“Doomsday” covers a lot of turf. A virus that is 100% communicable and 100% lethal against humans, and only humans, would be a perfectly acceptable “Doomsday,” even though the sudden extinction of Mankind would leave Earth just fine. From there you can move up to, say, a major global nuclear war or a substantial asteroid impact… not only humanity wiped out, but the biosphere of Earth damaged for a good long while. Then you can move up to things that would physically destroy the planet itself, and if you are really enthusiastic, you could blow up the Sun. But even then, blowing up the sun and vaporizing all the planets of the inner solar system is just going to be a pretty light show in the skies of planets orbiting Alpha Centauri, with maybe some damage to their ozone layers. And even if you could destroy the entire galaxy, there are still uncounted trillions of other galaxies that’ll get along just fine. If you want a really comprehensive “doomsday” you need to trash the observable universe.
There are not currently a whole lot of ways, even in theory, to do that. A Big Crunch. Wait long enough for Heat Death. Perhaps Phantom Energy will cause a Big Rip. A brane collision between this universe and another, resetting everything. These are all fine, but they are natural functions of the universe rather than something an evil genius might bring about… like threatening the world with “in 5 billion years the sun will expand and roast the world! MuAHAHAHAHA!!!” Not that much of a threat, as threats go. You need something that can be brought about, rather than something that will just eventually happen on its own. And here we get to “Vacuum Decay.”
The video below explains it nicely. But in short, it’s the idea that the Higgs Field (or some other basic value of the nature of reality) is not currently at its ground state, but is currently at a higher potential level. With a push, some small sector of space could be pushed out of the current “false vacuum” and collapse down into the true vacuum. In doing so, the fundamental constants would change in unpredictable ways. Chemistry wouldn’t work right anymore. Atoms could well fly apart. Particles could dissolve. In short, *everything* would be utterly destroyed. What makes this into a true Doomsday system is that this doesn’t just happen in some discrete volume of space, but it expands outwards as a spherical shell moving at the speed of light. If it began in a galaxy a hundred million light years away, that galaxy would appear to be perfectly fine for a hundred million years. The expanding shell moves at the same speed as photons, so there would be absolutely no warning. One second everything is fine; the next, the shell gets to you and you cease to exist in the time it takes for light to cross you. So there would be no way to detect it coming, and no way to protect yourself even if you knew it was happening.
From a science fiction perspective, here we have a Universal Doomsday Device that could, hypothetically, be the product of a sufficiently advanced Evil Genius. To destroy the universe you don’t need to actually destroy the entire universe. You just need to tick over a small spot of false vacuum. A spot no bigger than a proton would be perfectly adequate. And then the process will proceed from there, encompassing the observable universe.
But if you put this into a science fiction perspective, this is a doomsday that could be survived. If you run into the expanding shell, you’re boned. There’s no surviving it, and no reversing it. But you could outrun it.
Warp drive, hyperdrive, jump drive, etc. all allow science fiction spacecraft to travel faster than light. So you could fly away from the zone of destruction faster than the lightspeed expansion. Of course you’ll need to know to do this in the first place. If, say, the Klingons accidentally began a vacuum decay in one of their research facilities 200 light years from Earth, it will take 200 years for the shell to get to Earth. No radio messages will be able to alert you. Any ships you send towards the research base will not be able to alert you to the problem… they’ll simply vanish. But if you happen to have a large enough mesh of sensors in the area communication with FTL subspace radio, you should be able to get a heads up. The sensor net will simply start shutting down; the individual sensor probes won’t give you any useful data other than their subspace radio transmissions suddenly stopping. Probes sent to check on them will also vanish. But if you figure out that the probes are disappearing based on their distance from a central point, you should be able to guess what’s going on. From there you’ll send a ship loaded with “pingers” to somewhere ahead of the shell. Scatter these devices, simple subspace radio beacons, in advance of the shell, and listen to them shut down.
Once you have determined what’s up, you need to start planning. Earth has two hundred years. But you already have warp drive, capable of traveling hundreds or thousands of times faster than light. Spend then next one hundred years evacuating the population to a fallback position, say, 2,000 lightyears downrange. You now have 2,100 years before the shell gets to you. By which time you will doubtless have greatly improved warp drive. Perhaps capable of velocities millions of times the speed of light, moving whole worlds or even entire star systems. With this you can pull up stakes to another galaxy, tens of millions of lightyears away. If your propulsion systems are really good, you could move tens of billions of lightyears away, beyond the edge of the observable universe. Due to the expansion of the universe, this would put you out of reach of the expanding vacuum decay shell. The distance is so vast that the rate of expansion exceeds the speed of light. You are now permanently safe.
Well, maybe. If those dumbass Klingons were able to make a vacuum decay doomsday device a few thousand years ago, it’s a safe bet that the technology is now available to any morose emo teenager in your society. Vacuum decay bombs might be popping off with the regularity of DDOS attacks today. Additionally, you need to make sure that that far distant galaxy you want to colonize hasn’t already been consumed by its own vacuum collapse, and that there aren’t any in progress between here and there. It might be that the universe is filled with expanding vacuum decay bubbles, and that older civilizations are flying torturous paths between the galaxies in warp drive equipped Dyson spheres.
This, just maybe, might explain the Fermi Paradox. Where is everybody? By all rights the galaxy should be filled with networks of civilizations. Especially if they have FTL propulsion, the aliens should be everywhere. But they seem to be absent. Well… perhaps 30,000 years ago a vacuum decay bomb was set off in the galactic core. Over the next 10,000 years all the spacefaring races got their affairs in order and then took off for Andromeda. Civilizations millions and billions of years old vanished essentially overnight, leaving behind those who did not have starflight capability. You know, us.
One wonders how a vacuum decay and a wormhole would interact. Chances seem good that a vacuum decay event would cause a wormhole to simply collapse. But perhaps the event would transition through the wormhole, spitting out the other end and starting another expanding sphere, unknown lightyears away from the original. If the universe is filled with tiny but exceedingly long natural wormholes, then this would be a way for a vacuum collapse to wipe out the entire universe in relatively short order. It would also be an incentive for an escaping spacefaring race that uses wormholes to be pretty bloodthirsty. Once *you* are through, it’s in your interests to promptly shut down the wormhole behind you lest you let the vacuum decay through. Screw those schmoes behind you…