I don't know the answer to this, but looking at some LIGO charts (http://www.ligo.caltech.edu/advLIGO/images/refdes03.gif) they seem to be looking at 10-100Hz (roughly). Are there interesting or even expected sources in the frequency band investigated in this paper?
Gravitational waves are emitted at a wide spectrum of frequencies by different astronomical bodies. LIGO's frequency range is limited mostly by seismic activity at the low end and radiation pressure noise (essentially the momentum imparted by photons hitting mirrors) at the high end. It's about as well as we can do on Earth, currently. Indirect detections via astronomical techniques can avoid the issue of seismic activity disrupting measurements, and so it is possible to look at much lower frequencies. These frequencies, however, correspond to different sources to the ones LIGO can potentially see, so we can learn new information about different parts of our universe from both detection techniques.
Where does a gravity wave theoretically come from? All I can imagine is that they would come from a mass increasing or decreasing in magnitude, and I don't know of any way that happens.
The Guardian article refers to a detector which might have made an indirect detection of gravitational waves.
If two massive bodies such as neutron stars or black holes collide, the energy they lose in the form of gravitational energy is propagated away in waves. These waves are ripples in spacetime, and they are quadropolar in nature. This means that they stretch spacetime in one direction while squeezing it in the other.
Gravitational waves form part of the predictions of Einstein's Theory of General Relativity. They are the last piece of the theory yet to receive a direct detection. A notable indirect detection of gravitational waves is the measurements of the orbital decay of the PSR B1913+16 binary pulsar, for which Hulse and Taylor received the Nobel Prize in 1993.
For the purposes of direct detection of these waves, on Earth we've set up a network of laser interferometers (the major players are Advanced LIGO, Advanced Virgo, GEO-HF and KAGRA, though all but GEO are currently in the process of being commissioned). If we arrange our detectors on Earth at right angles, we become optimally sensitive to the majority of gravitational wave sources. If the masses of the bodies involved in the collision are big enough, the ripples in spacetime will be strong enough to change the time in which it takes light to travel along each arm of the interferometer - in one arm the light will take longer time to travel, and in the other it will take shorter time. If we recombine the light in each arm, we can sense via the interference pattern of the light whether a gravitational wave has passed through the detector. In practice there are loads of other signal sources present in the interferometer, and quantifying and eliminating these sources of noise are the major tasks facing these detectors. With these noise sources accounted for, the first direct detection of gravitational waves might be made in the next few years.
The LISA project referred to in the main article is dead since NASA pulled out funding. The project lives on in the form of the ELISA project, funded by European organisations. This has tentative approval for launch in the next 20 years. This mission is not intended to directly detect the 'first' gravitational wave, but rather to detect them in abundence. Indeed, the problem with this type of detector is dealing with the huge number of potential detections. ELISA is also designed to detect waves in a completely different spectrum from the ground detectors, and from different astronomical sources. By the time ELISA launches it is likely that the network of detectors as part of the LIGO Scientific Community will have made the first detection, here on Earth.
And it should be the law: If you use the word `paradigm' without knowing what the dictionary says it means, you go to jail. No exceptions. -- David Jones