Will new physics affect our Universe’s far future?

All throughout the Universe, we can see evidence for not only the “stuff like us” that’s out there, but additional forms of energy that take us beyond the Standard Model of elementary particles and forces. Sure, there’s plenty of normal matter: things like atoms and ions, made up fundamentally of quarks, gluons, and electrons, just like we are. There are stars and planets, but also gas, dust, plasma, and even black holes made from the same raw ingredients that make us up. There are also photons, or quanta of light, and the nearly invisible neutrinos and antineutrinos, all playing detectable, measurable roles in the evolution of our cosmos.

But that doesn’t explain everything we know is out there contributing to our Universe. We know, from observing galaxies, galaxy clusters, and the large-scale cosmic web, that the dominant form of mass in the Universe is not found in the Standard Model, but instead is a mysterious novel substance that we presently call “dark matter.” We know that the Standard Model, with all of its known laws and ingredients, cannot account for the matter-dominated Universe we have, and that some type of new physics must have created the matter-antimatter asymmetry. And we know, from observing the expanding Universe in a variety of ways, that the Universe is actually dominated by a novel form of energy that isn’t a type of matter or radiation at all: dark energy.

We think that we know the general properties of what dark matter is, of what caused baryogenesis, and how dark energy behaves. This creates a picture of our Universe’s far future that’s based in the best science we have today. But if any of those things are different from our simple (and perhaps naive) expectations, then our far future could turn out very, very differently from how we currently expect. Here’s how it breaks down.

If you want to know the fate of anything in the Universe, you have to understand the physics behind how it evolves over time. For the matter in our Universe, the physics that governs it is different depending on whether it’s part of a bound structure or not. If it is part of a bound structure, the spacetime that governs its evolution will be non-expanding, and the separation distance between that particle and any other particle that’s a part of that same system will not be compelled to increase due to the expansion of space in that region. But if that matter particle is not part of a bound structure, then it will recede away from all other bound structures, becoming colder, less dense, and more isolated as time goes on.

In the context of the expanding Universe, the clumps of matter that grow large enough rapidly enough can form these bound structures, and then it’s the space between those clumps that keeps on not only expanding, but whose expansion accelerates due to the presence of dark energy as time goes on. This leads to a web of cosmic structure, where matter clumps along filaments to produce galaxies and groups of galaxies, and then clusters more significantly at the nexus of various filaments, producing clusters and even multiple merging clusters of galaxies. The in-between regions, however, continue expanding, driving the various isolated galaxies, groups, and clusters apart from one another as time marches on.

Meanwhile, within any individual bound structure (including our own Local Group of galaxies, with the Milky Way, Andromeda, and the hundreds of smaller galaxies accompanying us), less and less matter flows into them from intergalactic space as the Universe accelerates, placing a cutoff on the total amount of normal matter within it. Over time, the star-formation rate has dropped ever since “cosmic noon” was achieved approximately 10-11 billion years ago; it’s now at just 3% of what it was at its peak, and continues to decline. As time continues to pass, not only will the star-formation rate drop further, but eventually, it will slow to a trickle and then stop altogether.

Star-formation requires neutral clouds of molecular hydrogen, and the more stars that form, the less hydrogen there is. Winds from new episodes of star-formation can expel atoms and ions from galaxies and send them into intergalactic space, and as the gas and dust population within a galaxy goes down, the prospects for forming new stars decline as well. Even though it may taken tens of billions, hundreds of billions, or even many trillions of years, eventually the star-formation rate in whatever our Local Group evolves into will fade away to zero. When the now-existing stars run out of fuel in their cores, they will cease to shine as well.

And then, over very long periods of time, everything else will fade away too. The distant, unbound galaxies, groups, and clusters will be accelerated away to beyond the limits of what’s reachable or communicable with an observer in any one bound structure, rendering them inaccessible. The stellar and planetary remnants within any bound structure will gravitationally interact, leading material to either fall into a central, supermassive black hole or to get ejected from the galaxy entirely: a process which should take between 1017 and 1019 years, or millions of times the present age of the Universe.

The persisting clumps of matter should remain stable, with the non-radioactive elements persisting for all eternity. After around 1067 years pass, the lightest black holes will completely decay away, and after around 10110 years, the heaviest ones will have decayed away also. This leads to a scenario for the far future Universe known as a heat death, where everything that remains is in its lowest-energy state (the ground state), and from which no further energy will be able to be extracted or used to do work.

The Universe will be nothing more an isolated smattering of bound clumps of mass — mostly dark matter but with occasional clumps of burned out stars and desolate planetary objects — all separated by vast, inaccessible distances that continue to increase as time goes on. That’s how the Universe ends.

But what if we’ve assumed something that’s incorrect? After all, this scenario only works under the best current description of reality as we understand it today. It assumes that:

dark matter is a cold, collisionless, non-interacting (except gravitationally) species of massive particle,
baryogenesis admitted baryon-violating interactions early on, to create the matter-antimatter asymmetry we observe in our Universe, but that conserves baryon number today and for all the time afterwards, leading to a stable proton,
and that dark energy is a cosmological constant, and that its energy density and contribution to the expansion rate of the Universe won’t change over time.

All three of these assumptions are consistent with every piece of information we’ve ever collected about the Universe, even if you include the pieces that don’t quite fit, such as the Hubble tension, the evidence for evolving dark energy, the lack of particle physics hints that take us beyond the Standard Model, or the constraints we have on a variety of plausible baryogenesis scenarios.

Nevertheless, there is a tremendous amount of wiggle-room here, and if any one of these three simple (but not necessarily well-established) properties turns out to be wrong, the fate of our whole Universe could be due for a profound cosmic shake-up. Here’s how new physics, on any of these three fronts, would impact our far future.

