With consistent, impressive leaps in observational technology, cosmology keeps running into all kinds of issues. One infamous problem is called the Hubble tension. Namely, the two main ways scientists measure the Hubble constant, which represents the universe’s expansion rate, don’t agree. Whether that’s a scientific skill issue or evidence of some unidentified physics, scientists also don’t agree.
To be clear, the numerical difference is not that big. The method that uses the cosmic microwave background (CMB), or the leftover radiation from the Big Bang, has the constant at 41 or 42 miles (67 or 68 kilometers) per second per megaparsec (a unit of distance about 3.3 million light-years). The other approach that uses local observations of galaxies and supernova puts it at 45 miles (73 kilometers).
But it’s certainly “far larger than can be explained by statistical uncertainty,” as NOIRLab notes in a recent statement on measuring the constant. So for this Giz Asks, we asked the experts for their takes on the Hubble tension. How has the academic debate over the tension progressed in recent times? What would it take to solve this problem, if that’s even feasible? Most importantly, how important is the tension to cosmology as a whole—and what’s really at stake?
The following responses may have been lightly edited and condensed for clarity.
Caroline Huang
Astrophysicist, Center for Astrophysics | Harvard & Smithsonian.
The Hubble constant is defined as how fast the universe is expanding at the current day. Right now, when we compare the Hubble constant measured through direct, local observations with the result that we derive by combining early-universe observations with our standard cosmological model, we find that these two values don’t agree.
This has definitely grown into a significant problem over the past decade. Despite all of our efforts, it doesn’t yet look like we have a satisfying modification of our cosmological model that brings the early and late observations into alignment. Many current theories require fine-tuning or create problems elsewhere. But as we showed in our recent paper, if we assume that the late universe measurements are wrong and all biased too high, we would also need to have something (or maybe many somethings) that can systematically offset over a dozen standard candle and standard ruler based probes in the same direction, which also looks increasingly daunting. None of the most robust late universe measurements scatter below the value inferred by observations of the early Universe, which you’d otherwise expect if they were both measuring the same number.
Either way, I think the resolution will be a big deal. If the model is missing something, we need new physics to augment it, but if the local measurements are wrong, it means that we have systematically misunderstood our astronomical observations across completely independent methods on a huge scale. Regardless of the outcome, solving this problem will fundamentally change how we understand the universe.
Adam Riess
Astrophysicist, Johns Hopkins University; co-winner of the 2011 Nobel Prize in Physics “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.”
The Hubble tension is one of the most significant unresolved problems in the field. For more than a decade, astronomers have tested whether the discrepancy arises from measurement errors, yet it has persisted through increasingly precise observations and independent methods. Most recently, the distance network has shown that it cannot be attributed to an error in any one tool, method, team, or telescope.
What makes the tension important is that measurements of the present-day expansion rate consistently differ from predictions based on observations of the early universe. Both approaches are precise and well-tested, but they do not agree. This could point to an unlikely confluence of subtle effects or it could be a clue that our standard cosmological model is incomplete.
Either way, the Hubble tension is doing exactly what good scientific puzzles should do: forcing us to question assumptions, improve measurements, and deepen our understanding of the universe.
Raul Jiménez
Cosmologist, University of Barcelona in Spain.
This is a very interesting question, because all current evidence points to a real tension between two ways of inferring the present expansion rate of the universe. On the one hand, local measurements based on the distance ladder find a value of the Hubble constant higher than the one inferred from the standard cosmological model, ΛCDM, when calibrated by the early universe. On the other hand, it is important to remember that almost all such measurements are, in one way or another, measurements of distance. At cosmological scales, distance is not a directly observed quantity: it is inferred within a space-time metric. Thus, what appears as a disagreement about the expansion rate may also contain hidden assumptions about geometry, calibration, astrophysical systematics, or the way we reconstruct distances across cosmic time.
This is why I think independent methods are essential. One approach that I helped pioneer, the cosmic chronometer method, does not rely on distances. Instead, it measures how cosmic time changes with redshift, providing a genuinely distance-free reconstruction of the expansion history of the universe. At present, this method does not yet have the precision to rule out the local measurements, but its results tend to agree more closely with ΛCDM. It is also true that recent large-scale-structure and CMB analyses have moved the ΛCDM-inferred value of [the Hubble constant] upward, to around 69 km/s/Mpc, closer to the local distance-ladder value. So the jury is still out. I would not yet call this a crisis, but rather a serious and fascinating problem in precision cosmology. The devil is in the details, and here the details are the systematic uncertainties. We hope to answer this question soon through our ERC Synergy Grant, RedH0t, which is designed precisely to test whether the Hubble tension is new physics or a subtle problem in the measurements.
Giz Asks is a recurring Gizmodo series in which experts answer big questions in their own words, offering a range of perspectives on the ideas, discoveries, and debates that affect our lives and shape our understanding of the world.
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