The Far Future of the Universe

Time Machine Control Panel
  1. Stelliferous Era - (You are here)
  2. Degenerate Era - 1014 to 1040 years in the future
  3. Black Hole Era - 1040 to ~10100 years in the future
  4. Dark Era - ~10100 years in the future to ∞
Modern physics has been able to make many fascinating predictions about the future of the universe by taking current natural processes and extrapolating them trillions of trillions of years into the future. In 1997, two physicists - Fred Adams and Gregory Laughlin - published a paper titled A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects. They later followed up with a popular science book based on their research, named The Five Ages of the Universe.

Adams and Laughlin split the history of the universe into five distinct eras: The Primordial Era, the Stelliferous Era, the Degenerate Era, the Black Hole Era, and the Dark Era. The fiery Primordial Era stretched from the Big Bang up until around 300,000 years ago and was marked by a torrid universe whose extreme conditions prevented even simple atoms from being able to exist. The second era, the Stelliferous Era, is the lively era of stars that we currently reside in, and that will serve as the beginning of our journey into the future.

The vast majority of the predictions in this book originate from the aforementioned work of Fred Adams and Gregory Laughlin, as well as the 1979 paper by Freeman Dyson: Time Without End: Physics and Biology in an Open Universe. I will try to provide links if possible when additional sources are used.

While the material contained herein is quite thought-provoking, please keep in mind that this is based on current scientific observations and knowledge. As new discoveries are made, a different model for the future of the universe may emerge. This article would be radically different were it written just a century ago. Nonetheless, the predictions below are as accurate as they can be based on scientists' current understanding of the world.

Without further ado, let us hop in the time machine and see what awaits us!

Stelliferous Era
It's good to take some time to appreciate the lively and fertile state of the universe that we reside in. Although outer space is often described as a barren void, and it indeed is far emptier than most of us realise or can even comprehend, the observable universe around is populated with trillions of galaxies, containing hundreds of millions or billions of stars. The vast majority of baryonic matter in the universe is still in the form of hydrogen - the element required to fuel stars - ensuring that the universe will continue to be a fertile place for new stars to be born for many eons to come.

Indeed, as we will see, the current state of our universe will seem like the feast of a particularly gluttonous Roman emperor to any life-forms that inhabit the universe of the distant future.

Space Colonisation

While space colonisation is something that I am neither knowledgeable nor particularly curious about, I thought I should at least give the subject a cursory mention here. I have heard it said that it would take at least 1 million years to fully colonise the Milky Way galaxy. It is theoretically possible that we could do not only that but also colonise the other galaxies in our Local Group, but anything beyond that is quite doubtful.

As we will soon see, the universe is not only expanding but actually doing so at an accelerating rate. Objects outside of our Hubble volume (a sphere around us that stretches 14.4 billion light-years in every direction) are already moving away from us faster than the speed of light, meaning that no information can ever reach us from beyond that point. This seemingly paradoxical situation is possible because space itself is expanding everywhere at once. While nothing can physically move faster than light, it is possible for space to expand faster than light between two objects.

Andromeda and all of the other galaxies in our Local Group are bound to the Milky Way by gravity, but nothing beyond this cluster is. While we can theoretically colonise areas outside of it, we will eventually permanently lose contact with those colonies when the galaxies they reside in begin to recede away from us faster than the speed of light.

I realise that space colonisation is a very exciting topic for many people, so I will leave a link here to the personal website of Isaac Arthur, a physicist who has made numerous accessible videos, many of which are centered around various aspects of space colonisation.

The Earth's Torrid Fate

Regardless of one's interest level in space colonisation, this will eventually become a field that we will have to master if we want to preserve ourselves and the other species on this planet.

