This is almost identical to an unanswered question already asked, but I want to put it a different way. The announced LIGO event was from a localized source. I assume that gravitational waves from the Big Bang would come from "everywhere" like the CMB, so I don't see how LIGO or the next generation observatory (ie. eLISA) would detect this.
One thing else I would like to know, if anyone is aware: I watched an hour long webcast on You Tube and the question I had during the whole time was asked just as the webcast went "off the air". My question is that, since this announcement is about five months after the detection and since it happened "right away", has anything else been detected over the last five months? (Has LIGO been continuously observing? I know that the upgrade is still in progress and sensitivity should get about three times better.)