Astronomy homework
Theory: what if we were living in a huge black hole
If there's any quantitative reasoning behind the question, it comes from comparing the amount
of matter within the observable universe to the radius of the observable universe, and noticing
that it looks a lot like the relationship between the mass of a black hole and its Schwarzschild
radius. Slightly more formally, it looks like the universe satisfies the hoop conjecture, so
shouldn't it form a black hole? Blackhole is a region of space from which nothing, including
light, can escape. The implication is that there is a region outside the black hole from which
things could at least imagine escaping to. For the universe, there is no such outside region. So,
at a pretty trivial level, the universe is not a black hole. You might say that this is picking nits,
and the existence of an outside region is beside the point if the inside of our universe resembles
a black hole. That's fine, except it doesn't. You may have noticed that the universe is expanding,
rather than contracting as you might expect the interior of a black hole to be. Our universe
according to conventional general relativity has a singularity in the past, out of which
everything emerged, not a singularity in the future into which everything is crashing. We call
that singularity the Big Bang, but it's very similar to what we would expect from a white hole,
which is just a time-reversed version of a black hole. The spacetime solution to Einstein's
equation that describes a universe expanding from the Big Bang is very similar to the timereversal of a black hole, but you don't learn much from making that statement, especially
because there is no outside; everything you wanted to know was already there in the original
cosmological language. Our universe is not going to collapse to a future singularity, even though
the mass is enough to allow that to happen, simply because it's expanding; the singularity
you're anticipating already happened. Still, some folks will stubbornly insist, there has to be
something deep and interesting about the fact that the radius of the observable universe is
comparable to the Schwarzschild radius of an equally-sized black hole. And there is! It means
the universe is spatially flat.
Done by: Hayder Alrubaye
Grade: 11BA