The Sun sustains life on Earth by providing the heat and light needed for almost all life-forms to exist. It does so by fusing hydrogen into helium inside of its core, releasing a great deal of energy in the process. Unfortunately, the helium that the Sun produces is also slowly poisoning it as more and more of it accumulates. As the helium supply of the Sun grows, so too does its power output. The Sun is already becoming imperceptibly warmer and brighter as time goes on.

While this is effectively meaningless right now, the Sun is expected to become approximately 1% brighter over the course of the next 100 or so million years, and by 10% in 1 billion years. 10% might not sound like much, but it will be more than enough to wipe out almost all life on the planet and even evaporate the oceans.

Worse yet, in around five to six billion years, the Sun will exhaust the last hydrogen in its core and begin to collapse under its own gravity. This will cause the core to contract and start fusing helium into heavier elements. The force of helium fusion will begin to push back against the gravitational collapse and cause the Sun's outer layers to start expanding. At this point, the Sun becomes a red giant star. It will continue to expand up until 7 or 8 billion years in the future, until it engulfs Mercury and Venus. It is not yet certain if Earth and Mars will survive this event.

Once this expansion stops due to the lack of hydrogen or helium in the Sun's core, the core finally collapses under its own gravity, blowing away its outer layers in a violent explosion. The Sun's collapsed core becomes a white dwarf - a torrid stellar remnant that will slowly cool down to the background temperature of the universe. I will go into more detail about white dwarfs in the Degenerate Era section as they do not have much relevance in the current era.

A New Night Sky

The familiar night sky will become more and more unrecognisable as we proceed through the Stelliferous Era. If you were watching a montage of the night sky changing into the future, the first change you would notice is the slow but steady rearrangement of the constellations as the stars that make them up move through space. This is something that would only take tens of thousands of years to begin unfolding.

As you move further and further into the future and watch our Sun begin to become more and more uncomfortably close to us, you will notice a steady increase in stars that are visible with the naked eye. In 2.4 billion years, the Magellanic Cloud dwarf galaxy will collide and merge with the Milky Way galaxy. This event will be followed by the far more spectacular merger between the Milky Way and Andromeda galaxies in 4 to 5 billion years. Although the diffuse nature of space means that we will likely not witness any collisions in our own solar system, these mergers will light up the night sky as the free hydrogen in the two galaxies triggers a new wave of star formation.

Over the course of approximately the next 450 billion years, all of the galaxies in our Local Group will have collided with each other, creating one massive galaxy. Before these consolidations are complete however, there will be another alarming change to the familiar night sky.

As mentioned earlier, the universe is both expanding and accelerating the rate at which it is doing so. Right now, everything over 14.4 billion light-years away from us is receding faster than the speed of light, but as time goes on, more and more objects outside of our Local Group will begin moving away from us faster than light and will disappear from the night sky. In 150 billion years, everything outside of our Local Group will have left our observable universe. An observer in our galaxy would never be able to see a single object beyond this point. As far as they will be able to tell, their little group of galaxies is all that there is or ever has been.

The Peculiar Twilight of the Stars

As the Stelliferous Era proceeds, the hydrogen supply of the universe is slowly exhausted as generations of stars live and die, fusing away hydrogen into helium in the process. As a result, star formation begins to taper off somewhere between 1 trillion to 10 trillion years in the future (1012 to 1013 years) and the presence of more massive stars in particular drops off dramatically. Around this time, the universe begins to see the births of two types of stars that have not yet been able to come into existence.

While larger stars such as our Sun and Betelgeuse swell up to become giants before exploding and shedding their outer layers, the smaller and fully convective red dwarfs go out in a more peaceful manner. Once a red dwarf enters the late stages of its life, it begins burning hotter, sometimes enough to emit light on the blue side of the electromagnetic spectrum. At this point in its life, it is called a blue dwarf. It will continue burning its remaining hydrogen before peacefully settling as a helium white dwarf once it has no more hydrogen left to fuse.

Due to the extreme longevity of red dwarfs (800 billion to 12 trillion years), not a single red dwarf has lived long enough to become a blue dwarf.

