In special relativity, where spacetime is a fixed, flat, Minkowskian geometry, we can pick a global reference frame and extend that rule to distant objects. There is a rule, valid both in special relativity and general relativity, that says two objects cannot pass by each other with relative velocities faster than the speed of light. There is no well-defined notion of “the velocity of distant objects” in general relativity. The idea of even talking about “the expansion velocity of the universe” is bizarre and never should have been entertained in the first place.Ģ. But different galaxies, manifestly, have different velocities. Individual galaxies are indeed associated with recession velocities. Not the same units! Comparing the two concepts is crazy.Īdmittedly, you can construct a quantity with units of velocity from the Hubble constant, using Hubble’s law, v = Hd (the apparent velocity of a galaxy is given by the Hubble constant times its distance). Speed, meanwhile, is measured in distance/time. That’s (distance divided by time) divided by distance, or simply 1/time.
You’re not doing good scientific explanation you’ve had too much to drink and should just go home.The expansion of the universe is quantified by the Hubble constant, which is typically quoted in crazy units of kilometers per second per megaparsec. Comparing the expansion rate of the universe to the speed of light is like comparing the height of a building to your weight. The great thing about the superluminal-expansion misconception is that it’s actually a mangle of several different problems, which sadly don’t cancel out to give you the right answer.ġ.The expansion of the universe doesn’t have a “speed.” Really the discussion should begin and end right there. “Inflation was a period of superluminal expansion” is repeated, for example, in these texts by by Tai-Peng Cheng, by Joel Primack, and by Lawrence Krauss, all of whom should certainly know better. This isn’t, by the way, one of those misconceptions that rattles around the popular-explanation sphere, while experts sit back silently and roll their eyes. I can recommend this nice article by Tamara Davis and Charlie Lineweaver, which tries to address this and several other cosmological misconceptions. (I just noticed it in this otherwise generally excellent post by Fraser Cain.) A Google search for “inflation superluminal expansion” reveals over 100,000 hits, although happily a few of the first ones are brave attempts to squelch the misconception. Breaking my radio silence here to get a little nitpick off my chest: the claim that during inflation, the universe “expanded faster than the speed of light.” It’s extraordinarily common, if utterly and hopelessly incorrect.
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