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A little less than 100 years ago, cosmologist Edwin Hubble made the discovery that led to the conclusion that the universe began in a gigantic explosion – the Big Bang.
Almost since the moment we discovered that everything we see had a beginning, scientists have wondered about how it might end.
Because we still don’t know the precise makeup of the cosmos, it’s impossible to predict which one of roughly half a dozen scenarios will play out, but it’s increasingly accepted that between 2.8 billion and 22 billion years from now, the universe we know will come to an end.
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Cosmologist Katie Mack has specialised in the study of the death of the universe and in her new book The End of Everything (Astrophysically Speaking) she outlines the five most likely possibilities.
The Big Crunch
This outcome is based on the idea that everything is symmetrical, and that the rapid expansion of the universe that began with the Big Bang will slowly run out of steam and eventually come to an end.
Then, assuming that there is enough mass left to hold the driving galaxies together, they will steadily be attracted back to a common centre. Probably quite slowly at first, but accelerating faster and faster until all the matter in the Universe slammed together in an unimaginable hot ball of energy that matches the original Big Bang.
Researchers from the University of Southern Denmark identified a “phase transition” that they believe makes this the most likely scenario back in 2013. "The universe will probably collapse , and a collapse is even more likely than the old calculations predicted," says Jens Frederik Colding Krog, a PhD student at the Centre for Cosmology and Particle Physics Phenomenology
But even if he’s right, there’s probably still plenty of time before you need to worry.
If there isn’t enough mass to hold the universe together, then the momentum from the Big Bang will carry galaxies so far apart from each other, and spread supplies of interstellar gas so thinly that not new stars can be born.
The universe will slowly decline into a cold dark state where black holes will outnumber glowing stars until they, themselves, evaporate into nothingness.
“Eventually," Katie told Quanta Magazine, “you get a universe where the only thing that’s left is a few strange particles and some radiation.”
The Big Rip
Just as the first two end scenarios depend on the amount of matter in the universe, so the Big Rip depends on the amount of energy.
Specifically, a mysterious concept called Phantom Dark Energy which could accelerate the expansion of Space Time until it becomes unstable.
Katie describes it as the “horrific destruction of the universe in a finite time.” Galaxies would begin to unravel from their orbits as the space they moved through warped and buckled.
Eventually, planets would fly off into space, before being pulled apart even at the atomic level.
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The precise conditions before the Big Bang are not known, and probably not knowable. Time as we understand it hadn’t started so there is no “before” as such.
One theory is that everything we can see and understand exists as a bubble between unimaginably vast membranes existing in higher dimensional realms.
The initial spark for the Big Bang occurred when two of these membranes came into contact and if they bounced together again and entire universe could be destroyed in an instant.
This, the most science-fictional sounding concept in Katie’s catalogue of disasters, is based on the value of a recently-discovered subatomic particle called the Higgs Boson.
This infinitesimally tiny entity supplies all matter with mass. But depending on the particle's exact makeup – which scientists are still arguing about – there could be nothing to prevent a bubble of a brand-new universe featuring its own set of natural laws from springing up at any moment.
The new baby cosmos could completely overwrite our own laws of physics like a virus in a computer.
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Unlike most of these end-of-everything scenarios, which would unfold over countless billions of years, Vacuum Decay could theoretically destroy the entire Universe at any moment.
“Vacuum decay is my favourite,” says Katie. “One, because it is a very dramatic kind of idea. But also because it seems to come right out of left field.”
The End of Everything: (Astrophysically Speaking) by Katie Mack is out now, published by Penguin
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