PBS space time has a good video on this - https://www.youtube.com/watch?v=msVuCEs8Ydo
It's like a blowing up a balloon. There's no limit to how big it gets or how fast it grows, so objects on opposing sides can move apart at greater than the speed of light even if they're not moving themselves.
This rate of expansion is accelerating too, driven by something called "dark" (as in unknown) energy which is overpowering all the gravity that should be pulling things together. It's fascinating to think that in 100s of billions of years, our local galaxy will be the only thing we can see in space. Any civilizations then would believe that there's nothing else out there.
Metric expansion does not result in causality issues because it's an isolating phenomenon (one that slows down interactions between things) rather than an interconnecting phenomenon (one that would speed up interactions between things). Therefore it's at least possible, and observations (which you can read a brief summary of in TFA) back it up.
No, in the specific case of the concordance cosmology (and more general expanding cases such as an expanding Robertson-Walker spacetime, or a 4-dimensional de Sitter spacetime), the manifold is still Lorentzian [1] and globally hyperbolic [2].
These features let one sort trajectories into spacelike, null, and timelike. If spacelike trajectories are forbidden to interacting matter and gravitational waves, then we get our usual idea of causality [3].
Collapsing Robertson-Walker spacetimes and anti-de Sitter spacetimes are defined on globally hyperbolic Lorentzian manifolds. They are defined as free of matter and gravitational waves. However we can perturb them slighly by adding a bit of matter, even matter that generates small amounts of gravitational radiation, and as long as that matter and radiation moves only on timelike trajectories or lightlike trajectories, we have the normal causality even though the interactions among separated bits of normal matter perturbing these collapsing spacetimes will generally become more frequent and stronger over time. Indeed, this remains the case even for very highly accelerated compactions, where at late times distances between objects contract faster than light between the same objects can move.
Other commonly encountered spacetimes -- the flat spacetime of Minkowski approximately describes laboratories on Earth and in the International Space Station, and the (exterior) Schwarzschild spacetime which approximately describes the spacetime in which Earth-orbiting satellites move -- are also globally hyperbolic and Lorentzian, and thus we only need to keep superluminality at bay to guarantee normal causality.
In short: our usual idea of causality comes from being able to forbid a certain class of trajectories, and in physically realistic spacetimes (i.e., ones which approximately match parts of our observed universe), those are all as far as we can tell free from things moving spacelike (i.e., nothing's faster than a massless particle, and light is made up of massless particles, so nothing is superluminal).
Taken the other way around: our local observations of causal interactions (both locally, like in laboratories, and astronomically) are strong evidence in favour of us inhabiting a manifold equipped with the requisite features [1][2]. Alternative explanations have generally either failed in some way (self-inconsistency, obvious incompleteness, or inconsistency with repeatable observations) or been more complicated than, but reduces to, General Relativity in some limit. None of the second variety of alternatives has yet to be demanded by actual observations that cannot be explained using General Relativity as the underlying spacetime theory.
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[1] https://en.wikipedia.org/wiki/Pseudo-Riemannian_manifold#Lor...
[2] https://en.wikipedia.org/wiki/Globally_hyperbolic_manifold
> If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light years in 13.8B years.
No. What we're seeing was 13.8BLY away (in our reference frame) when the light was emitted. Those things are 46BLY away now (again, in our reference frame) but we can't see them now, we can only see them as they were 138B years ago.
For example, for redshift z=9, light travel time is 13.17 Gy, proper distance at time of absorption (aka comoving distance) is 30.72 Gly and proper distance at time of emission (which, in a spatially flat universe, corresponds to angular diameter distance) is 3.07 Gly. The factor of 10 between these two distances is no coincidence as the ratio of the scale factor is given by 1+z.
( The values have been calculated via http://www.astro.ucla.edu/~wright/CosmoCalc.html )
I said what we're seeing was 13.8B light years away 13.8B years ago, since it took 13.8B years for the light to reach us from the point it was emitted. I never said we could see now where they currently are. What I failed to account for was the space expansion during the time that the light was traveling meaning that the light had to travel further to reach us than the point of emission actually was when it emitted.
So really, it wasn't 13.8B years ago; it was emitted later. I don't know by how much, but it seems like maybe the other commenter does.
But then that expansion ratio may of been different over time, but it is a bit like sampling the second hand on a clock in a photo and working out the entire motion and workings of the clock - at least on the scale of the data/measurements we have and the age of the universe. Currently we are still not 100% sure when midnight was, but as always moving in the right direction.
If space wouldn't have expanded sufficiently fast in the beginning of time, all matter would be still clumped together as in a giant black hole. The fact that space expanded "faster" than the speed of light allowed this black hole to become free matter.
Maybe I’m misunderstanding though.
EDIT: I guess you also have to take into account the expansion of the fabric of space as well and not just the movement of the galaxies. Still a bit difficult to grasp because I still intuitively want to say that those galaxies are traveling faster than the speed of light with the assistance of space expansion like the old Hollywood trick of actors running on a moving rug to fake superhuman speed. They’re still moving faster, just not on their own. I guess the key difference is that an actor on a sliding rug is moving through space faster even if their legs are moving at the same speed, but in the case of the galaxies, space itself is moving thus they’re not technically FTL because the speed of light is relative to its movement through space.