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.” It’s 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 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. 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 it’s actually a mangle of several different problems, which sadly don’t cancel out to give you the right answer.
1.The expansion of the universe doesn’t 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. 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. That’s (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 Hubble’s 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 can’t. There is simply no such thing as the “velocity” between two objects that aren’t 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 can’t be any rule that says that velocity can’t be greater than the speed of light. Period, full stop, end of story.
Except it’s not quite the end of the story, since under certain special circumstances it’s 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 it’s a Doppler shift, and work backwards to define an “apparent velocity.” Good for you, cosmologists! But that number you’ve defined shouldn’t be confused with the actual relative velocity between two objects passing by each other. In particular, there’s no reason whatsoever that this apparent velocity can’t be greater than the speed of light.
Sometimes this idea is mangled into something like “the rule against superluminal velocities doesn’t 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 can’t 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 what’s special about inflation is that the universe is expanding faster than light is a crime against comprehension and good taste.
What’s 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. That’s a big deal, with all sorts of interesting and important cosmological ramifications. And it’s not that hard to explain.
But it’s not superluminal expansion. If you’re sitting at a stoplight in your Tesla, kick it into insane mode, and accelerate to 60 mph in 3.5 seconds, you won’t get a ticket for speeding, as long as the speed limit itself is 60 mph or greater. You can still get a ticket — there’s such a thing as reckless driving, after all — but if you’re 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, isn’t like that; it’s just wrongity-wrong wrong. The only good quality of the phrase “inflation is a period of superluminal expansion” is that it’s short. It conveys the illusion of understanding, but that can be just as bad as straightforward misunderstanding. Every time it is repeated, people’s appreciation of how the universe works gets a little bit worse. We should be able to do better.
That is why we cannot see them.
But the science is settled. FTL expansion denier!
And the universe is 13.8 Billion years old. And began from a singularity, and some things are 24 billion light years apart. That alone tells you that yes, speed of light can be exceeded.
There are some very weak points in this article.
The entire universe is inside a wormhole.
The entire universe is inside a wormhole.
Electric Universe Theory
https://www.thunderbolts.info/wp/eu-guides/beginners-guide/
Universe is not expanding. Black hole don’t exist
OK.... Thanks for clearing that up.... I think.
Stephen Crothers Destroys the Quackademic “Black Hole” & Relativity
https://www.youtube.com/watch?v=wRsGPq77X0Q
I’m not even going to pretend I understand this, but I have a question. If two things left the point of the singularity in opposite directions traveling at the speed of light for 13.8 billion years, wouldn’t they now be 27.6 billion light years apart?
What if those objects are receding?
"If you’re sitting at a stoplight in your Tesla, kick it into insane mode, and accelerate to 60 mph in 3.5 seconds, you won’t get a ticket for speeding, as long as the speed limit itself is 60 mph or greater."
It essentially destroys everything the author is trying to say, and it has nothing to do with anything whatsoever. The correct analogy is: If you accelerate your Tesla to 60 mph in 3.5 seconds driving East, the cop cannot claim you were going ~1060 mph by adding the speed of the Tesla to the speed of the underlying medium (Earth) because comoving objects in General Relativity [the cop, you] do not have velocities that add that way relative to the frame of reference they're in.
And to answer your question: Yes, objects can be further from each other than their distance to the beginning of spacetime, and those objects essentially cannot see each other, they are beyond each other's event horizons.
Absolutely not! Well, maybe. The speed of light is only determined by relative points of reference. Some points of reference, maybe outside of relative relationship. Dimensional enthusiasm is not paid much attention, by well compensated “one size” standardist blowhards.
This is confirmed by observations since the 1990s of Type Ia supernovae compared to their observed redshifts. The ratio of the redshift to the brightness of the exploding star is not constant, but rather stars that are farther away seem to have much higher redshifts, indicating acceleration.
The end result will be the Big Rip.
DE and the Inflation Theory are two different theories. Inflation Theory is still speculative with no real observational confirmation (other than the large-scale anisotropy for which it was invented to explain in the first place).
Nice explanation, FredZ. Thank you.
#darkmatters
When I get to heaven I want to see the universe from it. Watching the movie Interstellar made me think about this. And I want to know if God created aliens on other solar systems as well...
If I’m not mistaken, that 24 billion light years distance between two objects would be the sum total of their movement away from each other. That means they each moved, assuming both are moving at the speed they were since time began, 12 billion light years in those 13.8 billion years of time.
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