Ask Ethan: Does the multiverse explain our fundamental constants?

Here in our Universe, there are three major properties that have led to it unfolding as it has:

• the laws of physics that govern all of nature,
• the initial conditions that our Universe began with,
• and the values of the fundamental constants that apply to the particles, fields, and forces in our Universe.

Over time, this has led to our modern cosmos: full of atoms, stars, planets, galaxies, galaxy clusters, and a grand cosmic web. On some of those planets, life has arisen, with at least one instance of intelligent, technologically advanced life arising on a planet known very well to us: Earth.

But what if things were just a little different? Perhaps, even with the same laws of nature and very similar initial conditions, a version of our own Universe that possessed different fundamental constants could have turned out vastly differently than our own. So why does our Universe have fundamental constants with the values that they do? That’s what Pierre Louw wants to know, following up on an earlier Starts With A Bang article to ask:

“Near the end of your Big Think article of 26 February ’25, you wrote: ‘But there’s a key thing to keep in mind: even though we know all of this and can weave a consistent story out of it, we still don’t know why nature has the values that it does.’
Could this be explained by bubble universes, created by inflation, each with different values?”

There are many possible explanations for why things are the way they are, and yes, that option is one worth considering. But is it a compelling explanation? Let’s find out.

This chart of particles and interactions details how the particles of the Standard Model interact according to the three fundamental forces that quantum field theory describes. When gravity is added into the mix, we obtain the observable Universe that we see, with the laws, parameters, and constants that we know of governing it. However, many of the parameters that nature obeys cannot be predicted by theory, they must be measured to be known, and those are “constants” that our Universe requires, to the best of our knowledge.

Here in our Universe, the laws of physics are known very well. The Standard Model of elementary particles and fields describes the electromagnetic and nuclear forces, as well as the quarks, leptons, and bosons that we know of. General Relativity is our theory of gravity, explaining everything from how the Universe expands to how orbits work to the properties of black holes and gravitational waves. We also know the particles and other forms of energy found within our Universe:

• the quarks and leptons, which include the electron, the neutrinos, and the constituents of protons and neutrons,
• the bosons which mediate the fundamental forces, including the photon, the W-and-Z bosons, the Higgs boson, and the gluons,
• whatever dark matter happens to be,
• plus whatever form of energy is behind the accelerated expansion of the Universe, often assumed to be a cosmological constant but always called dark energy.

You might think that if you knew all of this, plus the initial conditions of the Universe — that it was hot, dense, almost perfectly uniform, rapidly expanding, filled with matter and antimatter and radiation, and was seeded with density imperfections from inflation — that you could predict pretty much everything that would arise down the line in cosmic history. And that is precisely what we can do: that’s one of the major achievements of theoretical astrophysics, and of cosmology in particular, in the late 20th and early 21st centuries.

The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structures in the Universe today. This is the best picture we have of how the entire Universe behaves, where inflation precedes and sets up the Big Bang. Unfortunately, we can only access the information contained inside our cosmic horizon, which is all part of the same fraction of one region where inflation ended some 13.8 billion years ago.

But there’s also one more ingredient we need. You see, the Universe isn’t simply governed by physical laws and the ingredients within it; there are also important parameters that enter into our governing equations. These include:

• The strong, weak, electromagnetic, and gravitational coupling constants, which tell us the strength of the various fundamental interactions. If you’ve heard of the number 1/137 or the concept of the fine-structure constant, these refer to the strength of the electromagnetic interaction.
• The rest masses of every one of the fundamental, massive particles, from neutrinos to quarks to leptons and more, which can also be parametrized by a coupling to the Higgs. Some of them, like the electron’s rest mass, would lead to a very different Universe from what we recognize as our own if they were even a little bit different.
• The mixing parameters between the quarks (given by the CKM matrix) and the neutrinos (given by the PMNS matrix), which determine how particles with identical quantum numbers mix together, required to explain phenomena such as weak decays and neutrino oscillations.
• And there’s also one more parameter, at least, for the cosmological constant, which is our simplest approximation for what dark energy could be.

All told, that equates to 26 fundamental constants, and that still leaves puzzles like dark matter, baryogenesis, and any parameters related to inflation unaccounted for. Importantly, these fundamental constants are needed to reproduce the Universe we have; if they were different, our Universe would be correspondingly different as well.

The rest masses of the fundamental particles in the Universe determine when and under what conditions they can be created, and also relate to how long they can survive after their creation during the hot Big Bang. The more massive a particle is, the less time it can spontaneously be created in the early Universe, and the shorter its lifetime will be. Although we can explain particle masses through a coupling to the Higgs, we have no way of successfully predicting their values; they must be experimentally measured in order to be determined.

To the best of our knowledge, these constants really are constant and unchanging across space and throughout time; they do not seem to vary or change. But how do we know the values of these constants?

