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Microwave radiation traveling across the universe appears mostly uniform — as shown in the larger sphere — except for tiny temperature variations (red and blue) that indicate the locations of the seeds of future galaxies and other structures. Some cosmologists now say that if, in the formation of the early universe, another “bubble” universe — the smaller sphere — collided with our own, the collision might have left its mark on the cosmic microwave background radiation.
Credit: A. Aguirre