Physicists posit that at first our universe was infinitesimally small and infinitely hot and dense.
Topic
Starting conditions of the universe
Framework
Old/New
Main Point
Carroll and Chen are positing that the origins of our universe was a fluctuation of energy in cold, empty space, within a vast low entropy multiverse.
Summary
The first two paragraphs introduce some background information on the Big Bang inflationary period at the beginning of our universe, the view of C&C that this type of event isn't unique, and the emerging picture of the multiverse.
The third paragraph describes how C&C got to their view, via work on entropy, and then it explains what entropy is and provides an example.
The fourth paragraph brings together the two different threads: because of what we know about entropy, what we believe about the Big Bang seems like quite a mystery. The starting conditions for the Big Bang seem like extremely unlikely starting conditions. C&C's innovation (New) is saying that the most common starting condition would be cold, empty space.
The fifth paragraph, brings in the Recent (New) research of two other people, G&V, who are able to potentially explain how cold/empty space could give way to a fluctuation as big as the Big Bang.
The last paragraph ties it together: C&C think that our universe could have emerged from an energy fluctuation in cold, empty space, occurring in a high entropy multiverse, and they think this phenomenon would likely have occurred more than once.