Awareness of the size and age of the universe is hard-won knowledge that has taxed scientists for the past 2,500 years. To ancient cul­tures, the sky was a proximate canopy that circled overhead, and there was no sense of the vast distance to the stars, let alone the idea that something might lie beyond those pinpoints of light. The ancient Greeks were the first civilization to spawn a class of philosopher-scientists, who applied logic and mathematics to their observations of the sky.

Cosmology has its root in the Greek idea of “cosmos,” or an orderly and harmonious system. In the Greek view, the antitheti­cal concept of “chaos” referred to the initial state of the universe, which was darkness or an abyss.1 Thus, order emerged from disor­der when the universe was born. Pythagoras is believed to be the first to use the term cosmos, and the first to say that the universe was based on mathematics and numbers, although in truth, so lit­tle is known about Pythagoras and his followers that direct attri­bution of these ideas is impossible. Pythagoras is also credited with “harmony of the spheres,” a semi-mystical, semi-mathematical idea that simple numerical relationship or harmonics were mani­fested by celestial bodies, with the overall result having commonal­ity with music. Pythagoreans didn’t think the music of the spheres was literally audible.2

Aristotle’s geocentric cosmology dominated Western thought for nearly two millennia, but Aristarchus developed a heliocentric

cosmology that implied large distances to the stars, so as not to observe a parallax shift from one season to another. Mapping the stars in the third dimension didn’t become possible until parallax was measured in the nineteenth century, giving William Herschel an inkling of the extent of the system of stars that we inhabit. Twin foundational discoveries by Edwin Hubble early in the twentieth century—the distances to the nebulae, or galaxies, and the uni­versal recession velocities of galaxies—set the stage for modern cosmology. By the mid-twentieth century, the universe was known to be billions of years old and billions of light-years in extent.

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