Question: How can gravitational waves be described?
About the article "Gravitational wave", section "Sources of gravitational waves".

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In physics, a wave is a disturbance or oscillation that travels through space and matter, accompanied by a transfer of energy.
Wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass transport.
They consist, instead, of oscillations or vibrations around almost fixed locations.
Waves are described by a wave equation which sets out how the disturbance proceeds over time.
The mathematical form of this equation varies depending on the type of wave.
There are two main types of waves.
Mechanical waves propagate through a medium, and the substance of this medium is deformed.
The deformation reverses itself owing to restoring forces resulting from its deformation.
For example, sound waves propagate via air molecules colliding with their neighbors.
When air molecules collide, they also bounce away from each other (a restoring force).
This keeps the molecules from continuing to travel in the direction of the wave.
The second main type of wave, electromagnetic waves, do not require a medium.
Instead, they consist of periodic oscillations of electrical and magnetic fields generated by charged particles, and can therefore travel through a vacuum.
These types of waves vary in wavelength, and include radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.
Further, the behavior of particles in quantum mechanics are described by waves, and researchers believe that gravitational waves also travel through space, although gravitational waves have never been directly detected.
A wave can be transverse or longitudinal depending on the direction of its oscillation.
Transverse waves occur when a disturbance creates oscillations perpendicular (at right angles) to the propagation (the direction of energy transfer).
Longitudinal waves occur when the oscillations are parallel to the direction of propagation.
While mechanical waves can be both transverse and longitudinal, all electromagnetic waves are transverse. 
In physics, gravitational waves are ripples in the curvature of spacetime that propagate as a wave, travelling outward from the source.
Predicted in 1916 by Albert Einstein to exist on the basis of his theory of general relativity, gravitational waves theoretically transport energy as gravitational radiation.
Sources of detectable gravitational waves could possibly include binary star systems composed of white dwarfs, neutron stars, or black holes.
The existence of gravitational waves is a possible consequence of the Lorentz invariance of general relativity since it brings the concept of a limiting speed of propagation of the physical interactions with it.
Gravitational waves cannot exist in the Newtonian theory of gravitation, in which physical interactions propagate at infinite speed.
Although gravitational radiation has not been "directly" detected, there is "indirect" evidence for its existence.
For example, the 1993 Nobel Prize in Physics was awarded for measurements of the Hulse–Taylor binary system that suggests gravitational waves are more than mathematical anomalies.
Various gravitational wave detectors exist and on 17 March 2014, astronomers at the Harvard–Smithsonian Center for Astrophysics announced that they had detected and produced "the first direct image of gravitational waves across the primordial sky" within the cosmic microwave background, providing strong evidence for inflation and the Big Bang.
Peer review will be needed before there can be any scientific consensus about these new findings. 
A possible target of gravitational wave detection experiments is a stochastic background of gravitational waves.
This background is known as the gravitational wave background or the stochastic background.
The detection of such a background would have a profound impact on early universe cosmology and on high-energy physics, opening up a new window and exploring very early times in the evolution of the universe, and correspondingly high energies, that will never be accessible by other means.
The emission of gravitational waves from a large number of unresolved astrophysical sources can create a stochastic background of gravitational waves.
For instance, sufficiently massive stars, at the final stage of their evolution, collapse to form a black hole or a neutron star.
In this explosive supernova event gravitational waves are liberated.
Also, in rapidly rotating neutron stars there is a whole class of instabilities driven by the emission of gravitational waves.
A stochastic gravitational wave background is also of theoretical interest.
Given its stochastic nature, a coincidence of gravitational waves at a particular point could create stress-energy densities sufficient to produce an event horizon.
This would produce a relativistic explanation of nonlocal effects.
Efforts to detect the gravitational wave background is ongoing.
The first claimed detection of a gravitational wave background was reported on March 17, 2014.  
During the past century, astronomy has been revolutionized by the use of new methods for observing the universe.
Astronomical observations were originally made using visible light.
Galileo Galilei pioneered the use of telescopes to enhance these observations.
However, visible light is only a small portion of the electromagnetic spectrum, and not all objects in the distant universe shine strongly in this particular band.
More useful information may be found, for example, in radio wavelengths.
Using radio telescopes, astronomers have found pulsars, quasars, and other extreme objects that push the limits of our understanding of physics.
Observations in the microwave band have opened our eyes to the faint imprints of the Big Bang, a discovery Stephen Hawking called the "greatest discovery of the century, if not all time".
Similar advances in observations using gamma rays, x-rays, ultraviolet light, and infrared light have also brought new insights to astronomy.
As each of these regions of the spectrum has opened, new discoveries have been made that could not have been made otherwise.
Astronomers hope that the same holds true of gravitational waves.
Gravitational waves have two important and unique properties.
First, there is no need for any type of matter to be present nearby in order for the waves to be generated by a binary system of uncharged black holes, which would emit no electromagnetic radiation.
Second, gravitational waves can pass through any intervening matter without being scattered significantly.
Whereas light from distant stars may be blocked out by interstellar dust, for example, gravitational waves will pass through essentially unimpeded.
These two features allow gravitational waves to carry information about astronomical phenomena never before observed by humans.
The sources of gravitational waves described above are in the low-frequency end of the gravitational-wave spectrum (10−7 to 105 Hz).
An astrophysical source at the high-frequency end of the gravitational-wave spectrum (above 105 Hz and probably 1010 Hz) generates relic gravitational waves that are theorized to be faint imprints of the Big Bang like the cosmic microwave background (see gravitational wave background).
At these high frequencies it is potentially possible that the sources may be "man made" that is, gravitational waves generated and detected in the laboratory. 
One of several analogies between weak-field gravity and electromagnetism is that, analogous to electromagnetic waves, there are gravitational waves: ripples in the metric of spacetime that propagate at the speed of light.
The simplest type of such a wave can be visualized by its action on a ring of freely floating particles.
A sine wave propagating through such a ring towards the reader distorts the ring in a characteristic, rhythmic fashion (animated image to the right).
Since Einstein's equations are non-linear, arbitrarily strong gravitational waves do not obey linear superposition, making their description difficult.
However, for weak fields, a linear approximation can be made.
Such linearized gravitational waves are sufficiently accurate to describe the exceedingly weak waves that are expected to arrive here on Earth from far-off cosmic events, which typically result in relative distances increasing and decreasing by formula_8 or less.
Data-analysis methods routinely make use of the fact that these linearized waves can be Fourier decomposed.
Some exact solutions describe gravitational waves without any approximation, e.g., a wave train traveling through empty space or so-called Gowdy universes, varieties of an expanding cosmos filled with gravitational waves.
But for gravitational waves produced in astrophysically relevant situations, such as the merger of two black holes, numerical methods are presently the only way to construct appropriate models.