![]() You, standing in the middle, would correctly observe that your "universe" is expanding: any objects placed on that fabric would slowly move away from you.īecause stretchy stuff is stretchy, the objects on the fabric close to you would appear to move away with some speed, but the farther objects would appear to move faster. Let us assume they're choreographed well and are able to walk backward and pull at the same rate. Imagine a bunch of folks standing around the edges of a stretchy piece of fabric, tugging at it. But in general, in the biggest of pictures, the galaxies are getting farther away from each other.Ī key feature of this expansion is how uniform it is. There are slight motions on top of that general expansion, leading to instances such as the Andromeda Galaxy heading on a collision course for the Milky Way. Every day the galaxies get farther apart from each other - on average. What gives?įirst off, it's important to note that we live in an expanding universe. They come barging in with a simple observation: Some galaxies are moving away from us…wait for it…faster than the speed of light. Nothing further needs to be said about the issue.Īnd then come the astronomers, always excited by the chance to mess up your comfort zone. For one fairly natural definition of velocity, there are galaxies we observe that are now and always have been receding from us at a velocity greater than c.It seems like it should be illegal, doesn’t it? Over and over (and over and over) we're told the supreme iron law of the universe: Nothing - absolutely nothing - can go faster than the speed of light. The second statement is at best an oversimplification because relative velocities of distant objects are not well defined in general relativity. The first statement is incorrect because the expansion of the universe can’t be measured with a single velocity. You may sometimes see statements that cosmological inflation caused the universe to expand faster than c, or that the edge of the observable universe occurs at the place where the Hubble law gives a velocity equal to c. Locally, general relativity is the same as special relativity. Because relative velocities of distant objects aren’t well defined in general relativity, there is no way to extend special relativity’s prohibition on v>c to distant objects in general relativity. Therefore it doesn’t make sense to worry about whether such a velocity is greater than c. In fact, general relativity allows us to assign absolutely any value we like to A’s velocity relative to B it simply isn’t a well-defined thing. If we like, we can use certain measures of distance and time (see: How are time and distance measured in cosmology?) and verbally describe A and B as moving relative to one another at a rate found by taking the change in distance divided by the change in time. If we like, we can verbally describe the situation by saying that both galaxies are at rest, but the space between them is expanding. General relativity does not have a uniquely defined way of talking about the velocity of galaxy A relative to galaxy B if they are at cosmological distances from one another. Can they be receding from one another at a speed greater than c? This question requires relativity. Now suppose we fix our attention on two specific galaxies. For similar reasons, it doesn’t make sense to ask for “the” velocity of expansion of the universe. The velocity will be different if we pick a different pair of atoms. A velocity can only be defined if we first specify which two atoms in the metal we’re talking about. When a piece of metal expands, we can’t describe its overall expansion using a velocity in units of meters per second. This is exactly what happens, for example, when a piece of metal expands because it has been heated. ![]() All intergalactic distances are increasing by the same scaling factor in any given interval of time. Hubble’s observation therefore implies v=Hd, where v is the relative velocity of two galaxies, H is a number that is the same for all galaxies, and d is the distance between the two galaxies. ![]() A redshift of, say, 0.037% indicates that a galaxy is moving away from us at almost exactly 0.037% of the speed of light. To keep things simple, let’s start by thinking about how this would be interpreted if we didn’t know about relativity, so that velocity and distance can be defined as we expect in Newtonian mechanics. The expansion of the universe was originally discovered by Hubble, who found that the redshifts of galaxies were proportional to their distances from us. To see why, let’s start by thinking about how we know the universe is expanding. Neither of these questions actually makes sense in the form in which it was posed.
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