When we gaze out into the abyss of space, it seems like the most peaceful, serene of sights. Night after night, the planets and our Moon migrate predictably — like clockwork — while the stars, the Milky Way, and even extragalactic objects hardly change at all over time. Sure, there are occasional cataclysms, including stars that die and new lights that briefly appear before fading away, as the shining stars burn through their fuel, evolve, and sometimes even interact. But space itself, although it’s expanding, seems like it’s the most stable thing of all: the “stage” upon which the play of the Universe unfolds.

Things seem safe for us for two major reasons: everything that we know of that’s potentially dangerous is very, very far away, and that signals, even greatly energetic ones with the potential to cause harm, can only propagate at the speed of light. That doesn’t just include neutrinos, high-energy radiation, and the blast waves from events like supernovae, but potentially the greatest catastrophe of all: vacuum decay. Decaying from a false vacuum state of the Universe to a lower-energy state would be a world-ending (and more) catastrophe: one that we wouldn’t even see coming until it, and its consequences, arrived. For that reason, should we stop worrying about it entirely? That’s what Yair Givoni wants to know, writing in to ask:

“If a false vacuum decay bubble formed out there, outside of the observable Universe, and this bubble propagates no faster than the speed of light, it will never reach us. So why worry?”

Although it’s up to each individual to decide what is and isn’t worth worrying about, you asked me, and I think there are plenty of good reasons to not only worry, but to think about solutions to what might become the biggest problem we’d ever encounter. Here’s why.

This graph shows the 1550 supernovae that are a part of the Pantheon+ analysis, plotted as a function of magnitude versus redshift. The supernova data, for many decades now (ever since 1998), has pointed toward a Universe that expands in a particular fashion that requires something beyond matter, radiation, and/or spatial curvature: a new form of energy that drives the expansion, known as dark energy. The supernovae all fall along the line that our standard cosmological model predicts, with even the highest-redshift, most far-flung Type Ia supernovae adhering to this simple relation. The slight, upward curve to the graph beginning above a redshift of z = 0.2 provides strong evidence for dark energy.

Credit: D. Brout et al./Pantheon+, Astrophysical Journal, 2022

Back in the 20th century, one of the major goals of cosmology was to determine — once and for all — what the Universe itself was actually made of. We knew that matter and radiation were part of the story: the part that includes us. We knew that there was some type of dark matter out there, and far too much of it, from its gravitational effects, to just be “normal matter” that’s dark and non-luminous. We knew that neutrinos were out there, but were far too low in mass, as well as far too fast-moving in the early Universe, to account for what we saw.

Then, in the 1990s, the first strong evidence began to come in, from exploding stars located hundreds of millions or even billions of light-years away, that supported a picture of the Universe that didn’t just have dark matter within it, but a new form of energy that caused the expansion of the Universe to accelerate rather than slow down: dark energy. This dark energy, nearly 30 years after its initial discovery, remains consistent (despite tensions suggested by the latest large-scale structure data) with a cosmological constant: a constant, positive, non-zero form of energy, uniformly inherent to space itself at all locations.

The matter and energy content in the Universe at the present time (left) and at earlier times (right). Note how dark matter and dark energy dominate today, but that normal matter is still around. At early times, normal matter and dark matter were still important, but dark energy was negligible, while photons and neutrinos were also quite important. The expansion rate is determined by the actual, instantaneous value for density, not by the distribution of the pie chart.

Today, based on far superior data than was available in the 20th century and from many independent lines of evidence, we not only have validated the presence of dark energy, but have determined that it’s the dominant form of energy in the Universe: something, as Nobel Laureate Adam Riess reminded us in a 2024 interview, that you can only discover once. Initially, there were large uncertainties as to what the behavior of this dark energy was, including:

• whether it would weaken, or get less dense, as the Universe expands,
• whether it would rise in strength, or intensity, over time, leading to a Big Rip scenario,
• whether it was spatially homogeneous, or the same everywhere, or whether it would clump up the way matter does,
• whether it showed signs of evolution across either space or time,
• and whether it deviated from the predictions of a cosmological constant in any way.

Today, here in 2026 — nearly three full decades after the first evidence indicating its presence was revealed — the answer to all of these questions appears to be “no.”

That means that dark energy, in the context of our theory of gravity, Einstein’s general relativity, behaves as a cosmological constant. That term has a counterpart in our “other” way of making sense of the physical Universe: through quantum field theory. In every quantum system, there’s a lowest-energy state, also known as the system’s zero-point energy. You might think that the value of that zero-point energy would always be zero, but in fact that’s not the case at all.

The lowest energy level (1S) of hydrogen (top left) has a dense electron probability cloud. Higher energy levels have similar clouds, but with much more complicated configurations and covering a much larger volume of space. For the first excited state, there are two independent configurations: the 2S state and the 2P state, which have different energy levels due to a very subtle quantum effect. The ground state of hydrogen, at top left, has a zero-point energy that is finite, positive, and non-zero: a property it shares with many quantum systems.

