Our universe might be really, really big — but finite. Or it might be infinitely big.

Both cases, says physicist Brian Greene, are possibilities, but if the latter is true, so is another posit: There are only so many ways matter can arrange itself within that infinite universe. Eventually, matter has to repeat itself and arrange itself in similar ways. So if the universe is infinitely large, it is also home to infinite parallel universes.

Does that sound confusing? Try this:

Think of the universe like a deck of cards.

"Now, if you shuffle that deck, there's just so many orderings that can happen," Greene says. "If you shuffle that deck enough times, the orders will have to repeat. Similarly, with an infinite universe and only a finite number of complexions of matter, the way in which matter arranges itself has to repeat."

Greene, the author of The Elegant Universe and The Fabric of the Cosmos, tackles the existence of multiple universes in his latest book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos.

Recent discoveries in physics and astronomy, he says, point to the idea that our universe may be one of many universes populating a grander multiverse.

"You almost can't avoid having some version of the multiverse in your studies if you push deeply enough in the mathematical descriptions of the physical universe," he says. "There are many of us thinking of one version of parallel universe theory or another. If it's all a lot of nonsense, then it's a lot of wasted effort going into this far-out idea. But if this idea is correct, it is a fantastic upheaval in our understanding."

How Quantum Mechanics And General Relativity Play A Part

Greene thinks the key to understanding these multiverses comes from string theory, the area of physics he has studied for the past 25 years.

In a nutshell, string theory attempts to reconcile a mathematical conflict between two already accepted ideas in physics: quantum mechanics and the theory of relativity.

"Einstein's theory of relativity does a fantastic job for explaining big things," Greene says. "Quantum mechanics is fantastic for the other end of the spectrum — for small things. The big problem is that each theory is great for each realm, but when they confront each other, they are ferocious antagonists, and the mathematics falls apart."

String theory smooths out the mathematical inconsistencies that currently exist between quantum mechanics and the theory of relativity. It posits that the entire universe can be explained in terms of really, really small strings that vibrate in 10 or 11 dimensions — meaning dimensions we can't see. If it exists, it could explain literally everything in the universe — from subatomic particles to the laws of speed and gravity.

So what does this have to do with the possibility that a multiverse exists?

"There are a couple of multiverses that come out of our study of string theory," Greene says. "Within string theory, the strings that we're talking about are not the only entities that this theory allows. It also allows objects that look like large flying carpets, or membranes, which are two dimensional surfaces. And what that means, within string theory, is that we may be living on one of those gigantic surfaces, and there can be other surfaces floating out there in space."

That theory, he says, might be testable in the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research.

"If we are living on one of these giant membranes, then the following can happen: When you slam particles together — which is what happens at the LHC — some debris from those collisions can be ejected off of our membrane and be ejected into the greater cosmos in which our membrane floats," he says. "If that happens, that debris will take away some energy. So if we measure the amount of energy just before the protons collide and compare it with the amount of energy just after they collide, if there's a little less after — and it's less in just the right way — it would indicate that some had flown off, indicating that this membrane picture is correct."

Greene explains that when he began studying string theory and parallel universes, it wasn't because he could one day measure energy at CERN or develop new mathematical equations. He simply liked the idea, he says, of studying something on such a large scale.

"We're trying to talk about not just the universe but perhaps other universes — but all within a logical framework that allows us to make some definitive statements," he says. "To me, that's enormously exciting, to step outside the everyday and really look at the universe, within these mathematical terms, on its grandest scales."

Brian Green: The Hidden Reality