Besides #GravitationalWaves from pairs of black holes & neutron stars, @LIGO and @ego_virgo could detect the combined rumble of the many sources that are too far away (hence too faint) for us to distinguish individually. We call this a “stochastic background” (1/N)
This stochastic background of gravitational waves is almost like the pitter-patter of rain on a tin roof, or the drumming of popcorn: many individual short sounds adding up to a noisy whole (2/N)
As a more scientific analogy, this is also similar to the cosmic microwave background (CMB): noisy radiation produced in the early universe that reaches from all directions. Except instead of light, the stochastic background we look for is made of gravitational waves! (3/N) 
We search for stochastic GWs in LIGO/Virgo data by looking for “coherent” noise across detectors, i.e. a signal that shares many properties with instrumental noise, except it looks exactly the “same” in all detectors (4/N)
Although we have not detected evidence of the stochastic background yet, we can limit its magnitude as a function of frequency, e.g. journals.aps.org/prl/abstract/1…
Detecting a signal like this would be extremely important: it would not only allow us to better understand populations of black holes, but also explore exotic physics like cosmic strings and test Einstein’s theory gravity! link.aps.org/doi/10.1103/Ph… (6/N)
BUT you have to be careful! when we search for stochastic signals, we make *many* assumptions about the properties of gravitational waves (and space time in general) that need not be true if Einstein was wrong... this means it’s not that easy to interpret our measurements (7/N)
But fret not! If you’re trying to probe the nature of gravity with stochastic waves, @duetosymmetry and I have you covered: we review assumptions that go into LIGO searches (so you don’t have to!) and show how to correctly derive constraints w examples journals.aps.org/prd/abstract/1…
The bottom line is: tread carefully! Especially if you want to talk about the “energy carried by a gravitational wave, a notion that is v theory dependent (9/N)
End of plug! If you want to learn more about stochastic gravitational waves check out our nontechnical summary ligo.org/science/Public… OR, if so inclined, this technical review link.springer.com/article/10.100… (10/10)
