Most of the universe may not be as constant as physicists think it is.
Dark energy and dark matter—the two main ingredients of the universe, and two of the great mysteries of science—can turn into each other over time, according to cosmologist Elisa Ferreira at McGill University in Montreal and her colleagues. If further data supports this scenario, this discovery might controversially suggest that, as is largely thought now, dark energy is not an immutable force of nature—a cosmological constant that evenly controls the expansion of the universe—but instead changes over time.
If true, it could upend our understanding of the universe itself. Ever since the Big Bang, the universe has been expanding, a fact that astronomers first discovered nearly a century ago when they found that galaxies were hurtling away from us. Scientists had initially assumed that the attractive force of gravity would slow down the universe’s expansion over time to eventually either stop it or even collapse everything back together in a “Big Crunch.” However, about 25 years ago, researchers unexpectedly discovered that cosmic expansion was not only not slowing down, but that it was speeding up.
Shedding Light on Dark Energy
Scientists call the force driving this mysterious acceleration dark energy and suggest that it could make up roughly 70% of all matter and energy in universe. In comparison, they estimate that matter only makes up about 30% of the universe. However, physicists don’t really have a clue as to what dark energy is—so much remains unknown about dark energy that some researchers wonder if it even exists.
Dark matter, on the other hand, is thought to be an invisible material that makes up roughly five-sixths of all matter in the universe. This means that dark matter composes about 25% of the universe, while ordinary matter only makes up about 5%. Currently, the consensus among scientists is that dark matter is made of unknown particles that lie outside the Standard Model, which is the best description we have to date of how subatomic particles behave.
Ferreira’s new research that suggests a link between dark energy and dark matter relies on data from the Baryon Oscillation Spectroscopic Survey (BOSS), which was designed to map the history of the universe’s expansion. Baryons are particles such as protons and neutrons, the building blocks of atoms, and astronomers use the term as a shorthand way of referring to the ordinary matter that makes up stars, planets, and people. When the universe was very young and small, all of its matter was very hot and densely packed together, and sound waves zipping through it led to ripples of density, causing material to clump together in spots. These initial baryon acoustic oscillations are visible today as clusters of galaxies.
As the largest program in the third Sloan Digital Sky Survey (SDSS-III)—which seeks to create one of the most detailed maps of objects in the sky—BOSS has mapped the positions of roughly 1.5 million galaxies using the Sloan Foundation Telescope at the Apache Point Observatory in New Mexico. By pinpointing the locations of galaxies of different ages, scientists can deduce the rate at which the universe has expanded over time. This in turn helps explain the effects of dark energy over the course of cosmic history.
Beyond a distance of about 6 billion light years, galaxies become fainter and more difficult to see. To map baryon acoustic oscillations beyond this distance, BOSS relied on roughly 160,000 quasars, the brightest objects in the universe, whose light probably comes from supermassive black holes feeding on surrounding matter.
As light from distant quasars passes through hydrogen gas in the interstellar and intergalactic void, pockets of greater density absorb more light. The absorption lines that hydrogen leaves behind in the spectrum of light from these quasars are known as Lyman-alpha lines, which are so numerous that they resemble a forest. Astronomers can use this so-called “Lyman-alpha forest” to determine the locations of these pockets of hydrogen gas and help create a 3D map of the universe.
Ferreira and her colleagues say there are hints from BOSS’s data that undermines the current leading theory of how dark energy and dark matter should have forced the universe to evolve, technically known as Lambda-CDM. One potential explanation is that dark energy possesses a bizarre quality known as negative energy density, where dark energy stores more and more energy as it stretches out. However, Ferreira notes that both normal matter and dark matter, the other major components of the universe, have positive energy density, so it would be strange if dark energy did not.
Instead, the researchers suggest a simpler model to explain these BOSS findings—that dark energy and dark matter are linked. “The possibility of interaction between the two largest components of the universe is allowed and even favorable in some contexts in physics,” Ferreira says. “Interacting dark energy models, in which the dark components interact with each other and their evolution is linked, are possible and should be considered.”
