There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.
There is another theory which states that this has already happened.
-Douglas Adams, The Restaurant at the End of the Universe
As we’ve seen in other essays in Naturally Curious, science has long been trying to discover where our Universe came from, and it’s a question that cosmologists continue to grapple with. While we do know it started with the Big Bang, an even tougher and arguably more cogent question remains: where is it going? Clearly, theories about the end of the Universe remain untestable—it hasn’t happened yet, and we can’t run an experiment beforehand.
Nevertheless, new possibilities are arriving with surprising rapidity. As we make developmental leaps in science and technology, and are able to journey or peer further and further out into the cosmos, both physically and using higher resolution telescopes, new discoveries add to what we already know, changing our ideas about the Universe and its destination.
I don’t want to scare anyone unnecessarily, but it’s worth noting that long, long before the predicted end of the Universe our sun will die (in five billion years or so), taking Earth and our solar system with it. The planet is basically toast.
Whilst there is no scientific consensus for how the end of the Universe happens, there are ideas like the “Big Snap,” the “Big Freeze,” the “Big Crunch”, the “Big Bounce” and the “Big Rip.” (It seems there is little disagreement that the event will be big.)
But I’m getting ahead of the story. Two of the most important finds of the last hundred years, both critical to our current understanding of the cosmos, were Edward Hubble’s 1929 discovery that the Universe was expanding, and the 1998 discovery by Nobel-prize winning astronomers Saul Perlmutter, Brian Schmidt and Adam Riess, that the Universe’s expansion was increasing rapidly.
We’ve all no doubt heard that the force that was causing the Universe to speed up was given the name “dark energy” by enterprising, creative physicists. Whilst we know little about it, dark energy is believed to make up about 68.3% of the Universe. If we’re looking for certainty about the ultimate fate of our Universe we’ll need to figure out how this works, and where all this escalating, madly accelerating inflation is heading.
Before the 1998 discovery, the two prevailing theories about the fate of the Universe were the “Big Chill” and the “Big Crunch.” The “Big Chill,” also known as the “Big Freeze,” is an idea derived from the notion that while the Universe continues its increasingly accelerating expansion, it is simultaneously cooling. If you accept this, and there’s good evidence for it, it’s logical to assume that this cooling and expansion are primed to go on together, hand-in-hand, indefinitely. So, at some point the Universe will cool to such a degree that it will be impossible to sustain life.
That’s one view; a logical extrapolation of current trends. An inevitable Big Freeze. Here’s another: In the “Big Crunch” theory, the Universe is expanding, and it will do so until it reaches a finite point. Once it hits that point it will stop expanding and begin to contract. And eventually, everything in the Universe—everything that is the Universe, all of matter and space-time—will collapse back into itself; into a new spacetime singularity. All that we know and see, squished together in an incredibly dense point – not even the size of a pin-prick.
Theoretically this could happen an infinite number of times: a Big Bang will follow a Big Crunch, which will come after a Big Bang, which will arrive after the last Big Crunch and so on. Inevitably, cosmologists have labeled this the “Big Bounce”. So where does our own Universe, the one we live in, figure in this theory? As a reaction to the last Big Crunch. How many previous bouncing, yoyo-type Universes have there been? Who knows?
The “Big Crunch” account, while popular, has been seen as irreconcilable with a law of thermodynamics, which should predict the Universe will heat during the cycles. The Big Crunch theory also requires a closed Universe, which is not a proposition that is accepted by many astronomers.
After the discovery of dark energy in 1998, scientists had to re-think these scenarios. What are the possibilities if the Universe is expanding more and more rapidly? Will it just slowly freeze over? That’s what the Big Chill/Big Freeze account says. According to new theories that have emerged, this isn’t necessarily the case.
So here’s another mind-blowing idea. According to the “Big Rip” theory, the expansion of the Universe will continue until scale and distance gets close to infinity, at which point all matter in the Universe will be torn apart. Previously there were aspects of this theory that were irreconcilable with the laws of physics. For instance, if the Universe is moving apart faster and faster, as it gets to extremely high velocities it will bump up against a natural speed barrier: nothing can travel faster than the speed of light. As matter needs to travel faster than the speed of light for the “Big Rip” to occur, the theory doesn’t appear to hold up.
