freyar
Extradimensional Explorer
I was looking more into the amount of energy that is present in the universe as gravity waves, and found this:
http://www.tapir.caltech.edu/~teviet/Waves/index.html
In particular:
http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html
Relic background: A stochastic signal from the Big Bang itself, this consists of quantum fluctuations in the initial explosion that have been amplified by the early expansion of the Universe. While the spectral shape of this source can be predicted, its overall strength is highly uncertain, but is constrained by the fact that gravitational wave perturbations are one of several components contributing to the observed temperature fluctuations in the cosmic microwave background. This limits the maximum strength of gravitational waves at cosmological length scales. Two curves are shown: one at the upper limit of the observational constraints, and another an order of magnitude weaker.
Those TAPIR pages at Caltech are really great. Anyway, I just wanted to mention that relic background. There's a relic background of neutrinos (not yet observed), gravitational waves (not yet observed), and light (observed very precisely all the time). The gravitational wave background may eventually be detected by looking at pulsars, which are neutron stars that rotate and pulse with a well-determined period that would be affected by gravitational waves. The other way the gravitational wave background could be detected is through its influence on the polarization of the relic background light. If you remember a big fuss (from the BICEP2 collaboration) about two years ago, that was because one experiment thought they had detected this effect. However, another experiment showed that they just measured dust. Anyway, that's something LIGO can't measure but would be another very interesting.