Specifically, this new model suggests that dark energy changes over time, decaying to become dark matter. “There is less dark energy in the past than we have today,” Ferreira says. In contrast, the current leading of model of dark energy and dark matter suggests that dark energy is a cosmological constant, meaning its strength has remained the same over time, just like other constants such as the speed of light.
Einstein’s ‘Biggest Blunder’
Albert Einstein first proposed the existence of a cosmological constant in his equations for general relativity to support the accepted view at the time that the universe was static—while gravity would cause the universe to contract, a cosmological constant would push the universe apart and keep such a collapse from occurring. However, after Hubble discovered that the universe was expanding and not static, physicist George Gamow said that Einstein called the cosmological constant his “biggest blunder.” Still, interest in the concept of the cosmological constant revived when evidence began accumulating that dark energy might be one of the cosmological constants.
Ferreira notes that their interacting dark energy model is consistent with past data from the Wilkinson Microwave Anisotropy Probe (WMAP) launched in 2001 and the Planck satellite launched in 2009. These two spacecraft analyzed the cosmic microwave background, the heat left over from the Big Bang, to provide some of the most accurate measurements yet of a number of key cosmological parameters, such as what the universe is made of. “The consistency of this result with other observations, like Planck, WMAP, and others is very encouraging to me,” she says.
“I would say that the evidence now is overwhelming that dark energy cannot be a cosmological constant,” says theoretical astrophysicist Fulvio Melia at the University of Arizona at Tucson, who did not participate in this study.
However, the notion that dark energy is not a cosmological constant is a controversial one, and many disagree with it. “I think overall that the BOSS data is very nicely consistent with a cosmological constant model,” says cosmologist Daniel Eisenstein at Harvard University, director of SDSS-III. “We do find mild discrepancies, but I don’t see them as being statistically significant enough to argue against a cosmological constant.”
If dark energy is not a cosmological constant, “it’s a very important discovery,” Eisenstein says. “It would mean that whatever is causing the large-scale acceleration of the expansion is actually changing on cosmological measurable time scales. That could signal a breakdown in general relativity or the presence of a very pervasive low-energy component of the universe that is still evolving.”
In addition, if dark energy’s properties do change over time, that means that dark energy has to have particle-like characteristics, Melia says. “Dark energy would presumably be made up of particles like electrons or quarks or the Higgs,” Melia says. Though he does point out that dark energy particles, if they exist, would definitely lie beyond the Standard Model. Such dark energy particles might interact with other particles in ways besides gravity, perhaps through as-yet unknown forces, he adds.
Melia says that while the idea that dark energy is not a cosmological constant is controversial, the converse—that dark energy is a cosmological constant—is “very problematic. Its measured value is some 10120 times different from what it should be in the context of quantum mechanics. That’s 120 orders of magnitude. So it would not disappoint anyone if it turns out that there is no cosmological constant and that dark energy is instead something more understandable in the context of particle physics.”
Ferreira stresses that their conclusions are not definitive—the BOSS data that suggests a departure from the current leading model of dark energy and dark matter falls short of the five-sigma level of confidence that physicists often rely on to confirm a result. Plus, while the paper has been submitted to the journal Physical Review D, it has not yet been accepted. “We need other experiments to confirm this effect, and also, we need a better precision in the measurements so that we can say more precisely that such deviations from Lambda-CDM are real,” Ferreira says. Future models and data could also examine the strength and duration of these interactions, she says.
Ferreira notes there are now many projects investigating dark energy that could support or refute their interacting dark energy model, such as the Dark Energy Survey operating in Chile, the Javalambre Physics of the Accelerating Universe Survey operating in Spain, and the proposed BINGO telescope in Uruguay. “I think that the prospects of this area of research for the future are very good and promising,” Ferreira says.
While Eisenstein says that current astronomical observations suggest that dark energy is in fact a cosmological constant, he nevertheless supports research to rigorously test that idea.
“It’s very reasonable to keep exploring all the possibilities about the dark sector,” Eisenstein says. “We’re all very interested in testing the very simple model of the cosmological constant at higher precision. It’d be great if we could find something to disprove it.”