Expert: Courtney Seligman - 12/25/2009

Question
Hi wondered about your views on the following idea. We have 'calculated' expansion rates of the universe. We have observed black holes and learn so much. Its been suggested that the universe could be in a black hole. Could black holes be studied closer: their different evolutionary stages,how much matter they pull in when young to old,rates of spin at these stages etc. Maybe this could be correlated to factors in our universe. What if these things could be studied closely and it was found a black hole pulls in matter extremely fast when first formed,correlating to an inflation era in our universe. Then a black hole pulls in matter at a steady rate correlating to steady expansion in our universe and so on. Couldnt observations such as these go some way to proving the universe is in a black hole? This would then allow us to take our timeline back,not up to the big bang,but beyond it. Thanks   Richard

Answer
The idea that the Universe is or could become a black hole corresponds to the Closed Universe version of the Big Bang theory, in which the mass of the Universe is greater than a critical amount; but its actual mass is less than a third of the critical mass, and it is not like a black hole, at all.

For one thing, even if the Universe could be thought of as a black hole, it would only be in the sense that anything inside it could never escape. The ability of a "normal" black hole to accrete material from outside would not apply to the Universe, as there is nothing outside it. It exists in its own space time, completely separate from any other existence, and there is no "outside", let alone any connection to something outside, which could "fall" into it.

Also, when black holes gain mass, their "size" increases only slightly, while the expansion of the Universe is doubling its size every ten billion years or so, and will continue to do so, in less and less time, as it continues to increase in size.

Ignoring all that, I suppose that you could propose different rates of mass accumulation for a black hole at different stages in its existence, which would change its Schwarschild radius at different rates. But actual black holes could (and probably do) have extremely variable rates of mass gain, depending upon changing conditions in the regions surrounding them. So only one particular version of your theory would fit any particular history of our Universe, and theories which have a variety of possibilities, but are required to choose only one particular result in order to achieve their purpose, are generally doomed to failure or rejection.

So although your idea is interesting, I don't think it would appeal to any astronomer or cosmologist. Still, it is a clever way of looking at things. Unfortunately, all it proves is that analogies, although useful for explaining things when solid theory backs up the result, are wrong more often than not, when used on their own.