Perhaps disappointingly, we don’t “know” what these values are in any sort of predictive sense. We don’t have a way to calculate what they ought to be, and there isn’t any relationship or equation between them that holds up to any sort of scrutiny. There have been proposed relationships aplenty, but none of them have worked out to actually yield the values of any of these fundamental constants.

All we can do, if we want to determine what they are, is to put the question to nature in an experimental fashion: to go out and measure these parameters directly. That’s how we know their values: we’ve performed the critical measurements to determine what they are: not in any theoretical fashion, but purely empirically. When I talked about the Universe in an earlier piece, and I said, “even though we know all of this and can weave a consistent story out of it, we still don’t know why nature has the values that it does,” that’s precisely what I was referring to: the fact that we cannot predict these values, and can only measure them to determine what they are.

Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. Only around stars that have formed with a large enough fraction of heavy elements from the lives and deaths of previous generations of stars can rocky worlds, the only home for life that we know of, come into existence. Without those key ingredients, intelligent observers would not be possible in our Universe.

But what if we weren’t satisfied with that answer, as our question-asker this week is? What would we do; what options are there, and how would we go about determining whether there was any sort of underlying reason for these constants having the values that they do?

In other words, how did they get to have the values that we measure them to have?

In physics, we encounter this situation all the time: we observe something, and we learn how measure it very accurately and precisely, and we determine as much as we can about it. And once we do, then we go ahead and ask the next follow-up question, “how did it come to have the properties that it does?”

That’s where theorists thrive: in concocting plausible mechanisms that can explain and account for the phenomena that we see. In general, we call these classes of problems fine-tuning problems: as we can imagine that any such parameter, constant, or value could have been all over the map, and that significantly different possibilities would have led to vastly different outcomes. A Universe with:

would have never enabled the formation of stars and galaxies at all, much less rocky planets, complex molecules, and the possibility of life.

Balanced Rock, shown here in photos from before 1975 (black and white) and after 1976 (color), showcases how weathering and erosion can lead to the collapse of structures over time. At right, the structure once known as “Chip off the Old Block” fell during the winter of 1975-1976, and someday, Balanced Rock will fall as well. However, it was created by natural processes, even if it doesn’t appear so; understanding how is the key to solving what appears, on the surface, to be a fine-tuning problem.

The fact that something appears to be finely tuned for a specific outcome, at least on the surface, doesn’t necessarily indicate any fine-tuning at all, however. Take the above example of what looks like a naturally occurring example of a giant boulder in unstable equilibrium: where a small nudge could knock it over. If you found something like this, you might think:

• this was supremely unlikely to occur naturally,
• that it must’ve been placed there and carefully balanced deliberately,
• or that there must be some mechanism, even if it’s not immediately obvious what it was, to create this configuration.

Of those three thoughts, only the third one is considered a scientific approach to the problem. The idea that this couldn’t have occurred naturally is an easy thought to have, but it assumes that there isn’t a physical mechanism that would have created this configuration through natural processes. The second one is also possible; certainly humans (and, to a lesser extent, other creatures) do modify their environment in deliberate, and often profound, ways.

But this was not put there by a human or any intelligent creator; it did happen naturally: through the process of uneven erosion and weathering acting on sedimentary rock with different hardnesses to its various layers. The scientific approach demands that there be an underlying mechanism that explains how things came to be the way they are.

When a symmetry is restored (yellow ball at the top), everything is symmetric, and there is no preferred state. When the symmetry is broken at lower energies (blue ball, bottom), the same freedom, of all directions being the same, is no longer present. In the case of the electroweak (or Higgs) symmetry, when it breaks, there’s a spontaneous process that occurs, giving mass to the particles in the Universe.

So what are some possible “mechanisms” for determining the fundamental constants that give our Universe the properties we observe it to possess?

We can imagine it the way we imagine spontaneous symmetry breaking, illustrated above. That at some point, the Universe was in a more symmetric state than it is now, and such values were not only not determined, but possibly even the laws of physics were different back then. That’s what would correspond to a “restored” symmetry state, or where the ball is at the “top” of the potential shown above.

But clearly, it won’t stay that way forever; that’s an example of an unstable equilibrium point. What determines if something is stable or unstable is to simply imagine giving it a tiny little nudge: a push in any direction that moves it away from its current state. In any direction, a nudge would cause that ball to cascade down the slope of the side it gets nudged towards, where it will then roll down into the valley below.

That leads to a “broken symmetry” state: where it went down in one particular direction, and eventually settles in a valley that corresponds to a lower-energy state. In this state, the laws of physics may now be different, the fundamental constants may have taken on specific values that may not have even made sense before, and where a return to the “restored symmetry” state would take an enormous input of energy; it will not happen spontaneously.