However, scientists from Vanderbilt University in Nashville, Tennessee have recently developed a series of mathematical equations that could help to resolve this. I won’t go into the math, but there’s a paper in Physical Review D by Disconzi and colleagues that you can read if you want to follow up.
A more recent “Big” theory comes from Max Tegmark from the Massachusetts Institute of Technology. In a 2012 article for the Huffington Post, Tegmark says that if he were to bet, he’d put just “40% on the Big Chill, 9% on the Big Crunch and 1% on the Big Rip.” (Interesting that he can frame odds so accurately). What about the remaining 50%? Tegmark’s putting his money on the “Big Snap.” That is, the Universe just snaps suddenly out of existence. Here one moment, gone the next.
The rationale? There are notions drawn from quantum mechanics, analyzing the smallest states in physics, arguing that the cosmos must conserve the amount of information it holds. This idea, combined with the firm evidence of the rapid expansion of the Universe, would, according to Tegmark, cause a “snap.” Much like a rubber band. There goes the whole Universe again. None of us will know much about it, but it is a very abrupt end to it all compared to the Big Chill.
Thus far, thus scary. These are a few of the prevailing ideas, but discoveries and theories are coming thick and fast, and almost every day we are adding and expanding to our knowledge—and the range of possible applications of that knowledge. And if you thought the “Big” theories were a little bizarre, some others are, frankly, out of this world.
One of the most recent ideas about of the end of the Universe comes from Nemanja Kaloper from the University of California and Antonio Padilla from the University of Nottingham, UK.
Vacuum energy is the name we give to the simplest version of dark energy—the background energy permeating the Universe—responsible for accelerating its expansion. However, scientists have a problem with vacuum energy. If Einstein’s general theory of relativity is accurate, and energy makes space curve (stay with me here), the intensity of vacuum energy will force the Universe to collapse in on itself. So to get around this, and explain the continuing expansion that we see, Kaloper and Padilla have changed Einstein’s theory, just slightly, by proposing that space-time is finite.
“Talk about a cure that’s worse than the disease,” grumbles the New Scientist article reporting on the new theory: the catch is, if space-time is finite, at some point, according to Kaloper, “Cosmic expansion must eventually reverse … the Universe returns back to where it banged from.”
The two scientists published a second paper that attempts to explain what the cause of the collapse might be. They suggest a new quantum field whose energy slowly drops over time to negative, at which point the Universe would contract. Just before that contraction however, the field would rapidly expand.
According to Padilla and Kaloper, that is where we are at now, in, that final expansion before contraction. Indeed, Padilla says, “It’s as if the dark energy is the harbinger of doom.”
If this is too difficult to contemplate, at least we have a picture of what Kaloper and Padilla mean. The figure illustrates that, according to this theory, we are at the stage in the history of the Universe where we are nearing the end of the process of expansion, and not far from entering contraction.
Kaloper and Padilla and others are currently working on determining exactly when this contraction will begin, but they say they are poised to find it is much sooner than anyone anticipates. All I can say is, we should stay tuned.
The science of the Universe, you will have noticed, makes all sorts of amazing, and sometimes seemingly preposterous claims about all there is and what will happen to it, on the basis of meticulous measurements and with underlying mathematics and extensions of careful logic. One of the most peculiar, and complex, was proposed by physicists in 2009. This proposition suggests that physicists themselves might be the cause of the end of time.
Ben Freivogel, speaking at a seminar at the Massachusetts Institute of Technology in Cambridge, put it as simply as he could: “We (meaning us humans) could run into the end of time.”
Huh? Personally, I struggle with this idea. What would that be like? Running into a brick wall at full speed? Ouch.
But their longer explanation is even more bizarre. What this team is suggesting is that a mathematical theory that’s used to measure the multiverse, could have actual physical consequences.
