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Inflation?
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To say “it’s complicated” is way underselling it. First off, the universe is expanding in (at least) 3 dimensions, 14.6 something billion lightyears to the (4th dimensional) center, then the same again to the other side. But cosmic inflation throws that right out the window, when shortly after the birth of the universe space time expanded faster than the speed of light. The more distant galaxies are still retreating from us faster than the speed of light. You would need input from someone with a lot more knowledge than me to answer your question with anything more than 
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Thanks for this. I’m wondering what the curved line is. Is it some artifact of gravitational lensing?
The line is actually straight. It’s SPACE that is curved.
(J/K I have no freaking idea)
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(I mean, yeah, on some level I knew this, but for all I know the telescope is looking at the ass-end of ourselves from a year ago)
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I’ve read a few explanations of exactly how that works. They often try to make it comprehensible by saying things like “No, nothing inside the universe was travelling faster than light. But space was stretching faster than light.” And that’s the simple, easy-to-understand version. Which, to say the least, opens more questions than it answers.
I’ve walked away with the notion that I can either
- accept that peer review has ground away any shaky parts of the theory and take it at face value, or
- take the plunge and really wrap my head around General Relativity.
I passed enough math courses to get a degree in engineering, then a few more for fun. I haven’t wrapped my head around General Relativity, nor do I expect to. This is absolutely mind-blowing stuff, and I’m a happy for / jealous of people who understand it.
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I like to think that have a sorta-kinda “visual” (way wrong word) understanding of it, but the math is so far beyond me that it might as well be Romulan. But the field fascinates me, so i read obsessively about it. But mostly, it just reminds me that i am not all that smart. It’s good to be reminded of that now and then.
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The bit that absolutely blew my mind was in a Dr Becky video where she pointed out that expanding space doesn’t mean space is being created. She said words to the effect of “that point behind your left ear was present at the Big Bang”, and I had to go sit down and not think for a while.
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So the thing is that comparing the expansion to the speed of light is misleading, because the units are different. It’s not a certain length per second but a certain percentage. Every metre grows by some H m/s. If you have two metres, they grow by 2H m/s. If you have 100 metres, they grow by 100H m/s. And so on.
By the time you measure out to the observable edge that total may or may not pass the speed of light. But that’s a sum growing, nothing actually moving through spacetime. It’s like two ships drifting with the current…how fast they separate doesn’t depend on their top speed in the water. Or if you will forgive a very poor drawing:
The circle is the universe at a particular moment in time, and the two points on it are both moving forward in time (the outside is the future). As they do the total distance between them increases. However, the speed of light from them is represented by the little grey cones – and you can see neither is actually moving outside of those cones at all, just the cones are tilted differently.
I don’t know how easy I made that to follow, but that’s what the math of actually represents. In flat and static spacetime the cones would all be the same direction, but not in curved or expanding spacetime.
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Indeed, and (to clarify further) just because the distance between two points may increase faster than the speed of light, there’s nothing actually wrong with that, so long as no thing (or even information) is, itself, traveling faster than the speed of light.
Could you use the expansion of the universe to send a message to a planet that’s (currently) a million light years away in less than a million years? No. So no laws of physics are being broken.
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Rubber sheets are surprisingly helpful for visualizing cosmology. We probably all know the gravity metaphor, wherein you drop a bowling ball into the stretched sheet, and presto, gravity, because of how the mass distorts the sheet and causes things to roll around each other. Gravity happens because mass distorts space time, causing things to orbit because the space you’re moving straight through is now bent around the mass.
For expansion, imagine the sheet is stretching while a ball is rolling across it. The two together could exceed the speed of light without either of them doing so because the ball is rolling and the surface being rolled upon is pulling it forward. It’s not a perfect analogy, since the ball would actually slow down and it’s sort of an airplane-on-conveyor-belt thing, but it helps anyway.
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It’s not like the expansion caps out at twice the speed of light, though. The balls don’t have to be rolling at all, and in fact galaxies are not moving anywhere near the speed of light compared to their local comoving frame. It’s all from the stretching which has no such limit since it’s not really a set speed.
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Like I said, not a perfect analogy. Just something to help visualize it.
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Yeah, I find the “frictionless rubber sheet” analogy great for understanding how orbits and gravitational lensing happens. And it’s a semi-intuitive explainer about why Hubble’s constant can be a constant of 70 km/s/MPc and explain the speed of expansion of the distant universe and how it makes sense that the further they are, the faster they go away.
But it falls apart for me, around there. Like, why the fuck would a rubber sheet magically stretch itself? Why is it only stretching itself in the spaces between galaxy clusters and not everywhere? And what does “distance” mean when something like that is happening?
That’s the part where I’m content to let peer review take care of it for me (yay, bickering scientists!). And let Dr Becky occasionally hand me a lightning-in-a-bottle insight into how it works.
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Well, I don’t think “why” is ever a good question to ask for massively oversimplified analogies like rubber sheets standing in for cosmology. 
It’s more like a Hail Mary to maybe wrap one’s head around the gist of something that otherwise requires a whole lot of pretty intense math to grasp.
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Yep. I was still in uni when I realized I didn’t have the patience to get there. 
So I’m delighted people who get it can argue and collaborate and get it right, and I have the weirdest happy / jealous feeling about that.
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