What the Redshift Means

Not that Alvy's mother was wrong. The truth is that Brooklyn isn't expanding. But the universe that revolves around it (we're taking a Brooklynite's perspective here) is expanding. Much to the chagrin of New Yorkers, Brooklyn is not somehow exempt from the laws of the universe. This borough—like the earth, like a star, like a galaxy, or like a human being—is held together by its own internal forces. The expansion of the universe that is the echo of the Big Bang is only visible on the largest scales, the distances between clusters and superclusters of galaxies.

Let's return to our picture of the universe as the surface of a rubber balloon. As the balloon is inflated, the distance between any two dots drawn on its surface is increasing. Another way to think of it is that the photons that leave another galaxy are part of the fabric of the universe, and as this fabric is stretched, the photon gets stretched along with it—to a longer wavelength. This type of wavelength increase is called a cosmological redshift, and it is a direct measure of universal expansion.

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