Question: Why would cosmologists interested in gravitational waves?
About the article "Gravitational wave", section "Introduction".

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In Einstein's theory of general relativity, gravity is treated as a phenomenon resulting from the curvature of spacetime.
This curvature is caused by the presence of mass.
Generally, the more mass that is contained within a given volume of space, the greater the curvature of spacetime will be at the boundary of this volume.
As objects with mass move around in spacetime, the curvature changes to reflect the changed locations of those objects.
In certain circumstances, accelerating objects generate changes in this curvature, which propagate outwards at the speed of light in a wave-like manner.
These propagating phenomena are known as gravitational waves.
As a gravitational wave passes a distant observer, that observer will find spacetime distorted by the effects of strain.
Distances between free objects increase and decrease rhythmically as the wave passes, at a frequency corresponding to that of the wave.
This occurs despite such free objects never being subjected to an unbalanced force.
The magnitude of this effect decreases inversely with distance from the source.
Inspiralling binary neutron stars are predicted to be a powerful source of gravitational waves as they coalesce, due to the very large acceleration of their masses as they orbit close to one another.
However, due to the astronomical distances to these sources the effects when measured on Earth are predicted to be very small, having strains of less than 1 part in 1020.
Scientists are attempting to demonstrate the existence of these waves with ever more sensitive detectors.
The current most sensitive measurement is about one part in (as of 2012) provided by the LIGO and VIRGO observatories.
The lack of detection in these observatories provides an upper limit on the frequency of such powerful sources.
A space based observatory, the Laser Interferometer Space Antenna, is currently under development by ESA.
Gravitational waves should penetrate regions of space that electromagnetic waves cannot.
It is hypothesized that they will be able to provide observers on Earth with information about black holes and other exotic objects in the distant Universe.
Such systems cannot be observed with more traditional means such as optical telescopes and radio telescopes.
In particular, gravitational waves could be of interest to cosmologists as they offer a possible way of observing the very early universe.
This is not possible with conventional astronomy, since before recombination the universe was opaque to electromagnetic radiation.
Precise measurements of gravitational waves will also allow scientists to test the general theory of relativity more thoroughly.
In principle, gravitational waves could exist at any frequency.
However, very low frequency waves would be impossible to detect and there is no credible source for detectable waves of very high frequency.
Stephen W. Hawking and Werner Israel list different frequency bands for gravitational waves that could be plausibly detected, ranging from 10−7 Hz up to 1011 Hz.
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.  
Propagating waves.
Gravitational waves.
Gravitational radiation for two orbiting bodies in the low-speed limit. 
The Cosmic catastrophe is a thought experiment in which the sun were to instantaneously disappear.
The question is what would then happen to the earth and the other planets orbiting the sun.
According to Isaac Newton's classical theory of gravity, the planets would immediately cease to move in circular motion, and due to inertia would start traveling in a straight line.
Albert Einstein saw a deficiency in Newton's theory.
Due to the finiteness of the speed of light, it would take a certain amount of time before the darkness from the sun's absence would reach the orbiting planet.
Therefore, why would the planet instantaneously start traveling in a straight line before the information that the sun's disappearance has occurred.
The Cosmic catastrophe thought experiment led Einstein to the invention of the General theory of relativity and the creation of the concept of spacetime.
With the concept of spacetime, Einstein was able to fix the deficiency in Newton's theory.
In Einstein's spacetime model, the disappearance of the sun would create gravitational waves in the spacetime.
The gravitational waves travel at the speed of light, and an orbiting planet would not react to the sun's disappearance until after the gravitational wave has reached it.
Only after the gravitational wave has passed the planet, would the planet start traveling in a straight line. 
Researchers believe that gravitational waves also travel through space, although gravitational waves have never been directly detected.
Not to be confused with gravity waves, gravitational waves are disturbances in the curvature of spacetime, predicted by Einstein's theory of general relativity.