For an eternity, our universe lay dormant—a frozen, featureless netherworld. Then, about 15 billion years ago, the cosmos got an abrupt wake-up call.
In the ekpyrotic model, our universe is represented as a three-dimensional membrane embedded in a five-dimensional surface. That “brane” (blue at left) is known as a boundary brane because it lies at one boundary of the fifth dimension.
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