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Wednesday, June 19, 2024

‘Solar Twins’ Reveal the Consistency of the Universe

Sometimes we must look to the heavens to understand our own planet. In the 17th century, Johannes Kepler’s insight that planets move in elliptical orbits around the sun led to a deeper understanding of gravity, the force that determines Earth’s tides. In the 19th century, scientists studied the color of sunlight, whose distinctive properties helped reveal the quantum structure of the atoms that make up the star—and all matter around us. In 2017, the detection of gravitational waves showed that much of the gold, platinum, and other heavy elements on our planet are forged in the collisions of neutron stars. 

Michael Murphy studies stars in this tradition. An astrophysicist at Swinburne University of Technology in Australia, Murphy analyzes the color of the light emitted by stars similar to the sun in temperature, size, and elemental content—”solar twins,” as they are called. He wants to know what their properties reveal about the nature of the electromagnetic force, which attracts protons and electrons to form atoms—which then bind into molecules to form almost everything else. 

In particular, he wants to know if this force behaves consistently across the entire universe—or at least, among these stars. In a recent paper in Science, Murphy and his team used starlight to measure what’s known as the fine structure constant, a number that sets the strength of the electromagnetic force. “By comparing the stars to each other, we can learn if their fundamental physics is different,” says Murphy. If it is, that hints that something is wrong with the way we understand cosmology.

Standard physics theory, known as the Standard Model, assumes this constant should be the same everywhere—just as constants like the speed of light in a vacuum or the mass of the electron are. By measuring the fine structure constant in many settings, Murphy is challenging this assumption. If he finds discrepancies, it could help researchers amend the Standard Model. They already know the Standard Model is incomplete, as it does not explain the existence of dark matter.

To understand this constant, think of the electromagnetic force in analogy with the gravitational force, says Murphy. The strength of an object’s gravitational field depends on its mass. But it also depends on a number known as G, the gravitational constant, that remains the same regardless of the object. A similar mathematical law dictates the electromagnetic force between two charged objects. The two attract or repel each other based on their electric charge and their distance from each other. But that force also depends on a number—the fine structure constant—that stays the same regardless of the object. 

All experiments thus far have indicated that in our universe, that constant equals 0.0072973525693, with uncertainty less than one part per billion. But physicists have long considered this number a mystery because it seems totally random. No other part of physics theory explains why it is this value, and thus, why the electromagnetic field is the strength that it is. Despite the word “constant” in its name, physicists also don’t know if the fine structure constant has the same value everywhere in the universe for all time. Physicist Richard Feynman famously described it as “a magic number that comes to us with no understanding.” Murphy puts it this way: “We don't really understand where these numbers come from, even though they're in the back of textbooks.” 

Researchers study the fine structure constant because it offers “a very clean shortcut” to new physics, says astrophysicist Luke Barnes of Western Sydney University, who was not involved with the work. For example, some hypothesized forms of dark matter lead to variations in its value. “The values of the fundamental constants are a mystery, and we also don't know much about dark matter,” says Murphy. “It's quite possible that these phenomena are both connected by one underlying theory that we don't know yet.”

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Murphy’s team studied 17 stars within 160 light years of our solar system. These stars produce observable light of many colors by fusing atoms in their cores. That light travels through a star’s atmosphere as its atoms absorb certain colors, or wavelengths. Using telescope data, Murphy’s team identified the missing wavelengths, corresponding to light absorbed by sodium, calcium, iron, and other elements in each star’s atmosphere. The stars should be missing the exact same wavelengths of light. Any discrepancies could point to a variation in the fine structure constant, which could be an indication of dark matter or some other unknown physics. 

Murphy’s experiment shows that the constant is looking … pretty constant. Previous astronomical measurements, which focused on faraway galaxies, yielded precision in the parts per million. In Murphy’s study, the fine structure constant agreed with that value to about 50 parts per billion. Their result complements laboratory measurements of the constant using atomic clocks that achieve precision in the parts per quintillion (1018), but those are limited to earthly settings.

Given the limits of human-made tools, Murphy can’t say that the fine structure constant is definitively constant. Still, “it limits how big a variation can really be in the fine structure constant,” he says. “If you have ideas that are beyond the Standard Model of particle physics, then they have to obey this bound.” 

Why measure this number so painstakingly? Because the existence of the universe seems to depend on it. The value of the fine structure constant dictates the attraction between a negatively charged electron and its positive atomic nucleus. Take the simplest atom, hydrogen, which is a single electron bound to a single proton. If the constant had a bigger value, the electron and proton would be closer together. If this value were smaller, the electron and proton would be further apart. Change the fine structure constant, and all the atoms that we know of would be different, or may not even form. 

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For example, if the fine structure constant was double its current value, the positively charged protons would be significantly heavier, whereas the mass of neutrons would be less changed, says Barnes. In our universe, a free neutron will decay into a proton, an electron, and an antineutrino in about 15 minutes. Consequently, “we've got loads of protons hanging around,” says Barnes. “That's hydrogen. And when it collapses under its own gravity, it forms stars.” 

But in another universe, where protons are heavier than neutrons, neutrons would not be able to decay into protons. “Suddenly, you've got a universe in which there's [less] hydrogen, and probably no stars, either, with just a relatively minor change,” he says.

Writing with coauthor Geraint Lewis in A Fortunate Universe, Barnes likens the universe to a cake. “You can slightly vary the amount of each of the ingredients and end up with tasty cake,” they write. “But deviate too far and you’ll probably make an inedible mess.” The fine structure constant is an ingredient whose value seems to be in just the right narrow range to deliver a universe capable of sustaining stable matter and life.

Some physicists think the constant’s seemingly arbitrary value implies the existence of multiple universes, each with a different fine structure constant. The reasoning is similar to why Earth has the conditions to sustain life, says Barnes. “How did the Earth manage to be just the right distance from the sun to have liquid water?” he says. “The answer seems to be: There's lots of planets out there.” Our universe may have just the right fine structure constant for stable matter because there’s lots of universes out there. 

Barnes thinks that hypotheses about multiverses are worth exploring, but in the past, physicists have had trouble developing models that are complex enough, or that predict the right values for the fundamental constants of our universe.

The 17 stars in Murphy’s study deliver results that are consistent with previous findings. But these measurements are far from universal, as these stars are relatively nearby and there are so many other kinds. Now, Murphy has set his sights on analyzing more of them. “We want to go much further out now and use the same technique,” he says. And that may be the challenge of trying to pin down a universal constant. To prove that it’s truly universal, you would have to look everywhere.

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