Why isn’t the universe boring? It could be. The number of subatomic particles in the universe is about 1080, a 1 with 80 zeros after it. Scatter those particles at random, and the universe would just be a monotonous desert of sameness, a thin vacuum without any structure much larger than an atom for billions of light-years in any direction. Instead, we have a universe filled with stars and planets, canyons and waterfalls, pine trees and people. There is an exuberant plenty to nature. But why is any of this stuff here?
Cosmologists have pieced together an answer to this question over the past half-century, using a variety of increasingly complex experiments and observational instruments. But as is nearly always the case in science, that answer is incomplete. Now, with new experiments of breathtaking sensitivity, physicists are hoping to spot a never-before-seen event that could explain one of the great remaining mysteries in that story: why there was any matter around to form complicated things in the first place.
The interestingness of the world around us is all the more puzzling when you look at the universe on the largest scales. You find structured clumpiness for a while. Stars form galaxies, galaxies form galaxy clusters, and those clusters form superclusters and filaments and walls around great cosmic voids nearly empty of matter.
But when you zoom out even further, looking at chunks of the universe more than 300 million light-years wide, all that structure fades away. Past this point, the light from all the stars in the cosmos merges into an indistinct blur, and the universe does indeed look quite boringly similar in all directions, with no features or differences of note anywhere. Cosmologists call this the “end of greatness.”
This tedious cosmic landscape exists because the universe really was boring once. Shortly after the Big Bang, and for hundreds of thousands of years after that, it was relentlessly dull. All that existed was a thick red-hot haze of particles, stretching for trillions upon trillions of kilometers and filling every point in the universe almost evenly, with minuscule differences in the density of matter between one spot and another.
But as the universe expanded and cooled, gravity amplified those tiny differences. Slowly, over the following millions and billions of years, the places in the universe with slightly more stuff attracted even more stuff. And that’s where we came from—the profusion of things in the universe today eventually arose as more and more material accumulated, making those slightly over-dense regions into radically complicated places packed with enough matter to form stars, galaxies, and us. On the very largest scales, boredom still reigns, as it has since the beginning of time. But down here in the dirt, there’s ample variety.
This story still has some holes. For one thing, it is not clear where the matter came from in the first place. Particle physics demands that anything that creates matter must also create an equal amount of antimatter, carefully conserving the balance between the two. Every kind of matter particle has an antimatter twin that behaves like matter in nearly every way. But when a matter particle comes into contact with its antimatter counterpart, they annihilate each other, disappearing and leaving behind nothing but radiation.
That’s exactly what happened right after the Big Bang. Matter and antimatter annihilated, leaving our universe aglow with radiation—and a small amount of leftover matter, which had slightly exceeded the amount of antimatter at the start. This tiny mismatch made the difference between the universe we have today and an eternity of tedium, and we don’t know why it happened. “Somehow there was this little imbalance and it turned into everything—namely, us. I really care about us,” says Lindley Winslow, an experimental particle physicist at MIT. “We have a lot of questions about the universe and how it evolved. But this is a pretty basic kindergarten sort of question of, okay, why are we here?”
Caught in the act
To answer this question, Winslow and other physicists around the world have constructed several experiments to catch nature in the act of violating the balance between matter and antimatter. They hope to see that violation in the form of neutrinoless double-beta decay, a type of radioactive decay. At the moment, that process is theoretical—it may not happen at all. But if it does, it would provide a possible explanation for the imbalance between matter and antimatter in the early universe.
That explanation would rely on neutrinos, the ghostly oddballs of particle physics. These lightweight specters whiz about the universe, barely interacting with anything. Trillions of neutrinos are constantly streaming through every square centimeter of your body and the entire planet Earth, ignoring you just as completely as they ignore the iron core of our planet. Reliably stopping just one neutrino would take a slab of lead a light-year thick.
And neutrinos might perform an even more bizarre trick. The neutrino and its antimatter partner could be one and the same, making it different from every other known form of matter and capable of annihilating itself. “If we observed [neutrinoless double-beta decay], it would prove that the neutrino is its own antiparticle,” says Winslow. “It would also provide us a process that makes more matter than antimatter.”
That process starts in the heart of the atom. When some unstable atomic nuclei decay, they emit an electron along with an antineutrino to counterbalance it: one particle of matter and one of antimatter. This is a very common kind of radioactive decay, known for historical reasons as beta decay. Significantly less common is double-beta decay, when an atomic nucleus emits two electrons at once, along with two antineutrinos to balance them out.
