Accounting for the expansion of the universe when traveling between galaxies

[nhnifong]

In KSP interstellar warp drives cannot directly produce delta V. I assume these are modeled after Alcubierre drives, which warp space-time in order to move. After a hypothetical voyage from one star to another, a vessel may have to produce around 100,000 km/s of delta V by conventional means in order to make orbit. But when traveling between galaxies the difference in velocity is due to the expansion of the universe. Does this mean that Alcubierre drives are exempt from accounting for it?

[AngelLestat]

Hard question, I would said that in the alcubierre drive case you will have a slight benefic against normal way from the distance point of view (not time), This would become clear if you made the calculation using derivatives for each instant of propulsion that has into account the infinitesimal change of lenght for each infinitesimal moment.

But do not believe that using this method the difference in velocity due to expansion is void.. "Not even close".

Becouse all the remain distance for each moment will still had the expansion effect over you, it does not matter if you use the normal or the alcubierrre way.

And all this was just from the distance point of view. But expansion also has into account time. So the relative way has a benefic in this matter. Your trip is shorted from the time point of view.

How Carl Sagan said... If you had a ship with constant acceleration of 1g, you will reach the center of the galaxy in just 25 years. Also if you had a alcubierre drive ship able to achieve 100 times the speed of light, you will reach the center of the galaxy in 300 years.

[Nuke]

a relativistic trajectory might be better time wise. i certainly wouldn't want to have to depend on a 300 year old galaxy ship.

[TheGatesofLogic]

depends on how far the galaxies are apart from one another, in comparison to the size/distance ratio between individual stars, the size/distance ratio between galaxies is FAR smaller, and thus you must take that into account when factoring in space-time expansion. As such, galaxies in the local cluster, like andromeda for instance, are not expanding away from us very rapidly at all, in fact andromeda is EXPANDING away from us very slowly and is MOVING towards us comparatively rapidly.

Regardless, space-time expansion has nothing to do with relativistic velocities, and when you relativistically approach something that was expanding away from you slower than your velocity the rate of that expansion decreases exponentially, and ultimately the delta-v is the same as it would have been otherwise, disregarding any delta-v lost in the intermediary travel phase in the process of exceeding the expansion rate.

equally true is the knowledge that the alcubierre drive never actually moves, it just brings some things closer to it and pushes other things farther away, and as such if something's location has a vector pointing away from the drive's target vector (since when spacetime expansion is being taken into account it is useful to consider locations as vectors) it must then compress MORE spacetime, and thus expend more total energy total, either more energy per unit time, or the same power over a longer time, so it doesn't just "ignore" that expansion.

overall, the answer is a definite "sort-of"

[nhnifong]

Thank you for the interesting explanation TheGatesofLogic!