Consider, for example, the humble hydrogen atom, shown above. In its lowest energy state, known as the ground state, the electron orbits the atomic nucleus in a cloud-like configuration. However, the electron isn’t at rest at the atom’s center, co-located atop the nucleus itself, but instead has a rather rapid motion (indicating kinetic energy), an indeterminate position that’s “smeared out” due to Heisenberg uncertainty, and a finite, positive, non-zero amount of energy to it. This was discovered more than 100 years ago, and represented an early version of evidence that not every system can be reduced to a zero-energy state.

It turns out that this can be extended to even a system containing nothing more than empty space itself, as empty space still contains quantum fields (and the laws of physics) within it. There is nothing that mandates that the ground state, or zero-point energy, of empty space itself must be zero. That value could be finite and positive, zero, or finite and negative. It’s only by measuring the expansion of the Universe, and how that expansion rate evolves, that we can reconstruct what’s in our Universe, and determine the value of this zero-point energy.

As observations overwhelmingly indicate, that value is small — the energy equivalent of a handful of protons per cubic meter of space — but finite, positive, and non-zero.

A scalar field φ in a false vacuum. Note that the energy E is higher than that in the true vacuum or ground state, but there is a barrier preventing the field from classically rolling down to the true vacuum. Note also how the lowest-energy (true vacuum) state is allowed to have a finite, positive, non-zero value. The zero-point energy of many quantum systems is known to be greater than zero, and nobody knows if today’s observed dark energy is due to a true vacuum or false vacuum state.

That brings up a really big, existential question of whether our Universe is stable in its current configuration: whether this finite, positive, low-energy state that we’re in is truly the ground state of the Universe, or whether this is only a “local minimum” to all possible energy configurations, with at least one (and possibly more) lower-energy states also being possible. This can happen spontaneously: either through a direct transition or, if a direct transition is forbidden, through the process of quantum tunneling, where even forbidden transitions (like the hydrogen spin-flip transition) can occur.

For a system of particles, like an atom, transitioning from a higher-energy state to a lower-energy state results in the emission of energy: in that case, in the form of a photon (or, occasionally, multiple photons). If the transition requires quantum tunneling, that transition can take an extremely long time to occur, and still occurs randomly: without warning and on unpredictable timescales. When the transition does occur, the signals from that transition (i.e., photons) can only propagate outward at the speed of light.

For empty space itself — or for a quantum field in general — such a transition will result in something else entirely.

Visualization of a quantum field theory calculation showing virtual particles as field fluctuations in the quantum vacuum. (Specifically, for the strong interactions.) Even in empty space, this vacuum energy is non-zero. If there are additional particles or fields beyond what the Standard Model predicts, they will affect the quantum vacuum and will change the properties of many quantities away from their Standard Model predictions. However, the QCD contribution cannot be calculated perturbatively, the way electromagnetism can.

The zero-point energy of quantum fields in space, among other things, determines the coupling constants of those fields: the thing that’s responsible for the strength of the forces and interactions between all quanta in the Universe. Change the zero-point energy, and you change how the forces behave. That means:

• the structure of protons and neutrons will change,
• the rest masses of atoms and atomic nuclei will change,
• the transitions between different energy levels will change,
• the shapes and bonds that are present in all molecules and ions will change,
• and, in general, the rules that enabled the formation of all bound structures will be different from what they are today.

This implies that any stable structure that we have that’s made out of multiple fundamental particles bound together, from protons and neutrons to atomic nuclei to atoms to molecules to humans, will be unstable in their current configuration if the quantum vacuum successfully tunnels into a lower-energy, more stable state.

In other words, if the zero-point energy of empty space changes where we are — if we transition from what we currently experience as the lowest-energy state to an even lower-energy state, whether it’s the “true ground state” or just a lower “false minimum” in energy — then everything that makes us up will immediately disintegrate. The building blocks of atoms themselves will rearrange into a configuration that’s more stable under those new rules, and that will mark the end, practically instantaneously, of all the structures made from normal matter that presently exists today.

In a Universe that comes to be dominated by dark energy, there are four regions: one where everything within it is reachable, communicable, and observable, one where everything is observable but unreachable and incommunicable, one where things will someday be observable but aren’t today, and one where things will never be observable. The labeled numbers correspond to our consensus cosmology as of 2024, with boundaries of 18 billion light-years, 46 billion light-years, and 61 billion light-years separating the four regions. On scales of ~10 billion light-years and larger, the Universe is almost perfectly uniform.

Of course, space is large, and the odds of having such a transition if we aren’t in the lowest possible energy state of all are random: just as likely to occur anywhere else as it is to occur here, where we are. We live in a Universe that’s vast, where:

• our Solar System extends for up to 1-to-2 light-years in extent,
• our galaxy is over 100,000 light-years across,
• our Local Group, the largest bound structure that we’re a part of, extends for 3-to-5 million light-years away from us,
• the Universe beyond our Local Group is expanding,
• if we sent a signal today, at the speed of light, it could reach as far away as objects that are between 15-and-18 billion light-years at present,
• where the farthest thing we can observe, today, is at a distance that’s 46 billion light-years away from us,
• and where the unobservable Universe, beyond the limits of our observability, goes on for an unknown extent (that’s possibly infinite), but that’s likely much, much larger than the observable Universe.