This diagram displays the structure of the standard model (in a way that displays the key relationships and patterns more completely, and less misleadingly, than in the more familiar image based on a 4×4 square of particles). In particular, this diagram depicts all of the particles in the Standard Model (including their letter names, masses, spins, handedness, charges, and interactions with the gauge bosons: i.e., with the strong and electroweak forces). It also depicts the role of the Higgs boson, and the structure of electroweak symmetry breaking, indicating how the Higgs vacuum expectation value breaks electroweak symmetry and how the properties of the remaining particles change as a consequence. Neutrino masses remain unexplained.

There are many examples of this in physics: grand unified theories, string theory, other theories of everything, etc. We can always hypothesize additional symmetries in an earlier, higher-energy state, and then concoct mechanisms by which those symmetries break. If we came up with a good physical mechanism, it would not only explain why the fundamental constants took on the values that they did, but it would also lead to concrete predictions with directly observable consequences; that’s normally what we demand of a good physical theory.

What was asked, however, is about a different idea: a non-mechanism solution. Instead, the scenario that was proposed was that:

• cosmic inflation occurred,
• that inflation comes to an end in one particular region,
• giving rise to our Universe and our hot Big Bang,
• and that in that region we now inhabit, the fundamental constants took on the values they have here,
• whereas the other locations where inflation ends may produce wildly different values of the fundamental constants from our own.

This is not a well-defined mechanism with concrete predictions, unfortunately. It is only an idea based loosely on the idea of the string landscape: asserting that the fundamental constants are determined by the vacuum expectation values of the string vacua, that those values were undetermined prior to and during cosmic inflation, and that the act of inflation ending then determines those string vacua (and, by association, the values of the fundamental constants) randomly for each baby/bubble universe that gets created by inflation’s end.

The string landscape might be a fascinating idea that’s full of theoretical potential, but it cannot explain why the value of such a finely-tuned parameter like the cosmological constant, the initial expansion rate, or the total energy density have the values that they do. Each “pocket” of the string landscape has its own unique vacuum expectation value, which corresponds to a value for a fundamental constant in the Universe.

It’s by no means clear that this is the case, however. It is an assumption, and by many accounts, a rather poor one. String theory is expected to become relevant at energies close to the Planck scale: at energies of ~10¹⁹ GeV or so, perhaps as low as one or two orders of magnitude below that value. But there’s an upper limit to the maximum energies achieved in the hot Big Bang that’s more than a factor of 1000 below the Planck energy, suggesting that no, the dynamics of inflation shouldn’t be sufficient to select out various string vacua.

However, because we lack a quantum theory of gravity, and because we have only a rudimentary understanding of the dynamics that occurred at the end of inflation, any discussion of what “mechanism” could be at play is fundamentally speculative at the present time. Therefore, many physicists make a simplifying assumption — one that is not necessarily valid — that all possible values of all the various string vacua (and therefore, of the fundamental constants that arise) are equally likely, and therefore, the inflationary multiverse, with its various baby/bubble universes inside of it (only one of which becomes ours), take on all of those possible values.

And therefore, when it comes to the question of “why do the constants in our Universe have the values that they do,” the answer is simply, “because we, observers who can measure the Universe, could only have arisen in a universe that took on values that allowed the emergence of intelligent observers to be physically possible.”

This image, showing Einstein lecturing a group of students at Lincoln University in 1946, was a strong act of anti-racism at the time. A world in which humans of different races can come together is only possible in a Universe whose fundamental constants have values that admit the existence of intelligent observers as a possibility. However, that fact, although true, does not function as a compelling or satisfactory explanation for why the constants take on the values that they are observed to possess.

How does that explanation sit with you?

Some people are perfectly fine with it. It’s an example of anthropic reasoning: the recognition that if things were different enough that intelligent observers (such as us) were not a physically admissible possibility, there would be no one in such a Universe to ever observe it. While that is certainly true, it isn’t a very satisfying, nor a very scientific, approach to the problem from the perspective of most physicists.

If we want to understand the nature of reality — to truly understand how things came to be the way they are — that’s what’s truly required: understanding the mechanism by which things obtained the properties they have today. If we want to understand why we have protons and neutrons instead of a quark-gluon plasma, we need to understand the QCD phase transition; if we want to understand why we have the electromagnetic and the weak nuclear force, we need to understand electroweak symmetry breaking. Similarly, if we want to understand properties of our Universe that we don’t presently understand, like how our Universe came to be matter-dominated or how the fundamental constants got to have the values they presently possess, we shouldn’t be “satisfied” until we understand the underlying mechanism.

So in summary, yes, the values of the fundamental constants can be explained by bubble universes created by inflation, but that explanation is hardly satisfactory from a scientific point of view. It may turn out to be a correct component of the story, but until we understand how it happened, our physical understanding will remain dissatisfying and incomplete, even in the best-case scenario.

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