![]() ![]() (If you like, you can further imagine this is happening in a zero-gravity environment, like on the International Space Station.) Now, if you put your finger down on one raisin, what do you see the other raisins doing? Now imagine that the dough leavens, expanding in all directions. Imagine that you have a ball of dough with raisins all throughout it. The way I like to think about the expanding Universe is with the “raisin bread” model. The redshift-distance relation predicted by the expanding Universe is borne out in observations, and has been consistent with what’s been known all the way back since the 1920s. The farther away any two raisin are from one another, the greater the observed redshift will be by time the light is received. ![]() The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. If something is 20 Mpc away from us, we’d expect to see it moving away at the equivalent of 1320-1480 km/s from us if it’s 5000 Mpc away, we’d expect to see it moving away at ~330,000-370,000 km/s. ![]() We’ve even measured that rate precisely well, and can be certain, from all the measurements and observations we’ve taken, that the present-day rate of expansion is precisely between 66 and 74 km/s/Mpc: kilometers-per-second-per-megaparsec.īut what does it mean that space is expanding?įor every megaparsec (about 3.26 million light-years) away that a distant and unbound object is from us, we’ll see it recede from us as though it were moving away at the equivalent of 66-74 km/s. So, nothing can move faster than light through space, but what about the ways that space itself changes? You’ve likely heard that we live in an expanding Universe, and that we’ve measured the rate at which the fabric of space itself expands: the Hubble constant. Kirshner, PNAS, 2004) Space doesn’t expand at a speed In agreement with both observation and theory, the universe is expanding. Therefore, whether looking at a massive or massless object, I had better observe that the velocity I get never exceeds the speed of light, or that would violate the laws of relativity.Įdwin Hubble’s original plot of galaxy distances versus redshift (left), establishing the expanding universe, versus a more modern counterpart from approximately 70 years later (right).Then, by using the definition of velocity - that it’s a change in distance divided by a change in time - I can get its velocity.When I see it, I can record its observed position and the time at which I observe it.When I observe an object, I can track its motion, observing how its position changes over time.But what does that actually mean? Most people, when they hear it, think the following thoughts: It is true: Nothing can travel faster than the speed of light. (Credit: Christopher Vitale of Networkologies and the Pratt Institute.) What “nothing can travel faster than the speed of light” actually means In addition, the distances between unbound objects evolve with time, owing to the expansion of the universe. In General Relativity, we treat space and time as continuous, but all forms of energy, including but not limited to mass, contribute to spacetime curvature. Instead of an empty, blank, three-dimensional grid, putting a mass down causes what would have been ‘straight’ lines to instead become curved by a specific amount. The most distant galaxy we’ve seen so far is presently 32 billion light-years away the most distant light we see corresponds to a point presently 46.1 billion light-years away and galaxies beyond about 18 billion light-years away can never be reached by us, even if we sent a signal at the speed of light today. DOES THE UNIVERSE EXPAND FASTER THAN LIGHT SERIESThrough a series of precise observations, we’re confident that the Universe is precisely 13.8 billion years old. There’s no way around these facts, as they’re the fundamental principle on which relativity is based.Īnd yet, when we look out at distant objects in the Universe, they seem to defy our common-sense approach to logic. The faster your motion through space, the slower your motion through time, and vice versa. If you’re massless, you have no choice you can only move at one speed through spacetime: the speed of light if you’re in a vacuum, or some slower speed if you’re in a medium. If you’re a massive particle, not only can’t you exceed that speed, but you’ll never reach it you can only approach the speed of light. If there’s one rule that most people know about the Universe, it’s that there’s an ultimate speed limit that nothing can exceed: the speed of light in a vacuum. ![]()
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