You might wonder, then, why it’s even worth bothering to worry about vacuum decay. After all, if it’s going to happen somewhere, it’s probably going to happen — just based on volume arguments — somewhere far, far away from where we are.

Wherever it does happen, sure: the laws governing matter, and hence matter itself, will fundamentally change in its properties and structure, and that would wipe us out instantaneously. But that destruction would simply begin from a point and propagate outward, in a bubble of destruction, that only moved at the speed of light through the expanding Universe.

In a vacuum decay scenario, our Universe exists in a false minimum state, and it’s possible to arrive, either through quantum tunneling or an energetic kick that causes us to leave that state, to enter a true (or truer) vacuum state. If that happens anywhere, every bound structure, from protons on up, will be destroyed in a “bubble of destruction” propagating outward at the speed of light.

Unless it occurred in our own galaxy, it would take over 100,000 years to reach us. Unless it occurred within the limits of the reachable, communicable Universe (within about 15-18 billion light-years), it would never reach us at all. And if it occurred beyond the limits of the observable Universe, it’s very likely that nothing within our observable Universe would ever interact with it.

So why worry?

There are plenty of reasons. First, it’s possible that vacuum decay has already occurred somewhere, and portions of the Universe have already been destroyed and reconfigured. Only, until that “bubble of destruction” reaches us, we won’t have any ability to detect it, because it propagates at the speed of light: the maximum speed that any signal can propagate at through the Universe. Second, if vacuum decay can happen at all — if there’s a non-zero probability of it occurring — then someday it will happen in our region, inevitably, and the resulting “bubble of destruction” will then wipe us out as well. But the third reason to worry, or at least to think about it, is because if it is possible, then it’s also potentially possible to stop it, or prevent that transition from occurring.

This animation shows a simulation of a wave packet, traveling rightward, as it strikes a high but thin potential energy barrier. The solution is described by the time-dependent Schrödinger equation, which shows most of the wave packet’s probability function being reflected, but a small fraction of it being transmitted to the other side of the barrier, showcasing the phenomenon of quantum tunneling.

The possibility of preventing a random quantum transition from occurring might be foreign to you, but if your system is reliant on quantum tunneling, such prevention really is possible. It’s known as the quantum Zeno effect: a (true) version of the (untrue) old adage that “a watched pot never boils.” If you have a quantum system that can, potentially, tunnel into one or more other states (including more stable states), then over time, its wavefunction will spread out. There will be a non-zero probability that, after a certain amount of time has elapsed, the quantum state enters a more stable configuration, completing a quantum transition. This can be:

• the decay of a radioactive atom,
• the flipping of the relative spin of an electron to the spin of the atomic nucleus it orbits,
• or the decay of a system in a metastable (false minimum) state to a more stable (true minimum, or at least more stable false minimum) state.

However, the likelihood of those transitions — defined by things like “half-life” or “average time to decay/transition” — can change, and specifically can be extended, simply by making frequent, repeated measurements of the quantum state. In other words, if you can keep “watching” (or interacting with) the quantum vacuum in its current state, and if you can do it continuously, you can prevent that quantum tunneling from occurring at all.

This three-panel animation shows a depiction of the quantum Zeno effect. Initially, in all three systems, they’re in the same (central) state. Over time, the wavefunction is permitted to spread out. On the left, the spread is smooth and continuous, and quickly bleeds over into the other boxes, representing other possible quantum states. In the middle panel, a measurement is conducted after a certain amount of time has elapsed, and the wavefunction is likely to be (and happens to be) in a different box. However, by making a rapid series (or a continuous set) of measurement, as on the right, the wavefunction remains localized, slowing or even preventing the spread of the wavefunction that enables the possibility of other states.

Now, before you worry too much, it’s worth pointing out that we don’t know whether we exist in a false minimum of the quantum vacuum, or whether our Universe really is in the true “lowest energy” state. We know that the zero-point energy of empty space itself, from observations, is positive and non-zero, but we don’t know whether a lower-energy state is possible. If it isn’t, then there’s nothing to worry about at all: the vacuum is eternally stable, and this bubble-of-destruction scenario is one that’s theoretical only, and will not come to pass within our Universe.

But if we are in a false minimum state for our quantum vacuum, and vacuum decay is possible, then those quantum transitions — and the resultant bubbles-of-destruction that will arise and expand outward — are inevitable. Sure, because of the vastness of our Universe, the rapid, accelerated expansion of space, and the finite speed of light at which signals can propagate, the odds are any one particular transition won’t ever affect us: the bubble-of-destruction won’t ever reach or interact with us. But eventually, inevitably, one will. The fact that we can potentially prevent such a bubble from forming here, where we are, should be motivation enough to worry enough about it that we can prevent our own demise. After all, this is the only Universe we’ve ever known, and it made our emergence possible. For that reason alone, it’s worth preserving. If we can make a difference in doing so, that’s all the more reason to pursue it.

Send in your Ask Ethan questions to startswithabang at gmail dot com!

This article Ask Ethan: Should we stop worrying about vacuum decay? is featured on Big Think.

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