No weak force.

I haven’t written about my work much on this blog. I’ve mostly kept to interesting science stories in the news or the community, but since this is a website for science as well as science fiction, I wanted to talk about a new paper that I have written with fellow physicists Evan Grohs and Fred Adams here at the University of Michigan, exploring the idea of a “weakless universe”—a universe without the weak nuclear force. It has been accepted for publication by Physical Review D and is available to the public to read here.

Now, the first question you might be asking is, why would we study parallel universes? To be sure, this is a purely theoretical study of something that, even if it existed, would probably never be observable. However, there are good reasons for this. First, just as you will often learn more about your native tongue by studying a foreign language, thinking about what other universes might be like helps us better understand our own. And second, it addresses a small part of a longstanding philosophical question in physics: does the universe have to look the way it does in order for life to exist?

The “weakless universe” is a universe without the weak nuclear force (also called the “weak force”), one of the four fundamental forces of nature. Usually, we say that the weak force causes radioactive decay, but there’s more to it than that. The weak force drives two very important cosmic processes: the fusion of hydrogen to helium in the sun, and the explosion of supernovae, which distribute heavy elements across the universe. If you remove the weak force, it seems that there would be no stars, and even if there were stars, the universe wouldn’t have the necessary elements to produce life. Or would it?

The idea of a weakless universe was first studied by a team of particle physicists in Harnik, Kribs, & Perez (2006), who suggested that such a universe could support life, producing the necessary elements by different processes. The idea enjoyed a bit of popular attention in a 2009 Scientific American article, but their analysis was incomplete and didn’t dig deep into the mechanics of how stars would operate without a weak force. We decided to follow up on their work to create a more complete picture of such a universe, and we found that it would look different in some key ways, but it could still support life as we know it.

To start off, without the weak force, neutrons, which normally decay into protons on their own, are stable. Many radioactive isotopes—all those that decay by beta decay like carbon-14 and strontium-90, are also stable (but not uranium and plutonium, which decay by alpha decay). Because of this, instead of the Big Bang producing many more protons than neutrons, as in our universe, the weakless universe produces them in roughly equal numbers. By itself, this would result in nearly all the matter in the universe fusing into helium in the Big Bang, which is not good for life.

The solution is to change one other thing: the density of the universe—or more specifically, η (pronounced “eta”), the density of protons and neutrons in the universe. If there are fewer particles around, they can avoid colliding and fusing together in the early universe. Galaxies are big, so there’s still plenty of gas around to form stars, but it’s not all helium.

The next problem is that the weakless universe has a bunch of free protons and free neutrons flying around. A proton and a neutron can fuse together into deuterium, and because they don’t electrically repel each other, they don’t need a hot star to do it. It can happen in the cold of space. But space is also pretty empty. The question is how dense does this cosmic gas of protons and neutrons need to be to fuse into deuterium quickly. That’s something we can calculate, and it turns out to be the density of a forming protostar. Stars will be going through nuclear fusion before they form.

This early fusion doesn’t stop star formation because fusing protons and neutrons into deuterium only produces a little bit of energy, but it does mean that stars will be made almost entirely out of deuterium. In fact, this solves one of our original problems: the hydrogen fusion in the Sun doesn’t work without the weak force, but deuterium fusion does. It uses the strong force.

Deuterium burns much faster than normal hydrogen, and at a lower temperature. Deuterium stars in a weakless universe will switch on before they fully collapse, when they are still big and red. A star with the mass of the Sun in a weakless universe would look a lot like a red giant, and it would last about as long as a red giant—only a few hundred million years, too short for life to develop as it did on Earth.

However, deuterium stars can also burn if they are smaller than stars in our universe, and smaller stars live longer. In our paper, we created a model of a “Weakless Sun”, which is only 5.6% the mass of our Sun (stars in our universe have to be at least 8% the mass of the Sun) and lives for close to 10 billion years. The Weakless Sun would look like a red dwarf, but brighter—about as bright as a K8 or K9 star in our universe, which puts it out of that worrisome M-dwarf territory where habitable planets would be tidally locked.

So we have long-lived stars. What about planets and life? Our other problem is that core-collapse supernovae don’t work, and those are the main source of oxygen and several other important elements in our universe. Luckily, there are two other processes that produce elements heavier than helium that do work: Type Ia supernovae, and red giant winds.

Type Ia supernovae are caused by exploding white dwarfs, which will exist in a weakless universe, are the main source of iron. They undergo nuclear fusion using the strong force, producing lots of iron and nickel, and also silicon, sulfur, and calcium, among others. Meanwhile, red giant stellar winds blow huge amounts of gas out into space, including the elements produced in their cores (known as “dredge-up”). Helium burning in red giants produces mostly carbon, but also some oxygen—not as much as our universe, but some.

Now, we’ve got iron and silicon to form planets; and we also have carbon, hydrogen, and oxygen, the minimum elements needed to form life as we know it. We’re short on nitrogen; there’s some around, but a lot less than in our universe, which complicates matters, but it doesn’t forbid life from occurring. It might just have to evolve differently. The bottom line is that life would be harder to form in a weakless universe, but contrary to our initial guess, it’s still possible, and I think that’s pretty cool. As Fred Adams would say, it’s a lot harder than we think to “break the universe”, and that includes even getting rid of one of the fundamental forces.