One major, unsolved problem in astronomy involves the Hubble constant, a number representing how quickly the universe expands. Specifically, the two main ways to measure the constant keep contradicting each other.
To reconcile this Hubble “tension,” an international team of astronomers tried to combine these methods into a single framework in hopes that doing so would finally provide a definitive expansion rate. As it turns out, unifying these methods is feasible and results in what might be the most precise, direct measurement of the Hubble constant thus far. According to the new findings, that equates to an expansion rate of roughly 45.67 miles (73.50 ± 0.81 kilometers) per second per megaparsec.
But perhaps most importantly, the new, unified constant essentially solidifies the Hubble tension—adding to a body of growing evidence that our current cosmological models have many holes, reported the researchers, who published their findings in a recent Astronomy & Astrophysics study.
“Confirming the Hubble tension makes it even more important for us to reexamine the foundations of the current cosmological model and to identify any new phenomena that might modify the evolution of the universe,” Adam Riess, one of the authors of the study and a researcher at the Space Telescope Science Institute (STScI), told Phys.org.
A paradoxical universe
Empirical observations and theoretical calculations both strongly support the idea that our universe is expanding. Astronomers have generally depended on two methods to measure this expansion rate, or the Hubble constant. The first involves the cosmic microwave background, or the near-uniform glow of radiation left over from the Big Bang. In the second approach, researchers consider the various physical characteristics of galaxies and supernovas in the local universe.
The Hubble tension refers to a slight discord between what each approach says is the constant. The former maintains the Hubble constant is about 41 or 42 miles (67 or 68 kilometers) per second per megaparsec, while the latter says it’s 45 miles (73 kilometers).
“Although the numerical difference is modest, it is far larger than can be explained by statistical uncertainty,” according a statement from NOIRLab, which provided technical support for the new study. Researchers have long argued over the exact source of this tension, with some even proposing that the tension isn’t real. However, recent observational advances have provided strong evidence that the discrepancy exists and is an outstanding paradox that astronomers need to address.
A unified framework
For the new study, the team developed a “distance network” for bringing together different techniques for measuring distances in the local universe. The network primarily consolidates information from direct measurements of the local universe, which itself include various datasets of slightly different calculations. The goal was to determine whether any single method contributed to errors. The answer was no.
“This isn’t just a new value of the Hubble constant,” the team pointed out. “It’s a community-built framework that brings decades of independent distance measurements together, transparently and accessibly.”
The results have several implications. First, they give astronomers a relatively “standard” metric for the Hubble constant—meaning at the very least, humans aren’t using a faulty mechanism for deriving the expansion rate of the current universe. But it also confirms that the version of the same constant, calculated from the early universe, was slightly lower. In other words: the Hubble tension still exists.
What now?
In short, the unified constant further solidifies what astronomers suspected for a long time: the standard model of cosmology is far from complete. The team does have some ideas of what could be missing. For instance, the issue could come from our limited understanding of “dark energy, new particles, or modifications to gravity,” NOIRLab explained.
“If the tension is real, as the growing body of evidence suggests, it may point to new physics beyond the standard cosmological model,” the team added in a statement.
Then again, remember that we’re living through a particularly good time for cosmology. The researchers behind the new research appear to believe that as well, as they describe their contributions as an open-source framework for future investigations. We’ll just have to wait and see if next-generation telescopes will resolve the tension once and for all—or force physics to reconsider reality.
