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Comment by gcanyon | Hacker News Reader

The stars near us in the Milky Way are tens of light years away, and we’ll have to do some incredible science and technology to visit them.

How much worse would it be if you were an intelligent life form on a planet orbiting one of those orphans, and the nearest star was a thousand, ten thousand, even a million light years away?

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At some point, given the universe's expansion, galaxies will be moving away so fast that the observable universe will be just one galaxy. Already, the effective rate from galaxies far away is actually faster than the speed of light. Even at light speeds, we cannot reach 94% of them as we would never catch up if we left today. In cosmic scales of many billions of years, observers on every galaxy will eventually think that their galaxy is the only one that has ever existed.

Large distance and time scales are rather gloomy.

Indeed, the only hint they would have that they were not alone would be the "weak" gravitational effects of all other existing matter pulling at the fringes of their visible universe. Perhaps they'll refer to this phenomenon as "dark energy".

The galaxies in our local group are gravitationally bound.

Right, but isn't the expansion increasing? So if they don't merge into one big galaxy at some point the expansion would out run gravity?

> In cosmic scales of many billions of years, observers on every galaxy will eventually think that their galaxy is the only one that has ever existed.

Unless of course some of them manage to find ways to preserve information and keep records over those billions of years. The lost galaxies will be known from that.

But the information would still be forever locked away in its own galactic island. Even transmitting at the speed of light, the information could never reach any other galaxy. So no, in this far future, a galaxy would only be able to know itself.

We have records of other galaxies right now. Assuming we manage to preserve those and humanity over these kinds of time scales, our descendants would be able to access those records and learn that other galaxies once existed that are beyond their cosmic horizon now.

Might be much harder to bootstrap your astronomy in that situation as well.

As I understand, we built much of our picture of the universe on the fact that we can do precise parallax measurements of nearby stars.

Then we used that to see how far the nearby Cepheid variable stars are, and used the patterns those exhibit to measure distant galaxies. Then at some point we could determine the distance to a certain kind of supernova, and now we have a 3D model of the whole observable universe.

But it starts with parallax measurements which are limited in range.

https://www.jpl.nasa.gov/news/cosmology-standard-candle-not-...

Ooh, this is super interesting! This video by The Science Asylum lays out the "distance ladder" in great detail: https://www.youtube.com/watch?v=zrpdwJNNskk This wikipedia article also lays out the distance ladder: https://en.wikipedia.org/wiki/Cosmic_distance_ladder so if our hypothetical "intelligent species on a planet orbiting an orphan star" were more than 30,000 light years away from the nearest star, they wouldn't be able to measure that distance with accuracy using the technology we have now. That might not apply to ranging whole galaxies? And they could get better with time? In any case, my idea that they might take off for a "nearby" star "only" ~100 light years away because they don't know better is foolish.

There is a novel written with that premise as the background! I can share the title with you, if you'd like - that particular premise is a very small part of the novel.

I'd like to read it!

Iain Banks's "Against A Dark Background"?

Indeed, though I was hoping to avoid posting an explicit spoiler - I'd have rot13'd it or something.

One word. Time warps.

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