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Conditions:

  • The civilization is in a galaxy which is not the Milky Way. For the purposes of this question, assume the target galaxy is 0.5 Mpc away.
  • The civilization has elected, for whatever reason and regardless of practicality, to build full Dyson spheres. Assume for the purpose of the question that they had all the necessary mass within their galaxy to do this.
  • The Dyson spheres absorb 100% of all energy emitted from the stars and do not emit any heat. From the perspective of our understanding of science, that's probably impossible, but for the purpose of the question, please assume it's true.
  • The civilization has elected, for whatever reason and regardless of practicality, to enclose each and every energy-emitting source in their galaxy with the exception of black holes.

From a certain point of view, the galaxy is dark, with no visible-spectrum light emitted. This question does not embrace the effects of Dark Matter (we can't detect it today anyway). All the mass is there, but all the stellar-emitted light is gone.

It is my recommendation that unless you're absolutely sure about your answer that you give this question a half-day before answering for people to ask for clarifications. I'm expecting the requests.

Question: How could Earth, 2023, detect such a galaxy?

  • Do not worry about how long this might take. In other words, if it just so happens that some aspect of the James Webb telescope could do this, assume that serendipitously it just happened to sweep across the galaxy's location last night and explain how it succeeded.

  • I am not asking that we discover the presence of the Dyson spheres. I am only asking about detecting the presence of the galaxy. As in some lucky astronomer sending out a Tweet along the lines of, "we can't see it, but we know it's there, and we know it's the size of a galaxy."

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    $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Aug 18, 2023 at 2:17

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One of my favorite things about galaxies is their gas. It's a major source of baryonic matter; while the amount of gas varies between galaxies, in some cases it can be comparable to or greater than the stellar mass (see e.g. Magdis et al. 2012). While the Dyson spheres mean stars in the galaxy won't affect its emission, the gas absolutely will -- and there's a nice little loophole.

There are lots of ways to detect gas. Some of the more common or interesting examples:

  • Much of it is in the form of neutral hydrogen (HI), which forms the 21 cm line. 21 cm emission is easily detectable much further than 0.5 Mpc, up to a redshift of $z\approx0.3$ (see section 3.2.2 of Fabian et al. 2020), i.e. roughly 800 Mpc -- not high-redshift by most cosmologists' standards, but far enough! Beyond $z\approx0.3$, you can use Lyman-alpha lines to find neutral hydrogen.
  • Another common component is molecular hydrogen, H2. It's harder to find, but we can use a proxy, carbon monoxide (CO) -- the second most abundant gas component after HI. CO emits light through rotational transitions and can easily be found as far as $z\approx3$, or about 1.5 Gpc (Riechers et al. 2020).
  • If you don't want to detect the gas itself, you can look for its effects on background objects. Andromeda's massive gas halo was found by looking at how it creates silicon absorption lines from quasars far beyond it (Lehner et al. 2015).

So . . . how would this galaxy end up being detected? It would likely be found in an all-sky survey looking for HI or CO emission. Many such surveys specifically target high-redshift galaxies far, far beyond 0.5 Mpc (e.g. VLASPECS), but lower-redshift candidates, like our camouflaged galaxy, inevitably sneak in. There would then be targeted follow-up observations looking for other emission or absorption lines. You don't even need JWST; ground-based surveys should be able to do the job. They can detect much fainter galaxies farther away; this one should be no problem.

This galaxy would appear to be full of gas but with no stars. It might be possible to calculate its mass by looking at the rotation curves from sets of HI regions in the galaxy. This would make it clear that there's additional, unseen matter beyond the gas. Much of that would be chalked up to dark matter, but astronomers would see that the matter distribution was inconsistent with a typical dark matter halo. At some point, someone would propose a non-dark-matter dark component.

I don't know whether we'd ever stumble upon the truth -- it's just too wacky. We astronomers tend to be very cautious about making big claims. Since there's so much we don't know about the universe (and dark matter in particular), theories would probably spring up trying to explain how the rotation curves could be created using just dark matter. We're (mostly) modest enough to attribute weird phenomena to our lack of understanding of complex processes, rather than aliens.

But -- getting back to your actual question -- regardless of what explanation the astronomy community settled on, we'd definitely detect the galaxy. It would be one more mystery in a very mysterious universe.

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Gravity.

Dyson spheres can’t hide that. Clear gravitational lensing would be observed at that weirdly dark spot.

Energy isn’t the only thing galaxies emit.

Why isn’t it just a blob of “dark matter?” Because unlike dark matter, stuff behind it is also black. The galaxy is made of opaque stuff.

Not worrying about how long it takes:

It takes several observations spanning a long time during different points along our elliptic. Likely a few years would show us there’s a gravitational anomaly that isn’t dark matter. But the timeframe isn’t relevant.

It’s a galaxy

because the mass can be measured by the lensing of candle points behind it (everything in the visible universe has something behind it).

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short answer for limited time. If this galaxy interposed itself between us and another galaxy it would create a gravitational lense. Obviously it would present as an anomaly since the measured mass would not match the observable mass, thus inviting closer scrutiny. How we could possibly determine it was a galaxy full of dyson spheres at intergalactic distances.... that would be the true puzzle.

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    $\begingroup$ Discovering the Dyson sphere's isn't part of the question, merely galactic detection. I'll clarify that in the question. $\endgroup$
    – JBH
    Aug 16, 2023 at 16:28

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