In a multiverse theory of the Universe (see the Naturally Curious essay of January 12, “More than just a doppelgänger: multiple yous and mes everywhere”), where ours is only one of many universes, areas of space can grow rapidly and balloon out, creating new universes. But because in this theory anything that can happen will happen, and will happen an infinite number of times, it is impossible for scientists to calculate probabilities. So, physicists have been using something called a geometrical cut-off to take “samples” of the multiverse and count the number of universes within that.
What a Multiverse might look like
However, in 2011, theoretical physicist Raphael Bousso and his colleagues from the University of California published a paper arguing that the mathematical tool could have dire physical consequences. “You cannot use [cutoffs] as mere mathematical tools that leave no imprint,” states Bousso. “The same cutoff that gave you these nice and possibly correct predictions also predicts the end of time ... . In other words, if you use a cutoff to compute probabilities in eternal inflation, the cutoff itself becomes an event that can happen.”
So, it’s possible that the Universe—and all of us in it—will simply get to a point where we bump into the end of time – just because physicists have thought of this as an event and it is possible in some universe, it becomes one in this Universe. And it’s probably not the case that this will horrify us any more than all the other “Big” alternatives, because the next cutoff is forecast to happen in five billion years, just around the same time that our sun is due to die. Some physicists report that it would be like coming to the edge of a black hole, although for most of us that doesn’t mean much, visually, either.
Not you and me of course—we’ll be long gone. But Homo future-us will be there to experience the final conflagration when the Universe hits the edge of space-time. It’s not something for which you’d line up to buy tickets.
Until we can definitively answer questions about the shape and structure of the Universe, about dark energy, dark matter, and inflation, there’s no way we can be certain about any of the current theories of the Universe. We still have so much to learn.
I’m intrigued about these options for the end of the Universe beyond almost anything I write about in Naturally Curious. Perhaps the most interesting thing about theories of the Universe are the incredibly complex ideas and math that scientists grapple with or invent in order to come up with them. You could say that some, like Boussos’s, stretch our imaginations further than Tegmark’s rubber band.
An equation that proves that mathematical formulations that physicists compile in the very act of them compiling them, could causally bring about the end of the Universe? No one could make that stuff up.
All except Douglas Adams, of course.
Further reading:
Adams, Douglas (1980) The Restaurant at the End of the Universe. London, UK: Pan Books
Becker, Adam (2015). When will the Universe end and could anything survive. BBC, Earth [Online]. June 2. http://www.bbc.com/earth/story/20150602-how-will-the-Universe-end
Bousso, Raphael, Freivogel, Ben, Leichenauer, Stefan, Rosenhaus, Vladimir (2011). Eternal inflation predicts that time will end. Physical Review D, 83: 023525
Courtland, Rachel (2010). ‘Countdown to oblivion: Why Time Itself could end. New Scientist. Issue 2780. October 2. https://www.newscientist.com/article/dn19513-countdown-to-oblivion-why-time-itself-could-end/
Disconzi, Marcelo M, Kephart, Thomas W, Scherrer, Robert J (2015). New approach to cosmological bulk viscosity. Physical Review D, 91 (4). DOI: 10.1103/PhysRevD.91.043532.
Kaloper, Nemanja, Padilla, Antonio (2014). Sequestering the Standard Model Vacuum Energy. Physical Review Letters, 112 (9). DOI: 10.1103/PhysRevLett.112.091304.
Kaloper, Nemanja, Padilla, Antonio (2015). Sequestration of Vacuum Energy and the End of the Universe. Physical Review Letters, 114 (10). DOI: 10.1103/PhysRevLett.114.101302.
McKee, Maggie (2015). Dark energy, the harbinger of doom. New Scientist. Issue 3015. April 4, 2015.
Tegmark, Max (2012). The Big Snap. Huffington Post. The Blog. May 1. http://www.huffingtonpost.com/max-tegmark/the-big-snap_b_1184800.html
Than, Ker (2010). Time Will End in Five Billion Years, Physicists Predict. National Geographic. October 29. http://news.nationalgeographic.com/news/2010/10/101027-science-space-Universe-end-of-time-multiverse-inflation/