Even stranger are the so-called frozen stars whose existence was proposed by Fred Adams and Gregory Laughlin. Once the average hydrogen content of the star-forming gas clouds in the universe reaches a sufficiently low level, these bizarre objects will begin to form. These stars will have masses well below that of any star today - only 4% that of the Sun - and will have surface temperatures low enough for ice to form on them. Inside their core however, they will in fact be actual hydrogen-burning stars.

The bizarre properties of this star will be afforded by the insulation granted to it by the amounts of metals (elements other than hydrogen or helium) present inside it, which will insulate the core and trap sufficient heat to allow for hydrogen fusion to occur in spite of the lack of gravitational pressure. These strange objects are expected to continue fusing hydrogen for up to a quadrillion years (1015 years).

Degenerate Era
It is now 100 trillion years in the future (1014 years), and the universe seems utterly unrecognisable compared to its vibrant and fertile younger self. What was once a vast and mystical cosmic quilt, is now a series of exhausted galaxies littered with zombie stars and floating incomprehensibly far away from each other. Aside from the frozen stars clandestinely and slowly burning their hydrogen away under the cover of their ice clouds, there will likely still be a few conventional stars chugging away in our galaxy at the start of the Degenerate Era. As we will see, this will continue for a surprisingly long time.

The somewhat unfortunate term for the Degenerate Era has nothing to do with moral decay amongst celestial bodies. In this era, the universe is dominated by degenerate stellar remnants such as white dwarfs, neutron stars, and theoretical quark stars. In contrast to stars, which actively fuse hydrogen into helium in their core to push back against gravitational collapse, degenerate stellar remnants are held together by degeneracy pressure.

Essentially, the Pauli exclusion principle does not allow two particles to occupy the same quantum state, and all of the particles pushing against each other keeps the star from collapsing into a black hole. Which particles specifically are involved depends on how massive the object is and how tightly packed together its matter is as a result. White dwarfs are held together by electron degeneracy pressure, neutron stars by neutron degeneracy pressure, and quark stars by quark degeneracy pressure.

Regardless, with star formation having tailed off almost entirely, these strange objects - cooling relics of a grand cosmic orgy that once was - take the spotlight, and their interactions become the main events in an ever-evolving universe.

Stars of the Degenerate Era

As mentioned earlier, the Degenerate Era is sworn in with a handful of stalwart red (or blue) dwarf stars still lighting up their galaxies with their meager power output. These too will die as trillions more years pass by, but unorthodox star formation processes will ensure that new ones continue to slowly be born for quadrillions of years to come. One commonly mentioned scenario involves the formation of a red dwarf star as a result of two brown dwarfs colliding. Brown dwarfs are objects that are dozens of times more massive than Jupiter but not quite massive enough to become a hydrogen-burning star, and the collisions of two such objects can produce a new body with enough mass to start fusion reactions.

A second scenario is one that was hypothesised by Koh Xuan Yang on his blog. Galaxies are filled with vast, diffuse clouds of mostly hydrogen gas, and brown dwarfs attract and collect this gas at a very slow rate as they move through space, slowly increasing their own mass. After perhaps dozens of quadrillions of years of slow accretion, a brown dwarf can finally obtain enough mass to begin burning its hydrogen, and become a star. Both this and the aforementioned brown dwarf collision scenario will ensure that the universe will continue to shine moderately for long after the last typically-formed star dies 100 or so trillion years in the future.

White Dwarf Evolution

Although white dwarfs are the stellar equivalent of derelict furnaces, they will continue to heat and enlighten the universe around them for much longer than the stars they were born from. As a result of the low surface area and high density of a white dwarf, they radiate their heat away very slowly. Furthermore, a white dwarf cools slower and slower the cooler it becomes. It's estimated that the average white dwarf will only reach the background temperature of the universe, thus becoming a black dwarf, within a quadrillion years after its birth. That is, if it does nothing but sit there and cool off.

