January 17, 2021

Multiverso

Artistic impression of a Multiverse — where our Universe is only one of many. According to the... [+] research, varying amounts of dark energy have little effect on star formation. This raises the prospect of life in other universes — if the Multiverse exists.

Jaime Salcido/simulations by the EAGLE Collaboration

 

The Universe is all there ever was, all there is, and all there will ever be. At least, that's what we're told, and that's what's implied by the word "Universe" itself. But whatever the true nature of the Universe actually is, our ability to gather information about it is fundamentally limited.

It's only been 13.8 billion years since the Big Bang, and the top speed at which any information can travel — the speed of light — is finite. Even though the entire Universe itself may truly be infinite, the observable Universe is limited. According to the leading ideas of theoretical physics, however, our Universe may be just one minuscule region of a much larger multiverse, within which many Universes, perhaps even an infinite number, are contained. Some of this is actual science, but some is nothing more than speculative, wishful thinking. Here's how to tell which is which. But first, a little background.

There is a large suite of scientific evidence that supports the picture of the expanding Universe... [+] and the Big Bang. The entire mass-energy of the Universe was released in an event lasting less than 10^-30 seconds in duration; the most energetic thing ever to occur in our Universe's history.
 
There is a large suite of scientific evidence that supports the picture of the expanding Universe and the Big Bang. The entire mass-energy of the Universe was released in an event lasting less than 10^-30 seconds in duration; the most energetic thing ever to occur in our Universe's history. NASA / GSFC

The Universe today has a few facts about it that are relatively easy, at least with world-class scientific facilities, to observe. We know the Universe is expanding: we can measure properties about galaxies that teach us both their distance and how fast they appear to move away from us. The farther away they are, the faster they appear to recede. In the context of General Relativity, that means the Universe is expanding.

And if the Universe is expanding today, that means it was smaller and denser in the past. Extrapolate back far enough, and you'll find that things are also more uniform (because gravity takes time to make things clump together) and hotter (because smaller wavelengths for light mean higher energies/temperatures). This leads us back to the Big Bang.

There are a number of things we observe in the Universe that the Big Bang can't explain, but a new theory that sets up the Big Bang — cosmic inflation — can.

Inflation tells us that, prior to the Big Bang, the Universe wasn't filled with particles, antiparticles and radiation. Instead, it was filled with energy inherent to space itself, and that energy caused space to expand at a rapid, relentless, and exponential rate. At some point, inflation ends, and all (or almost all) of that energy gets converted into matter and energy, giving rise to the hot Big Bang. The end of inflation, and what's known as the reheating of our Universe, marks the start of the hot Big Bang. The Big Bang still happens, but it isn't the very beginning.

If this were the full story, all we'd have was one extremely large Universe. It would have the same properties everywhere, the same laws everywhere, and the parts that were beyond our visible horizon would be similar to where we are, but it wouldn't be justifiably called the multiverse.

Until, that is, you remember that everything that physically exists must be inherently quantum in nature. Even inflation, with all the unknowns surrounding it, must be a quantum field.

If you then require inflation to have the properties that all quantum fields have:

  • that its properties have uncertainties inherent to them,
  • that the field is described by a wavefunction,
  • and the values of that field can spread out over time,

you reach a surprising conclusion.

Inflation doesn't end everywhere at once, but rather in select, disconnected locations at any given time, while the space between those locations continues to inflate. There should be multiple, enormous regions of space where inflation ends and a hot Big Bang begins, but they can never encounter one another, as they're separated by regions of inflating space. Wherever inflation begins, it is all but guaranteed to continue for an eternity, at least in places.

Where inflation ends for us, we get a hot Big Bang. The part of the Universe we observe is just one part of this region where inflation ended, with more unobservable Universe beyond that. But there are countlessly many regions, all disconnected from one another, with the same exact story.

 

An illustration of multiple, independent Universes, causally disconnected from one another in an... [+] ever-expanding cosmic ocean, is one depiction of the Multiverse idea. In a region where the Big Bang begins and inflation ends, the expansion rate will drop, while inflation continues in between two such regions, forever separating them.  

An illustration of multiple, independent Universes, causally disconnected from one another in an ever-expanding cosmic ocean, is one depiction of the Multiverse idea. In a region where the Big Bang begins and inflation ends, the expansion rate will drop, while inflation continues in between two such regions, forever separating them. Ozytive / Public domain

 
That's the idea of the multiverse. As you can see, it's based on two independent, well-established, and widely-accepted aspects of theoretical physics: the quantum nature of everything and the properties of cosmic inflation. There's no known way to measure it, just as there's no way to measure the unobservable part of our Universe. But the two theories that underlie it, inflation and quantum physics, have been demonstrated to be valid. If they're right, then the multiverse is an inescapable consequence of that, and we're living in it.
the Multiverse is not a scientific theory on its own. Rather, it’s a theoretical consequence of the laws of physics as they’re best understood today. It’s perhaps even an inevitable consequence of those laws: if you have an inflationary Universe governed by quantum physics, this is something you’re pretty much bound to wind up with. 
 
 
 

 

 

 

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