What Happens After the Universe Ends?

**SPEAKER_1** (0:00)
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**Matt O'Dowd** (0:56)
Our universe began in a state of ultimate heat and compression in what we call the Big Bang. And it'll end by expanding forever towards a state of perfect cold and emptiness.
It's incredible that we could figure this much out, but we should not get too cocky. Big questions remain open, like what happened before the Big Bang? What if anything happens after our universe?
I'm about to give you the most outrageous hypothesis so far that may actually be right.
Conformal cyclic cosmology is a story of the origin and the end of our universe from the great mathematical physicist Sir Roger Penrose. It goes like this. The infinitely far future when the universe has expanded exponentially to an unthinkably large size and every black hole and particle has decayed into faint radiation. That infinite stretch of space and time is identically the same thing as the infinitesimal and instantaneous big bang of a new universe. And our universe is just one in an endless chain. We know this is an outrageous proposal because Roger Penrose himself called it that. And as with all outrageous proposals, it's probably wrong. But if there's a faint chance that it's right, it's so bizarre that we should definitely know about it. Okay so, Conformal Cyclic Cosmology, CCC. Here, conformal is for the conformal scaling needed to turn your gigantic end of the universe into a tiny new big bang. A conformal transformation is just some mathematical function that you apply to a geometric space which preserves all of the angles in that space. An example would be if you had a sheet of rubber and drew some lines on it, then expanded the rubber evenly in all directions. The lines would get longer and further apart, but the angles at their intersections would stay the same. This is perhaps the simplest conformal transformation, just multiplying or dividing all dimensions by the same scaling factor. We would say that our universe has conformal invariants under scale changes, so the angles don't change if you change the size smoothly. But other things sure do change. It makes a big difference whether every atom in the universe is right next to each other or a billion light years apart. But let me try to give you a sense of a situation where scale might not matter. A conformal scaling of space time means scaling both space and time. For example, consider a universe that's one light second across, and it exists for the span of a single second. Light has time to travel across it once.
Scale it up by around 30 quintillion times to describe a second universe that's a billion light years across and lasts a billion years. Again, light crosses it once in that time. Let's say these universes contain no matter, only photons, light.

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