Why the Hubble Tension Remains So PuzzlingAndriy Onufriyenko – Getty Images
Here’s what you’ll learn when you read this story:
Scientists rely on two methods to measure the expansion of the universe: extrapolating from Cosmic Microwave Background (CMB) radiation, and making direct astronomical measurements.
The mismatch between the results of these two methods has created what’s known as the Hubble tension, which suggests that there’s a missing piece in the standard model of cosmology.
An international collaboration has created the most accurate astronomical observation of the Hubble constant to date, once again confirming that the tension is more than observational error.
Humanity’s understanding of the universe gradually came into focus over the course of millennia. But things really became clear in the 1920s when American astronomer Edwin Hubble discovered the first galaxy beyond our own, and with it, the fact that our universe was actually expanding (as evidenced by studying the “redshift” of distant galaxies). It wasn’t until the late 1990s that by studying Type Ia Supernovae—which serve as a type of “standard candle,” thanks to their consistent peak luminosity—scientists discovered the universe wasn’t just expanding, but accelerating.
However, nailing down the universe’s exact expansion rate (known as the Hubble constant, or H₀) hasn’t been easy. Today scientists rely on two methods. The first is late-universe observation (or observation of the universe as we see it today), which builds a “cosmic distance ladder” to measure large distances by connecting smaller ones and using consistent celestial objects or events to calibrate those measurements. The second is early-universe observation (or observation of the universe as it looked shortly after the Big Bang), which uses the standard model to deduce expansion rates from the Cosmic Microwave Background (CMB). Unfortunately, these two methods yield different expansion rates: late-universe observations produce a Hubble constant of about 73 kilometers per second per megaparsec (km/s/Mpc), while early-universe observations produce a value of about 67 km/s/Mpc. This difference is known as the Hubble Tension, and it’s a fundamental mismatch between what we expect (from calculations made using the standard model) versus what we see.
Now, an international scientific collaboration has conducted precision astronomy to produce the most accurate measurement of the expansion rate yet. The team, led by the H0 Distance Network (H0DN) Collaboration, published a paper in the journal Astronomy & Astrophysics arguing that the universe is indeed expanding at 73.50 ± 0.81 km/s/Mpc—a measurement completed with a margin of error of less than 1 percent. The team relied on data from a global network of observatories, including the National Science Foundation’s (NSF) Cerro Tololo Inter-American Observatory in Chile and NSF Kitt Peak National Observatory in Arizona. Instead of relying on just supernovae to make their measurements, scientists created a “distance network” using several overlapping methods to ensure accuracy, including Cepheid variable stars (also used by Hubble back in the day), red giant stars, and certain luminous galaxies. By using multiple methods, scientists were able to determine that when they removed one individual technique from the analysis, the results remained almost unchanged and consistent with one another. This further suggests that the Hubble tension is far from a measurement fluke.
“The power of this work is that it doesn’t depend on any single method,” coauthor Adam Riess, from Johns Hopkins University, told NASA. “When multiple, independent measurements all point to the same answer, it strengthens the case that we’re seeing a real feature of the universe, not a flaw in one technique. Right now, those measurements suggest the universe today is expanding faster than we would expect based on how it looked shortly after the Big Bang.”
The H0DN collaboration also made their data publicly available, so future studies can improve upon this precise measurement. This will be especially important as data streams in from new space-based observatories like the Nancy Grace Roman Space Telescope—an infrared observatory launching in 2027 that will investigate not only cosmic distance, but dark energy, dark matter, and exoplanets.
Of course, this new result doesn’t resolve the decades-long Hubble tension. But it does reinforce the idea that the tension is very real, and that there must be something we’re missing in our current understanding of the universe. That’s because early-universe predictions don’t account fully for dark energy, new particles, or modifications to gravity, according to NSF’s NOIRLab (a member of the H0DN collaboration). As a result, extrapolation from the CMB to our modern universe would be impacted by those unknown omissions.
“This work effectively rules out explanations of the Hubble tension that rely on a single overlooked error in local distance measurements,” the authors wrote. “If the tension is real, as the growing body of evidence suggests, it may point to new physics beyond the standard cosmological model.”
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