# Travel Delays due to the Expansion of the Universe

So, I know that the universe is expanding, and that that means that things are generally getting further apart as time goes on. What I'm having difficulty with, however, is figuring out just how much this might affect things like interstellar travel.

The 'world' I'm building is essentially a series of colonies on planets in star systems no more than 1 thousand light years apart, in the same galaxy. I plan on having FTL where a journey of that length would take about 60 days (assume some sort of Alcubierre drive, or if that complicates things too much, assume the same distance travelled at sublight speeds).

What I'm wondering is if someone were to start one of these 1000-light-year journeys, would the destination drift away from them at a measurable/meaningful way? If a shipping company continuously sends ships on this journey for a few centuries, would this effect become more important then? If numbers can be found for the change in distance over time, I would love to know them. Alternately, if the galaxy has enough gravity or other forces affecting its stars to nullify the universe's expansion, then evidence of that would also be sufficient.

• The expansion does not affect bodies within the same galaxy. 1000 ly is nothing like an intergalactic distance. Commented Mar 23, 2017 at 6:56
• @JDługosz, how does space time know when two bodies are in the same galaxy to prevent affecting them? probably you mean that within the same galaxy the effect is negligible...
– L.Dutch
Commented Mar 23, 2017 at 10:08
• No, the galaxy is not expanding in the same manner as the distances between them! The effect is a minor correction to what it means to orbit, effectivly weakening the attraction between them. Two stars 1000 ly apart will not move farther apart due to cosmic expansion. Commented Mar 23, 2017 at 15:53

## 2 Answers

The universe is expanding over large distances. Over short distances (1000 ly is a short distance in this sense) the universe is gravitationally bound and does not expand at all:

Once objects are bound by gravity, they no longer recede from each other. Thus, the Andromeda galaxy, which is bound to the Milky Way galaxy, is actually falling towards us and is not expanding away. Within the Local Group, the gravitational interactions have changed the inertial patterns of objects such that there is no cosmological expansion taking place. Once one goes beyond the Local Group, the inertial expansion is measurable, though systematic gravitational effects imply that larger and larger parts of space will eventually fall out of the "Hubble Flow" and end up as bound, non-expanding objects up to the scales of superclusters of galaxies.

The Local Group, about 10 million ly across, is considered to be the largest object which will always be with us. Only larger objects will be affected by the expansion of the universe.

Over a 60 day journey the expansion will be negligible.

The rate of expansion is 67.15 ± 1.2 (km/s)/Mpc

There are 3.26 light years to the parsec.

Over the distances you are talking about that amounts to 20 m/s of expansion. On on a 60 day journey the distance will extend by 103680000m, which is less than one light second.

• Not to mention that on such short scales the Universe is gravitationally bound and does not expand at all... Commented Mar 22, 2017 at 21:33
• @AlexP That sounds like something that could be turned into an excellent answer. Commented Mar 23, 2017 at 12:43
• I have turned the comment into an answer. Commented Mar 23, 2017 at 14:11