Energy fluctuations and the origin of our universe

Was the Big Bang the beginning of everything, or the beginning of our part of something much larger? That is the distinction Carroll and Chen want us to consider. The opening describes our universe as initially tiny, hot, and dense. It then expanded rapidly and has continued expanding and cooling. Carroll and Chen accept that event. Their proposal concerns where it fits in the larger picture.

Perhaps our Big Bang wasn't the one and only Big Bang. They suggest that events like it occur repeatedly over an enormous time scale. Our universe would then be one universe within a larger multiverse, which includes everything, even what lies beyond our ability to see. Keep those two levels separate: an event that starts our universe needn't start the whole multiverse. Our “cosmic bubble” is the smaller setting we inhabit.

Why propose this? Carroll and Chen originally wanted to understand why time flows in one direction. The author connects that question to the second law of thermodynamics: entropy, a measure of disorder, naturally increases with time. The useful idea is that there are more ways for a system to be disordered than ordered. Random changes are therefore more likely to move it toward greater disorder.

The room example makes that idea easier to grasp. Imagine a pen that belongs in a drawer. If you toss it randomly into the room, it could end up on the floor, behind furniture, or in any number of other inconvenient places. Putting it back in the drawer restores order; the many scattered locations don't. If objects keep moving randomly, we wouldn't expect the room to become tidier and tidier. There are many more ways to make a mess than to arrange everything neatly.

That creates a puzzle about our beginning. The expansion of our universe and what followed fit the second law. But why did we get such a small, hot, dense universe in the first place? The author identifies that beginning as a low-entropy state—an unusually ordered configuration. Explaining how a system changes after an improbable starting point doesn't yet explain how it got that starting point.

Carroll and Chen propose a more common initial condition resembling cold, empty space. But how does that help? If we start with something cold and empty, we still need to explain how it could produce the hot, dense beginning and rapid expansion of our universe. Replacing one starting description with another isn't a complete solution.

The next paragraph gives us the proposed connection. Even empty space contains faint traces of energy, which fluctuate on a subatomic scale. Garriga and Vilenkin suggest that these fluctuations can generate big bangs in tiny areas, widely separated in space and time. Carroll and Chen take our universe to be one such event within a high-entropy multiverse.

Now distinguish the larger background from the particular event within it. The multiverse can have high entropy while a fluctuation produces the unusual low-entropy beginning of an individual universe. The proposal doesn't require our own initial state to be cold and empty; it uses the larger setting to explain how our hot, dense beginning could arise.

The final paragraph changes the scale on which we consider the improbability. Taken on its own, a beginning like ours looks extremely unlikely. Across a vast multiverse and an immense time scale, there are many opportunities for such fluctuations. An unusual event is less surprising in that larger setting, and it may occur more than once. That's how the account returns to the opening suggestion that our Big Bang was not unique.

We are being shown a theory and its rationale, rather than asked to settle cosmology ourselves. The writer explains the puzzle, the proposed starting condition, and the mechanism connecting it to our universe. Our main idea should keep that explanation intact: our universe may have arisen through an energy fluctuation in a larger multiverse. Simply saying that other universes exist would leave out how this account explains ours.