Will an Expanding Bubble of Alien Matter Consume the Universe?
What happens at the end of the universe? Cosmologists have attempted to answer this question for centuries, but the strangest answer may be the newest one: that the universe will end when a bubble of vacuum expands and consumes our entire universe, converting all matter into an alien substance of which we cannot conceive.
In an interview with Slate, cosmologist Alex Vilenkin explained this relatively new theory of the end of times: "All of a sudden a tiny little bubble will appear. It can appear anywhere-under your chair, or somewhere in Andromeda, very far away-and this little tiny thing starts growing at a speed that's pretty close to the speed of light. And as it expands, all things that it engulfs turn into an alien form of matter... Inside of this bubble, ordinary matter as we know it does not exist. It's made up of different kinds of particles. So everything will be turned into some other stuff that we just don't know about."
Vilenkin's theory has some high-profile supporters; Stephen Hawking recently claimed that the recently discovered Higgs Boson's inherent instability puts our universe in danger of catastrophic vacuum decay, in which the entire universe becomes one large vacuum. (He even used the bubble analogy!)
Both scientists agree that this catastrophic vacuum decay could technically occur at any time, although Valenkin clarifies that the probability of its occurrence at any given moment in the present day, or even in the foreseeable future, is very low. Indeed, the chances that it will happen before the sun has died are almost negligible, which means we will likely never have to worry about this particular doomsday scenario, as it will happen after we've already gone extinct. (Although, as the interviewer pointed out, this could lead to a whole host of other existential crises.)
Valenkin points out that this scenario, while in keeping with our current knowledge of physics and cosmology, is not necessarily a given, as several previous theories of the end of the universe have been debunked. For example, physicists once believed that the end of the universe would essentially be a reversal of the Big Bang; where the Big Bang began with an extremely condensed and hot pinpoint and then rapidly expanded and cooled, the Big Crunch theory asserted that gravity would eventually cause the expansion of the universe to stop and reverse until we were all consumed by fire. This theory was invalidated by the discovery of dark energy, which produces a repulsive gravitational force that sustains the expansion of the universe.
But then, the concept of the perpetual expansion of the universe led to the Big Chill theory, in which the universe expands to the point that it reaches absolute zero, causing stars to die and the universe to become a vast, empty space. Valenkin asserted that this theory is more correct, as it would, in fact happen eventually, except for the fact that the bubble scenario will simply occur first.
But what does the "end of the universe" even mean? Or more specifically, what happens after? Just as there was "something" before the Big Bang, there must be "something" after our universe is gone. Valenkin explained, "If the 'bubble' theory of how the universe ends is correct, then although we and our surroundings will sooner or later be engulfed, beyond that, the vacuum will continue expanding and there will be other bubbles forming. So this kind of 'boiling' of the vacuum never ends, it continues forever."
But he also clarifies that the question of what happens "after" the end of the universe may be the wrong question to begin with, as time may simply be a construct that exists in our universe that will cease to exist: "St. Augustine wrote a wonderful book called Confessions in the fourth century. He was trying to answer the question of what God was doing before he created Heaven and Earth. Augustine was wondering: If God was just hanging out doing nothing, why did he suddenly burst into action after an eternity of idleness? And the answer he came up with was that when God created the universe, he created time as well. So there was no concept of 'time' before the creation of the universe. Some cosmological theories have this property as well, that space, time, and matter all came into being at the same time. As for the end … we don't really know what happens after that. Or if there is any 'after.'"
And finally, when asked if he found the insignificance of humanity in the context of cosmology "depressing," he opined that our smallness is less depressing than our lack of uniqueness in the universe: "According to these modern theories the universe keeps going forever. And even though our local region will kind of succumb to an evil bubble, in different places there will be different Earths and in that scenario, things will repeat themselves. So there will be other Earths that are pretty much exact copies of ours. So of course most of the different civilizations will be nothing like ours, but there will also be ones just like ours. So what I'm sad about is that we're not unique in the universe."