While the nature of dark matter is currently a mystery, some physicists believe that it consists of weakly-interacting massive particles (WIMPs), which only interact with other matter via the force of gravity and the weak nuclear force. If this theory is indeed true, then dark matter will eventually slowly be accreted by white dwarfs and annihilated inside of their cores, slightly heating the stellar remnants up in the process. Given enough time, all of the dark matter in the galaxy is expected to be gobbled up by white dwarfs in this fashion, until the dark matter supply is depleted and the white dwarf finally cools into a black dwarf in approximately 1024 years (1 septillion years.)

Regardless of how the process ultimately goes, not every white dwarf will spend its twilight years in such tedium. Some white dwarfs will go out in supernova explosions after either colliding with another white dwarf, or accreting enough gas from space to override the electron degeneracy pressure supporting them and undergo gravitational collapse.

Goodbye, Galaxy!

Although the movements of planets and stars across the heavens can seem timeless and eternal, they are in fact neither of these. The orbits of planets and stars can and are occasionally disrupted by close contacts with other massive objects. When an object in space flies too close to a more massive object, it "borrows" some of its orbital energy and accelerates. This can eject planets from the orbits of their stars and turn them into rogue planets, or even eject planets or stars from the galaxy. Freeman Dyson calculated that in a mere quadrillion years (1015 years), all of the planets in the galaxy will be ejected from the orbits of their stars.

This process does of course affect stars and stellar remnants, and it's estimated that over 90% of all stellar remnants will eventually be ejected out of the galaxy, likely due to close encounters with the supermassive black hole at the center of the galaxy. The other ~10% will eventually be devoured by said monster black hole as their orbits slowly decay over the eons like an object circling a drain, and they spiral into the black hole. By 1024 years (1 septillion) in the future, our once vibrant galaxy will be nothing more than a monster black hole orbited by smaller black holes.

Dyson Organisms and Black Hole Civilisations

When examining the desolation of the Degenerate Era, it can seem intuitive that life cannot possibly continue to exist in the face of such bleak conditions. This is not necessarily so.

One of the most fantastical and well-known constructions that have been proposed in hard science fiction is the idea of a Dyson Sphere - a sphere constructed around a star in order to make use of all of the energy that it produces. Although Freeman Dyson - the man who invented the idea - only intended the idea as a joke, and it is quite doubtful whether such a thing could actually be pulled off, the idea could be made to work with a white dwarf instead of a regular star.

As mentioned earlier, white dwarfs radiate away their heat at very slow rates. It is highly unlikely that any habitable planet could exist around one as it would have to orbit in so closely that it would be tidally locked and would risk being destroyed by the dead star's gravitational tidal forces. A sphere constructed around such an object could sustain a fairly large population of aliens, animals, and/or people, and a scientific paper has even been written about how such an object might work. Such a construction could sustain a population for at least a quadrillion years, with the hopes that another brown dwarf would ignite and become a red dwarf and later white dwarf in the meantime, to serve as the next home of the species.

A much stranger future home for anyone still around in the far future would be a black hole. As I will discuss in the next section, black holes actually slowly radiate energy. While no naturally forming black hole radiates enough to sustain life, artificially-formed smaller black holes could. Isaac Arthur has produced multiple fascinating videos on the subject, but the gist is that a future civilisation could create a small black hole and slowly feed it with matter to sustain its energy output.

Failing this, black holes can also be used as an energy source by either harnessing the kinetic energy from its accretion disc by dropping items into it, or by slowly decreasing its spin and extracting its rotational energy via the Penrose process, named after and invented by Roger Penrose.

Freeman Dyson put forth a fascinating scaling hypothesis for life, which was also touched upon in Fred Adams and Gregory Laughlin's Five Ages of the Universe. Essentially, the idea is that we can imagine life-forms that exist at any particular temperature, and that the life-form's energy consumption rate is directly proportional to its temperature. So if there can exist a critter that thrives at half the temperature that we thrive in, it would conduct its natural processes at half the speed that we do ours.

