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The death of our universe is an undeniable fact. Among the most accepted theories for its end is the concept of eternal expansion, leading to its ultimate demise due to entropy. As the universe keeps expanding, entropy grows until everything we know is gone. But what does life look like as the end draws near? This question has sparked intriguing theories about the universe and the nature of life itself.
10. The Night Sky Will Be Starless
In about 150 billion years, the view of the night sky from Earth will be radically different. As the universe hurtles toward its heat death, space itself will expand faster than the speed of light. Many understand that light speed is the ultimate speed limit for objects in the universe, but this applies only to physical objects, not to the very fabric of space-time itself. It’s a difficult concept to grasp, yet the expansion of space-time is already happening faster than light, and this will have strange consequences in the distant future.
As space continues to expand faster than light, a cosmological horizon comes into play. Any object beyond this horizon would require us to detect particles traveling faster than light, but no such particles exist. Once objects cross this boundary, they are unreachable. Attempting to communicate or interact with galaxies beyond this horizon would need technology capable of surpassing the expansion of space itself. Currently, only a few objects lie outside of our cosmological horizon. However, as dark energy speeds up the universe’s expansion, everything will eventually cross this observational threshold.
What does this mean for Earth? Picture yourself gazing at the night sky in 150 billion years. The only visible stars will be those within the cosmological horizon. Eventually, even these will fade. The sky will turn completely dark. Future astronomers will have no evidence of any other celestial bodies beyond our system. All the stars and galaxies visible today will be unreachable by telescopes. For all we could know, our solar system could be the sole remnant in the entire universe.
9. Our Sun Will Fade Into a Black Dwarf
Currently, our universe is home to a variety of stars. Red dwarfs—cool stars emitting red light—are among the most common types. White dwarfs, which are related semantically, are also widespread. These are the remnants of dead stars, composed of degenerate matter, and are held together by quantum forces. Presently, astronomers believe white dwarfs have effectively infinite lifespans, as the universe hasn’t aged enough for them to have perished. However, with enough time, even white dwarfs will eventually die, evolving into exotic stars called black dwarfs.
Our Sun is headed down a similar path. In the far future, it will shed its outer layers, transforming into a white dwarf. It will remain in this state for billions of years. As the universe winds down, our Sun’s white dwarf will gradually cool. After approximately 10 years, its temperature will drop to match the ambient background microwave radiation, just a few degrees Kelvin above absolute zero.
Once that occurs, it will become a black dwarf. Due to its extreme cold, this type of star will be invisible to the naked eye. Anyone attempting to locate the Sun, which once nourished life on Earth, will be unable to see it with optical instruments. Instead, its presence will only be detectable through its gravitational influence. Most of the stars we see in the night sky will eventually turn into black dwarfs, but the fact that our once-warm Sun will fade into a dark, cold remnant feels more personal.
8. Strange Stars
By the time our Sun becomes a black dwarf, the process of stellar evolution will have come to an end. No new stars will be born. Instead, the universe will be filled with the cold remnants of stars. This will set the stage for the emergence of peculiar stars, quite unlike anything we currently know.
One example is the frozen star. As the stars burn through their nuclear fuel, they will accumulate more metals. In astronomy, 'metallicity' refers to the concentration of elements heavier than helium—essentially, everything from lithium onward. As the metallicity in stars increases, they will cool down because heavier elements produce less energy through fusion. Eventually, stars will become so cold that they will reach a temperature of 273 Kelvin, the freezing point of water.
Looking far into the future, an even stranger star will arise. About 10 years from now, entropy will have taken its toll, and the universe will be nearly dead. In this cold era, quantum effects will dominate the universe.
Quantum tunneling will enable light elements to fuse into an unstable form of iron. This unstable form will then decay into a more stable isotope, releasing small amounts of energy. These so-called iron stars will be the only type of star left in that time. However, this idea is based on models where astronomers assume that protons will not decay, meaning it is not a widely accepted concept.
7. The Decay of Nucleons
Fast forward from 10 years after the Big Bang to the distant future. If humanity has not already perished, we certainly won’t survive this period. As previously mentioned, astronomers frequently debate whether proton decay will occur by the end of the universe. For the sake of this discussion, we will follow this particular model.
Nucleons are the particles—protons and neutrons—that make up an atomic nucleus. Free neutrons are known to decay with a half-life of about 10 minutes, but protons are exceptionally stable. No evidence has been observed that protons decay, but this will change when the universe reaches its end.
