Monday, August 8, 2016

The End of the Milky Way

An alien civilization that is motivated to stay in existence might be able to do so by migrating from solar system to solar system. With a small population and the mandatory high-percentage recycling, they could hope to get on the order of a million years from each solar system they colonize, and then move on. There are so many solar systems in the Milky Way, and more being born every gigayear, that they could expect to keep going for billions of years, provided nothing else gets them.

They would have asymptotic technology from the very first solar system, their home world and origin planet, and so could avoid parking near any imminent supernovas, and could arrange to stay in quiet parts of the galaxy. By doing mapping of nearby stars, they could find any nearby solar systems that are moving on a collision course on one they were contemplating as a future home, and avoid anything that would be disrupted by a stellar encounter. They would stay in the disk of the Milky Way, avoiding the ambient radiation present in the bulge and bar. So, if they chose to do so, they could merrily await the end of the Milky Way. Just when is that, anyway?

Not much thought has be given to the fate of galaxies. They don’t participate in the Hubble expansion of the universe, which is simply groups of galaxies parting from each other. Even the local group, Andromeda, Triangulum, and all the dwarfs like the two Magellanic Clouds, doesn’t participate in that expansion, but keeps on orbiting around one another at roughly the same distances, in a kind of ball of galaxies. It’s not even sure that the nearest neighbor Local Group of galaxies is going to be able to escape the clutches of our Local Group. But that collision is truly a long time out. Let’s try and think out what might happen in a shorter time scale.

One known event is the collision of galaxies, which is understood because we have figured out how to measure the proper motion of them, and can do some simulations given that as input data. Small dwarf galaxies are crashing into the Milky Way all the time, but their minimal size doesn’t disrupt it, except locally. The time scales of galactic encounters is measured in the scale of gigayears, a fraction of a gigayear to several gigayears. It would be so utterly simply for an alien civilization to figure out where, for example, the Canis Dwarf Galaxy is going to disrupt some region of the Milky Way, and avoid that area. If they travel something like a thousand light years from colonizable world to colonizable world, which we call alpha-habitable worlds, in a gigayear they would have crossed the Milky Way many times over.

The big collision, expected between Andromeda and the Milky Way, is forecast to happen about four gigayears out, and all alien civilizations would be able to predict it. At this point in time on Earth, we don’t know what will happen, except that there will be major disruption. Andromeda has a bit more mass than the Milky Way, according to our current estimates, so the gravitational pull of it on stars would be comparable to the gravitational pull that keeps stars in orbit around the Milky Way center. The disk may simply be torn into pieces and dispersed, thence to form some dwarf galaxies or even some rogue stars, ones without a galaxy to call home. The majority of the stars might coalesce into a giant bulge, except without a disk around it, it’s called an elliptical galaxy.

This collision will happen slowly, so if an alien civilization is jumping from solar system to solar system every million years or so, they can probably figure out where to jump to avoid being cast into the center of the bulge, where radiation is intense and stellar encounters run rampant, or being thrown into the great dark void to be a lonely rogue solar system. The latter is a problem, not because life would be unpleasant on a planet of a rogue star, but because there’s nowhere nearby to go to. They would be able to migrate out, perhaps on an earlier than normal schedule, if that was to happen. A nice dwarf galaxy would be a good place to go to, provided the Milky Way was being ripped apart into many of these. A large dwarf galaxy might have a billion stars, which means that there would be good hunting grounds for a lot more iterations of the alien civilization’s colonization attempts.

On a colonized planet, occupied by an alien civilization with asymptotic technology, they would have computational resources far in excess of what we can imagine now here on Earth, and would be able to actually predict the motion of the stars in both the colliding galaxies. Finding which stars were going to form a dwarf would be fairly easy with this much computing power, and so many years to do the calculations, thousands upon thousands at a minimum. No problem.

Another fate might be the important one. There is a black hole at the center of major galaxies, as far as we know, and they slowly eat the stars surrounding it. Consumption is not rapid, as angular momentum keeps stars orbiting it. Obviously, a stellar encounter can mix up the orbits of different stars, allowing one of them to be swept into the black hole. This would be noticable because of the X-ray emission that would result, so we know that, for the last few years, this hasn’t been regularly happening. But in the aftermath of a galactic collision, the rate of consumption could rise, for both of the two or more black holes involved in the collision. It would be unlikely that the two of them would come close enough to interact with one another, but if they did collide, there might be an explosion even able to extinguish life in the galaxies involved. Little is known about the history and future of the black holes in our galaxy, so that at least is a source of some finality to the history of a migrating alien civilization.

Just as scarcity drives the alien civilization from solar system to solar system, the galaxy can suffer from it. It runs on hydrogen gas, which is used to make stars, for example, in the spiral arms. As that gas is consumed, it is only partially replaced by hydrogen that is emitted during the lifetime of a star. It gradually disappears, leaving no raw material to make new stars. How many gigayears in the future the Milky Way has before it becomes bereft of gas and starts to turn off is not known, but there is some limit. Once it does so, the brightest stars will disappear first, then the mid-level stars, leaving only the red dwarfs, which last extremely long. What would an enterprising alien civilization do at this point? Maybe call it quits.

This harks back to the question of cosmological philosophy. If all the members of an alien civilization know their culture, lifestyle, history, and existence will be eradicated from the universe, irretrievably, at some time in the future, maybe ten gigayears, what effect does that have on their motivation to endure until the very last moment? Do they think, much, much earlier than that about voluntary extinction? Do they even think about it before they make their first colonization effort, or only after ten or a hundred of them? Is this an inevitable byproduct of asymptotic technology?

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