The Council of Giants and Rare Earth Theory
I recently read some new astrophysics.
We (the Milky Way galaxy) are part of region called the Local Sheet. (Why can't scientists think of better names?) The Milky Way, Andromeda, Triangulum, and our associated minor galaxies are in middle. Outside, there is a rough ring of 14 galaxies, called the Council of Giants. All these galaxies are more-or-less in a flat plane.
To me, this seems connected to the question of the frequency of life in the universe (i.e. are there aliens to find?). There are two main ideas in play here The first is called the Anthropic Principle, which means that we can only exist to ask these questions in special circumstances (e.g. on Earth vs. in space). The second idea is the Mediocrity Principle, which means we should assume our circumstances are ordinary unless proven otherwise (e.g. you could swap our star with a similar one with no harmful effects). In most cases, the reality probably lies somewhere between the two. For example, I could use any computer to type this in, but I do need an available computer.
Now, we need to remember that galaxies move around (we are due to hit Andromeda in about 4 billion years). They just do it over millions to billions of years instead of human timescales. In a common form of movement, galaxies (especially the more-numerous dwarf galaxies) form in deep space and them move into clusters, following strands of dark matter. The same dark matter would have been involved in the formation of the Council of Giants, so the incoming galaxies would have to pass through (or near) the ring. I would guess that Council of Giants would capture (or eject?) most of these before they reached us.
To me, this sounds like a good thing. While galaxy collisions involve almost no actual star-to-star collisions (because space is so empty), they do involve a lot of changing gravitational tides, which are bad for orbits. Earth is pretty close to the sun, and would only be affected by a relatively near miss (estimated at a 1-in-a-million chance). However, there are a lot of other things in larger (and thus less stable) orbits, such as the Kuiper Belt and the (hypothesized) Oort Cloud, and whatever equivalent Andromedian stars bring. As these contain billions of dinosaur-killer-sized (or larger) "asteroids", we are much better off if they stay in the orbits they have now, where they never come near us. Disrupted orbits would be scary, which means that a collision with another galaxy would probably be bad.
In my opinion (I am not an expert), it seems reasonable to assume that we only have particularly unusual things (like our moon) if we need them to survive. If the Council of Giants really is blocking the incoming galaxies, it raises the question: Is life on Earth so fragile that we actually need a whole ring of "guard" galaxies to protect us?
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