Cosmology deals with the big questions of the universe, often the same questions that keep philosophers up at night. When did the universe begin? How did it start? Has the universe always been expanding? (For the record, the answers are: about 13.8 billion years ago, in a high-density state that rapidly expanded called the Big Bang and yes, but not always at the same speed.) But here’s a question they haven’t figured out yet: How’s it all going to end?
It’s a big question all right, but we’ve made surprising headway toward an answer. In the last years of the 20th century, the astrophysical community was stunned to learn that the universe was driving itself apart. For decades, scientists had known that distant galaxies all move away from us, with the farther ones moving the fastest. The only way this makes sense is if the universe itself is expanding. Given all the matter in the cosmos, the force of gravity should be slowing down that expansion. But when cosmologists calculated just how much it’s slowed down, they got a negative result — the expansion of the universe is speeding up!
Nobody knows what’s driving the acceleration, so cosmologists have dubbed that mystery dark energy. It is so dominant (about 69 percent the total content of the entire cosmos) that dark energy quickly became a part of any discussions about the final end of the universe. And while there are no definite answers yet, those discussions have come up with a few interesting possibilities.
The Big Rip
No one knows what dark energy is, so we can’t be sure how it will behave in the future. In 2003, Robert Caldwell of Dartmouth College proposed a new theory of the expansion of the universe where the rate of acceleration keeps increasing over time.
Imagine a driver who keeps a foot on the gas pedal of a car with no top acceleration. As the car goes faster and faster — the speed of the velocity change itself increasing over time — the car would eventually fly apart in pieces as friction took its toll.
A similar thing happens to a universe with relentless acceleration: Galaxies would be destroyed, the solar system would unbind and eventually all the planets would burst asunder as the rapid expansion of space rips apart its very atoms. Finally, our universe would end in an explosion, a singularity of literally infinite energy.
Current theories predict that if this so-called Big Rip is in our future, it will take another 22 billion years to arrive. But there are still many details to fill in, and scientists like Vanderbilt University mathematician Marcelo Disconzi will provide those details. His work originally focused on bulk viscosity — the measurement of a fluid’s resistance to expansion or contraction — and how moving fluids behave when approaching the speed of light. (That can happen in extreme astronomical situations, such as an exploding star.) No one had successfully modeled how a viscous fluid would act at relativistic speeds, but working with colleagues in the Vanderbilt physics department, Disconzi successfully did it.
For decades, scientists had been trying to link mainstream physics’ understanding of viscosity with a related concept: cosmic viscosity, which tells us about the universe’s resistance to accelerating expansion. The universe is modeled as a fluid, in large scales, so Disconzi’s work proved instrumental in starting to understand cosmic viscosity. He showed for the first time that the driving force behind the Big Rip — if it happens — could be the universe’s cosmic viscosity.
The results are interesting, but Disconzi acknowledges that Big Rip theories still require a bit of work to make sense, particularly the part about infinite energy being released.
“In physics, finding an infinity is likely an indication that we are missing part of the puzzle,” he says.
An improved theory might show that the universe’s expansion rate doesn’t really go all the way to infinity, no matter how large the Big Rip becomes. For now, as with many cosmological theories, we’ll have to wait and see.
The Big Freeze
Many current theories suggest dark energy is a cosmological constant, a kind of uniform energy that exists throughout space. “If the cosmological constant is the dominant thing in the universe,” explains Mark Trodden, a co-director for the Penn Center for Particle Cosmology at the University of Pennsylvania, then instead of speeding up unsustainably and tearing itself apart, the universe would just keep expanding — forever. “If the cosmological constant continues to dominate, the universe will continue to accelerate and that’s it.”
“That’s it” might not sound like much, but it’s a bleak way to go. As the cosmological constant continues to drive the acceleration of the universe, all the galaxies outside our immediate neighborhood will be too far away to be visible in a few trillion years — their light will simply never reach us. Entropy would continue to increase as well: Star formation itself will end in 100 trillion years as all the matter to fuel them is exhausted. Black holes will evaporate, matter itself will eventually decay into radiation and the universe will be a cold, lightless, lifeless place for the rest of eternity. This dark future is known as the Big Freeze.
Dark energy is so baffling that it has taken the better part of two decades to understand the theory enough to even design experiments to study it, let alone figure out if the Big Freeze lies in our future. (Not that we could do anything about it if it did, of course.) Trodden is a theorist for one such planned experiment at the Large Synoptic Survey Telescope, currently under construction, that will image the entire sky every few nights at unprecedented detail. When it “switches on” in 2022, the telescope will study billions of galaxies every night, giving astrophysicists an unparalleled ability to test galactic behavior.
By analyzing how gravity affects galaxies’ light, and analyzing their three-dimensional positions and movements in space, the scientists should have enough information to finally unlock some of dark energy’s secrets. Does it arise from quantum fluctuations in the vacuum of space? Is it just a part of gravity we’ll have to add to the laws of physics? Understanding such details is crucial if we want to figure out whether we’re heading for the Big Freeze — or something else.
The False Vacuum
Of course, there’s always the chance that dark energy won’t actually matter. The other scenarios we’ve considered assume that our universe is all there is. But what if there’s more out there, and our universe is but a small part of a multiverse? Could these other universes affect the ultimate fate of our own?
The short answer is yes. “Imagine you go to a very, very large scale — much larger than our current observable universe,” explains Jonathan Braden, a cosmologist at University College London. While our own universe is homogenous and roughly the same everywhere, taking such a large view might reveal that it’s just a tiny pocket with its own physical parameters and laws, different from the larger multiverse.
If so, our universe would exist in a state known as a false vacuum, where we wrongly suppose that we exist in the most stable state, but it’s still possible to drop to another one suddenly. Braden explains this would result in a phase transition, a change similar to how water changes from liquid to gas at its boiling point — only for the entire universe. Basically, our cosmos might be like one of the bubbles boiling in a pot of water, he says, just one of many with their own sets of laws and constants. “Eventually these bubbles can run into each other, and from our viewpoint it would be like our universe … collided with another universe.”
That would be as bad as it sounds. Such a collision would spell an immediate end to our universe as everything changes to a new state. It could result in a combination of the two universes, or it could create something new entirely. There would be no warning, and nothing we could do about it.
What scientists can do, however, is see whether a false vacuum ever existed in our own universe in the very distant past, which could help tell us whether such a collision is likely to happen again. Such a prehistoric false vacuum would necessarily be much smaller than the scale of our universe (just a bubble within our bubble), but it might still leave behind a hint that something curious happened long ago.
Braden is part of a research team trying to predict what such a signature would look like. Their prime target is the cosmic microwave background (CMB), the oldest light scientists can see, which dates back to when the universe was just 380,000 years old. The team hopes that perhaps a little blip on the otherwise uniform CMB would betray the existence of such a false vacuum in the distant past, as well as provide the first concrete evidence for a multiverse.
If we can’t ward off the end of our universe, the least we can do is try to understand it.