Physicists have proposed that protons have a half-life of 10 years. We haven’t witnessed their decay yet because the universe is still too young. As we enter the Degenerate Era (10 years to 10), protons will begin decaying into positrons and pions. By the end of the Degenerate Era, all protons and neutrons in the universe will be gone.
This has significant implications for life in the universe. If humanity has managed to survive the Sun’s transformation and migrated to more habitable regions, this is when the laws of physics will dictate the end of human life. Our bodies and all interstellar matter are composed of nucleons. Once those decay, life as we know it will cease because the very atoms that make up our bodies will no longer exist. Life cannot survive beyond this point, and the universe will enter the age of black holes.
6. Black Holes Will Take Over the Universe
When nucleons are no longer present, black holes will finally become the dominant forces in the universe, reigning from 10 years after the Big Bang to 10 years. At this stage, we’re discussing timescales that are nearly impossible for our minds to comprehend. For a period longer than the entire history of the universe, black holes will be the only structures left.
With nucleons gone, the universe's primary subatomic particles will be leptons, like electrons and positrons. These particles will fuel the black holes. As the black holes consume the last of the universe’s matter, they will emit radiation, which will replenish the universe with photons and hypothetical gravitons. However, as Stephen Hawking demonstrated, black holes, too, will eventually cease to exist.
According to Hawking, black holes lose mass due to their radiation. As they continue to emit energy, they lose mass. This process is slow, which is why it’s difficult for us to grasp. It takes around 10 years for a black hole to fully evaporate, a duration far exceeding the current age of the universe. But in the end, even black holes will vanish. The only remnants will be a mix of massless particles and a few scattered leptons that will interact as they gradually lose their energy.
5. A New Type of Atom Forms
After the universe has broken down into scattered subatomic particles, it may seem like there’s nothing left to discuss. However, life might still emerge in this improbable state.
For years, particle researchers have speculated about positronium, a bond-like structure between a positron and an electron. These two particles carry opposite charges, with the positron being the electron’s antiparticle. Their opposing charges cause them to be electromagnetically attracted, drawing them closer. As they interact, they could form basic orbits and behave much like the atoms we know.
Since positronium is rare, we don't yet have a complete model of its 'chemistry.' However, a few intriguing properties arise from these peculiar 'atoms.' One of the most interesting is that they can exist with incredibly large orbits, even spanning across vast interstellar distances. As long as the two particles are interacting, they can form a pair, no matter how far apart they are.
During the Black Hole Era, some of these 'atoms' will have diameters extending beyond the current observable universe. Made from leptons, positronium atoms will survive proton decay and endure through the Black Hole Era. In fact, black holes will generate these atoms through radiation. Even they, however, will eventually decay, as the positron-electron pair spirals closer to mutual annihilation. But before that, the universe may witness the emergence of life in ways we’ve never imagined.
4. Everything Happens Extremely Slowly, Including Thought
As the Black Hole Era reaches its conclusion and even these cosmic giants vanish into the void, only a handful of things will remain in the universe, primarily dispersed subatomic particles and the last surviving positronium atoms. Once this state is reached, time itself will slow down immensely, with every action stretching over eons. According to some theoretical physicists, including Freeman Dyson, life may reemerge during this period.
Given the immense stretch of time, organic evolution might take hold among the positronium. The life forms that evolve could be radically different from anything we know. For example, they might grow to such enormous sizes that they span entire interstellar distances. With so little remaining in the universe, they would have endless space to inhabit. However, these creatures would be so vast that their thought processes would occur at a pace many orders of magnitude slower than ours. In fact, forming a single thought could take them trillions of years.
While this concept might seem absurd to us, for these beings, thoughts would feel instantaneous because of their enormous time frames. If these life forms came into being as the universe neared its end, they wouldn't be capable of contemplating thinking faster—just as we cannot imagine speeding up our own thought processes. For them, 'spontaneous thought' would unfold on incomprehensibly vast timescales, but in their perception, it would occur instantly. These entities would exist for inconceivably long durations, witnessing the universe's slow decline, yet eventually, even they would fade away.
3. Random Quantum Tunneling May Start It All Over Again
What of the universe we leave behind? After an unfathomable length of time, it will reach its maximum entropy, making it a barren wasteland. But even in this lifeless state, there might still be a possibility for life to emerge again. Quantum mechanics researchers are familiar with a phenomenon known as quantum tunneling, in which subatomic particles can access energy states that would not be achievable according to classical physics.
