We live in a strange universe filled with unexplained phenomena that have perplexed humans since time immemorial. Scientists have pieced together a rough guide to the cosmos—known as the Lambda cold dark matter model (ΛCDM), or more simply, the standard model of cosmology—but many mysteries don’t seem to fit into this otherwise well-corroborated framework, especially as our view of space has gotten ever more precise in recent years.
Scientists are now especially preoccupied with intractable tensions that have emerged from different measurements of two cosmic properties: The rate at which our universe is expanding, known as the Hubble constant (Ho), and a value called sigma-8 (σ8), which describes variations in how matter clumps together across large cosmic scales.
Efforts to measure these properties in space have puzzlingly returned different values. When the Hubble constant is measured based on observations of brilliant stars that act as yardsticks in space, its speed is clocked as about 50,400 miles per hour per million light years. However, when it is measured using the cosmic microwave background (CMB), the oldest light in the universe, it is 46,200 miles per hour per million light years. Meanwhile, the value of sigma-8 is different when measured using the CMB, compared to other observational techniques.
What this means, essentially, is that there may be a potentially serious flaw in our basic understanding of the universe and the fabric of reality. In response, scientists around the world are now trying to resolve these tensions.
Indeed, the incongruities have inspired a flurry of research into possible ways to solve the problems, especially the Hubble tension, which attracts much more attention. Cosmologists are constantly imagining possible universes with variable parameters in the hope that, one day, somehow, they might stumble on a model where it all adds up.
These efforts are fueled in part by an innate human need to comprehend our surroundings, but they also represent a broader quest to finetune, or ultimately replace, ΛCDM with an even more complete portrait of the universe.
“The [Hubble] discrepancy, and maybe even the sigma-8 discrepancy, might be our first hint at looking at these cracks in the ΛCDM model,” said Arsalan Adil, a PhD student in theoretical physics at the University of California, Davis, in a call. “But having said that, it’s proven to be so resilient that I’m not sure where the solution will come from.”
Adil and his colleagues recently took a new stab at “alleviating both these tensions simultaneously” using a theoretical quantum field called quintessence, according to the team’s study, which was posted last month on the preprint site arXiv and has not been peer-reviewed. Quintessence is one explanation for dark energy, the term for the unknown force driving the expansion of the universe, and has been invoked in previous research into these tensions.
The researchers cautioned that their findings fall short as an overarching solution, but said the approach “found interesting ramifications of adding a quintessence component to the Universe,” according to the study. Adil also noted that their specific model avoids the common outcome of relieving one tension only to exacerbate the other in what his professor, the UC Davis cosmologist Nemanja Kaloper, calls a game of “cosmological Whac-A-Mole.”
“You fix one problem and another problem pops up,” Adil said. “It actually speaks to the strength of experimentalists. We have such amazing data available today. The CMB is the first light, emitted 13 billion years ago, and we can make this elaborate map of this light and from that, infer these fine details—the fingerprints of our universe. There’s many many proposals out there that fix this [Hubble] discrepancy, but then they make the sigma-8 discrepancy much worse.”
Eleonora Di Valentino, a Royal Society Dorothy Hodgkin Research Fellow at the University of Sheffield who led comprehensive reviews of these tensions—and has also explored quintessence as a solution—noted that the problem arises because “all the parameters are correlated.”
“Once you modify one of them, the others change accordingly,” Di Valentino said in an email. “In most of the models, increasing [Hubble] means increasing also [sigma-8] and vice versa. For this reason it is really difficult to find a model that behaves differently.”
While Adil and his colleagues produced a lower Hubble constant and a higher sigma-8 value, offering a promising glimpse of a solution, he said the new portrait didn’t cohere with other observational data.
“This paper, as many others, attempts to solve the tensions with an alternative scenario,” Di Valentino said of the study by Adil’s team. “Unfortunately at the moment none of the hundreds of models proposed is able to really solve them” with enough accuracy to “convince people.”
She added that gravitational waves, which are ripples in spacetime, may provide another independent measurement of the Hubble constant that could shed light on this long-standing problem. This new data will help scientists assess the possible pitfalls of ΛCDM, and yield insights into other major enigmas, such as the nature of dark energy and dark matter, an unidentified substance that makes up most matter in the universe.
“I consider the ΛCDM model a fitting model of the current cosmological probes, based on completely unknown quantities, like dark matter and dark energy,” Di Valentino said. “I consider the [Hubble] and [sigma-8] tensions a first indication that the ΛCDM model is good in first approximation, but it is not the final conclusive scenario.
To that point, these tensions are just two of the many glitches that scientists have found in the ΛCDM matrix. They represent some of the discrepancies that exist on large scales, a category that also includes head-scratchers like the strange absence of the element lithium in the observable universe, compared to expectations. Scientists have also identified a number of challenges to the model on smaller scales, such as the oddly synced-up orbits of satellite galaxies that orbit larger galaxies, which clashes with predictions of messier and more varied orbits.
These enigmas can be alternately frustrating and invigorating to cosmologists, but as Adil noted, they ultimately stem from the incredible advances in observational techniques that have enabled scientists to measure elusive cosmic properties with unprecedented accuracy.
Cosmologist Tanvi Karwal, postdoc at the Center for Particle Cosmology at the University of Pennsylvania who has puzzled over the Hubble and sigma-8 tensions and developed fascinating potential solutions to them, also emphasized this point.
“If there’s anything slightly off, we can see it now,” said Karwal in a call. “I hope that’s happening with these tensions, because it would be fantastic if we’re starting to get a hint of what dark matter and dark energy really are, and how the universe really functions. Are we actually right with ΛCDM? Are we way off?”
At the same time, she cautioned against framing the tensions solely as challenges to ΛCDM, because this doesn’t capture the full picture of the quest to comprehend the universe. After all, many models of the cosmos have come and gone, and ΛCDM is just the next one up to bat.
“We should always go in with the understanding that ΛCDM was never meant to be the long-term goal,” Karwal said. “This is the logical next step. Nothing is being destroyed. This is where we want to go next.”
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