Time, as we understand it, moves from the past to the future irreversibly. But now, an international trio of theoretical physicists is suggesting that there’s more than one future. Two parallel universes were produced by the Big Bang: ours, which moves forward in time (pictured above), and another where time moves backwards. These findings were published in Physical Review Letters in October.

In the 1920s, British astronomer Arthur Eddington coined the term “arrow of time” (sometimes “time’s arrow”), which describes the asymmetrical, one-way direction of time. Many physicists today accept that time moves in the direction of increasing entropy—or disorder, randomness, and even chaos—in an effort to approach some equilibrium among all of the things. According to this thermodynamic arrow of time, things increasingly fall apart. If that’s the case, then our universe must have began in a low-entropy, highly ordered initial state.

But why would there have been this rare moment of low entropy in our past? One century-old idea developed by Austrian physicist Ludwig Boltzmann is that our visible universe is a temporary, low-entropy statistical fluctuation that affects only a small portion of a much larger equilibrium system, APS Viewpoint explains.

Here’s another possible explanation. Oxford’s Julian Barbour, Tim Koslowski from the University of New Brunswick, and Flavio Mercati of the Perimeter Institute for Theoretical Physics are introducing a new arrow of time, one that’s based not on thermodynamics, but on gravity. "Time is a mystery,” Barbour tells the Daily Mail. “Basically, all the known laws of physics look exactly the same whichever way time runs."

To arrive at this “gravitational arrow of time,” they used a simple proxy for our universe, Scientific American explains: a computer simulation of 1,000 particles interacting under the influence of Newtonian gravity. They found that every particle configuration evolves into a state of low-complexity—like a chaotic swarm of bees that settles into a more orderly structure analogous to Boltzmann's low-entropy fluctuation. And from there, the particles expanded outward in two distinct, symmetrical, and opposite arrows of time.

“If you look at a simple model with a swarm of bees in the middle [the Big Bang] but breaking up in either direction, then you would say there are two arrows of time, pointing in opposite directions from the swarm of bees,” he tells Daily Mail. In this diagram, you can see how a unique moment of lowest complexity, or a past from which two futures emerge:

“This two-futures situation would exhibit a single, chaotic past in both directions, meaning that there would be essentially two universes, one on either side of this central state,” Barbour tells Scientific american. “Both sides could sustain observers who would perceive time going in opposite directions. Any intelligent beings there would define their arrow of time as moving away from this central state. They would think we now live in their deepest past.”

Images: NASA via Goddard Media Studios (top), APS/Alan Stonebraker via APS Viewpoint (middle)