Such a "Dyson organism", as Adams and Laughlin referred to it, could experience life at rates vastly slower than we do, but could survive in the bleak world of the Degenerate Era and perhaps even beyond.

Isaac Arthur further extended this idea by exploring the concept of intelligent life converting themselves to computers. If intelligent life could indeed exist in such a form, it would be free to modify its own "biology" and slow its natural processes down to conserve energy. Perhaps such computerised beings could even exist on the meager energy output of a typical naturally-formed black hole.

The End of Matter as We Know It

All good things must come to an end, and some models of particle physics suggest that they will come to an end sooner rather than later. The atoms that make up us and everything around us are composed of protons, neutrons, and electrons. Neutrons are stable when bound to protons, but decay within 10 minutes if they are alone. While the Standard Model of particle physics predicts that protons are eternal, other popular models of physics suggest that they will decay into elementary particles in 1040 years or less.

Proton decay is expected to be a slow and tedious process, not a sudden complete disintegration of all baryonic matter. If you were to observe a black dwarf floating in the empty void 1037 years in the future and somehow take a census of all of its constituent protons, and then travel to 1038 years in the future, you would find that 1% of the protons are now missing. Travel to 1039 and you would find 10% missing, travel one more order of magnitude into the future and there would be nothing left at all.

Now, unlike all of the other predictions mentioned on this page, there is no actual evidence that this will occur aside from predictions from some unproven particle models. Experiments have been done to attempt to prove proton decay by monitoring a large pool of water with sensitive particle detectors meant to detect neutrino emissions from decaying protons, but none of these experiments have so far borne fruit.

I personally do not believe that proton decay will actually occur, but since it is still a popular notion, I decided it was worth including.

Either way, if proton decay does indeed occur, all baryonic matter - black dwarfs, planets, brown dwarfs - will cease to exist after around 1040 years.

Black Hole Era
The year is 1040, alternatively 10 duodecillion or 10,000,000,000,000,000,000,000,000,000,000,000,000,000. The universe is a bleak void with occasional elementary particles and clusters of overgrown black holes. If protons have not decayed, then there are also occasional frigid stellar remnants and rogue planets separated by almost infinite expanses of space.

If proton decay is indeed not in the cards, there may still be some stalwart life-forms desperately clinging to existence in this desolate and distant era. If they do exist, then they are computer-based and survive by orbiting and harnessing the energy of a smaller black hole. There is almost no chance of organic life surviving into the Black Hole Era.

Cosmic Consolidations

With all other macroscopic matter having either decayed away or been banished to the infinite void, black holes become the protagonists of the story in this era. In The Five Ages of the Universe, Fred Adams and Gregory Laughlin estimate that the galaxies that comprise our observable universe, will together have 1040 galactic core black holes and 1046 regular black holes by the start of this era. Bereft of other matter on which to engorge themselves, they will slowly seek out each other before colliding and merging into larger black holes.

When black holes merge together, they release a titanic amount of their energy in the form of gravitational radiation. One black hole merger that scientists observed was between a black hole with 85 solar masses and one with 66 solar masses. The resulting black hole came out to be of 142 solar masses; the remaining 8-9 solar masses being released in gravitational waves. These occasional outbursts will be the sole source of liveliness in the otherwise bland first act of the Black Hole Era.

Hawking Radiation

One of the most fascinating scientific discoveries of the past century is that empty space is not actually empty. No matter where you may be in outer space (or on Earth), if you were to look deeply enough at the space around you, you would see that it is populated by an endless sea of particles and anti-particles that are steadily popping into existence and annihilating with each other. A sort-of quantum foam. To quote Lawrence Krauss: "nothing is something".

