located near Pisa at the European Gravitational Observatory (EGO). Virgo is slightly smaller than the LIGO detectors with only 3 km long arms. Also in Europe is the German-British GEO600. This is a much smaller detector and is mostly used for developing new technologies for the bigger detectors. Japan is also building a detector underground in the Kamioka mine known as KAGRA. The detector will pioneer the use of cryogenic cooling, amongst other technologies, to enhance its performance, and was formerly known as the Large Scale Cryogenic Gravitational Wave Telescope (LCGT). It should be operational in 2018.

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