The latest news says that scientists detected gravitational waves from the Big Bang. My question is how do they know the waves originated in the big bang verses any number of supernovae and or spinning black holes?
After the Universe became transparent to electromagnetic waves, the first wave produced from whole mass lump was Cosmic Microwave Background (CMB) radiation which we can still receive from farthest point of the observable universe (CMB causes your radio to hiss on blank stations). Universe has expanded since then, so CMB is still running trapped inside Spacetime. CMB is also called Afterglow of Big Bang.
By analyzing CMB, we can visualize the Universe of past. That was the time when astronomical bodies started to take shape due to Gravity.
But, the problem was: CMB seemed uniform throughout the universe. It didn't contain any non-uniformity which is essentially Gravity signature of that time. Without non-uniformity, the universe we know today wouldn't exist.
The recent discovery found this non-uniformity in CMB with high accuracy.
Yes, the CMB came from electromagnetic energy that started traveling (more or less) freely after the decoupling time (when the universe became transparent to electromagnetic waves, which is when photons and charged particles decoupled, which is when protons and electrons combined to form hydrogen, and the charged plasma that existed before that absorbed and interacted strongly with photons became rare enough) at 380,000 years (more or less) after the Big Bang.
But of course that is NOT gravitational radiation, it's your grandfather's electromagnetic radiation.
There might have been black holes that were formed in the early time of the universe, but we can't see electromagnetic radiation before that. Gravitational waves from that epoch were expected to be emitted, from the matter and radiation turbulence around and all kinds of very strong gravitational effects. We cannot detect them now because they have much longer wavelengths than what we can see now. We need interferometers that are thousands or millions of kilometers to see those. There is a plan, that depending on funding, will build a 2 or 3 satellite system in space separated by a million kilometers, that should be able to see, through gravitational waves which are not blocked by the wall that blocks photons at 380,000 years after the Big Bang (gravitational waves penetrate that), gravitational events much bigger than we can see from earth-sized interferometers.