To be clear, this is not a theoretical construct. This is a phenomenon that has both been observed and that was needed to take into account to accurately calculate the weight of a hydrogen atom. Although this strange phenomenon has little bearing on our daily lives, it becomes a major player in both the Black Hole Era and later the Dark Era.

Stephen Hawking deduced that when this process occurs around a black hole, there is an infinitesimal chance that one of these "virtual" particles appears within the event horizon of a black hole, while the other one appears outside of it. Since nothing can escape a black hole's event horizon, the doomed virtual particle falls into the black hole, while its suicide pact partner wanders off demoralised. Since the masses of the two virtual particle originated from the mass of the black hole, the escaped particle is essentially being emitted by the black hole.

Not only is the chance of this event occurring near a typical black hole extremely miniscule, but it becomes increasingly more improbable the larger the black hole is. Moreover, black holes have temperatures far below that of even the frigid background temperature of the universe. Until the universe cools below that of the temperature of a regular black hole, typically-formed black holes will not emit any Hawking radiation and instead will actually continue to grow by absorbing the heat from the space around them.

Nonetheless, given the immense time-scales of the Black Hole Era, even the largest black holes will not only eventually start to give off Hawking radiation, but will eventually completely evaporate. In The Five Ages of the Universe, it was predicted a black hole with the size of the smallest black hole of today (somewhere around 3-5 solar masses) would take 1067 years to evaporate. Larger black holes will take significantly longer, but by somewhere around 10100 even the most titanic supermassive galactic core black holes should be extinct.

As a black hole loses mass, its temperature and luminosity steadily increase. There's a fun black hole temperature calculator available here if you want to see how a black hole's temperature changes as its mass shrinks. Towards the end of a black hole's life, it becomes bright enough to be visible with the human eye, and eventually almost as bright as a dim star. At the very end of the evaporation process, the now meager black hole simply explodes, releasing all of its remaining mass energy in a spectacular display.

Dark Era
The explosive death of the final stalwart supermassive black hole heralds the beginning of the universe's melancholic final era. An exact timeframe for when this is expected to occur is impossible to predict because it is impossible to predict how large that last black hole will become when it reaches its peak. The most massive known black hole today is TON-618, which weighs in at an estimated 66 billion solar masses. This could take 699 years to evaporate if we assume that it miraculously does not gain any more mass from now until then. A black hole that consumes the vast majority of the matter in its local galatic cluster could grow to be trillions of times more massive than the Sun and would take over 10110 years to finally decay.

Exotic Atoms at the End of Time

With black holes having gone extinct, and protons also having decayed long ago (not!), the Dark Era is marked by a complete absence of macroscopic matter. Nonetheless, the diffuse nature of the universe is expected to allow for the domination of a bizarre new form of matter: positronium. The atoms that make up us and everything around us consist of hadrons - protons and neutrons - and electrons. Protons and neutrons are composed of the elementary particles known as quarks, while electrons are themselves elementary particles. In contrast, positronium is an exotic atom that consists of an electron and an antimatter electron known as a positron.

Although positronium atoms and even molecules have been created and studied in modern laboratories, they are extremely unstable. The electromagnetic attraction that allows positronium to exist also results in its untimely demise as the electron and positron smash into each other and annihilate. However, the diffuse state of the universe during the Black Hole Era and Dark Era allows for the existence of positronium atoms with unfathomably massive orbits, and by extension equally unfathomably long lifespans due to the time required for its constituent particles to annihilate.

The average diameter of a positronium atom born in the Black Hole Era from positrons and electrons released by evaporating black holes is expected to be trillions of light-years, vastly exceeding the size of the entire observable universe around us today. These strange atoms could live for as long as 10145 years. This is a period of time that dwarfs even the nigh-eternal Black Hole Era by exponentially more than the entire star-filled age of the universe dwarfs a millisecond.

