simply nebulas (gas clouds) within the Milky Way itself. In other words, astronomers believed that the Milky Way constituted the entire universe and that all celestial bodies belonged to it. The cosmos, they thought, was a homogeneous sea of stars (and other formations) that had remained roughly the same since the beginning of time.

Before Hubble’s discoveries, Einstein shared this early perception, believing that the overall distribution of material in space was essentially static. Therefore, when he applied general relativity toward the universe, he was astonished to discover that his result was highly unstable. Like an acrobat teetering on a wire, a slight push in any direction would send his model flying. A bit too much matter and his solution collapsed. A bit too little and it blew up. In either case, the universe seemed a fleeting creation, not a rock of the ages.

Reluctantly, the German physicist felt compelled to supplement his elegant equation with an extra term, known as the cosmological constant or the Greek letter (lambda). This addition served to stabilize his model of the universe by counteracting gravitational attraction with a kind of antigravitational repulsion. It effectively offered a balancing pole to the teetering acrobat. Where the anti-gravity came from, Einstein couldn’t say. Finding it a bit crazy, he informed his friend Paul Ehrenfest that he had “committed something in the theory of gravitation that threatens to get me interned in a lunatic asylum.”

The geometry Einstein had chosen for his model of the universe too was rather unusual. Instead of a stretched-out, speckled sheet, as we often imagine the canopy of the heavens to be, it resembled a polka-dot balloon. Rather than infinite, it was closed and finite. A beam of light heading in any direction would circumnavigate the entire universe and eventually return to its starting place.

Einstein selected a bounded, rather than unlimited, cosmos purely for philosophical reasons. He ardently wanted general relativity to obey Mach’s principle—with the distant stars guiding

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