![]() Einstein's theory that nothing can travel faster than the speed of light in a vacuum still holds true, because space itself is stretching, and space is nothing. There is no limit to how fast the universe can expand, says physicist Charles Bennett of Johns Hopkins University. It's just easier to see Hydra's redshift because the farther a galaxy is from our own, the faster it is moving away. Everything is shifting, because the universe is expanding. Hydra isn't the only distant cluster of galaxies that displays a redshift, though. The size of the shift is called the redshift, and it helps scientists figure out the movement of stars in space. The farther the bands shift toward the red end, the farther the light has traveled. The farther the light travels, the more stretched it gets. On their journey across the universe, the wavelengths of light have stretched. But during the time it takes Hydra's light to reach us, the bands of color have shifted down toward the red end-the low-energy end-of the spectrum. Through a prism, Hydra's hydrogen looks like four strips of red, blue-green, blue-violet and violet. Astronomers have measured the distance from the Earth to Hydra by looking at the light coming from the cluster. Take Hydra, a cluster of galaxies about three billion light years away. The wavelengths of light from other galaxies shift as they move away from us, just as the pitch of an ambulance siren changes as it moves past. Scientists know this because of the Doppler effect, among other reasons. That first explosion is still pushing galaxies outward. Around 14 billion years ago, all matter in the universe was thrown in every direction. ![]() Just how this will happen is a bit complicated, so let's begin at the very beginning: the big bang. Yes, the universe itself will eventually outpace the speed of light. ![]()
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