Seeking good physical candidates for total collapse of the universe we live in

Question

Bringing this question over from Physics SE where it was closed:

I'm looking for an 'as close to our understanding of current physics' doomsday scenario that results from something about the physics of the world changing subtly.

The scenario ideally has three attributes:

1. The effect should be detectible/measurable
2. The effect should be progressive - not catastrophic until it goes past a certain point - whereupon the whole of existence basically unravels, or the universe is otherwise rendered inert or sterile.
3. The effect should have a runaway positive feedback loop, which is to say that once you've begun making this modification to physical reality, it becomes easier and easier to make MORE of that modification.
4. The effect MUST be cosmological in scale, hence why I'm looking at physical properties of the universe that, if tweaked, would catastrophically alter how physics functions.

I am NOT asking for whatever technology should be the culprit, that much is decided upon (it's FTL travel). I'm asking for the specific attribute(s) of physics that it should be screwing with to plausibly present an existential threat to the cosmos.

Answer 1

An idea that gets tossed around a lot in this sort of scenario is false vacuum decay.

Let's say we have a tiny ball on the end of an ideal spring, moving back and forth as a harmonic oscillator. The ball and spring have some amount of energy $E$, and knowing the value of $E$ tells us something about the dynamics of the system. There's a quantum analog to this, the more abstract quantum harmonic oscillator. Again, if we know the energy of a quantum harmonic oscillator, we can say something about its dynamics. The one difference is that for the quantum system, the energy must be quantized in some discrete chunks, and we say that each possible amount of energy corresponds to a quantum state.

We can order the possible quantum states like $|0\rangle, |1\rangle, |2\rangle,...$, where $|n\rangle$ represents the $n$th state. Note that there is a lowest-energy state, $|0\rangle$, which we call the ground state. In the case of a harmonic oscillator, it turns out to have a non-zero energy.$^{\dagger}$

Quantum field theory describes the universe as a set of interacting quantum fields, with each one describing fundamental particles (this is a slight simplification, but not by much). Each field has its own ground state, which is usually referred to as a vacuum state - and as above, we expect this state to be the lowest-energy state.

Ideally, the vacuum state of a field is stable, but it's possible for it to be only metastable. This can happen if the vacuum state isn't actually the lowest energy state, but a so-called false vacuum. In this case, there actually is a stable lower-energy state, the true vacuum, but a field in the false vacuum state would need a nudge to get there, and without any sort of nudge, it'll stay in the false vacuum state.

The aforementioned false vacuum decay is a scenario where, at some point in space, the false vacuum decays to the true vacuum through some mechanism like quantum tunneling. A bubble or sorts forms around this point; inside is the true vacuum and outside is the false vacuum. It is quite possible - thought not guaranteed - that a required critical condition will be reached where the bubble expands outwards at an increasing speed approaching the speed of light. Within the bubble, the laws of physics may be different in ways that prevent any significant structures like atoms from remaining stable; conversely, there may be minimal impact at all.

Your conditions could be satisfied if we have a series of false vacuums, where the highest-energy false vacuum is actually several states above the true vacuum. At some point in space, this false vacuum decays into a slightly lower-energy false vacuum, causing tiny, non-catastrophic changes but also making it extremely likely that further vacuum decays will happen, each causing more and more severe effects. By the time the true vacuum is reached, the universe as we know it is no more.

---

$^{\dagger}$The one-dimensional quantum harmonic oscillator has states where state $|n\rangle$ has an energy $$E_n=\hbar\omega\left(n+\frac{1}{2}\right)$$ where $\hbar$ is the reduced Planck constant and $\omega$ is some frequency, so clearly the ground state has a non-zero energy of $E_0=\hbar\omega/2$.

Answer 2

Big Rip is another good candidate. Dark energy is going to increase to the point that all matter in the universe is going to be ripped apart. Its downside is that if it's indeed going to happen, it will be billions or trillions of years in the future.

But let's say we have strongly underestimated the dark energy. Let's say the dark energy is going to increase exponentially within our lifetimes. Then we are are going to see strange cosmological effects which progressively be out of compliance with existing science. The only explanation would be that dark energy is actually much stronger than we had assumed by observing distant (millions and billions of light years away) objects, and the universe as we know it would soon end.

Answer 3

You don't need to change anything in physics to have a runaway death of the universe scenario. You just need a strangelet to come into existence.

A strangelet is a hypothetical particle consisting of a bound state of roughly equal numbers of up, down, and strange quarks. An equivalent description is that a strangelet is a small fragment of strange matter, small enough to be considered a particle. The size of an object composed of strange matter could, theoretically, range from a few femtometers across (with the mass of a light nucleus) to arbitrarily large. Once the size becomes macroscopic (on the order of metres across), such an object is usually called a strange star. The term "strangelet" originates with Edward Farhi and Robert Jaffe in 1984. Strangelets can convert matter to strange matter on contact.

Once a strangelet appears anywhere, all mass around it will eventually be converted into strange matter - even intergallactically. It's just a matter of time. The only thing keeping the universe as a whole from actually disintegrating thus is its expansion.

For more information, see this video by Kurzgesagt, starting at 3:52 (or watch the whole thing to learn even more about this bizarre thing).