In classical mechanics, for example, a ball cannot spontaneously roll uphill. This is considered a forbidden energy state. Similarly, subatomic particles also have forbidden energy states in classical mechanics, but quantum mechanics flips that notion on its head. Occasionally, particles can 'tunnel' to these otherwise inaccessible energy states.
This phenomenon already occurs within stars. But when considering the end of the universe, an intriguing possibility arises. According to classical statistical mechanics, particles cannot transition from a higher entropy state to a lower one. However, through quantum tunneling, they can and will. Physicists Sean Carroll and Jennifer Chen have suggested that given enough time, quantum tunneling could spontaneously reduce the entropy in a dying universe, potentially triggering a new Big Bang and rebooting the universe. But don't expect this to happen soon; such an entropy decrease would require about 10 years.
A different theory, rooted in mathematics, might offer a glimmer of hope for a new universe. In 1890, Henri Poincaré introduced his recurrence theorem, which posits that over an extraordinarily long period, any system will return to a state very close to its original form. This could apply to thermodynamics, where random thermal fluctuations in a high-entropy universe could drive it back to its initial state, essentially starting everything anew. After countless eons, our universe could reform, and any future inhabitants would be oblivious to its origins in the universe we know.
2. There Might Be A Way Out
Our exploration of the universe's ultimate fate has been a series of increasingly grim scenarios. However, physicists are not without hope, and they have proposed potential ways for humanity to survive the end of time and perhaps even reboot our universe.
The greatest opportunity for escaping our universe's eventual maximum entropy lies in utilizing black holes before proton decay makes life unsustainable. Black holes remain one of the greatest mysteries in physics, and theorists like Stephen Hawking have suggested that these massive objects could provide gateways to entirely new universes.
Contemporary theory suggests that our universe is constantly giving rise to 'bubble' universes, creating entire new realms of matter and the potential for life. Hawking believes that black holes may serve as portals to these new universes. There's just one catch: once you pass the event horizon of a black hole, there's no turning back. This is a fundamental idea in physics. So, should humanity choose to venture into a black hole, it would be a one-way journey.
The first challenge would be finding a sufficiently massive, rotating black hole that could allow for survival while crossing the event horizon. (Contrary to popular belief, larger black holes are actually safer to traverse.) Then, space travelers would have to hope that the journey leaves them intact, but they would never be able to communicate with anyone on the other side of the black hole to confirm their success. Every journey would become a leap of faith.
However, there is a way to ensure that a new universe awaits on the other side. According to Alan Guth, only a small amount of matter would be needed to create a new universe: 10 photons, 10 electrons, 10 positrons, 10 neutrinos, 10 antineutrinos, 10 protons, and 10 neutrons. While this may sound like a lot, it only adds up to a few ounces of material.
In the distant future, humans could potentially generate a false vacuum—an area of space capable of expansion—using an extraordinarily strong gravitational field. With the right technology, future humans could create this false vacuum and initiate a new universe. As the universe undergoes rapid inflation in the first fraction of a second, the new universe would expand almost immediately, providing a new home for humanity. A quick trip through a wormhole could lead us to a safe universe where our species could continue.
1. No More ‘Macro-Physics’
At this point, the universe will have reached a nearly maximum entropy state, essentially becoming a vast, uniform field of energy with a scattering of subatomic particles. This will occur long after the Black Hole Era, extending far into the future, well beyond 10 years from now. Space will have expanded so greatly, and dark energy will be so dominant, that even black holes will have disappeared, leaving the universe devoid of any large stellar objects.
Envisioning such a universe is challenging. The expansion will have stretched so far that stars, as we understand them, will no longer form. The subatomic particles that constitute matter will be too widely dispersed, preventing them from interacting unless they exceed the speed of light. What will remain are only a few scattered particles, adrift in the vast emptiness, unable to even interact and form positronium atoms.
This signifies the end of physics as we know it. Only quantum mechanics will hold sway over this new reality. Quantum phenomena will span vast distances and incredibly long time periods, unlike anything we observe in our current universe. Eventually, the temperature of the universe will plummet to absolute zero, leaving no energy capable of doing work. In some models, the universe's expansion will continue accelerating, ultimately ripping space-time apart. At this point, the universe as we know it will cease to exist.
