Sean Carroll

or demands explanation that our universe is smooth and homogeneous at early times, or if you think it's weird that our universe is spatially very close to flat, and you have some dynamical mechanism that makes that happen, the universe gets either smoother or flatter over time, then if you go backwards, the opposite happens.

So if you just have spatial curvature, for example...

In inflationary cosmology, where you don't try to make it cyclic, you just say, well, there was some curvature, but I inflate it away.

And that actually makes sense.

But if you inflate curvature away again and again, OK, over the course of many, many cycles, then as you go backwards, the curvature gets bigger and bigger.

And eventually, the curvature gets to be infinity, and you have a singularity.

And you have to explain why things are like that.

You're not really benefiting from the cyclic nature of the universe.

So because this fine-tuning into the distant past seems to be worse in a traditional cyclic model than in just a model with a regular Big Bang, they've never been my favorites.

Now, I have been interested in models where the universe lasts forever but is not cyclic.

Models where the far, far past and the far, far future expand forever in their respective directions.

So opposite directions in time from each other.

But locally, from their point of view, it looks like the universe is always expanding.

And it never repeats.

There is sort of a minimum entropy, minimum size stage.

But that size doesn't need to be small.

That entropy doesn't need to be low.

as long as there is some point of the universe existing and having an entropy generically in the models that i have thought about with jennifer chen and other people the universe expands in either direction of time and gets bigger and bigger once and for all there's no repetition or anything like that okay all of that is sort of traditional cosmological talk by which i mean it's sort of semi-classical

That is to say, we imagine that we can describe spacetime using the classical equations that Einstein gave us in general relativity.

And there might be some quantum fields living on that classical spacetime, but it's basically a mishmash, mishmash,