For any plausible model of our expanding universe, there exists a relatively simple conversion to translate redshift into recessional velocity. The higher redshift of a galaxy, the faster it is receding from us. (That is, its wavelength increases.) This so-called cosmological redshift is measured by astronomers, so distant galaxies can be labeled by their redshift. How can we tell the universe is expanding faster than the speed of light in the first place? The wavelength of a light pulse traveling the universe is stretched as space expands, so the light gets redder. The conclusion is that we still can observe galaxies receding faster than light! Put another way, the Hubble sphere is not the limit of our observable universe. Of course, as long as the pulse is traveling a region receding from us at a velocity smaller than the speed of light, it will eventually reach us. Take a look at this video, which transforms these words into a cool animation. Now, if the rate at which the Hubble sphere expands is larger than the net velocity at which the photon recedes from us, the pulse will eventually pass from a superluminal region into a region receding from us slower than the speed of light. It looks like we will never receive this pulse - but wait a sec! As the universe expands, the Hubble sphere gets bigger, too. The pulse tries to makes its way to us, but it is "dragged" away from Earth by a region of space receding faster than light. Imagine a galaxy outside the Hubble sphere, which emits a light pulse towards Earth. Note that, since the universe expands at an accelerated rate, the Hubble sphere increases its radius as time goes by.Ĭan we see light coming from galaxies outside the Hubble sphere? Receiving light from a source moving faster than light might seem odd, but this is actually possible. This distance defines the "Hubble sphere", an imaginary sphere centered at us, outside which everything recedes faster than the speed of light. It turns out, galaxies 4300 megaparsecs away from us recede faster than light. Following the same logic, one could do the math to compute how far a galaxy has to be in order to move away at the speed of light. This means that a galaxy 1 megaparsec away from us is receding at about 70 km/s, another galaxy 2 megaparsecs away from us is receding at 140 km/s, and so on. As you read this, the universe expands at about 70 kilometers per second per megaparsec. Is there any proof that space expands faster than the speed of light, such as the sudden disappearance of stars or galaxies? If that hypothesis is true, shouldn't there be some stars and galaxies near the cosmic horizon that are disappearing from our observations?Ĭurrently, we are certain that we live in a universe that is expanding at an increasing rate.
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