1.) What if dark matter has a self-interaction?

For as long as it’s been proposed, people have been looking for ways to detect dark matter. They have looked indirectly, for astrophysical signals and consequences, finding strong evidence in how matter clumps, clusters, and moves within individually bound structures. They’ve also looked directly, for signs of dark matter particle annihilation (at galactic centers, for example), for interactions with normal matter and with light (both cosmically and within dedicated detectors such as ADMX), and for signatures of dark matter colliding with normal matter particles (through direct detection/recoil experiments).

The indirect signatures are overwhelmingly strong; the direct signatures are consistent with a null effect. This means that it’s eminently plausible, as we currently assume, that dark matter has no interaction with light, normal matter, or itself other than through gravitation alone. If that’s the case, then sure, some dark matter particles will be ejected from the bound galaxies, groups, and clusters that they form giant, diffuse halos around, but most will persist for what’s effectively an eternity. These clumpy halos of matter, even after the last black hole evaporates, should persist.

But if dark matter does have a self-interaction, the story of our far future could change dramatically. Sure, we might have constraints on direct dark matter signatures, but they could show up at levels that are simply below the threshold of what’s presently observable. One fascinating possibility is that dark matter does interact with itself to form a whole “dark universe” of structure: the dark matter equivalent of baryons, atoms, or some other structure that could serve as the building blocks for larger, more complex structure.

The key is recognizing that all our constraints imply is that there’s an upper limit to the rate, or timescale, on which these processes can occur. Our longest-running experiments have lasted for decades, or around 109 seconds. Planet Earth and the Sun have been around 4.5 billion years, or around 1017 seconds. But what if dark matter makes structures on long timescales: scales far longer than the age of the Universe, like sextillions or even googols of years?

It would mean that the “heat death” scenario will be all wrong, because there will be a new way of extracting energy and performing work in the Universe. Even if its small, the discovery of any way that dark matter isn’t purely collisionless or non-interacting through a force other than gravity would profoundly change our ultimate cosmic fate.

2.) What if baryon violating interactions still occur, and the proton is inherently unstable?

Just as we have constraints that dark matter doesn’t interact, we also have strong constraints on the stability of the proton. Through giant, long-term experiments that are sensitive to proton decay (often with the same experiments that are sensitive to neutrino detection or possible dark matter detection), we’ve constrained the lifetime of the proton to be greater than around 1034 years, or a septillion times longer than the present age of the Universe. This has been enough to rule out many scenarios for baryogenesis: the ones that would lead to an unstable proton with a lifetime of less than the constrained value, including the standard SU(5) Georgi-Glashow unification scenario, both with and without supersymmetry.

But again, this is just an upper limit. It’s possible that the proton is unstable, that there are actually no truly stable atomic nuclei on the periodic table, and that all of the atom-and-ion-based normal matter we know of will someday decay.

And decays are great for the Universe’s potential, because all decays liberate energy and leave you in a lower-energy state. Liberated energy is energy that can be used to do work, to power processes, or to extend the lifetime of — and what’s possible within — the Universe. The surviving bound structures, even long-term, could someday again house physically interesting, perhaps even metabolic, processes.

3.) What if dark energy turns out not to be a cosmological constant, but rather is evolving?

This one is particularly exciting if you want to change the fate of the Universe, because it doesn’t just change the “heat death” part by pushing it out or giving us a new source of energy to draw upon for increasing the entropy of the Universe further than our presently known physics allows us to go. Instead, if we allowed dark energy to evolve — rather than insisting that it remain as a cosmological constant — then it wouldn’t just be the individual clumps whose fates would be different from the standard picture, but the Universe as a whole. After all, dark energy represents the majority of the Universe’s energy, and changing how that energy behaves could change the entire behavior of the Universe.

The reason that the Universe is going to end as a cold series of disconnected, isolated clumps of matter is because of dark energy. As the matter density drops (because the volume of the Universe increases), dark energy — which behaves as a form of energy inherent to space itself in the cosmological constant case — becomes relatively more and more important. Because dark energy has both an energy density and a strong, negative pressure, it ensures that the Universe will always expand, and that distant “bound clumps” will speed away, faster and faster, from one another as time goes on. And therefore, that’s how the Universe will end.

But if dark energy isn’t a constant, all bets are off. There is circumstantial (but not conclusive) evidence that dark energy may evolve over time, and if so, then many new possible fates suddenly arise.

Dark energy could strengthen, and if it’s pressure becomes more negative than a cosmological constant’s, then the cosmic acceleration will intensify, and either a Big Rip (where space tears itself apart) or a rejuvenated scenario (where the increased energy density triggers a new hot, dense, Big Bang-like state) could ensue.
Dark energy could weaken and disappear entirely, causing the cosmic expansion to asymptote towards zero, rendering the distant groups reachable and communicable to one another.
Or dark energy could weaken and then flip signs, leading to a cosmic recollapse and a Big Crunch, with a potentially cyclic scenario (and a new Big Bang-like state) emerging in the future.

These are three of the big puzzles facing modern cosmology: what the nature, behavior, and properties of dark matter are, how the matter-antimatter asymmetry arose and whether atomic nuclei are truly stable, and what the nature and future evolution of dark energy are. In the simplest, most “vanilla” scenarios, dark matter is non-interacting, baryogenesis occurred only once early on and leaves eternally stable atomic nuclei behind, and dark energy is a pure cosmological constant, leading to our current “consensus” fate. But if the solution to any (or all) of these puzzles turn out to be different from our present ideas, our ultimate cosmic fate is suddenly up for grabs. In many ways, that’s the most exciting exploratory frontier of all!

This article Will new physics affect our Universe’s far future? is featured on Big Think.

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