Could The Universe Be Inside A Black Hole?

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**Matt O'Dowd** (0:58)
What is inside a black hole? Inevitable crushing doom? Gateways to other universes? Weird multi-dimensional libraries? If you've ever wanted to know, then you might be in luck. Some physicists have argued that you are inside a black hole right now.
Einstein's ridiculously successful general theory of relativity has never failed. But we know it must fail. At the centers of black holes and at the Big Bang, both of these involve matter being packed to infinite densities. They are singularities where the mathematics of GR breaks down. Well, it turns out that black holes and the Big Bang have more in common than vexing Einstein. They have mathematical similarities to each other that have led some physicists to believe that the Big Bang is in fact the singularity of an absurdly gigantic black hole. Let's figure out how crazy this proposition really is.
First up, black holes. Collapse any chunk of matter far enough and it gets stuck in its own gravity. In general relativity, that looks like a point of infinite density surrounded by an event horizon. We can think of the event horizon as the surface where the flow of space is like a river moving at the speed of light. Nothing can swim against that flow. From the outside, the black hole looks like an orb of blackness at a particular location in space due to the fact that nothing, not even light, can travel back out from below the event horizon. The universe also has a singularity and an event horizon. The singularity is the Big Bang, which we think of as a point in time at the beginning of the universe when all matter was compressed to infinite density and all points in space overlapped.
From the Big Bang, space expanded and it's still expanding. That expansion gives us our event horizon. If space is expanding evenly everywhere, then there are distant regions of the universe that are being propelled away from us faster than the speed of light. That means there's a surface at a particular distance that represents the limit of our observations in that we can never know about any event that happens beyond that surface.
This is the cosmological event horizon. Now it's somewhat more complicated. For example, we're still receiving light from objects that are now beyond that horizon because it was emitted by those objects before they crossed the horizon. Also the accelerating expansion of the universe means that the cosmological event horizon is closer to us than the spot where the recession equals the speed of light. But the effects are the same. No events that happen now beyond that horizon can ever be seen. Complications aside, there are striking similarities between the black hole and the universe. The obvious difference is that the black hole singularity seems to us to be a point of infinite density in space, while the big bang singularity is a time of infinite density that included all space.
Actually that difference isn't as different as it sounds. Both the big bang and the black hole singularities do occupy all space. The difference is that the big bang singularity exists in the past for all of space, while the black hole singularity exists in the future for all of the black hole space. Okay, now that last part probably needs a little more explanation. We've talked about an idea called geodesic incompleteness in a couple of episodes. In our recent one on the center of the universe. And also when we asked what happened before the big bang. We're going to need that idea again, so here's a refresher. In general relativity, objects that are not being acted on by a force follow something called a geodesic. These are just the straightest paths that can be taken through a curved spacetime.