Here’s what you’ll learn when you read this story:
The universe is expanding faster every second, and physicists still can’t fully explain why.
A new study connects the force binding quarks together to the mystery of dark energy.
Upcoming telescopes could soon reveal whether this nuclear-scale idea works at cosmic scale.
What humanity has learned about the cosmos within just the past century is difficult to overstate. A little more than a century ago, American astronomer Edwin Hubble used the 100-inch Hooker Telescope at California’s Mount Wilson Observatory to identify a Cepheid variable star far outside our Milky Way in the Andromeda Galaxy. Six years later, Hubble also discerned that the known universe—already growing in the minds of scientists year by year—was literally expanding.
But that wasn’t the end of this head-scratching revelation, because in 1998 two independent teams of astronomers confirmed that the expansion Hubble had discovered was also accelerating. Exploring these two ideas, scientists developed the Hubble constant, which measures the current rate of expansion, and the cosmological constant, a theoretical prediction of space-time that explains this acceleration. Albert Einstein first introduced the idea of the cosmological constant in 1917, and it’s the cornerstone of Lambda Cold Dark Matter, or ΛCDM, our current best guess at how the universe works. In this model, the cosmological constant represents a mysterious dark energy that permeates all of space and drives the universe’s accelerating expansion, while cold dark matter accounts for the invisible mass that shapes the large-scale structure of galaxies.
The ΛCDM theory has proven successful in describing cosmological observations, but it has some shortcomings, particularly one known as the “cosmological constant problem.” Simply put, quantum field theories predict that vacuum energy densities should exceed what we observe in the universe by roughly 120 orders of magnitude. So yeah, it’s a big problem. These mathematical inconsistencies have led scientists to pursue other ideas. One of them, published last month in the journal Universe, asks if the mechanics behind the strong nuclear force—which holds atoms together and is one of the four fundamental forces in the universe—could play a much larger role than scientists realize.
“Results from the Dark Energy Spectroscopic Instrument (DESI) have hinted at deviations from a pure ΛCDM expansion, favoring scenarios with a mildly dynamical dark energy component,” the authors write in the study. “This ongoing debate highlights the need for novel perspectives grounded in well-established physics.”
This “novel perspective” looks at the expansion rate of the universe and its interaction with the realm of quantum chromodynamics, the overarching theory of how quarks and gluons form protons and neutrons, and specifically an idea known as “quark confinement.” This simply means quarks and gluons are never found in isolation; they’re bound together to form these various hadrons. Using a model of nuclear theory called the “Polyakov–Nambu–Jona-Lasinio model,” the authors effectively linked the quark vacuum to the rate at which the universe expands, and found that the overall effect mimics dark energy.
“Quantum Chromodynamics, the gauge theory of the strong nuclear force, governs the behavior of strongly interacting matter and features a rich vacuum structure shaped by phenomena such as confinement and spontaneous chiral symmetry breaking,” the authors write. “The expansion of the Universe could influence the QCD vacuum structure, potentially inducing effective contributions to dark energy.”
Not content with just theoretical musings, the team then tested this new framework against low-redshift cosmological objects, including quasars, hydrogen-II galaxies (useful for studying the universe’s structure), and Type Ia supernovae—a common “standard candle” for measuring universal distances. Using Bayesian statistical methods, the team found that this new exponent (which they termed “d”) came close to zero, suggesting that the model closely mimicked the observable universe.
Of course, this QCD alternative to long-standing ΛCDM is one among many similar alternative theories, and scientists need more data to find out what’s really going on. Luckily, new observatories such as the European Space Agency’s Euclid telescope and the Vera C. Rubin Observatory are both designed for hyper-accurate readings of universal expansion. The glue that holds atoms together could be responsible for pulling the universe apart—and hopefully, we’ll soon know for sure.
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