This essay will discuss multiverse scientists’ strongest support for an infinite multiverse. This support comes from the theory of inflation and, in particular, from a model for this theory known as eternal inflation. We discuss to what extent eternal inflation supports the infinite multiverse premise and doesn’t support the varied multiverse premise (that the laws and constants of nature change from universe to universe). As a bonus, you’ll also learn a lot of fascinating physics along the way!
Highlights of this essay:
Below is an essay version of the ideas presented in Episode 3 of Season 2 of the Physics to God podcast. You can hear the audio version above.
The Theory of Inflation
Eternal inflation is one of the most popular scientific supports for the multiverse. To lay the groundwork for understanding this support, we’ll begin with the basic theory of inflation - a theory first proposed by Alan Guth in 1979 as an important modification to the ordinary way physicists understood the beginning of our universe (known as standard big bang cosmology).
According to standard big bang cosmology, the universe started with all its energy concentrated at one point in space. After that first moment, the universe began expanding. Scientists assumed that the universe has expanded at a relatively constant rate since the first moment of the big bang.
This intuitive premise led to a few problems. We'll explain some of these problems in physics terminology and then give simple analogies to help you understand the main ideas. You should know in advance that you don’t need to follow the details of these problems or their solutions. We just wanted you to have a basic idea of what inflation means and the problems that motivate it.
Let's begin by clearly explaining two of these problems.
One of the difficulties is the horizon problem. Whenever scientists examine two distant places in the universe on large enough scales, they always find them to be around the same temperature. This would make sense if these places, which are currently billions of light years apart, were once in contact with each other; such contact would have allowed them to acquire comparable properties. This would be akin to a cup of hot water eventually becoming the same temperature as the room it’s in. The problem is that according to standard big bang cosmology, there was never a point in time after the big bang when they were in contact.
To help appreciate this problem, consider an analogy involving temperature on Earth. We can readily understand why two very close places on Earth would have approximately the same temperature. However, we would be very surprised if we were to discover two places on opposite sides of the globe that were always the same temperature. Yet, this is comparable to what we observe when looking at the universe in all different directions.
It’s a bit more complicated than this, because you may be wondering why the common origin at the big bang doesn’t suffice to coordinate their temperatures. Going into the details to answer that would take us too far afield but the short answer is that quantum gravity effects in disconnected regions immediately after the big bang should have caused them to have randomly different temperatures.
For our purposes, it’s enough to understand the basic problem cosmologists face: Why is it that, on large enough scales, we observe all places in the universe to be at roughly the same temperature, no matter where we look? What caused the temperature to be so nearly uniform?
Let's now consider the second problem: the flatness problem. The basic problem is to explain why the shape of the universe is so close to being perfectly flat, without any random bumps or curves. It’s almost like it was intentionally made to be perfectly flat, something that physicists wouldn’t have expected without observing it to be so.
It’s difficult to fully comprehend this problem because it relates to the geometrical shape of the 3 dimensions of space. For the sake of simplification, you can picture the problem by thinking about a 2-dimensional space instead. For example, consider a bed sheet. If it were randomly thrown down on a bed, we would expect it to have wrinkles and bumps. If we instead found it to be perfectly flat, we would look for a cause that made it that way. Analogously, physicists were looking for the cause of the universe being so flat.
The theory of inflation proposed by Alan Guth modified standard big bang cosmology to solve both of these problems. The main idea is that the very early universe underwent a short period of very rapid expansion, and only afterward slowed down to the moderate expansion we observe today. This inflation is proposed to have occurred within the first second of the universe’s existence and to have lasted for only a fraction of a second. Nevertheless, the tremendous expansion in that short time had significant implications.
You can think of the universe’s expansion like you’re inflating a big balloon. The theory of inflation suggests that instead of inflating the balloon at a constant rate, you inflate it a tremendous amount in a very short period of time when you first start blowing it up. After that, you continue to inflate it at a relatively slow rate.
This helped answer the two problems. First, inflation answered the horizon problem - why do any two places in our observable universe have the same temperature? This is because all places in the universe we observe, no matter how far apart they are now, were in close enough contact after the big bang but before inflation occurred. The time they were in contact was sufficient for them to attain the same temperature. Even after inflation occurred, this equivalence was roughly retained until today.
To help visualize this, imagine you have an uninflated balloon and you mark off two points separated by a centimeter. Then, you inflate the balloon to be a million times bigger. Those two points would now be much farther apart than before. Similarly, two points in the universe that are now very far apart, were much closer together before inflation occurred. At the earlier time, they would naturally attain the same temperature - just like a hot cup and the room it’s in. Even though the universe’s rapid inflation subsequently caused a rapid increase in the distance between them, it’s perfectly reasonable that they would retain the same temperatures.
