Where did we come from? Where are we going? Why do we exist?
These are questions that we tend to associate with philosophers and theologians, but they’re also critically important to certain fields of physics. Theorists speculate on what sort of weird parallel universes we could or could not have existed in. Cosmologists study the shape of space, hoping to glean a peek into the future and determine whether the Universe will end with a big crunch or a big freeze—or not at all. And particle physicists? “We haven’t seen any physical processes in what we’ve looked at so far that could explain dark matter or that could explain matter antimatter asymmetry,” says Dr. David Weir of the University of Helsinki. In other words, we don’t know what makes up 95% of the Universe—and we don’t know where it, or any other matter, came from at all.
It’s generally accepted that for every particle of matter, there exists its complete structural opposite: antimatter. In the lab, matter can be created from pure energy, but it always comes into existence with a corresponding antiparticle—equal in mass but opposite in charge and other properties. When a particle and its corresponding antiparticle meet, they annihilate in a flash of light (gamma rays, not the visible kind). The very fact that the Universe is populated with stuff—gas, dust, stars, people—at all indicates that there is more matter than antimatter; otherwise the entire Universe would have self-annihilated at its very beginning. So why this imbalance? Why do we exist?
The currently accepted model of physics, known as the Standard Model, doesn’t explain the matter/antimatter asymmetry—obviously, or it wouldn’t be an open question. That’s why researchers like Weir turn to more exotic models, adding in extra particles that haven’t been observed yet. (This isn’t as crazy as it sounds, since many particles were predicted theoretically before being confirmed experimentally.)
Weir works with a number of models, developing testable predictions and studying their implications, but his most recent work concerns the Two Higgs Doublet Model (2HDM). This model is identical to the Standard Model but it includes four additional particles, all heavier than the current Higgs boson. Their additional mass means that they would have an extraordinarily high energy, impossible to produce even state-of-the-art particle accelerators like the Large Hadron Collider. That doesn’t mean it can’t be confirmed (or discredited), though: “Because these additional Higgs bosons easily decay into other particles and stuff, it is in principle quite testable,” Weir assures us. In fact, a number of researchers are actively working on just such tests.
But Weir has a slightly different focus: the beginning of the Universe. The 2HDM has been shown to be compatible with observations so far, so he wanted to take it a step further and see if it could—theoretically speaking—account for the cosmic mysteries outlined above.
As it turns out, if we make certain assumptions about the 12 parameters the 2HDM embodies (namely, that the extra particles are very heavy), the math is very similar to models developed decades earlier by researchers at the same university. This allowed Weir to take advantage of simulations originally developed for the other model. “It’s like recycling—it’s like, you have these results but you’re making use of them again,” he says with a grin.
|This is a conceptual sketch of the dimensional reduction technique employed by Weir and his colleagues. Effectively, by making certain assumptions about the masses of particles, they were able to eliminate them mathematically, creating a much simpler model to simulate.
Image Credit: David Weir
Of course, he is quick to admit that the study is limited by these assumptions and he wants to develop more complex simulations that examine the full parameter space of the 2HDM. But that introduces new complications, since lighter particles can’t be canceled out (mathematically speaking). This in turn requires more complex simulations that take these extra parameters into account. Weir is hopeful that this can be done—after all, he points out, computers have come a long way since the ‘90s, when the other simulation was developed. But before sinking the time and energy into that project, he wanted to run a faster check on pen-and-paper calculations and see if the area was worth exploring at all.
His team’s findings paint a new picture of the Universe’s early moments—one that actually allows for the matter/antimatter asymmetry in a way that the Standard Model doesn’t. His work agrees with mainstream physics in requiring a period of exponential growth known as inflation immediately following the Big Bang, but it’s a little different once we get to 10 picoseconds (that’s 10-11 seconds). At that point, Weir says, the Higgs boson “turned on”, giving fermions mass and solidifying matter as we know it. This allowed the weak nuclear force to kick into effect, differentiating itself from the electromagnetic force (transmitted by massless photons). It’s complicated, but the point is that there was a fundamental change in the state of the Universe around this time.
The thing is, Weir doesn’t think it was homogeneous. “You know when you have one of those hand warmers, where the hard bits start to form in the hand warmer when you squeeze it?” he asks. “That’s like a new phase, a crystalline phase of the heating stuff… matter is a new phase of the Universe, a phase when the Higgs boson is turned on.” He envisions the Universe nucleating in much the same way, from many different sites at once. This would create countless “bubbles” of matter surrounded by the primordial plasma.
Despite the visual that accompanies the word, these bubbles weren’t delicate. Instead, they were explosive, expanding outward at a very large fraction of the speed of light and creating a burning front at their peripheries caused by friction between the new matter and the surrounding plasma. As the bubbles grew and grew, the tumult could have unmixed matter and antimatter within the plasma, and then the bubbles would expand to encompass areas with a larger amount of matter, trapping it inside.
This simulation shows a 2-dimensional slice of 3D space in what could have been the early moments after the Big Bang. The bubbles of the new “Higgs phase” expand outward, heating a shell of plasma around them. They collide and continue to expand even after the bubbles have merged, creating gravitational waves that potentially could still be measured today.
As exotic as it sounds, Weir is hopeful that these predictions of the 2HDM model can actually be tested experimentally. “You’re burning up energy which is liberated by these bubbles expanding. And that creates quite a large amount of kinetic energy around the bubbles,” he says. As the bubbles grew into each other and collided, they would have release powerful gravitational waves that potentially could still be measured today. While they can’t be picked up by ground-based interferometers, Weir is looking forward to the launch of the Laser Interferometer Space Antenna (LISA) in 2034. As it happens, LISA is most sensitive at the frequency he expects to see from the bubble collisions, so there’s a good chance that this theory will either be backed up or discredited. This is important: to many scientists, an untestable hypothesis isn’t worth debating, and a lot of the more exotic theories that seek to explain the existence of our universe don’t offer many testable predictions. (We’re looking at you, string theory.)
In the meantime, there’s plenty of work to be done with the 2HDM and beyond. Weir is anxious to expand the simulation to give a full picture of the theory across its parameter space. It is also quite possible that the model could give us a better understanding of dark matter, but that’s another avenue to be explored.
Personally, Weir plans to continue studying other alternative models with a special eye towards any gravitational wave fingerprint they might create. He says that he’s not married to the 2HDM model in particular, although he thinks it does show some promise. Instead, he wants to make the best possible predictions for LISA so that as soon as the first data comes back we’ll know how to interpret it and which model(s) it supports.
It’s a long path for sure, but he has a few decades before the launch. And then—who knows? Maybe he will uncover why we exist… at least from a material point of view.