For such humble beings as we, the timelines of trillions of years seem unbearable. We are struggling with a lifespan of mere 80 years and for a considerable amount of the human population even 13.8 billion years of the existence of the Universe seem so hard to imagine that they find comfort in thinking that our world hasn’t been around for longer than 10 000 years.

And still, scientists venture to ask what will happen in a million, billion, trillion, quadrillion years. This way we know that in approximately one billion years from now, the Sun will steadily change from burning hydrogen at its core to burning hydrogen around its shell, its core will start to contract and the outer envelope will expand. It will engulf the orbits of Mercury, Venus and perhaps even Earth. But long before that its increased heat will eradicate terrestrial oceans and blow most of the atmosphere into space, transforming the planet into a melted magma ocean with floating continents of metals and metal oxides.

And still in cosmological sense this will not even be noticeable. The death of our Sun will just put down a scarcely noticeable dot among hundreds of billions of other dots in an ordinary galaxy among the hundreds of billions of galaxies. Humanity (hopefully we’ll still be around by that time) will probably be long gone to a find a new safe home among all those stars. From the ruins of the dead suns, new ones will be born, but eventually they will perish to exist as well.

What’s next? What is the future of the Universe? How will it end? Will it?

Big Freeze or heat death

Before 1998 the scientific consensus was that the Universe, although definitely expanding, will eventually come to a point when it will start to crunch back. But the empirical data surprisingly showed that the Universe was far from slowing down in its expansion. On the contrary, it is constantly accelerating. It was explained by the role of a mysterious dark energy, about which we have only a theoretical and very blurred guess.

The Big Freeze scenario predicts that the continued expansion will result in a Universe that will slowly approach absolute zero temperature. Like everything else the Universe is trying to reach the point of highest entropy (i.e. if you leave a hot water in a room, it will slowly lose its heat so that it comes in thermal balance with the temperature of its surroundings). In this hypothesis, stars will continue to be born for 1–100 trillion years but eventually the gas clouds that are the basis for their formation will be exhausted, so after the death of last stars the Universe will go dark, leaving it full of only black holes, which themselves will disappear over trillions of years as they emit Hawking radiation. The universe will reach the state of maximum entropy in which everything is evenly distributed everywhere and there are no gradients (which are needed to sustain information processing, one form of which is life). This way Universe will stay forever dead, unless at one point something known as random quantum fluctuations or quantum tunneling will produce another Big Bang.

Big Rip

The Big Rip is another cosmological scenario in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, is progressively torn apart by the expansion of the universe at a certain time in the future. According to the hypothesis, the acceleration will be so high that it will break the bounds of gravitation forces first of galaxies, throwing solar systems spreading at the speeds close to speed of light in all directions. Than the same fate will break solar systems, stars and planets themselves, until eventually even the smallest particles can’t withstand the pull, leaving unbound elementary particles and radiation ripping apart and shooting apart from each other in all directions in an infinite Universe.

Eternal inflation

The eternal inflation hypothesis states that our observable universe is just one among an infinite number of expanding regions of “normal” space within a larger volume of inflationary space.

The different parts of the universe undergo vacuum decay from inflationary to non-inflationary states at different times. Ultimately it produces many regions of normal space surrounded by still-expanding regions of inflationary space where the vacuum has not yet decayed.

These regions of normal space cannot contact each other, and so can be considered separate universes. While any given universe eventually reaches heat death, there are always other regions that haven’t, and new universes being produced within the inflationary volume, so the multiverse as a whole never ends.

False vacuum

Among other ideas the false vacuum scenario assumes that if the vacuum is not in its lowest energy state (a false vacuum), it could tunnel into a lower energy state, something called the vacuum metastability event. It has the potential to fundamentally alter our universe, either destroying all the matter instantaneously or changing the nature of physical constants like matter, energy or spacetime. Studies of a particle similar to the Higgs boson support the theory of a false vacuum collapse billions of years from now.

According to the many-worlds interpretation of quantum mechanics, the Universe will not literally end this way. Each time a quantum event happens that causes the Universe to decay from a false vacuum to a true vacuum state, the Universe will split into several new worlds. In some of them the Universe decays, in others it continues as before.

Big Crunch and Big Bounce

Big Crunch is a symmetric view of the fate of the Universe. It assumes that the amount and nature of the dark energy is not what we currently think and thus it will decrease over time making gravity the primary force of the Universe. Thus the expansion of space will eventually reverse and all the galaxies will race at each other in a dwindling and hotter Universe. Ultimately it will end as a black hole singularity (when all the supermassive black holes absorbing all the matter and pulling closer together will merge into one mega blackhole with the mass of the whole Universe) or causing a reformation of the Universe starting with another Big Bang.

The Big Bounce is a scientific model based on the Big Crunch theory. It basically predicts that the Big Crunch leads to the new Big Bang and the beginning of a new Universe, made of the same matter as previous one. And what is more, it continuously repeats this cycles of Big Bang followed up with Big Crunch. The whole Universe we live is an infinite string of finite universes happening again and again. So, Buddhist.

It is important to note that each possibility of the eventual fate of the Universe is based on the assumptions we have about the nature of the dark energy. In so many ways, although being so far ahead of all the people of the past in our understanding of the Universe, we are still in the infancy regarding our knowledge of its true nature and thus, its ultimate fate.

 It shouldn’t discourage us from searching though. The trillions of years timeline of the majority of the described above events are unbearable for our lifespans. But as a form of life, deriving from the matter cooked in the Big Bang and being based upon the 3.6 billion years of evolution and still carrying imprints of everyone who came before us in our DNA, huge numbers should not scare us away. Quite the opposite, they should inspire us and give us strength in our quest for ultimate knowledge.

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