The Bulk Theory or String Theory
Evgeny Buchbinde originated bulk theory. It seeks to tackle the question, "What happened before the big bang, and how did time originate?" The string theory points to seven hidden dimensions beyond the three we experience. Scientists Paul Steinhardt and Neil Turok expanded on string theory in 1999. They proposed that the universe was composed of "branes" (membranes). These branes are three-dimensional worlds, which exist in a higher-dimensional space. Our universe is just one 3-D brane moving through a larger 4-D background called "the bulk." If these branes can move, they can collide, unleashing a torrent of energies, similar to the Big Bang. This led to the search for cycles in the universe. A collision of branes would create the kind of universe we live in, with galaxies, planets, and stars. The space within the brane expands creating more bulk in which other brane collisions occur. This cyclical theory suggests that the universe is constantly creating and recreating itself.
Time's Arrow Theory
The bulk theory fails to explain the concept of time. In 2004, Sean Carrol explained time's one-way progression. He pointed to the concepts of entropy and equilibrium. Entropy describes a state of equilibrium (the even distribution of matter). Low entropy means far from equilibrium, while high entropy means close to equilibrium. Carrol suggests that our universe began with low entropy, allowing our universe to expand to equilibrium. A universe created with high entropy couldn't expand and evolve, thus the inert environment wouldn't support the birth of new stars or life. The fact that our universe started at low entropy is why we experience time as a straight arrow.
Multiverse
Carrol uses "time's arrow theory" to suggests a multiverse concept derived from an aspect of the Big Bang called inflation. When our universe was born, it went through an accelerated period of expansion or "inflation." This inflation blew-up a small corner of another universe. Carrol suggests through "eternal inflation" that this process of inflation is happening continually. This process works in both directions, creating inverse arrows of time.
Now Theory
Physicist Julian Barbour suggests something completely different - that there is no need to worry about what happened before the Big Bang - time did not exist prior to the event. Barbour argues that time does not exist. It is intangible. To Barbour, change creates the illusion of time. And inversely, the concept of time exists to measure change. Instead, Barbour subscribes to the idea of "nows." No matter how the nows are arranged, they are complete and independent of each other. In this theory, everything exists simultaneously. It is just the arrangement of our perspective on the "nows" that changes.