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CHAPTER 2

Endless Doppelgängers

The Quilted Multiverse

If you were to head out into the cosmos, traveling ever farther, would you find that space goes on indefinitely, or that it abruptly ends? Or, perhaps, would you ultimately circle back to your starting point, like Sir Francis Drake when he circumnavigated the earth? Both possibilities—a cosmos that stretches infinitely far, and one that is huge but finite—are compatible with all our observations, and over the past few decades leading researchers have vigorously studied each. But for all that detailed scrutiny, if the universe is infinite there’s a breathtaking conclusion that has received relatively scant attention.

In the far reaches of an infinite cosmos, there’s a galaxy that looks just like the Milky Way, with a solar system that’s the spitting image of ours, with a planet that’s a dead ringer for earth, with a house that’s indistinguishable from yours, inhabited by someone who looks just like you, who is right now reading this very book and imagining you, in a distant galaxy, just reaching the end of this sentence. And there’s not just one such copy. In an infinite universe, there are infinitely many. In some, your doppelgänger is now reading this sentence, along with you. In others, he or she has skipped ahead, or feels in need of a snack and has put the book down. In others still, he or she has, well, a less than felicitous disposition and is someone you’d rather not meet in a dark alley.

And you won’t. These copies would inhabit realms so distant that light traveling since the big bang wouldn’t have had time to cross the spatial expanse that separates us. But even without the capacity to observe these realms, we’ll see that basic physical principles establish that if the cosmos is infinitely large, it is home to infinitely many parallel worlds—some identical to ours, some differing from ours, many bearing no resemblance to our world at all.

En route to these parallel worlds, we must first develop the essential framework of cosmology, the scientific study of the origin and evolution of the cosmos as a whole.

Let’s head in.

The Father of the Big Bang

“Your mathematics is correct, but your physics is abominable.” The 1927 Solvay Conference on Physics was in full swing, and this was Albert Einstein’s reaction when the Belgian Georges Lemaître informed him that the equations of general relativity, which Einstein had published more than a decade earlier, entailed a dramatic rewriting of the story of creation. According to Lemaître’s calculations, the universe began as a tiny speck of astounding density, a “primeval atom” as he would come to call it, which swelled over the vastness of time to become the observable cosmos.

Lemaître cut an unusual figure among the dozens of renowned physicists, in addition to Einstein, who had descended on the Hotel Metropole in Brussels for a week of intense debate on quantum theory. By 1923, he had not only completed his work for a doctorate, but he’d also finished his studies at the Saint-Rombaut seminary and been ordained a Jesuit priest. During a break in the conference, Lemaître, clerical collar in place, approached the man whose equations, he believed, were the basis for a new scientific theory of cosmic origin. Einstein knew of Lemaître’s theory, having read his paper on the subject some months earlier, and could find no fault with his manipulations of general relativity’s equations. In fact, this was not the first time someone had presented Einstein with this result. In 1921, the Russian mathematician and meteorologist Alexander Friedmann had come upon a variety of solutions to Einstein’s equations in which space would stretch, causing the universe to expand. Einstein balked at those solutions, at first suggesting that Friedmann’s calculations were marred by errors. In this, Einstein was wrong; he later retracted the claim. But Einstein refused to be mathematics’ pawn. He bucked the equations in favor of his intuition about how the cosmos should be, his deep-seated