Physicists, mathematicians, and scientists can't seem to agree on how the universe will end. In some ways, they can barely agree on how the universe came to be. Nonetheless, most leading thinkers in the scientific field agree that the universe is not an infinite place and someday, it will cease to exist.
The theories run the gamut from a deathly freeze to getting ripped apart, to the universe crunching into itself. As a barely understood aspect of the universe, many leaders in the scientific community view dark energy as a key player in the universe's ultimate demise. Luckily for us, most predictions are in the unfathomable future, billions or even trillions of years from now.
And while it causes us very little real concern today, overall, it's disconcerting to consider that someday, even if it's billions or trillions of years from today, all that is the universe, all that is being, will one day, not be. And there will be no one left to study what once was, nor will there be anyone to remember us or the universe fondly. Someday, there will be no evidence that our universe ever existed.
Unless that is, before the universe expires, human beings figure out how to move to a new universe. And then, billions or trillions of years from today, if humanity manages to survive, our universe as we know it, and everything that ever existed within it, will seem more distant and more foreign than our current understanding of what kicked it all off...the "Big Bang."
Most scientists agree that the universe will eventually end. One such theory of how it will happen is called the "Big Freeze," a hypothesis which holds that ultimately, the entire cosmos will become empty and the temperature will hover around zero. Scientists who support this theory believe that all of the stars in the universe will burn out eventually; even the protons in the universe will decay. With only electrons left, the universe will eventually approach absolute zero. What this means in simple terms is that scientists who support the Big Freeze see the universe breaking down slowly.
When people hear about the Big Freeze, one of the first questions they ask is how scientists know that the universe will break down. The answer to that question hinges on accepting the second law of thermodynamics, which virtually all reputable scientists agree, governs the universe. The second law of thermodynamics, in a nutshell, means that entropy - or chaos - is the order of the universe, and entropy is always increasing in the universe. Entropy, in the scientific vocabulary, doesn't necessarily mean the disorder we experience when, for instance, driving in busy city traffic. This type of chaos, in the scientific lexicon, more or less says that all of the matter in the universe tends to fall apart slowly. Just like a piece of fruit left untouched will rot, so too is the matter in the universe rotting, or better stated, decaying. What that means for our universe is that eventually (trillions of years from now, actually), all matter will decay, all motion will stop, and the universe will succumb to a Big Freeze.
So what? Can't modern technology keep us warm in the midst of this Big Freeze? Not exactly. What is important to understand with this theory is that the Big Freeze is synonymous with "Heat Death." There will be no differences in temperature... anywhere. For example, you charge your phone in the electrical outlet, which gets its supply from the local power plant. The power plant uses heated water to power a turbine to generate electricity. Heat Death means there is no difference in temperature; that water won't be able to heat to generate electricity. We won't be able to grow food, due to the decay of the sun. Nothing will be able to survive.
Another, more violent end to the universe could come in the form of the "Big Rip." The Big Rip theory relies on dark energy's gravitational repulsion. What that means in simple terms is that force - in this case, dark energy - will overpower other forces that hold the objects of the universe together. When that happens, that dark energy's gravitational repulsion will rip apart all objects and matter in the universe. Scientists who support the Big Rip as theoretically possible hold that nothing in the universe, not even the smallest unit of matter, will survive getting torn apart.
Now, before you start getting anxious about dark energy's repulsion force tearing you apart, know that scientist don't expect this scenario to occur earlier than 16 billion years from now. The Big Rip will arrive, according to scientists that support this theory, because dark energy is growing, and it will keep growing infinitely. In doing so, it's causing everything in the universe to move and pull apart. And, in truth, even scientists, mathematicians, and physicists do not entirely understand dark energy. Some scientists have described dark energy as a "cosmological placeholder," a constant that we still do not understand. The scientific community overall accepts that the universe is expanding and the rate of expansion is accelerating, but they only understand that dark energy operates in the universe, and it's causing that expansion. They do not understand what it is or how it works.
Professor Carlos Frenk, a cosmologist at the University of Durham, has explained that dark energy is “the physicists’ way to hide their ignorance by giving it [this phenomenon] a mysterious name. We don’t have any physically compelling way to explain it.”
So, while the scientific community continues to develop their understanding of dark energy, we do know one thing: dark energy is a significant force, and it is slowly tearing our universe apart.