What Is (And Isn’t) Scientific About The Multiverse

Our best physical theories predict that a multiverse exists. But if we can’t test it, is it really scientific?

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. (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.

An illustration of our cosmic history, from the Big Bang until the present, within the context of the expanding Universe. The first Friedmann equation describes all of these epochs, from inflation to the Big Bang to the present and far into the future, perfectly accurately, even today. (NASA / WMAP SCIENCE TEAM)

But the Big Bang wasn’t the very beginning of the Universe! We can only extrapolate back to a certain epoch in time before the Big Bang’s predictions break down. 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.

The quantum fluctuations that occur during inflation get stretched across the Universe, and when inflation ends, they become density fluctuations. This leads, over time, to the large-scale structure in the Universe today, as well as the fluctuations in temperature observed in the CMB. (E. SIEGEL, WITH IMAGES DERIVED FROM ESA/PLANCK AND THE DOE/NASA/ NSF INTERAGENCY TASK FORCE ON CMB RESEARCH)

In the 1980s, a large number of theoretical consequences of inflation were worked out, including:

what the seeds for large-scale structure should look like,
that temperature and density fluctuations should exist on scales larger than the cosmic horizon,
that all regions of space, even with fluctuations, should have constant entropy,
and that there should be a maximum temperature achieved by the hot Big Bang.

In the 1990s, 2000s and 2010s, these four predictions were observationally confirmed to great precision. Cosmic inflation is a winner.

Inflation causes space to expand exponentially, which can very quickly result in any pre-existing curved or non-smooth space appearing flat. If the Universe is curved, it has a radius of curvature that is at minimum hundreds of times larger than what we can observe. (E. SIEGEL (L); NED WRIGHT’S COSMOLOGY TUTORIAL (R))

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.

*Inflation predicts the existence of a huge volume of unobservable Universe beyond the part we can observe. But it gives us even more than that.* (E. SIEGEL / BEYOND THE GALAXY)

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.

The quantum nature of inflation means that it ends in some “pockets” of the Universe and continues in others. It needs to roll down the metaphorical hill and into the valley, but if it’s a quantum field, the spreading-out means it will end in some regions while continuing in others. (E. SIEGEL / BEYOND THE GALAXY)

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.

Wherever inflation occurs (blue cubes), it gives rise to exponentially more regions of space with each step forward in time. Even if there are many cubes where inflation ends (red Xs), there are far more regions where inflation will continue on into the future. The fact that this never comes to an end is what makes inflation ‘eternal’ once it begins. (E. SIEGEL / BEYOND THE GALAXY)

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. (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 idea states that there are an arbitrarily large number of Universes like our own, but that doesn’t necessarily mean there’s another version of us out there, and it certainly doesn’t mean there’s any chance of running into an alternate version of yourself… or anything from another Universe at all. (LEE DAVY / FLICKR)

So what? That’s not a whole lot, is it? There are plenty of theoretical consequences that are inevitable, but that we cannot know about for certain because we can’t test them. The multiverse is one in a long line of those. It’s not particularly a useful realization, just an interesting prediction that falls out of these theories.

So why do so many theoretical physicists write papers about the multiverse? About parallel Universes and their connection to our own through this multiverse? Why do they claim that the multiverse is connected to the string landscape, the cosmological constant, and even to the fact that our Universe is finely-tuned for life?

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Because even though it’s obviously a bad idea, they don’t have any better ones.

The string landscape might be a fascinating idea that’s full of theoretical potential, but it doesn’t predict anything that we can observe in our Universe. This idea of beauty, motivated by solving ‘unnatural’ problems, is not enough on its own to rise to the level required by science. (UNIVERSITY OF CAMBRIDGE)

In the context of string theory, there are a huge set of parameters that could, in principle, take on almost any value. The theory makes no predictions for them, so we have to put them in by hand: the expectation values of the string vacua. If you’ve heard of incredibly large numbers like the famed 10⁵⁰⁰ which appears in string theory, the possible values of the string vacua are what they’re referring to. We don’t know what they are, or why they have the values that they do. No one knows how to calculate them.

*A representation of the different parallel “worlds” that might exist in other pockets of the multiverse.* (PUBLIC DOMAIN)

So, instead, some people say “it’s the multiverse!” The line of thinking goes like this:

We don’t know why the fundamental constants have the values they do.
We don’t know why the laws of physics are what they are.
String theory is a framework that could give us our laws of physics with our fundamental constants, but it could give us other laws and/or other constants.
Therefore, if we have an enormous multiverse, where lots of different regions have different laws and/or constants, one of them could be ours.

The big problem is that not only is this enormously speculative, but there’s no reason, given the inflation and quantum physics we know, to presume that an inflating spacetime has different laws or constants in different regions.

Not impressed with this line of reasoning? Neither is practically anyone else.

How likely or unlikely was our Universe to produce a world like Earth? And how plausible would those odds be if the fundamental constants or laws governing our Universe were different? A Fortunate Universe, from whose cover this image was taken, is one such book that explores these issues. (GERAINT LEWIS AND LUKE BARNES)

As I’ve explained before, 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. But — much like String Theory — it has some big problems: it doesn’t predict anything we either have observed and can’t explain without it, and it doesn’t predict anything definitive we can go and look for.

Visualization of a quantum field theory calculation showing virtual particles in the quantum vacuum. Even in empty space, this vacuum energy is non-zero. Whether it has the same, constant value in other regions of the multiverse is something we cannot know, but there is no motivation for it to be that way. (DEREK LEINWEBER)

In this physical Universe, it’s important to observe all that we can, and to measure every bit of knowledge we can glean. Only from the full suite of data available can we hope to ever draw valid, scientific conclusions about the nature of our Universe. Some of those conclusions will have implications that we may not be able to measure: the existence of the multiverse arises from that. But when people then contend that they can draw conclusions about fundamental constants, the laws of physics, or the values of string vacua, they’re no longer doing science; they’re speculating. Wishful thinking is no substitute for data, experiments, or observables. Until we have those, be aware that the multiverse is a consequence of the best science we have available today, but it doesn’t make any scientific predictions we can put to the test.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.