Double-beta decay is “one of the longest processes that we’ve ever measured,” says Winslow. To see a single atom undergo double-beta decay, she continues, we would typically have to wait a billion times longer than the current age of the universe. But if the neutrino is its own antiparticle, there’s the possibility of something even more rare than that: a double-beta decay where the two neutrinos annihilate each other immediately, leaving only the two electrons without any antimatter to counterbalance them. This is neutrinoless double-beta decay.
Spotting such a rare process would be difficult—but not impossible, thanks to the phenomenally huge number of atoms in objects of everyday size. There are nearly a trillion trillion atoms in a few grams of material. “So if you just pile up a bunch of stuff, you just have the possibility of seeing something that happens in timelines even longer than the age of the universe,” says Winslow.
This is the approach taken by the Cryogenic Underground Observatory for Rare Events (CUORE, Italian for “heart”), a detector under a mountain in Italy that is waiting for evidence of neutrinoless double-beta decay. A certain isotope of tellurium is one of the nuclei susceptible to double-beta decay. CUORE watches for it in a set of 988 five-centimeter-wide cubic crystals of tellurium dioxide, each connected to a highly sensitive thermometer. The combined energy of the two electrons emitted in neutrinoless double-beta decay is the same every time, so if the decay occurs anywhere within one of these crystals, that specific amount of energy will be deposited into the crystal as heat, raising its temperature by one ten-thousandth of a degree Celsius.
But a signal that small is hard to see against all the other things that could change the temperature of a crystal. That’s why CUORE is under a mountain—the bulk of the rock above it shields it from nearly all cosmic rays. And that’s also why CUORE needs to be kept phenomenally cold, just a few thousandths of a degree above absolute zero—it “wins the award for coldest cubic meter in the known universe,” says Winslow. The sensors are so exquisitely sensitive that they can even pick up vibrations from waves crashing on the beach, 60 kilometers away.
CUORE isn’t alone. There are other experiments looking for neutrinoless double-beta decay, including KamLAND-Zen, an experiment—also under a mountain—in Japan, using gaseous xenon in place of tellurium crystals. But none of the experiments searching for the decay have seen it yet, despite years of waiting. There are plans to upgrade the sensors at CUORE and increase the number of crystals being used; there are also plans to increase the size and sensitivity of KamLAND-Zen. But the future of these experiments is uncertain.
“In principle, we could make bigger, better experiments,” says Reina Maruyama, a physicist at Yale who is also part of the CUORE collaboration. “You could make 10 of what we have. And so I think it just becomes a matter of how much resources humankind wants to put into this experiment.” Winslow estimates that a full search would require two more rounds of improvements to existing experiments. If those are done and they come up empty-handed, she says, “then we will have pretty much eliminated the possibility of the neutrino being its own antiparticle.”
If that happens, it’s the end of a promising theory, but not the end of the search. Physicists have plenty of other ideas about how matter and antimatter could have become imbalanced. But finding evidence for those ideas is hard. Some could be confirmed if the Large Hadron Collider, the largest particle collider in the world, finds something unexpected over the next few years; other theories depend on sensitive searches for dark matter, an invisible and hypothetical substance, strongly suggested by decades of evidence, which is believed to constitute more than 80% of the matter in the universe.
And some theories exact a high price for explaining the imbalance: they suggest that protons, one of the key components of atomic nuclei, are unstable. These theories say that proton decay takes even longer than neutrinoless double-beta decay, on average about a trillion trillion times longer than the current age of the universe. Super-Kamiokande (aka “Super-K”), in Japan, is the largest experiment watching for proton decay, using an underground vat of 50,220 metric tons of ultra-pure water surrounded by 13,031 light sensors. At the limits of knowledge, Super-K waits for a faint flash in the darkness. It has yet to catch a proton in the act.
But whatever caused the matter-antimatter imbalance in the early universe, there’s one thing that physicists are sure of: eventually, the show will end. Over time, all interesting structures will fade away as the universe’s matter and energy are scattered about increasingly at random. Eons from now, this will lead once more to a fully featureless void—and this time, it will be far less dense and far more uniform than the primordial haze. This state, known as heat death, is likely to be the final fate of the universe, myriad quadrillions of years in the future.
So we’re lucky—we live at a time when the universe is filled with complexity and beauty, even if we don’t fully understand why.
Adam Becker is a freelance journalist based in Berkeley, California. He has written for the New York Times, the BBC, Scientific American, Quanta, New Scientist, and other outlets. He is also the author of What Is Real?, an affable account of the sordid untold history of quantum physics.
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