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samuelcook t1_j0o3dyw wrote

Reply to comment by pfildozer12 in Is the expansion of the universe significant enough to be included when calculating the trajectory of spacecrafts? by andreasdagen

How far would we need to travel in order for us to start including expansion in our calculations? Is it further than our obversable universe?

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TheStarsFell t1_j0o9td8 wrote

I don't really think that the expansion of the Universe would ever need to be considered, even if we traveled between, say, our galaxy and Andromeda (which will likely never be possible). Even if we traveled between five or six galaxies in our local galactic cluster, that's still a very small distance when you consider that the observable Universe is 90+ billion light years across.

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Thocc-a-block t1_j0oup7l wrote

Isn’t the expansion of the universe faster than the speed of light 🤔

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midnight_mechanic t1_j0p0sm3 wrote

Over very large distances it is. That's literally the boundary of the "observable universe". Everything past that is moving away from us faster than the speed of light.

The rate of expansion of the universe is increasing as well, so stars and galaxies are constantly moving beyond this boundary. In the far far distant future, whoever lives on earth, if it even still exists, won't have any way to tell that anything beyond the local group exists or ever did exist.

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midnight_mechanic t1_j0p2zg4 wrote

We can't travel beyond the edge of the observable universe. That space is expanding away from us faster than the speed of light.

There's only three conceivable ways that we could ever travel outside our local group

  1. we create some generation ship that travels at a significant fraction of the speed of light. It would take 50 or 100 million years for that ship to travel to the Vergo Cluster of Galaxies, which is the next closest large galaxy group to us.

  2. we create some faster than light warp drive mechanism. Recent calculations have suggested that it might be possible if we could convert the entire mass of Jupiter into energy. For this we would be limited by the speed the warp drive is able to provide. If it was even some single digit multiple of the speed of light, it would still take 5 to 10 million years to reach the Vergo Cluster.

  3. we create a worm hole that allows us to travel any distance very quickly. This would also require negative energy or negative mass to hold the wormhole open. We don't have a theory of quantum gravity that could even tell us if this is possible, but most current theories say that any wormhole would collapse immediately, or not allow mass (or possibly even light) to travel across it.

Keep in mind that due to relativistic time dilation, traveling faster than light is basically the same as traveling back in time. Also wormholes could theoretically be manipulated to create time portals that allow forward and backward movement in time.

Anyways, the Vergo Cluster is loosely gravitationally bound to our own local group. If you wanted to travel to a galaxy that was not gravitationally bound to us in some way, that might take 50 to 500 million years of travel (depending if faster than light travel is possible), mostly through voids of interstellar space. That would require taking the expansion of the universe into account for trajectory calculations.

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