If the universe is expanding spacetime faster than light (FTL), is FTL travel no longer completely impossible?
Do not care about energy requirements or needing new tech, just if it is NOT physically impossible given this natural, actual observation/phenomena.
The expansion of the universe is not measured in units of speed, so it cannot really be compared to c in the first place. Saying that it is faster than the speed of light is “comparing apples and oranges”.
The expansion of the universe is currently about 70 (km/s)/Mpc. It was much larger in the inflationary epoch, but would still have the same units. So even then it does not make sense to compare the inflation rate to the speed of light. There is always a distance where the expansion between two points separated by that distance is less than c.
In contrast, the speed of light is an actual speed. Even on a local scale a light wave travels at c. This is important because in GR only local speeds are physically meaningful. Speeds of things that are not colocated are not even well defined in a curved spacetime.
The speed of a light wave is local, and therefore meaningful, and is c. The expansion of the universe is not a speed and cannot be converted into a local speed other than 0, so it is not meaningful and therefore cannot meaningfully be compared to c.
Now, you asked specifically about superluminal travel. In curved spacetime it is possible for there to be multiple paths through spacetime and for one of them to be shorter than the other such that matter (always traveling slower than light locally) taking the short path can arrive before light taking the long path. These can be arranged into a type of superluminal travel, and are often called wormholes or warp drives. Wormholes and warp drives are permissible according to general relativity, but they require matter with negative energy density. No such matter has ever been found and there is no reason to suppose it exists.
Compare this to an ant walking on a very stretchy rubber band. The ants walking speed is limited, it can never exceed a certain speed (let's call it $c$). Now if we continuously stretch the rubber band it is possible for two points on the rubber band to move apart faster than $c$. But any ant moving on the band can still never move faster than $c$ with respect to the rubber band.
The rubber band is spacetime. Let's translate this to real life. At any point in spacetime we can assign a light cone, the cone that would be traced out if we send a light pulse in all directions. The future of any observer is always constrained to be inside this light cone. But two points in spacetime that are far apart can in principle move at any speed with respect to each other because space can stretch.
The limit on speed, the speed of light, affects objects and information within spacetime. It doesn't apply to changes to spacetime itself, which is what causes the universe's own expansion.
Wikipedia has a decent article on this:
The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric (which governs the size and geometry of spacetime itself) that changes in scale. As the spatial part of the universe's spacetime metric increases in scale, objects become more distant from one another at ever-increasing speeds. To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer.
While objects within space cannot travel faster than light, this limitation does not apply to the effects of changes in the metric itself. Therefore objects existing at a great enough distance from a potential observer are receding at a "speed" (in terms of distance/time, not motion) which exceeds even the speed of light, and they cannot be observed (due to the impossibility of a signal ever being able to traverse the ever-increasing distance between), limiting the size of our observable universe.
As an effect of general relativity, the expansion of the universe is different from the expansions and explosions seen in daily life. It is a property of the universe as a whole and occurs throughout the universe, rather than happening just to one part of the universe. Therefore, unlike other expansions and explosions, it cannot be observed from "outside" of it; it is believed that there is no "outside" to observe from.
There's a standard explanation for all "how can this be FTL" questions. Consider a laser beam that you point at a wall a couple light-years away. Ignore diffraction and all that for this Gedanken experiment . Then rotate the laser source and the spot moves along the wall. The rate at which the illuminated position moves is easily much greater than $c$. However, no energy and no information is actually travelling at the spot's speed.
Situations can change faster than $c$ but not information or energy (or mass, should that need to be said).
The expansion of the universe is not associated with any faster-than-light motion. We sometimes speak of faster-than-light recession rates (for example, in the context of Hubble's law), but these recession rates are not relative velocities, so there's no reason to compare them to the speed of light.
Cosmological recession rates of distant objects can be defined as follows. Conceptually, imagine a chain of galaxies that leads to some target galaxy. Each galaxy along the chain has some small velocity relative to the galaxy before it. If you add all of those relative velocities together, you might expect that to give you the velocity of the target galaxy. However, velocities in relativity add in a special way; see the relativistic velocity addition formula. The cosmological recession rate is computed by instead just adding the relative velocities naively, without using relativistic velocity addition. That's why we should not be concerned that it can exceed the speed of light. Cosmological recession rates are more like rapidity than speed.
According to standard physics, it's not possible to move through space faster than c. With the expansion of space, space isn't moving through space, it's just expanding. In relativity, no matter what local reference frame you're using, the velocity of an object next to you can't be greater than c. But it is possible for an object far away to have its distance from you to be increasing at a rate that is "faster than c". One way of thinking about it is to imagine a bunch of yardsticks between you and another object. It's not possible for the number of meter sticks you pass divided by the time you pass them to be greater than c, but it is possible to cram more meter sticks in between the objects, which means that it is possible for the number of meter sticks between you and your starting point divided by the elapsed time to be greater than c (because more meter sticks were crammed into the space after you passed through it).
In basic understanding the Universe is not an "Object" by which travels, its' the medium by which Objects Move. The subsequent "speed limit" does not apply to the expansion of space itself, but only to the objects that move thru that space.
