Question: Why can gravitational waves be of interest to cosmologists?
About the article "Gravitational wave", section "Introduction".

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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.  
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.
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. 
LIGO's mission is to directly observe gravitational waves of cosmic origin.
These waves were first predicted by Einstein's general theory of relativity in 1916, when the technology necessary for their detection did not yet exist.
Their existence was indirectly confirmed when observations of the binary pulsar PSR 1913+16 in 1974 showed an orbital decay which matched Einstein's predictions of energy loss by gravitational radiation.
The Nobel Prize in Physics 1993 was awarded to Hulse and Taylor for this discovery.
Direct detection of gravitational waves has long been sought.
Their discovery would launch a new branch of astronomy to complement electromagnetic telescopes and neutrino observatories.
Joseph Weber pioneered the effort to detect gravitational waves in the 1960s through his work on resonant mass bar detectors.
Bar detectors continue to be used at six sites worldwide.
By the 1970s, scientists including Rainer Weiss realized the applicability of laser interferometry to gravitational wave measurements.
Robert Forward operated an interferometric detector at Hughes in the early 1970s.
In fact as early as the 1960s, and perhaps before that, there were papers published on wave resonance of light and gravitational waves (V.B.
Braginsky, L.P. Grishchuck, A.G. Doroshkevieh, M.B.
Mensky, I.D.
Novikov, M.V.
Sazhin and Y.B.
Based on this phenomenon work was published in 1971 on methods to exploit this resonance for the detection of high-frequency gravitational waves.
In 1962, M.E.
Gertsenshtein and V.I.
Pustovoit published the very first paper describing the principles for using interferometers for the detection of very long wavelength gravitational waves, "On the detection of low frequency gravitational waves," M.E.
Gertsenshtein and V.I.
Pustovoit, JETP Vol.43, p. 605-607 (August 1962).
The authors argued that by using interferometers the sensitivity can be 107-1010 times better than by using electromechanical experiments.
Later, in 1965, Braginsky, extensively discussed gravitational-wave sources and their possible detection.
He pointed out the 1962 paper and mentioned the possibility of detecting gravitational waves if the interferometric technology and measuring techniques improved.
In August 2002, LIGO began its search for cosmic gravitational waves.
Measurable emissions of gravitational waves are expected from binary systems (collisions and coalescences of neutron stars or black holes), supernova of massive stars (which form neutron stars and black holes), accreting neutron stars, rotations of neutron stars with deformed crusts, and the remnants of gravitational radiation created by the birth of the universe.
The observatory may in theory also observe more exotic currently hypothetical phenomena, such as gravitational waves caused by oscillating cosmic strings or colliding domain walls.
Since the early 1990s, physicists have believed that technology has evolved to the point where detection of gravitational waves—of significant astrophysical interest—is now possible. 
B-mode polarization is a polarization signal in the cosmic microwave background radiation.
The pattern of polarization in the cosmic microwave background can be broken into two components.
One, a curl-free, gradient-only component, the E-mode (named in analogy to electrostatic fields), was first seen in 2002 by the Degree Angular Scale Interferometer (DASI).
The second component is divergence-free, curl only, and is known as the B-mode (named in analogy to magnetic fields).
Cosmologists predict two types of B-modes, the first generated during cosmic inflation shortly after the big bang, and the second generated by gravitational lensing at later times.BICEP2 announced in March 2014 that they had detected the first type of B-modes, consistent with inflation and gravitational waves in the early universe at the level of The second type of B-modes was discovered earlier using National Science Foundation's South Pole Telescope with help from the Herschel Space Observatory.
This discovery may help test theories on the origin of the universe.
Scientists are using data from the Planck mission, a collaboration between the European Space Agency and NASA JPL, to gain a better understanding of these waves.