Reacción a "Reactions: evidence found of a cosmic background of gravitational waves possibly produced by supermassive black hole collisions"

This is undoubtedly a very important finding in the field of gravitational-wave astronomy. In 2015 the LIGO and Virgo collaboration initiated this field of research with the first observation of the gravitational signal from the collision of two black holes, GW150914. Since then, LIGO and Virgo have observed around a hundred signals, all of them associated with collisions of binary systems consisting of two black holes, two neutron stars, or mixed binary systems consisting of a black hole and a neutron star.  

Now, results from these new experiments, collectively called Pulsar Timing Arrays (PTAs), have uncovered very clear evidence for the existence of a cosmic background of gravitational waves produced by black hole collisions.  

What is the difference between what LIGO/Virgo observe and what PTA observe, if it appears that in both cases they are collisions of black hole binaries? There are several differences:  

1. The first difference is that the LIGO/Virgo black holes are of stellar origin, with masses typically less than 100 times the mass of the Sun. Being of stellar origin means that they typically form at the end of the lives of very massive stars, when they gravitationally implode as they run out of fuel. However, PTA black holes are supermassive (and not of stellar origin), with masses typically in the range of hundreds of thousands of Suns and billions of Suns. These supermassive black holes must form by aggregation of other, less massive black holes. 
2. The second difference is that the frequency of the gravitational waves detected by LIGO/Virgo and by the PTAs are very different (due to the huge disparity in the mass of the sources involved in each case). In the case of LIGO/Virgo, the detected signals have frequencies (at the time of the collision) around tens of Hz or a few kHz. In the case of the PTAs, the detected signals have frequencies in the micro Hz range and even lower, down to nano Hz (hence one of the experiments is called nanoGrav). Thus, the PTA observations open the low-frequency window in the gravitational spectrum.   
3. The third difference is that, just as LIGO/Virgo detect signals emitted by individual collisions (for example, GW150914 is the signal produced by two black holes and GW170817 is the signal produced by two neutron stars), PTAs detect the "sum" of multiple individual signals, technically known as the "stochastic background of gravitational radiation". The adjective "stochastic" indicates the statistically random character of the individual signals that give rise to the total background. That is, what happens is that every collision of two supermassive black holes that has occurred in the history of our universe has produced a gravitational signal. When the signals from all the collisions are combined (added together), that produces a background. That's what the PTAs have detected, and it's a really phenomenal finding.