Posted on 10/13/2015 11:04:06 PM PDT by LibWhacker
The Universe Never Expands Faster Than the Speed of Light
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. Its extraordinarily common, if utterly and hopelessly incorrect. (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. I can recommend this nice article by Tamara Davis and Charlie Lineweaver, which tries to address this and several other cosmological misconceptions.
This isnt, by the way, one of those misconceptions that rattles around the popular-explanation sphere, while experts sit back silently and roll their eyes. Experts get this one wrong all the time. 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.
The great thing about the superluminal-expansion misconception is that its actually a mangle of several different problems, which sadly dont cancel out to give you the right answer.
1.The expansion of the universe doesnt have a speed. Really the discussion should begin and end right there. Comparing the expansion rate of the universe to the speed of light is like comparing the height of a building to your weight. Youre not doing good scientific explanation; youve 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. Thats (distance divided by time) divided by distance, or simply 1/time. Speed, meanwhile, is measured in distance/time. Not the same units! Comparing the two concepts is crazy.
Admittedly, you can construct a quantity with units of velocity from the Hubble constant, using Hubbles law, v = Hd (the apparent velocity of a galaxy is given by the Hubble constant times its distance). Individual galaxies are indeed associated with recession velocities. But different galaxies, manifestly, have different velocities. The idea of even talking about the expansion velocity of the universe is bizarre and never should have been entertained in the first place.
2. There is no well-defined notion of the velocity of distant objects in general relativity. 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. 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. In general relativity, we just cant. There is simply no such thing as the velocity between two objects that arent located in the same place. If you tried to measure such a velocity, you would have to parallel transport the motion of one object to the location of the other one, and your answer would completely depend on the path that you took to do that. So there cant be any rule that says that velocity cant be greater than the speed of light. Period, full stop, end of story.
Except its not quite the end of the story, since under certain special circumstances its possible to define quantities that are kind-of sort-of like a velocity between distant objects. Cosmology, where we model the universe as having a preferred reference frame defined by the matter filling space, is one such circumstance. When galaxies are not too far away, we can measure their cosmological redshifts, pretend that its a Doppler shift, and work backwards to define an apparent velocity. Good for you, cosmologists! But that number youve defined shouldnt be confused with the actual relative velocity between two objects passing by each other. In particular, theres no reason whatsoever that this apparent velocity cant be greater than the speed of light.
Sometimes this idea is mangled into something like the rule against superluminal velocities doesnt refer to the expansion of space. A good try, certainly well-intentioned, but the problem is deeper than that. The rule against superluminal velocities only refers to relative velocities between two objects passing right by each other.
3. There is nothing special about the expansion rate during inflation. If you want to stubbornly insist on treating the cosmological apparent velocity as a real velocity, just so you can then go and confuse people by saying that sometimes that velocity can be greater than the speed of light, I cant stop you. But it can be and is! greater than the speed of light at any time in the history of the universe, not just during inflation. There are galaxies sufficiently distant that their apparent recession velocities today are greater than the speed of light. To give people the impression that whats special about inflation is that the universe is expanding faster than light is a crime against comprehension and good taste.
Whats special about inflation is that the universe is accelerating. During inflation (as well as today, since dark energy has taken over), the scale factor, which characterizes the relative distance between comoving points in space, is increasing faster and faster, rather than increasing but at a gradually diminishing rate. As a result, if you looked at one particular galaxy over time, its apparent recession velocity would be increasing. Thats a big deal, with all sorts of interesting and important cosmological ramifications. And its not that hard to explain.
But its not superluminal expansion. If youre sitting at a stoplight in your Tesla, kick it into insane mode, and accelerate to 60 mph in 3.5 seconds, you wont get a ticket for speeding, as long as the speed limit itself is 60 mph or greater. You can still get a ticket theres such a thing as reckless driving, after all but if youre hauled before the traffic judge on a count of speeding, you should be able to get off scot-free.
Many misconceptions in physics stem from an honest attempt to explain technical concepts in natural language, and I try to be very forgiving about those. This one, I believe, isnt like that; its just wrongity-wrong wrong. The only good quality of the phrase inflation is a period of superluminal expansion is that its short. It conveys the illusion of understanding, but that can be just as bad as straightforward misunderstanding. Every time it is repeated, peoples appreciation of how the universe works gets a little bit worse. We should be able to do better.
no it isn’t, it’s hanging around the neck of a cat (or was it a dog?)- I saw the documentary “Men in Black” that proves it
It did hang around a cats neck plus the universe is inside an alien marble.
http://ak-hdl.buzzfed.com/static/enhanced/webdr06/2013/4/10/4/enhanced-buzz-21157-1365582040-9.jpg
https://s-media-cache-ak0.pinimg.com/736x/ab/f1/bb/abf1bbace2eeeb08cdbec2b9c98639f4.jpg
Alan Guth might disagree with this ‘never’ assertion. On the other hand, the speed of light is slowing down as the expansion-caused energy in the vacuum is increasing, so maybe ...
I don't understand this. In theory, if everything started as a singularity at the time of the Big Bang, then how can I have Hubble images on my hard drive of galaxies colliding?
In theory everything started in the singularity and exploded into on a direct path away from everything else at the same speed and trajectory??
Everything should be flying away from everything else and collisions between galaxies should be impossible.
Thanks :)
The edges of the observable Universe are now [emphasis added] moving away from us considerably faster than the speed of light. The remotest galaxies weren't going that fast when their photons were originally emitted circa 10-12 billion years ago. That is what the Hubble is seeing.
Any photons emitted today from those same sources will never reach us in the future.
Remember, it is the fabric of space itself that is stretching apart at an ever accelerating rate. Relativity applies to objects moving through space, not space itself. (Think of it like an ant crawling along the outside of inflating balloon, where the balloon is blowing up faster than the ant can walk.)
Collisions are local (see post 15).
>> Just when you think you know something, you don’t
A controversy about what we didn’t know...
I love the inordinate vacuum of thought... thanks for the reference.
TYVM!
Bump for a reread later.
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