There are many hypotheses about the end of the universe, such as the great tear torn into subatomic fragments by the ever-increasing dark energy, the great freeze that increases entropy to the highest level of heat silence, and the great rebound that re-contracts to the singularity. It is unclear which hypothesis will eventually become a reality. However, even if none of these hypotheses are ultimately realized, the universe will slowly fall into boundless darkness and dead silence over the long years.
Astrophysicists Fred Adams and Gregory Loveline, in The Five Ages of The Universe, divide life in the universe into five epochs. We can follow these five epochs to see the final outcome of the universe.
1. Primordial Era
The Proto-Epoch ranges from the Big Bang to 400 million years later, when the first star was born. This era has passed.
2. Stelliferous Era
Stars dominate the universe, and stars fill the night sky. Living in this era is undoubtedly our luck.
When will the Stellar Age end? Or when will the last star go out? The less massive the stars, the longer they live because they convert hydrogen into helium more slowly in nuclear fusion. The smallest red dwarfs can last up to 1 trillion years. Red dwarfs that formed in the early years of the universe now use only 1% hydrogen. From a human perspective, they are still babies.
Now, galaxies are still stirring up nebulae, creating new stars out of nebula gas. However, the gas will eventually run out one day. It is estimated that billions of years later, the supply of nebula gas will be weak, there will be fewer and fewer new stars, and the stars that have been born will run out of fuel, one by one. After that, events such as galaxy collisions create new stars, prolonging the age of star creation. However, compared with the trillion-year lifespan of red dwarfs, it is not much different to extend the lifespan of 50 billion years or even 100 billion years.
When a large number of stars go out, galaxies will change color. Most galaxies now exhibit a dazzling blue-white color, and large, bright stars are the main source of light within galaxies. When these stars die, the less massive dark stars become the protagonists, and the color of the galaxy will also darken and turn red.
Billions of years after the nebula gas ran out, only long-lived red dwarfs in the galaxy were still glowing. This will continue for 1 trillion years, perhaps longer. Probably the lowest-mass red dwarf can last 10 trillion years, but 10 times here is not too important a number. In the long course of life in the universe, they are just a few statistical ups and downs.
3. Degeneracy Era
When the last star is extinguished, only the remains of the stars are released in the universe: white dwarfs, neutron stars, and black holes. In addition, there are brown dwarfs whose mass is between stars and planets. In addition to black holes, white dwarfs and neutron stars are supported by various degeneracy pressures, that is, in trillions of years, the stellar era will end. The Age of Degeneracy has arrived.
The universe would have become very dark, at least to the human eye, if there were any humans back then. If we could see infrared, the universe would be a little brighter. These objects are still relatively warm, so there will be somewhat less infrared radiation. Neutron stars and white dwarfs are born very hot and then slowly cool. The rate of cooling is related to their size, but after trillions of years, they should all drop to room temperature. So, in the Age of Degeneracy, the universe slowly cooled down over time.
However, there will also be a flash in the pan in this epoch. The white dwarf stars that dance together in a lonely universe will eventually come together. Over trillions of years, the orbits of white dwarf binary stars will decay due to gravitational wave energy radiation, and the binary stars will burst into dazzling brilliance at the moment of merger, becoming supernovae. Similarly, neutron binary stars will merge, erupt, and form gamma-ray bursts, and their brightness will exceed 1,000 galaxies. However, this was only a short-lived event, and the universe soon returned to darkness.
Interestingly, brown dwarfs are a better source of energy. Brown dwarf binary stars merge to form low-mass stars that then bring a little light to the dark universe over tens of billions of years.
But in the face of time, nothing can be maintained into eternity. This is true not only for the universe and stars, but also for protons in the microscopic world. The half-life of a proton is 10^34 years, perhaps longer. When protons begin to decay, material will decompose, and white dwarfs, neutron stars, brown dwarfs, and planets will all melt in space. Thankfully, though, there is one bright spot in the process. White dwarfs release energy when they decompose, up to 400 watts (a fraction of the power of a microwave oven).
Note: The white dwarf should have completely cooled down and become a black dwarf by this time
4. The Age of Black Holes
After 10^40 years, the last degenerate object also disappeared, leaving only the black hole in the universe.
The universe entered the era of black holes.
In our minds, black holes are bottomless pits that swallow everything. However, black holes can produce energy in the same way – evaporation. In the 1970s, Stephen Hawking used quantum mechanics to study black holes and discovered that black holes do radiate energy outward, losing mass. The larger the black hole, the slower it loses mass. A 3 times the mass of the Sun (the smallest black hole born from a supernova) takes 10^68 years to completely evaporate. It's a time that looks almost ridiculous, but the universe can wait. It will take 10^92 years for the black hole at the center of the galaxy to completely evaporate. 1 is followed by 92 zeros, which is the following number.
This number is so large that we can't even find an analogy because it's even larger than the number of elementary particles in the known universe (10^85).
When a black hole becomes less massive, its evaporation rate accelerates. Finally, the black hole disappears without a trace after a flash of light. This is the only light in the Black Hole Age.
5. Dark Ages
In the end, everything disappeared. There are only small subatomic particles and photons with extremely low energies left in the universe. This is 10^92 to 10^93 years later. This is, we can safely say, that the universe is dead.
The universe has entered the Dark Age. If time were still meaningful, this epoch would continue to infinity.
6. Other possible outcomes
If dark energy continues to grow as is now speculated, the universe will end in a different direction: the Great Rip. In this ending, there is no force to fight against the expansion of space, and all matter will disintegrate, and even atoms will be torn to pieces. The Great Rift will happen before the Dark Age, but the ending is no brighter than the Dark Age.
Finally, we have a very small hope of getting a bright new universe. There is a conjecture in quantum physics that perhaps our vacuum is not a state of minimum energy. It's like you're standing on a step and there's another step below you.
When the universe has been sleeping for countless years in the Dark Ages, perhaps a small piece of space, for whatever reason, has fallen to the next energy level— a real vacuum. It will cause the surrounding space to fall. At this time, something strange happened, and in these falling spaces, the laws of physics would be rewritten and time and space would be erased. This area expanded rapidly, leaving a new universe behind. Maybe that's where our universe comes from too.
This conjecture gives us hope for a new universe. Although this hope is dim, there is a glimmer of hope, and it is better than sleeping in the darkness and cold to infinity.