The end of time in our universe might be the beginning of time in another universe, and vice versa. This is the hypothesis put forth by an international team of astrophysicists in the current issue of Physical Review Letters: that fundamental questions of time, structure, and entropy can be answered if we assume that our universe has a bizarro-universe twin where time runs backward, relative to us—each universe splitting off from the Big Bang in opposite temporal directions, serving not as the beginning of the time but rather as a halfway point between the two universes.
It’s a complicated explanation, but maybe not as complicated as the theory we’re currently using. And the reason for that complexity is entropy.
As Scientific American’s Lee Billings explains:
The thermodynamic arrow of time suggests our observable universe began in an exceptionally special state of high order and low entropy, like a pristine cosmic egg materializing at the beginning of time to be broken and scrambled for all eternity … [Ludwig] Boltzmann, believing the universe to be eternal in accordance with Newton’s laws, thought that eternity could explain a low-entropy origin for time’s arrow. Given enough time—endless time, in fact—anything that can happen will happen, including the emergence of a large region of very low entropy as a statistical fluctuation from an ageless, high-entropy universe in a state of near-equilibrium ...
Today’s cosmologists have a tougher task, because the universe as we now know it isn’t ageless and unmoving: They have to explain the emergence of time’s arrow within a dynamic, relativistic universe that apparently began some 14 billion years ago in the fiery conflagration of the big bang.
The team behind the mirror-universe theory—led by Oxford’s eccentric but brilliant Julian Barbour—has hypothesized that gravity could be the force that ultimately reorganizes the universe into a relatively pristine low-entropy state suitable for a Big Bang. The problem is that gravity would literally have no time to do this unless it already existed in a high-entropy state, which would imply deterioration from another low-entropy state, which would imply another Big Bang, and so on. It’s not impossible that the universe consists of an infinite series of oscillating Big Bangs, but it’s turtles all the way down, and unfalsifiable theories of (literally) infinite complexity don’t tend to make physicists very happy.
So when you consider that alternative, the mirror-universe hypothesis might actually turn out to be the least complex explanation for the Big Bang and the nature of time: two universes bouncing outward into high-entropy states like marbles on a Newton’s Cradle, with a low-entropy Big Bang resting in the center as the low-entropy origin point of both timelines. It’s a pretty wild idea, but we’re dealing with pretty wild data—and just as extraordinary claims require extraordinary evidence, extraordinary evidence may warrant extraordinary claims. Proving those claims is, of course, another matter entirely.