Given the utterly unimaginable timescales involved, some physicists have gone as far as to wonder if life-forms forged out of positronium may come to exist in some corner of the universe. Although there is no way to prove or disprove this fanciful idea, any life that comes to be in the desolate conditions of the Dark Era would have to be very simple. They would also be inconceivably slow compared to us, both as a result of the almost complete lack of energy-generating processes, and due to the vast distances that information would have to travel between its individual constituent particles.

The slowness is of course relative. While we cannot imagine existing such that it takes tens or hundreds of trillions of years to formulate a simple thought, a diffuse organism stretching out across billions or trillions of light-years, in a completely empty and frozen void would not see anything odd about its situation. Not that such an organism would even be afforded the energy resources to have the complicated brain it would need in order to contemplate its existence. I have nonethless pondered many-a-time on how a thinking being existing in such a moribund and inscrutable world would try to explain how it and the universe around it came to exist.

Alternative History: Iron Stars

If proton decay does not occur, then the Black Hole Era will herald an equally bizarre new era of the universe. After the last black hole finally dies of starvation, the remaining denizens of the universe will consist of the sea of frigid stellar remnants that avoided being consumed by escaping their galaxies during the long-gone days of the Degenerate Era. As mentioned, over 90% of stars and stellar remnants are expected to be ejected from their galaxy as a result of close encounters with more massive objects, with the other ~10% being devoured by a black hole.

One would assume that in such a scenario, the universe would end as an increasingly diffuse cosmic quilt marked with frozen black dwarfs, brown dwarfs, neutron stars, and planets infinitely far apart from each other, but things are rarely so simple.

As time goes on, all of the heavy elements inside these stellar remnants would decay into lighter elements via radioactive decay processes, until they settle as iron. Concurrently, over periods of time that make even the entire Black Hole Era seem utterly insignificant, elements lighter than iron undergo cold fusion via quantum tunneling, until they fuse together into iron.

Quantum tunneling is a bizarre and rare phenomenon that allows a particle to overcome a barrier to reach a lower energy state, like a ball passing through a wall. Since lighter elements have potential energy that can be unleashed by undergoing fusion, quantum tunneling allows for an infinitesimal chance of them fusing together into heavier elements. This only allows fusion up to iron because fusing elements heavier than iron consumes more energy than it unleashes.

Once these processes have run their course, all of the remaining black dwarfs, brown dwarfs, and planets will have become frozen spheres composed of pure iron-56. This is a process that Freeman Dyson estimated would take an utterly staggering 101500 years. That is one with 1,500 zeroes after it!! It is beyond me to even attempt to make a comparison that could make the sheer scale of that number remotely comprehensible.

I used the word "spheres" because quantum tunneling will ensure that all matter in the universe assumes a spherical form long before the advent of iron stars. Due to every particle in a body wanting to be in a lower state of energy (in the center of said body), quantum tunneling will slowly rearrange all cosmic bodies into perfect spheres via the slow and steady nudges each particle makes to attempt to reach the center of its host. According to Freeman Dyson, this will take approximately 1065 years.

Not every stellar remnant that survived the Black Hole Era will become an iron star. A recent paper by Michael Caplan postulates that some particularly massive black dwarfs would instead go out in supernova explosions, starting 101100 years in the future. This will occur as a result of the Chandrasekhar limit of the stellar remannt decreasing due to the fusion processes changing its composition. The Chandrasekhar limit is the maximum size of a stable white/black dwarf, and depends on the ratio of protons and neutrons. As lighter elements fuse into iron, black dwarfs with masses between 1.2 and 1.4 solar masses eventually succumb to gravitational collapse and explode.

Stellar remnants small enough to become iron stars also face a bright and explosive future, just one much further down the line. Although the exact final fate of iron stars is still uncertain, it is agreed that quantum tunneling will eventually cause them to undergo gravitational collapse and reach their final and lowest energy state as either a neutron star or a black hole. In the former case, iron stars will collapse into neutron stars, releasing a massive amount of energy in the process, in 101076years, alternatively they will collapse into black holes and summarily evaporate via Hawking radiation in 101026 years.