The theory of inflation also explains the apparent flatness of space. This is because no matter how bumpy and jagged the initial shape of space was, extremely rapid inflation would smooth it out so that space would look almost perfectly flat today.
Returning to the balloon analogy, compare two balloons - one slightly inflated and one inflated to a massive size. On the slightly inflated balloon, a small area on its surface will appear curved. But on the massively inflated balloon, a small area on its surface will appear like a flat bed sheet - this is like the fact that we don’t see the curvature of the earth when we walk down a city street. According to the theory of inflation, the universe is like a massively inflated balloon. It looks flat because everywhere we can see is only a small portion of the massively inflated universe.
Besides the theory of inflation’s solution to these problems, it has also made quantitative predictions that were confirmed by observation. That is, it predicted the distribution of energy that we observe in the cosmic microwave background radiation with impressive accuracy.
The background radiation is a faint non-visible light that scientists can observe now but originated at a very early stage of the universe. This light is one of the main data points scientists use to compare their theories of the early universe against real data.
The theory of inflation predicted the observed patterns in the background radiation by applying quantum mechanics to the early universe. Before the rapid expansion, the universe was very small, with all its energy close together, and therefore approximately the same temperature and homogenous. However, due to quantum mechanical fluctuations in the very small early universe, the theory predicted that the energy density wasn’t exactly the same everywhere. And it turned out that the patterns expected from those fluctuations were precisely the patterns scientists observed in the background radiation. The main point is that the theory of inflation made a quantitative prediction that was confirmed by observation.
We want to mention that this evidence is not a smoking gun in favor of inflation because there could be other ways to explain these patterns (and we’ll even discuss some of those alternatives in later essays). Nevertheless, it’s considered significant support and shows that inflation is a real scientific theory capable of making testable predictions.
Models of Inflation
You might be wondering what our one universe’s rapid inflation has anything to do with the infinitely many universes of the multiverse. We’re about to get there.
If you noticed, the theory of inflation as we’ve presented it so far is just a description of something that happened to the early universe - that it expanded a lot very quickly. However, that description doesn’t tell us what caused inflation to occur in the first place. In other words, what was the mechanism that caused the early universe to expand so quickly?
Scientists don’t exactly know the answer to this question but they have a lot of different ways of modeling possible mechanisms that caused inflation. And it’s only when we consider these models that the multiverse becomes relevant.
So there are two different things here. First is the theory of inflation which is a description of something that happened in the early universe. Second, are the various ways to model the mechanism that caused this inflation. It's important to realize that the theory of inflation has supporting evidence from observations, but the are various models of inflation that don’t.
To illustrate this crucial difference between a theory and the various models for a mechanism that explains the theory, consider the following analogy. We take a cup of hot water and place it in a cold room. After a sufficient amount of time, we’ll find that the water is at the same temperature as the room. Generalizing this observation, we can develop a theory: whenever two objects of different temperatures come into contact with each other for a sufficient amount of time, they end up at the same temperature - this is called thermal equilibrium.
Despite this basic description of the theory, we still haven’t provided the mechanism that brings the water and the room to the same temperature. We can propose several possible models, each describing a mechanism for producing this result: (1) the heat in the cup flows out of the cup into the air of the room, causing the cup to lose its heat; (2) the reverse - the coldness of the room flows out of the air into the cup, causing the cup to cool down; (3) some combination of heat and cold exchange between the cup and the room cause them to attain the same temperature; (4) temperature is related to motion, and the particles in the cup and the room bump into each other many times until they arrive at the same amount of average motion per particle (which happens to be the actual explanation of modern physics). These represent four of the many possible models consistent with the observation that the cup and the room eventually attain the same temperature.
Now, let’s assume that we perform many experiments to test the validity of the theory that whenever two objects of different temperatures come into contact for a sufficient amount of time, they end up at the same temperature. For instance, we can place many hot objects next to many cold objects to determine if they always end up at the same temperature. If they repeatedly do, then we have empirical evidence that our theory is correct.
All these experiments support the theory that whenever two objects of different temperatures come into contact for a sufficient amount of time, they end up at the same temperature. The question becomes: which of the four models does all of this evidence support?
The answer is: None. Any of the models can equally explain all of these experiments. The crucial point is that proof for a theory is not proof for any particular model.
To find support for a particular model, we would need to construct some sort of critical experiment that would differentiate between the different models. Only then can we determine if there’s evidence for any particular model of the theory as opposed to all the other models. Just as there are many models for the mechanism that brings about thermal equilibrium, there are many models for the mechanism that brings about inflation. In fact, there are around 100 competing models for inflation.
And just like in my analogy with the hot cup in the room, the evidence in favor of the theory of inflation doesn’t constitute evidence for any particular model of inflation.
