What is a universe? If ‘the universe’ just means ‘all that exists’, then there is, by definition, just one universe—one collection of all that exists—and that collection may or may not contain, what might be called, ‘many worlds’. But how many of them exist? Is the universe in an infinite cycle of rebirth?
Definitions of Universe and Multiverse
What definition could capture the relevant concept? It might be said that a universe is simply a collection of matter that is closed off from outside causal interaction. But by that definition, it would be impossible to travel from one universe to the other; the travel would be a causal interaction.
A better definition of ‘universe’ would seem to be a ‘space-time continuum and all the entities it contains’. This would keep us from identifying some subset of matter in our universe as a universe itself, but would also seem to allow for the possibility of other universes and travel between them.
The most common way to understand a multiverse is to imagine that our universe lives on a membrane, or brane, along with many other universes (that also live on branes) in what is called ‘the bulk’. It’s like the multiverse is a giant closed book, and there is a universe on every page.
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Singularity That Started It All
It’s widely acknowledged that our universe began when a Singularity exploded with a Big Bang—and many physicists, like Ed Tryon, suspect that the Singularity (which was the source of the Big Bang) was the result of a quantum fluctuation in the vacuum that existed prior to our universe. How is that possible?
Well, simply put: not even vacuums are empty. They contain no matter, sure, but they are still filled with quantum activity. And such quantum activity can, randomly, generate matter. Give a large enough vacuum a long enough time, and a large amount of matter could be produced. Such matter would briefly be contained in a tiny dense hot point that explodes, causing that matter to expand out. Something very much like this is what generated our universe.
Now, people used to wonder whether gravity would eventually pull all the matter of our universe back in—the big crunch, it was called. But now it is known that the force of gravity is not strong enough to do this and that indeed the expansion of the universe is accelerating. What many scientists now expect is that the universe will end in what is called a ‘heat death’.
All entropy will stop, every atom will decay, and things will be back down to a vacuum. But that vacuum will still be teeming with quantum activity. According to the work of physicists Sean Carroll and Jennifer Chen, given long enough, that could produce another big bang. Indeed, this could have already happened many times over, and happen many times over in the future.
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Two Different Kinds of Multiverses
Each one of these new collections of matter should be described as a new universe. Each one of these consecutive collections of matter would be in its own space-time continuum. If so, ours is just the latest in an infinite series of universes, and all of reality consists of what might be called an oscillating multiverse.
At this point, one might argue that an oscillating multiverse wouldn’t really count as a multiverse, because all of the universes aren’t happening at the same time. To really be a multiverse, the events of each universe have to be happening simultaneously—like they are on the previously discussed view that has multiple universes all stacked up in the bulk.
But, according to Einstein’s Theory of Relativity, simultaneity is relative to the reference frame. So, only two events in the same universe can be simultaneous, and even then only in certain reference frames.
It’s actually meaningless to think of two events in two different universes in the bulk as happening at the same time, just like words on two different pages of a book can’t be on the same line.
Indeed, since (on both models) each universe would be contained in its own space-time continuum, the only difference between a bulk multiverse and an oscillating multiverse is how it is pictured in the mind. The former’s universes are stacked vertically, the latter’s are laid out horizontally.
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Infinite Cycle of Inflation and the Rebirth of the Universe
An oscillating multiverse is not that different from Alexander Vilenkin’s conception of the multiverse, based on the theory of inflation. Inflation is a period of super-fast expansion of space that can be slowed locally by specific instances of quantum activity.
This is how quantum fluctuations can produce a big bang. When a fluctuation slows the inflation, the energy that drove inflation spills over igniting a ‘hot fireball of particles and radiation’. A big bang.
But inflation continues elsewhere, where that quantum activity had no effect. This continually creates more space for the same thing to happen again. Somewhere else, some other quantum activity slows inflation, and boom—another expanding bubble.
And since inflation is eternal, this would happen an infinite amount of times. It’s not clear whether such universes should be pictured as being stacked or laid out consecutively. But regardless, it seems this would rightly be called a multiverse.
Common Questions about the Infinite Rebirth of the Universe
The best simple definition of ‘universe’ would seem to be a ‘space-time continuum and all the entities it contains’. This would seem to allow for the possibility of other universes and travel between them.
Many scientists believe the universe will end in a ‘heat death’. Because gravity isn’t strong enough to pull everything together, the universe will expand until all entropy stops and all atoms decay. This will leave a vacuum that still has quantum activity. However, according to the work of physicists Sean Carroll and Jennifer Chen, given long enough, that could produce another big bang.
Both have infinite cycles of rebirth. However, how they are pictured in the mind is different. In the bulk multiverse, the universes are stacked vertically, and in the oscillating multiverse, they are laid out horizontally.