While this is obviously naught but pure fanciful speculation, Koh Xuan Yang has proposed a fascinating yet plausible scenario wherein the collapse of an iron star to a neutron star results in the formation of a planet that contains an underground ocean that harbours life for billions of years in the iron stars article on his blog here.

Quantum Hope

As you can see, regardless of the ultimate fate of protons, the universe is destined to eventually end as a frozen, bleak void with nary a particle to be found in any particular expanse, let alone any planets, stars, or life forms.

...Or is it?

Brain floating in space.

As explained in the Black Hole Era section, our universe does not and cannot actually have a true vacuum anywhere, even in the ostensibly empty void of space. In every single patch of space, there exists a so-called quantum foam consisting of tiny particles popping into existence. This is significant in understanding the lifecycle of black holes, and is also significant in understanding the events that may unfold in the Dark Era.

One of the strangest ideas that has ever come out of science is the idea of a Boltzmann brain. The term was initially invented to disparage Ludwig Boltzmann, a renown physicist who postulated that our universe may have arisen from one with much higher entropy via random fluctuations. A Boltzmann brain is a hypothetical human brain that comes into existence via a series of many highly improbable quantum fluctuations in empty space. Essentially like a monkey banging on a typewriter accidentally writing out a coherent and thought-provoking novel, or a monkey playing with paint accidentally producing a perfect recreation of the Mona Lisa.

A Boltzmann brain would be filled with false memories, and could theoretically even mimic the brain of a real person at any given point in time. It could even perfectly mirror your brain at the moment you're reading this article! Fortunately any Boltzmann brain would expire immediately after coming into existence, so you need not worry about your hypothetical twin having to contemplate this existential horror.

Andrei Linde calculated that it could take 101050 years for a Boltzmann brain to come into existence. Lamentably, I lack the mathematical knowledge to understand his explanation and cannot shed any light on how exactly this was calculated. Either way, if protons don't decay, this means a hypothetical Boltzmann brain could at least have a frigid iron star or neutron star to keep it company during its extremely fleeting life.

The notion of a Boltzmann brain is a popular one, but there's nothing that says that quantum fluctuations cannot produce other things. It's vastly more likely that they would create individual elementary particles, atoms, or molecules for instance. Given enough time, planets; stars; or even galaxies could eventually come to be.

Even more interestingly, it's possible that the quantum foam that permeates space is a harbinger of a whole new universe that will arise one day. Physicists now suspect that this strange property of our universe suggests that we are living in a false vacuum. That is, the vacuum in our universe is not at its lowest possible energy state. This means that, much like the elements inside iron stars will eventually fuse into iron via quantum tunneling in order to reach a lower energy state, the vacuum itself could eventually tunnel towards a lower energy state.

What exactly that entails is anybody's guess right now. It is believed that if such an event were to occur at some point in the universe, it would start spreading out in all directions at the speed of light, annihilating everything in its path and leaving behind a brand new universe in its wake. This may already have occurred somewhere in our universe. Since the event would travel at the speed of light, we would have no way of knowing because it would annihilate us at the same moment that the first bits of information about the event reached us.

Nonetheless, such an event could bring about an explosive end to the Dark Era, and start a fertile new beginning for this universe. It has even been hypothesised that our own universe came about as a result of such an event! Even before modern physics brought all of these ideas to the table, philosophers and scientists have speculated on the idea that the world goes through cycles of death and rebirth. Notably, the idea is a key concept in Ancient Egyptian religion.

This is as far as the time machine can currently take you. The ideas presented here are based on current observations and scientific knowledge, and may become outdated as new discoveries are made. While there is currently no reason to believe that these ideas are inaccurate, it's worth keeping in mind how much scientific knowledge has evolved in just the past century.