Keeping in mind this distinction between the theory of inflation and its many models, we’re finally ready to see how the multiverse becomes relevant.
Eternal Inflation Multiverse
The main idea is that some models of inflation lead to one universe and others lead to an infinite multiverse.
Many models of inflation – in fact, most of them - lead to the surprising conclusion that if inflation happens once, it will keep happening in different regions of space over and over again. As those separate regions experience inflation, this naturally leads to an infinite number of separate bubble universes, most with no possibility of ever coming into contact with the others.
These models of inflation are collectively called eternal inflation. Roger Penrose in Fashion, Faith, and Fantasy (page 326) explains:
There are various extensions of the original inflationary ideas, the most influential going under names like eternal inflation...The general idea of these proposals seems to be that inflation can be set off at various places throughout space-time…Such regions are often referred to as bubbles and it is supposed that our own perceived universe region is such a bubble. In some versions of this kind of proposal, it is envisaged that there is actually no beginning to this activity, and it is normally taken that there is to be no end.
If the eternal inflation models are correct, then scientists have strong support for the Infinite Multiverse Premise. However, other non-eternal models of inflation only involve finite inflation and only have one universe that inflates shortly after the big bang. If these models are correct, then inflation has nothing at all to do with a multiverse.
The point is that both types of models, eternal and non-eternal inflation, are entirely consistent with the theory of inflation described earlier. All the problems that the theory of inflation solved and all the observations that support the theory of inflation apply equally to all models of inflation - those that generate an infinite number of universes and those that only yield our one universe. As such, it would be inaccurate to claim that the theory of inflation itself establishes the Infinite Multiverse Premise.
Brian Greene, in The Hidden Reality (pg. 196), expands on this point as follows:
Astronomical observations over the past decade have bolstered the physics community’s confidence in inflationary cosmology but have nothing to say about whether the inflationary expansion is eternal. Theoretical studies show that although many versions are eternal, yielding bubble universe upon bubble universe, some entail but a single ballooning spatial expanse.
Nevertheless, supporters of eternal inflation argue that the simplest, most natural, and least fine tuned way to model inflation inevitably leads to eternal inflation and an infinite number of bubble universes. Therefore, even though all the evidence for inflation equally supports both finite and infinite models, they argue that we should choose an infinite model over a highly contrived finite model. Greene (ibid, pg. 67) explains as follows:
There are versions of the inflationary theory in which inflation is not eternal. By fiddling with details such as the number of inflaton fields and their potential energy curves, clever theorists can arrange things so that the inflaton would, in due course, be knocked off its perch everywhere. But these proposals are the exception rather than the rule. Garden-variety inflationary models yield a gargantuan number of bubble universes carved into an eternally expanding spatial expanse. And so, if the inflationary theory is on the mark, and if, as many theoretical investigations conclude, its physically relevant realization is eternal, the existence of an Inflationary Multiverse would be an inevitable consequence.
There is certainly merit to this argument. Assuming one accepts the theory of inflation, it’s reasonable to extrapolate from the laws of physics in our universe by modeling inflation in a manner that is mathematically simple and minimizes extra assumptions.
This is proper methodological thinking and is a legitimate application of Occam's razor. If there are many different ways to model something, the simplest model is the best and often the correct model to choose.
Since the simplest models of inflation lead to an infinite number of bubble universes, eternal inflation provides multiverse scientists with their best support for the Infinite Multiverse Premise.
Problems With Eternal Inflation Multiverse
Even though the theory of inflation is a scientific theory supported by observations, and even though the simplest way to model inflation leads to eternal inflation, positing an actual infinite number of intrinsically unobservable universes to explain our one universe is a dubious proposition.
As we discussed last essay, eternal inflation takes a stand on the controversial issue regarding the existence of an actual physical infinity. More importantly, an infinite number of actual universes creates major problems for multiverse scientists’ attempts to establish the third premise - that our universe is typical - and leads directly to something called the measure problem. But don’t worry about this for now. We’ll discuss these issues much more in later essays.
While most cosmologists accept the theory of inflation and the infinite number of universes that come along with eternal inflation, there are a few notable scientists, among them Roger Penrose and Paul Steinhardt, who maintain that there are better alternatives to explain the observations that the theory of inflation claims as supporting evidence. We’ll discuss their theories in more depth in a later essay.
The point is that even the basic theory of inflation hasn’t been conclusively proven. At this point, it’s merely mainstream cosmologists' best explanation for our observations. Physicist Paul Steinhardt, one of the originators of the theory of inflation, (page 238) in his book, Endless Universe (2007) elaborates on this point. He says as follows:
This raises a provocative question: Is the inflationary model, as it currently stands, a valid scientific theory? Or is the discovery of infinite possibilities a sign of a serious flaw, both logically and scientifically? A poll of cosmologists today would probably reveal that an overwhelming majority believe inflation to be not only a valid idea but the most likely explanation for the universe we observe. But if the poll went further to ask whether the infinite number of possible universes is a problem, it would probably reveal a split into very different camps.
It’s important to note that no scientists posit an extremely large, but finite, number of universes - a functional infinity - to rescue inflation from the problems of actual infinities that we’ll discuss in detail later. This is because one can’t legitimately posit other universes besides our own without demonstrating that the known laws of physics of our universe naturally lead to the generation of these other universes.
But, as Brain Greene pointed out, “garden-variety inflationary models” yield an actually infinite number of universes, and there’s no way to make the number of universes finite without artificially rigging it in a totally ad-hoc manner. Since no known law of physics naturally predicts a large but finite number of parallel universes, and because non-eternal inflation models with only one universe are highly contrived, the only reasonable scientific choices are eternal inflation with its infinite universes or a different theory besides inflation.
Inflationary Multiverse
Notwithstanding the prior points, let us grant multiverse scientists that the theory of inflation has been well established and that the best way to model inflation results in an infinite number of universes.
Would this alone provide sufficient support for multiverse theory as a complete explanation for our designed, fine tuned, and ordered universe without the need for an intelligent cause?
The short answer is “no” – at best, inflation supports the Infinite Multiverse Premise and points to the existence of many fine tuned universes, but does nothing to support the existence of a varied multiverse.
Recall, for a multiverse to explain fine tuning and design, multiverse scientists must show that the laws and constants of nature vary from universe to universe. Not only does eternal inflation fail in this regard, it seems to undermine the Varied Multiverse Premise, and instead indicates that each of the bubble universes is designed and fine tuned just like ours.
The reason is as follows: Since eternal inflation relies on the known laws of physics to derive the existence of other universes, it naturally follows that the unobservable universes suggested by eternal inflation would have the same designed laws and fine tuned constants as our universe.
Brian Greene explains this point (The Hidden Reality - page 72) as follows:
All the same, we can still contemplate an imaginary voyage to one or more of the other bubble universes. On such a journey, what would you find? Well, because each bubble universe results from the same process - the inflation is knocked from its perch, yielding a region that drops out of the inflationary expansion - they are all governed by the same physical theory and so are all subject to the same set of physical laws.
Despite the general point that the laws of physics and almost all the constants should naturally be the same in the different bubble universes, Greene argues that there is one exception to this point: the Higgs field that allows particles to have a mass. He writes, “in many versions of inflationary cosmology, a Higgs field would naturally have different values in different bubble universes.” This wouldn’t technically be a change in the fundamental laws of nature because the Higgs value is different from the other constants of nature - it gets its particular value from spontaneous symmetry breaking. Therefore, it’s not logically necessary that the Higgs value remains the same in all universes.
Don't worry if you don't follow that precisely. The point is that even assuming eternal inflation’s infinitely many universes, the laws and all the other fine tuned constants that we mentioned in essay 3 of series one should naturally be the same in all the universes. Therefore, to explain fine tuning without an intelligent cause, multiverse scientists must still provide support for their premise that the laws and constants vary from universe to universe.
Before moving on, we’d like to point out that even though eternal inflation cannot address the design of the laws and the fine tuning of the constants unless it also shows that the laws and constants vary from universe to universe, eternal inflation does make progress toward explaining the ordered initial conditions of our universe.
This is based on the fact that even in our universe itself, the specific arrangement of spacetime and matter isn’t fixed but constantly changes. This being the case, there’s no reason to think that all of the bubble universes in eternal inflation’s infinite multiverse started in the exact same initial state. Rather, it’s reasonable to assume that they each started with different initial conditions. Since some of these would be as highly ordered as our universe, the infinite universes of eternal inflation would seem to avoid the indication of an intelligent orderer regarding the initial conditions.
That being said, in later essays, we’ll show how problems involving infinities and the Typical Universe Premise lead to the eternal inflationary multiverse’s failure regarding our universe’s highly ordered initial conditions as well.
In conclusion, while we have our reservations about eternal inflation, for the purpose of our argument we’ll grant that the theory of inflation is correct as opposed to one of the alternative theories. We’ll also grant that if the theory of inflation is true, then insofar as eternal inflation is the simplest model for inflation, it’s also the best model. Given those two points, multiverse scientists can use eternal inflation to meet the burden of proof for the Infinite Multiverse Premise.
Nevertheless, eternal inflation only points to infinitely many designed and fine tuned universes, all of which imply an intelligent designer and fine tuner. To provide evidence that the laws and constants vary from universe to universe and thereby explain design and fine tuning without an intelligent cause, multiverse scientists need to turn to other arguments. We’ll discuss these in the next essay.
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